On the disappearance of the gem-dimethyl effect: the base-catalysed cyclization of ethyl hydantoates

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1 On the disppernce of the gem-dimethyl effect: the bse-ctlysed cycliztion of ethyl hydntotes Ergun Aty, Iv B. Blgoev, Anthony J. Kirby b nd Ivn G. Pojrlieff* Institute of Orgnic Chemistry, Bulgrin Acdemy of Sciences, ul. Acd. G. Bonchev block 9, Sofi 1113, Bulgri b University Chemicl Lbortory, Cmbridge, UK CB2 1EW Received (in Cmbridge) 17th April 1998, Revised 31st July 1998, Accepted 3rd August 1998 Buffer ctlysis nd solvent kinetic isotope effects in the cycliztion of methyl-substituted ethyl hydntotes with ω- N-methyl (MUE) nd ω-n-phenyl groups (PUE) hve been studied in n ttempt to elucidte the chnges in mechnism nd eventul reversl of reltive rectivities cused by incresing the number of substituent methyl groups (the gem-dimethyl effect). (Rte constnts for the hydroxide-ctlysed cycliztion re ctully reduced on going to the fully methylted compounds.) For ctlysis by hydroxide inverse isotope effects re generlly consistent with specific bse-ctlysed cycliztion, but the cycliztions of the most hevily substituted compounds show norml isotope effects, indicting chnge of mechnism. It is suggested tht proton trnsfer to the ethoxide leving group is slowed by steric hindrnce, nd becomes rte determining for the most substituted compounds 3. Eclipsing strin in the tetrhedrl intermedite my lso be involved. The Brönsted plots for generl bse ctlysis show distinctly different slopes of 0.2 for MUE nd 0.6 for PUE, respectively. These re ssigned to hydrogen bond pressocition nd concerted ctlysis mechnisms, of the ttck of the ureido group nitrogen on the ester group. Rte constnts for the rection of the substrte nion would fll in the region of s 1 for esters 3 if the gem-dimethyl effect were fully expressed. The crboxyltion of biotin by hydrogen crbonte involves the rection of ure nitrogen with crboxylte group, nd the nucleophilic rectivity of ures is of prticulr interest in this respect. 1 The introduction of methyl groups in positions 2 nd 3 of hydntoic cids ccelertes the cycliztion to hydntoins up to fold ( mnifesttion of the gem-dimethyl effect): n extreme cse is the irreversible bse-ctlysed cycliztion of the tetrmethylhydntote nion (Scheme 1). 2 Scheme 1 Thus vrying the number of methyl groups in the side-chin of hydntoic cid derivtive nd the polrity of the substituent t the nucleophilic nitrogen provides model systems for the systemtic study of vrious spects of the rectivity of ures. Centrl to the nlysis is the prtitioning of the tetrhedrl intermedite, T. In rections not involving cycliztion vrying the cyl substituent generlly hs little effect on the prtitioning rtio, k 2 /k 1 (Scheme 2), becuse both leving groups re subject to similr effects. And the effects of structure on leving group cpbilities re redily predictble, except tht steric effects involving the leving group seem to hve little effect on the prtitioning rtio in the lkline hydrolysis of secondry mides. 3 The gem-dimethyl effect superimposes new element in cycliztion rections, fvouring ring formtion but slowing steps involved in ring opening. For exmple, the lkline Supplementry mteril is vilble for this pper (SUPPL. NO. 722, 8 pp.) For detils of the Supplementry Publictions Scheme see Instructions for Authors, J. Chem. Soc., Perkin Trns. 2 vilble vi the RSC web pge ( hydrolysis of 1,-dimethyl-3-phenylhydntoin (Scheme 2, R = ) t low p hs k 2 rte-determining, while 3-phenylhydntoin (R = ) is hydrolysed with rte-determining k 1. Typiclly in such rections flexibility is lost progressively between the open chin nd the finl ring: the consequences for the prtitioning of tetrhedrl intermedites hve been discussed recently in some detil. 6 But we hd no convincing explntion for the pprent reversl of the gem-dimethyl effect observed in the O -ctlysed ring closure of ω-phenylhydntoic esters. 7 This prompted this more detiled study of the cycliztion of the two series of ethyl ω-methyl nd ω- phenylhydntotes, 1 3-MUE nd PUE. 1-MUE 2-MUE 3-MUE 1-PUE 2-PUE 3-PUE Scheme 2 New buffer ctlysis nd kinetic solvent isotope effect dt llow more definitive explntion of the chnge in the prtitioning rtio, to rte determining brekdown of the tetrhedrl intermedite in the most highly substituted compounds. Experimentl Mterils Inorgnic regents nd buffer components were of nlyticl R 1 R 2 R 3 R Ph Ph Ph J. Chem. Soc., Perkin Trns. 2, 1998,

2 Tble 1 Rte constnts for, O nd 2 O ctlysed ring closure of ethyl hydntotes Compound k /dm 3 mol 1 s 1 k w /s 1 k O /dm 3 mol 1 s 1 1-MUE 2-MUE c 3-MUE 1-PUE 2-PUE 3-PUE ( ) (2.6 ± 0.26) ± 0.07 (1.03 ± 0.1) 10 (3.2 ± 0.13) 10 3 (1.28 ± 0.02) 10 1 (7.1 ± 1.) 10 6 (. ± 0.19) 10 (1.3 ± 0.1) 10 d (1.68 ± 0.1) 10 d.1 ± 0.9 b 7±63 10 ± 3 (8.3 ± 0.) 10 e (9.1 ± 0.3) 10 (2.17 ± 0.0) 10 Observed rte in 1 M Cl. b From severl experiments in dilute crbonte buffers; ref. 1 reports 2 dm 3 mol 1 s 1. c Dt from ref. 9. d Vlues less precise becuse of indistinct plteux in the rte profile. e dm 3 mol 1 s 1 ccording to ref. 1. grde nd used without further purifiction. Potssium hydroxide nd buffer solutions were prepred with CO 2 -free distilled wter. D 2 O nd DCl ( wt% in D 2 O), 99 tom% D were from Aldrich. The preprtion of ethyl hydntotes 1-MUE nd (1 3)- PUE ws described previously. 8 All stock solutions of the substituted ethyl -phenylhydntotes were prepred in dry TF. 3-MUE ws prepred in situ s described for 2-MUE. 9 The ester cyclized redily in the presence of trces of moisture in the TF solution, so stock solutions were prepred before ech series of kinetic runs by injecting methyl isocynte (10. µl, 0.16 mmol) through septum into 1 ml dry TF contining ethyl 2-mino-2-methylpropionte (0.1 mmol). Smples were withdrwn in the sme wy. Stock solutions were stored frozen between kinetic runs nd were kept for no more thn 6 hours. The hydntoins used in the product nlysis were prepred by known procedures nd their mp greed with literture vlues: 1,3-dimethylhydntoin, 10 1,3,,-tetrmethylhydntoin, 2 1-methyl-3-phenylhydntoin, 11 1,-dimethyl-3-phenylhydntoin, 3-phenyl-1,,-trimethylhydntoin. 12 Product nlysis The ethyl hydntotes cyclized quntittively to the corresponding hydntoins. The end-points for the kinetic runs were identicl within experimentl error with the bsorbnces of solutions of the uthentic hydntoins. For 2-PUE nd 3-PUE the rection products resulting from bse-ctlysed ring closure were isolted s follows. A solution of 2-PUE or 3-PUE (0.0 mmol in 0. ml TF) ws dded to 28 ml phosphte buffer, p 6., nd the rection mixture kept for 3 min t room temperture. The solution ws then evported to dryness nd the residue extrcted with 3 10 ml C 2 Cl 2. The combined orgnic lyers were dried over MgSO nd evported to give hydntoin (78 80%). Products were identicl (mp, NMR) with smples of the respective hydntoins prepred by literture methods. Kinetic mesurements Rte constnts were determined t 2.0 ± 0.01 C under pseudo-first-order conditions in the thermosttted cell comprtment of Unicm SP-800 spectrophotometer. The rtes for cycliztion of the substituted esters were followed by monitoring the chnges of the bsorbnces t wvelengths in the 27 nm rnge. In the cse of substrtes with ω-phenylureido groups the decrese of the bsorbnce due to phenylhydntoic ester ws monitored, while for ω-methylureido compounds only the increse of bsorbnce due to the hydntoin ring formtion could be followed. Rections were initited by injecting µl of M stock solutions of (1 3)-PUE nd 2 µl of M stock solutions of (1 3)-MUE into 2.7 ml preheted buffer solution. Ionic strength ws mintined constnt (1 M) with KCl. surements of p-vlues nd clcultions of first-order rte constnts (k obs ) were s previously described. Good first-order liner plots (r > 0.999) were obtined over three hlf-lives nd k obs -vlues were reproducible to within ±3%. Fig. 1 Rte profiles for the cycliztion of ethyl -methylhydntotes 2- MUE () nd 3-MUE (); lines drwn re clculted from eqn. (1) using prmeters from Tble 1. Kinetic solvent isotope effects The determintion of KSIE for the hydroxide-ctlysed ring closure of the substrtes ws complicted by the occurrence of buffer ctlysis. The procedure dopted for ll six ethyl hydntotes consisted of mesuring k obs -vlues t three buffer concentrtions in the rnge , t p vlues where the rection is clerly first order in O : in phosphte buffer ( nd % free bse) with 1 3-PUE nd in crbonte (10 nd % free bse) with 1 3-MUE. k O -vlues were obtined by extrpolting to zero buffer concentrtion. For rections where k obs chnged with buffer concentrtion by less thn 6% the men vlue ws tken, nd divided by the ctivity of O or DO to obtin k O or k OD. pd-vlues were obtined by dding 0. to the p-meter redings nd the OD -vlues clculted using pk w = Results Rte constnts for ctlysis by [ ], [O ] nd wter were obtined by fitting the rte profiles to eqn. (1). The pseudo-firstk obs = k k O O k w (1) order rte constnts, k obs, re vlues extrpolted to zero buffer concentrtion. The ctivities of nd O were obtined from the p-vlues mesured in the most dilute buffer in the series of given buffer rtio. For runs in Cl the concentrtions of the cid t ionic strength of 1 M (KCl) were converted to ctivities using n ctivity coefficient of For the slower rections of 1-PUE nd 1-MUE only limited numbers of dt points were obtined, for evlution of the gemdimethyl effect. The rte constnts re summrized in Tble 1 nd the fits to experimentl dt shown on Figs. 1 nd 2. Second-order rte constnts for ctlysis by buffer components were obtined by fitting the dt to eqn. (2). The k obs = k A [A] k B [B] k k O O k w (2) preferred procedure ws to tret k, k O nd k w s known constnts using the vlues from Tble 1, omitting s pproprite 2290 J. Chem. Soc., Perkin Trns. 2, 1998,

3 Tble 2 Buffer ctlysis dt for the cycliztion of ethyl 2,2,3,-tetrmethylhydntote (3-MUE) t 2 C nd ionic strength 1.0 M Buffer cid pk A,b Conc. rnge/mol dm 3 Runs % Bse k A /10 dm 3 mol 1 s 1 b k B /10 dm 3 mol 1 s 1 b 3 O 1.7 (1.26) 1.78 (2.26) 000 ± 0 (66) 3 ± 1 (11). ± 0.19 c 3 PO ±39 d N C 2 CO ±11.1 ± CO ± ± C 3 CO ± ±1 80 (C 3 )AsO ± 3.3 6± PO ±7 (176) 81±10 (07) CO ± O 1.7 (1.0 ± 0.0) 10 7 pk A -vlues tken from ref.. b Sttisticlly corrected vlues in prentheses. c First-order rte constnt. d Not included in the Brönsted correltion. Fig. 2 Rte profiles for the cycliztion of ethyl -phenylhydntotes 1- PUE (), 2-PUE (), nd 3-PUE (). Lines drwn re clculted from eqn. (1) using prmeters from Tble 1. (Reproduced from ref. 7 with permission of the Royl Society of Chemistry.) terms tht do not contribute significntly. These results re summrized in Tbles 2, nd full detils re given s supplementry dt. As observed previously 9 with 2-MUE, the rtes of cycliztion of 3-MUE in TRIS nd crbonte incresed with buffer concentrtion (up to 0.10 dm 3 mol 1 ) by only c. %. Nevertheless good fit for the crbonte dt ws obtined with the eqution contining the terms for crbonte nd O ctlysis only. On the other hnd, n extended set of 19 dt points in TRIS buffers (0.0 to 0. dm 3 mol 1 ) gve no stble solution for k B nd k A. Similr complictions with buffers contining free mines hve been observed previously in the cycliztion of hydntoic cids nd esters nd were ttributed to inhibition by the free mine. 2,9 Tht generl bse ctlysis is possible t these p-vlues is demonstrted by experiments using sodium ccodylte in 0.0 M TRIS s crrier buffer t p 8.. k obs - vlues were more thn tripled in the presence of 0.32 M ccodylte. The k B found (.9 ± dm 3 mol 1 s 1 ) is in good greement with the k B -vlue found in ccodylte buffers (Tble 2). Though only similrly wek buffer ctlysis ws observed with 1-PUE nd 2-PUE, the cycliztion of 3-PUE showed strong buffer ctlysis. Formlly this might be becuse buffer ctlysis for the former compounds is superimposed on reltively greter bckground rte. In our preliminry communiction 7 we considered buffer ctlysis not to be significnt for the rections of 1 nd 2-PUE. owever, further mesurements using more buffers nd detiled regression nlysis ccording to eqn. (2) gve wht we consider to be meningful vlues of k B (k A were too smll to be estimted ccurtely). Thus ctlysis of the ring closure of 2-PUE by K 2 PO in glycine crrier buffer of p c. 3 is ttributed to the nion cting s generl bse. In the cse of 3-PUE both cid nd bse ctlysis re clerly expressed (the former in the more cidic buffers, up to nd including cetic cid). In 2 PO /PO buffers plots of the second-order constnts for buffer ctlysis, k buffer, ginst frction bse for 1-PUE nd 3-PUE were curved, the slope rising shrply t higher frctions of free bse (2-PUE ws studied only t lower % free bse) (Fig. 3). The observed rte constnts were well-fitted (Fig. 3b) by including the cross-term, eqn. (3): k obs = k B [B] k BO [B][O] k O [O] k w (3) The constnt k BO is best explined in terms of ctlysis by PO 3 : its contribution becomes significnt t higher p nd bse concentrtions becuse of the higher Brönsted β with the PUE series (v.i.). Since the rection PO O PO 3 2 O; gives eqn. () then the rte coefficient for ctlysis by the phosphte trinion is given by eqn. (). J. Chem. Soc., Perkin Trns. 2, 1998,

4 Tble 3 Buffer ctlysis dt for the cycliztion of ethyl 3-methyl--phenylhydntote (1-PUE) t 2 C nd ionic strength 1.0 M Buffer cid pk A,b Conc. rnge/mol dm 3 c % Bse k A /10 dm 3 mol 1 s 1 b k B /10 dm 3 mol 1 s 1 b 3 O C 3 CO 2 2 PO PO 2 O 1.7 (1.26) d (11.8) 1.7 (16.0) ± 0.1 (0.33) 1.02 ± ± 18 (16.2) ( ± 17) 10 (80) 10 (8.3) ± 0.) 10 8 pk A -vlues tken from ref.. b Sttisticlly corrected vlues in prentheses. c Four runs were crried out within ech concentrtion rnge. d Clculted from k BO = (1.1 ± 0.08) 10. Fig. Brönsted plots for GBC of the cycliztion of ethyl - methylhydntotes 2-MUE (), dt from ref. 9, 3-MUE (). The squre is the devint point for ctlysis in TRIS with 3-MUE. The lines drwn exclude the points for wter nd O. Fig. 3 Plot of k buffer ginst frction bse for 1-PUE in 2 PO / PO buffers. 3b. Dt for 1-PUE in 2 PO /PO buffers; experimentl points (), clculted by mens of eqn. (3) nd prmeters of Tble () presented s function of frction bse only. K PO [PO 3 ] = K w [PO ][O ] k PO 3 = k BO K w K PO () () Fig. Brönsted plot for GBC of the cycliztion of ethyl - phenylhydntotes 3-PUE () nd 1-PUE ( ). The line drwn for 3- PUE excludes the point for O. The line for 1-PUE is drwn through the two points for cette nd PO ctlysis. The ctlytic constnt k PO 3 obtined for 3-PUE fits the Brönsted plot (Fig. ) well. Brönsted plots for the cycliztion of the ureido esters re shown s Figs. 6. Kinetic solvent isotope effects k O /k OD (KSIE, Tble 6) were determined from runs in lkline buffers s described in the Experimentl section. Both generl cid nd generl bse ctlysis were observed with 3-MUE nd 3-PUE, s shown previously for 2-MUE. The rte constnts k B obtined for the wekest bse, 2 PO (lso for NCC 2 CO 2 for 3-PUE), showed lrge positive devitions from the line defined by the remining bses, suggesting tht the mjor contribution comes from the kineticlly equivlent generl cid ctlysis by the phosphte mononion. With 2-PUE the second point in the plot of Fig. 6, for 2 PO, lso shows positive though smll devition. Constructing the Brönsted plots in the cse of 2- nd 3-MUE presents two possibilities, s discussed previously 9 in the cse of 2-MUE. The inverse isotope effect is consistent with specific ctlysis for the O rection of 2-MUE, nd so comptible with positive devition from the Brönsted plot for generl bse ctlysis: the devition (bsed on the low β) is some log 2292 J. Chem. Soc., Perkin Trns. 2, 1998,

5 for hydntoin itself ccording to which O leves t lest 100 times more redily thn ureide nion. With N-ryl derivtives, however, the rte ws only first order in O. The cidity of the phenylureido group is some three orders of mgnitude greter thn tht of the methylureido group, 8 nd thus similr to tht of ethnol. This nd other evidence led to the conclusion tht hydrolysis proceeds by rte-determining CO bond formtion (equivlent to rte determining loss of ethoxide in ester cycliztion). The gem-dimethyl effect fcilittes cycliztion but hinders ring opening, effectively mking ureide poorer leving group. A result is the switch to rte-determining CN clevge observed in the lkline hydrolysis of the hydntoin derived from 2-PUE. With this bckground we expect the specific bse-ctlysed cycliztions of compounds 1 nd 2 to involve rte-determining formtion of the tetrhedrl intermedite. Scheme 3 shows the Fig. 6 Brönsted plot for GBC of the cycliztion of ethyl - phenylhydntote 2-PUE. The line drwn excludes the point for O - ctlysis. units. The O rection of 3-MUE shows norml isotope effect (k /k D = 2.2) nd the devition is reduced to 2 log units: s discussed below we believe this rection differs from tht ctlysed by the less bsic nions. The sitution with 2- nd 3- PUE is similr; lthough becuse of the lrger Brönsted β the positive devitions of the O -points re smller. While the points for wter ctlysis devite strongly in the cse of 2- nd 3-MUE, they fll on the Brönsted line in the cse of 2- nd 3- PUE. (These k w -vlues re less ccurte becuse of the lck of cler plteux for the wter rection in the rte profiles. owever, omission of the wter points for 2- nd 3-PUE does not chnge the slopes significntly.) Liner regression nlysis yields the following β-vlues. Discussion Compound β r 2-MUE MUE PUE PUE The min object of the present study ws to rtionlize the effects of steric strin on the mechnism of the bse-ctlysed cycliztion of hydntoic esters which led to the chnge of mechnism nd pprent reversl of the gem-dimethyl effect with 3-PUE. As shown by the p-rte profiles of Figs. 1 nd 2, nd quntittively by the dt for reltive rtes of the vrious rections in Tble 7, the ccelertion in k O due to the gemdimethyl effect is much reduced lso for the ω-n-methyl esters when compred to the ccelertions observed in the cid-ctlysed rection, nd the sme fctors re likely to be operting. Specific bse ctlysis The mechnism of the bse-ctlysed ring closure of hydntoic cid esters ws discussed by Sterb nd co-workers in their comprehensive kinetic study. 1 These uthors ssumed the formtion of the C N bond to be rte determining since the less bsic ethoxy group should leve more redily from the tetrhedrl intermedite thn ureide nitrogen. They do not report generl bse ctlysis, in contrst to the cycliztion of thioureidoesters. Rte-determining formtion of the tetrhedrl intermedite in the cycliztion of typicl ω-n-methyl esters is strongly supported by studies of the relted reverse rection, the lkline hydrolysis of hydntoins: rte-determining clevge of the CN bond t lower p is relibly indicted by chnge in rte determining step nd by the ppernce of terms second order in O. 1 The kinetics yielded n estimted prtitioning rtio Scheme 3 mechnism for specific bse ctlysis, where k 2 is rte determining. The rte constnts k 1 for deprotontion of N by O (recently reported s hydrogen exchnge rtes 8 ) re some two orders of mgnitude greter thn k O for cycliztion for ll the compounds discussed in this work (Tble 1). Thus the formtion of U from U is preliminry equilibrium, consistent with the inverse isotope effect observed for the rections of compounds 1 nd 2. Tble 8 shows vlues for k U, the rte constnt for the cycliztion of U (Scheme 3) clculted from equilibrium concentrtions of U (bsed on the pk-vlues recently determined for these esters 8 ). The rte rtios for compounds 1 nd 2 re similr to those shown below (Tble 7) for the overll hydroxidectlysed rections, consistent with the rpid pre-equilibrium formtion of U. owever, if the gem-dimethyl effect were fully expressed for compounds 3 (i.e. the rtes incresed c. -fold compred with compounds 2 s they do in cid), k U would be in the region of s 1. In fct the observed rte constnts for the cycliztion of the fully methylted nions 3U re smller thn for 2, nd the rection shows norml solvent kinetic isotope effect of round 2. Evidently different step, involving proton trnsfer, hs become rte determining. Of totl of four proton trnsfers which might be involved the deprotontion of the ureido-group (k 1 ) is too fst to be rte determining, s discussed bove; nd we rule out proton trnsfer to nd from the O of T on the bsis of the second-order term observed in the lkline hydrolysis of hydntoins. There remins slow proton trnsfer to deprting ethoxide (k 3 of Scheme 3); i.e. generl cid ctlysis by solvent wter of the brekdown of T. A mechnism of this sort ws suggested in Grvitz nd Jencks detiled study 16 of the GBC brekdown vi T of stble tetrhedrl intermedites contining n lkoxy nd mido group (though in their exmple the lkoxy group ws intrmoleculr nd the mine exocyclic). These uthors doubted its J. Chem. Soc., Perkin Trns. 2, 1998,

6 Tble Buffer ctlysis dt for the cycliztion of ethyl 2,3-dimethyl--phenylhydntote (2-PUE) t 2 C nd ionic strength 1.0 M Buffer cid pk A,b Conc. rnge/mol dm 3 c % Bse k B /10 dm 3 mol 1 s 1 b 3 O 3 PO 3 N C 2 CO 2 CO 2 C 3 CO 2 2 PO 2 O 1.7 (1.26) 1.78 (2.26) ± 0.01 d 0.1 ± e ± ± ± ±19 (11) (9.1 ± 0.3) 10 9 pk A -vlues tken from ref.. b Sttisticlly corrected vlues in prentheses. c Four runs were crried out within ech concentrtion rnge. d Firstorder rte constnt. e In crrier buffer 0.1 M cid glycine % bse. Tble Buffer ctlysis dt for the cycliztion of ethyl 2,2,3-trimethyl--phenylhydntote (3-PUE) t 2 C nd ionic strength 1.0 M Buffer cid pk A,b Conc. rnge/mol dm 3c % Bse k A /10 dm 3 mol 1 s 1 b k B /10 dm 3 mol 1 s 1 b 3 O 3 PO NCC 2 CO 2 3 N C 2 CO 2 C 3 OC 2 CO 2 CO 2 C 3 CO 2 (C 3 )AsO 2 2 PO PO 2 O 1.7 (1.26) 1.78 (2.26) e (11.8) 1.7 (16.0) (1.27 ± 0.02) 10 3 (23) 19.0 ± 0. (6.33).78 ± ± ± ± ± 0.1 Uncertin ± 0.01 d. ± ± ± ± ± ± ± 6± (103) (.3 ± 0.38) 10 6 ( ) (2.17 ± 0.0) 10 9 pk A -vlues tken from ref.. b Sttisticlly corrected vlues in prentheses. c Four runs crried out within ech concentrtion rnge. d First-order rte constnt. e Clculted from k BO = (2.27 ± 0.18) 10. pplicbility for O cting s the generl bse becuse wter is little more cidic thn ethnol, but the sme mechnism hs been proposed recently by Nunez et l. 17 for the reverse rection of similr compounds, the intrmoleculr ddition of lcohol to phthlimide: the point for ctlysis by O fell on the Brönsted plot. This work, prticulrly the vlue of β nuc, suggests lte trnsition stte for the brekdown step, where proton trnsfer from wter cn become fvourble. The best evidence in our system is the mgnitude of the solvent kinetic isotope effect, which is in the region of two for the rections of 3-MUE nd 3- PUE, but inverse for the less highly methylted systems (Tble 6). We cn estimte the SKIE for lkline hydrolysis in the cse of compounds 3 s follows, using the frctiontion fctors Scheme shown in Scheme. 18 (Ground stte frctiontion fctors re from Quinn nd Sutton: is the usul vlue for the primry 229 J. Chem. Soc., Perkin Trns. 2, 1998,

7 Tble 6 (KCl) Kinetic solvent isotope effects for the hydroxide ion ctlysed cycliztion of substituted ethyl hydntotes t 2 C nd ionic strength 1.0 M Compound p rnge ( 2 O) k O /k OD Compound p rnge ( 2 O) k O /k OD 1-MUE 2-MUE 3-MUE ± ± ± PUE 2-PUE 3-PUE ± ± ± 0.12 Tble 7 Rtios of cycliztion rte coefficients for ethyl hydntotes crrying different numbers of methyl groups Rte rtio k k w k AcO k O k U 2/1-MUE 3/1-MUE 3/2-MUE /1-PUE 3/1-PUE 3/2-PUE Rte constnts for cycliztion of the ureide nion re given in Tble 8. Tble 8 Rtes of cycliztion of ureide nions U Compound pk N k U /s 1 b 1-MUE 2-MUE 3-MUE 1-PUE 2-PUE 3-PUE (16.6) c (16.0) d Dt from ref. 8. b According to our conclusions bout rte-determining step (see text) this rte constnt corresponds to k 2 of Scheme 3 for 1 2-MUE nd PUE, but to k 3 k 2 /k 2 for 3-MUE nd PUE. c Estimted, ssuming tht the difference between (1 3)-PUE is the sme s for (1 3)-MUE. d Estimted by ssuming tht the difference in pk is the sme in 1:1 s in :1 wter:cetonitrile. isotope effect (the proton in flight) nd 0.66 is the secondry effect clculted ssuming α = 0. [eqn. (6)]). φ* = φ R 1 α φ Pα = = 0.66 (6) The observed SKIE of 1.79 ± 0.12 nd 2.19 ± 0.27 (Tble 6) for the rections of 3-PUE nd 3-MUE, respectively re mtched (clculted vlues k /k D = 1.82 nd 2.17) using α- vlues for the specific bse-generl cid-ctlysed process of 0. nd 0.7. These vlues indicte reltively lte trnsition stte for the leving of the ethoxy group. Since the chnge in rte-determining step which results in the brekdown of T becoming slower thn its reversion to products (k 3 < k 2 in Scheme 3) is ccompnied by slower rtes of cycliztion for compounds 3 (Tble 8), it must involve slower k 3 step (rther thn fster k 2 ). We suggest tht this is result of steric hindrnce to the proton trnsfer process. The leving ethoxy groups of 3-MUE nd 3-PUE re in very crowded environments: the ethyl chin will dopt the lest hindered conformtion (Fig. 7), blocking the esy ccess of solvting wter molecules. Thus in the hlf-chir conformtion of the presumed trnsition stte of the 3-MUE rection the lone pirs re shielded by the methyl groups on C() nd N(2), nd with 3- PUE screening by the N-phenyl group is more effective still (Fig. 7). Where one or both of the C() methyl groups is bsent this comprehensive shielding disppers. Although proton trnsfers between N nd O toms re not generlly sensitive to steric effects these hve been invoked to explin the slow rtes of proton removl from the open form of Fig. 7 Steric hindrnce of proton trnsfer to T for 3-PUE: compred with -bstrction by hindered minoxyl rdicl (). protonted 1,8-bis(dilkylmino)nphthlenes by hydroxide ion 21 nd the protontion of 2,6-di-tert-butylpyridine by 3 O ; 22 s well s (in less severely hindered sitution) the slower rte of exchnge of the 3-N of biotin. 23 Generl cid ctlysis of cetl hydrolysis is lso known to be subject to substntil steric hindrnce in hindered systems. 2 Steric hindrnce in the cse of 3-T is comprble with tht in minoxyl rdicls such s, which re protected by the djcent methyl substituents (s well s stbilized electroniclly) to such n extent tht they do not redily bstrct hydrogen toms from crbon. 2 It seems unlikely tht this fctor lone cn ccount for the reversl of the gem-dimethyl effect for the cycliztion of the nion of 3, becuse k 3 is some wy from being rte determining for compounds 2, yet clerly rte-determining for compounds 3. So though reduction of n order of mgnitude or so could explin the chnge in rte-determining step, reduction of severl hundred-fold in k 3 is needed to overcome the expected ccelerting effect (bout 0 ccording to the dt in Tble 7) of the dditionl methyl group. The most likely dditionl fctor is reduced increse in the equilibrium constnt for the formtion of the tetrhedrl intermedite T. The two djcent quternry centres in the five-membered ring (Fig. 7) induce eclipsing interctions between the exocyclic bonds which re bsent in the ground stte nd the product, nd most significnt for compounds 3. This effect cn explin the smller kinetic thn thermodynmic effective molrities for cycliztion rections of crboxylic cid derivtives: the disprity increses significntly for systems with gem-dimethyl groups in the α- position. 26 For both explntions key question is why the sme fctors do not show up in the cid-ctlysed cycliztion, where the ddition of the lst methyl in 3 cuses ccelertions in k comprble to ddition of the penultimte methyl to give 2 (Tble 7). In cid the leving group in the cycliztion rection is EtO (from the prototropiclly-equilibrted intermedite T, with proton on the ethoxy oxygen), nd CO clevge is not rte determining. The eclipsing interctions in the tetrhedrl J. Chem. Soc., Perkin Trns. 2, 1998,

8 intermedite will be similrly unfvourble for both rections, but less-developed, nd thus smller, in the trnsition stte for CN bond formtion (rte determining in the cid-ctlysed rection). In the rections of 3 the CN bond is fully formed in the rte-determining trnsition stte (Fig. 7). Generl bse ctlysis The cycliztion rections of the six compounds studied in this work re subject to both generl cid nd generl bse ctlysis, nd dt for both rections re shown in the Tbles. The disppernce of the gem-dimethyl effect is property specificlly of the bse-ctlysed rection, so we discuss here only generl bse ctlysis. The ppernce of GBC shows tht process involving rtedetermining proton trnsfer competes with the specific bsectlysed rection t lower p. There re only two proton trnsfers involved in the cycliztion rections of MUE nd PUE, the deprotontion of the nucleophilic N nd the protontion of the leving ethoxide nion. Generl cid ctlysis of the deprture of ethoxide (kineticlly equivlent to GBC) is certinly to be expected, since we hve concluded tht prtil proton trnsfer from solvent wter (see Fig. 7 bove) ccounts for the solvent kinetic isotope effect observed in the specific bsectlysed cycliztion of compounds 3. owever, GAC of this step will mke the formtion of the tetrhedrl intermedite less likely to be the slow step of the rection, so it is not expected to be observed unless it is securely rte determining. We conclude below tht the deprotontion of the nucleophilic nitrogen is the step which shows generl bse ctlysis in the generl cse. The clerest experimentl evidence comes from the Brönsted β-vlues observed for GBC. These fll into two well-defined ctegories: c. 0.2 for 2 nd 3-MUE nd c. 0.6 for 2 nd 3- PUE. This indictes tht no chnge of mechnism tkes plce between compounds 2 nd 3: the significnt difference ppers between the -N- nd -N-Ph esters. A β-vlue of 0.6 for PUE is consistent with rtedetermining trnsition stte corresponding to the trnsition stte TS of Scheme, with stronger generl cid replcing 2 O. The observed β-vlue corresponds to (1 α) for SB/GAC, nd the chnges in the mgnitude of the SKIE for 3-MUE vs. 3- PUE, which re reproduced by α-vlues of 0.7 nd 0., s discussed bove, re consistent with the direction of chnges of the β-vlues. owever, two rguments support the involvement of deprotontion of the ureido group in the rte-determining trnsition stte for the cycliztions of the MUE series. The rtes for deprotontion by the generl bse cn be clculted from the mesured pk-vlues 8 nd the vlue of k d, the diffusion controlled rte constnt, tken s dm 3 mol 1 s 1 [eqn. (7)]: Scheme We suggest tht this is the mechnism observed when the lifetime of the intermedite T ± is shorter thn the time for diffusion prt of T ± nd A. (A negtive devition from the Brönsted line for generl bse ctlysis is often observed for the wter points: this my reflect the fvourble electrosttic interction between the other, nionic, bses nd the positively chrged nitrogen tom. 28 ) In the cse of the ω-phenyl derivtives 2 nd 3-PUE the Brönsted β-vlue of c. 0.6 indictes tht proton trnsfer is well under wy in the rte-determining trnsition stte. The simplest mechnism, involving generl cid ctlysis of the brekdown of T (cf. Fig. 7, bove), cn be ruled out with some confidence becuse the rte constnts k B for generl bse ctlysis re higher for 3-PUE, in contrst to the hydroxide-ctlysed rection. We conclude tht the lifetime of T ± hs reched the point where concerted mechnism is enforced, 29 becuse phenylureido is better leving group compred to methylureido. The process is illustrted by the lterntive route digrm of k 1 = K N K A k d (7) In the cse of 2 nd 3-MUE the rte constnts estimted for deprotontion by bses weker thn the phosphte dinion re smller thn the rte constnts ctully observed for ctlysis of the cycliztion rection. This mens tht for 2 nd 3-MUE t lest ctlysis by the generl bses by-psses the high energy ureide nion t low p. The second rgument is bsed on rectivity selectivity reltionships, which cn mke it possible to distinguish between kineticlly equivlent mechnisms. A More O Ferrll Jencks digrm (see below) ccommodtes the chnges in the β coefficients for deprotontion of the ureido group, but not for proton trnsfer to the leving group. The Brönsted β coefficient of 0.2 observed for the rections of 2 nd 3-MUE is chrcteristic of pressocition mechnism involving hydrogen bond formtion (Scheme ). 27 Scheme 6 Scheme 6. Chnging R = for Ph rises the energy of T ± nd lowers tht of U, cusing perpendiculr shift of the trnsition stte from the bottom right-hnd corner towrds the centre of the digrm. Note tht concerted mechnism is not possible from the ground stte conformtion, which would require the ester crbonyl nd the generl bse to pproch the recting N simultneously from the sme direction. In the rective conformtion 6 9 the terminl nitrogen must hve rotted out of plne (nd conjugtion) to llow it to ct s nucleophile in the plne of the incipient five-membered ring, which lso mkes the recting N vilble to generl bse (Scheme 7). The generl bse is necessry for the less nucleophilic niline N (R = Ph) but not for R = J. Chem. Soc., Perkin Trns. 2, 1998,

9 nd the Royl Society of Chemistry for Journls Grnt for Interntionl Authors (to I. G. P.). Scheme 7 Scheme 8 Structure, conformtion nd rectivity 1 nd 2-PUE rect 00 times fster thn 1 nd 2-MUE in the hydroxide-ctlysed rection, k O, but re 7 times less rective in cid (k, Tble 1). Rectivity in the specific hydroxidectlysed cycliztions is governed by the pks of the N protons, which re c. 3 pk units lower for the N-phenyl compounds, modulted by the gem-dimethyl effect. The rte constnts for cycliztion of the nions U show little sensitivity to electronic effects (3 times lrger for 1 nd 2-PUE thn for 1 nd 2-MUE) but re subject to the gemdimethyl effect. The greter prt of the brrier to these very fst rections probbly consists in dopting the proper rective conformtion 6, s discussed previously. 9 For the nions U the rective conformtion bout the terminl CON bond is obtined directly upon deprotontion from the preferred Z- conformtion (Scheme 8). On the other hnd, of the two substituents on the centrl N, the bulkier one crrying the ester group will prefer the Z conformtion nd rection cn tke plce only fter rottion to the E conformtion. Thus for the min popultion of molecules in the ground stte the rottionl brrier round the CN prtil double bond hs to be overcome. The rte constnts (k U ) re of the order of 10 8 s 1, corresponding to free energy of ctivtion of 6.6 kcl mol 1 t 0 K: the conformtionl brriers in the nions re likely to be of the sme order of mgnitude (smller thn in the ure becuse of cross conjugtion with the outer CON ). Importntly these brriers decrese upon substitution becuse of relese of ground stte strin in the trnsition stte for the formtion of 6: brriers of 11.2 kcl mol 1 nd 6.3 kcl mol 1 hve been reported for ure nd tetrmethylure, respectively. 31 The low sensitivity to electronic effects cn be ttributed to the high rectivity of the nions, thus simple mnifesttion of the rectivity selectivity principle. Acknowledgements We thnk Professors Andrew Willims, Frnk ibbert nd Michel Pge for useful discussions, the Ntionl Foundtion for Scientific Reserch of Bulgri for funding this reserch References 1 J. R. Knowles, Ann. R. Biochem., 1989, 8, I. B. Blgoev, I. G. Pojrlieff nd A. J. Kirby, J. Chem. Soc., Perkin Trns. 2, 198, Slebock-Tilk, A. J. Bennet, J. W. Keillor, R. S. Brown, J. Peter Guthrie nd A. Jodhn, J. Am. Chem. Soc., 1990, 112, 87. I. B. Blgoev, J. Chem. Soc., Perkin Trns. 2, 1987, 127. I. B. Blgoev nd I. G. Pojrlieff, CR. Acd. Bulg. Sci., 1977,, 103; M. Bergon nd J.-P. Clmon, J. Chem. Soc., Perkin Trns. 2, 1978, A.. Koedjikov, I. B. Blgoev, I. G. Pojrlieff nd A. J. Kirby, J. Chem. Soc., Perkin Trns. 2, 1996, I. B. Blgoev, D. T. Tshev nd A. J. Kirby, J. Chem. Soc., Perkin Trns. 1, 1989, I. G. Pojrlieff, I. B. Blgoev, A. J. Kirby, B. P. Mikhov nd E. Aty, J. Chem. Res. (S), 1997, 2. 9 I. B. Blgoev, I. G. Pojrlieff, D. T. Tshev nd A. J. Kirby, J. Chem. Soc., Perkin Trns. 2, 1989, Biltz nd K. Slott, J. Prkt. Chem., 1926, 113, L. Cuneo, Ber., 1892, 2, E. Friedmn, Beitr. Chem. Physiol. Pthol., 1906, 11, A. K. Covington, R. A. Robinson nd R. G. Btes, J. Phys. Chem., 1966,, 38; J. C. Fishbein nd W. P. Jencks, J. Am. Chem. Soc., 1988, 110, 7. 1 J. Kvlek, V. Mchcek, G. Svobodov nd V. Sterb, Collect. Czech. Chem. Commun., 1986, 1, I. B. Blgoev, I. G. Pojrlieff nd V. Dimitrov, J. Chem. Soc., Perkin Trns. 2, 1978, N. Grvitz nd W. P. Jencks, J. Am. Chem. Soc., 197, 96, O. Nunez, J. Rodrigues nd L. Angulo, J. Phys. Org. Chem., 1993, 7, R. L. Schowen, Prog. Phys. Org. Chem., 1972, 9, 27; K. B. J. Schowen in Trnsition Sttes of Biochemicl Processes, eds. R. D. Gndour nd R. L. Schowen, Plenum Press, New York, 1978, p D. M. Quinn nd L. D. Sutton in Enzyme chnism from Isotope Effects, ed. P. F. Cook, CRC Press, Boc Rton, 1991; quoted from ref. 16, p. 27. F. ibbert, Adv. Phys. Org. Chem., 1986, 22, A. Awwl nd F. ibbert, J. Chem. Soc., Perkin Trns. 2, 1977, 189; F. ibbert nd K. P. P. unte, J. Chem. Soc., Perkin Trns. 2, 1983, C. F. Bernsconi nd D. J. Crre, J. Am. Chem. Soc., 1979, 101, C. L. Perrin nd T. J. Dwyer, J. Am. Chem. Soc., 1987, 109, E. Anderson nd T.. Fife, J. Am. Chem. Soc., 1971, 93, L. R. Mhoney, G. D. ndenhll nd K. U. Ingold, J. Am. Chem. Soc., 1973, 9, A. J. Kirby, Adv. Phys. Org. Chem., 1980, 17, M. I. Pge nd A. Willims, Orgnic & Bio-orgnic chnisms, Longmn, London, 1997, p Fischer, F. X. DeCndis, S. D. Ogden nd W. P. Jencks, J. Am. Chem. Soc., 1980, 102, W. P. Jencks, Chem. Rev., 1972, 72, 2. R. A. More O Ferrll, J. Chem. Soc., B, 19, M. Oki, Applictions of Dynmic NMR Spectroscopy to Orgnic Chemistry, thods in Stereochemicl Anlysis, vol., VC Publishers, Florid, 198, p.. Pper 8/0288B J. Chem. Soc., Perkin Trns. 2, 1998,