π-alkyne metal complex and vinylidene metal complex in organic synthesis

Transcription

1 Literature Seminar Kenzo YAMATSUGU (D1) π-alkyne metal complex and vinylidene metal complex in organic synthesis 0. Introduction ' ' = π-alkyne metal complex vinylidene metal complex ecently, electrophilic activation of alkynes through π-alkyne complex formation has been studied extensively. ex. Au(I), Au(III), Pt(II), u(ii), and so on. More recently, there has been a large number of reactions involving metal vinylidenes as catalytic species. ex. Cr 0, Mn 0, W 0, u(ii), h(i), Fe(II), and so on. In this literature seminar, mainly three reports will be introduced in full detail. Contents 1. W(C) 5 (thf)-catalyzed generation of carbonyl ylide and tandem reaction with alkene 2. Cpu + L 3 -catalyzed anti-markovnikov hydration of terminal alkynes 3. Control of π-alkyne and vinylidene complex 1. W(C) 5 (thf)-catalyzed generation of carbonyl ylide and tandem reaction with alkene * generally studied reactions involving π-alkyne metal complex For review of Pt, Au, see ; Angew. Chem. Int. Ed. 2007, 46, other metals ; J. rg. Chem. 2007, 72, X M X X X = 2 etc M X M M X X X = etc 1st. report. (Pd) (pyrrole) Utimoto et al. Tetrahedron Lett. 1981, 22, Iwasawa et al. JACS 2001, 123, (commun.) JACS 2005, 125, (full) x-ray 1 / 13

2 + postulated mechanism Scheme 6 + Tungsten-containing carbonyl ylide + Minute mechanistic consideration is presented below Table ot only ketene acetal but also alkyl vinyl ether is applicable entry 7, 8 + aryl aldehyde is applicable entry 8 + internal alkyne is applicable ot all the alkenes have a suitable carbon-hydrogen bond for tungsten-carbenoid intermediate to insert. It is well known that metal-carbenoid can readily insert into a silicon-hydrogen bond of silanes. + silicon trapping of intermediate tungsten-carbenoid E + Pr 5g 17 % yield 2 / 13

3 Table 2. eaction of 1d with Various Alkenes in the Presence of Silanes a entry 1-4 large silane : C- insertion occurred predominantly. entry 5 Small EtMe 2 Si was effective. entry 6-9 silyl enol ether, allyl silane, styrene were applicable. 1-octene and methyl acrylate : o xn. expansion of substrate scope to ones having no C- bond to insert + To gain insight into the reactivity of the tungsten-containing carbonyl ylide Ab initio calculation (lan12dz basis set was used for the W atom, and the 6-31 basis sets were used for the other atoms) *LUM of ylide A is considerably lower than that of ylide D. (probably because of the strong electron-withdrawing nature of the tungsten carbonyl) normal carbonyl ylide : react with electron deficient alkene preferentially tungsten-containing ylide : react with electron rich alkene preferentially + Mechanistic consideration cf. Shceme 6 in page 2 1. in the presence of 2 endo-attack isolation of alkylidene complex exo-attack 12b was directrly observed in M. Uemura et al. J. rganomet. Chem. 2002, 645, 228. In the presence of 2 instead of alkene, 13b was obtained probably through 11b. 3 / 13

4 3. in the absence of alkene & 2 vinylidene-metal complex Scheme 1 2 was isolated by silica gel chromatography and characterized by 13 C M (typical carbene carbon δ = ppm) and elemental analysis. In the absence of alkene and 2, 2 was obtained probably through metalvinylidene intermediate. Iwasawa et al. JACS 2000, 122, Dynamic equilibria to account for these experimental results + Path (i) and (ii) would be faster than path (iii). + Path (i) and path (ii) would be under rapid equilibrium. exo-attack + Therefore, in the presence of a reagent capable of trapping intermediate 11 or 7 ( 2 or alkene), the reaction proceeds through path (i) or (ii). + In the absence of such trapping reagents, vinylidene complex 2 would form slowly and to give 3 through an irreversible electrocyclization. endo-attack 4 / 13

5 + bservation of the tungsten-containing carbonyl ylide 1. 1 M time course *in the absence of alkene 1.7 eq. * ew species have much lower electron density than the SM ketone SM ketone * X disappeared and 3 c became a major product after 3.5 h. * singlet at 8.92 ppm envisioned three X δ ppm matched with isolated 3c see Scheme 1 2. MBC 1.7 eq. 30 min MBC 40 o C δ ppm ( 13 C) : correlation with (e) and (f) C 3 C 3 : correlation with singlet at δ 8.92 ppm ( 1 ) = (a) X would be endo-cyclized carbonyl ylide 7 c C- 183 W coupling (20% 13 C) * 183 W has 1/2 nuclear spin (other isotopes of tungsten have no nulear spin) * A set of satelite peaks should be observed in 13 C M spectrum at the carbon directly bonded to tungsten. * 76.4 z ( 1 J C-W ) (C1 - W) in 13 C M spectrum and 17.2 z ( 2 J C- ) (a - C1) in 1 M spectrum were observed. : direct bonding between C1 and W 5 / 13

6 + ther application TBS W(C) 6, hν Et Toluene W(C) 5 Tungsten-containing azomethine ylide Iwasawa et al. JACS 2002, 124, Cpu + L 3 -catalyzed anti-markovnikov hydration of terminal alkynes * general features of metal vinylidene complex M : u II, Cr 0,Mo 0, W 0, h I, etc.. ' ' = π-alkyne metal complex vinylidene metal complex Three types of reactions are commonly investigated. review ; Chem. Asian J. 2008, 3, nucleophilic addition to the α-carbon atom 1st example of a metal vinylidene complex as a catalytic species Bu Et 2 2% [u 3 (C) 12 ] C 2 (50 atom) toluene, 140 o C 20 h, y. 36% Bu ul n Bu Et 2 Et 2 Et 2 Bu Bu 6 : 5 : 1 Y. Sasaki, P.. Dixneuf J. Chem. Soc. Chem. Commun. 1986, 790. other nucleophiles ; carboxylic acid, alcohol, epoxide, carbonyl, thiol, amine, phosphine, enolate, enamine 2. [1,2]-alkyl migration from the metal center to the α-carbon atom alkyne dimerization 1% [u(-c C){P(C 2 C 2 P 2 ) 3 }]B 4 TF, reflux, 6h, y. 90% L L u 7 : 93 Bianchini et al. JACS 1991, 113, For earlier work by Yamazaki, see ; J. Chem. Soc. Chem. Commun. 1976, Pericyclic reactions 1st example of 6π electrocyclization onto a catalytic vinylidene intermediate 4% [ucl 2 (p-cymene)(p 3 )] C 2 Cl 2, reflux, 10 h, y. 89% ul n C. A. Merlic, M. E. Pauly JACS 1996, 118, / 13

7 *Cpu + L 3 -catalyzed anti-markovnikov hydration of terminal alkynes 2 anti-markovnikov hydration aldehyde 2 Markovnikov hydration methyl ketone = acid, mercury, etc.. known 1st report : M. Tokunaga, Y. Wakatsuki Angew. Chem. Int. Ed. 1998, 37, * By tuning phosphine ligand, they could produce aldehyde or ketone. * not so good selectivity * high catalyst loading * enylacetylene, tert-butylacetylene were not applicable. Et or 2-propanol Improvement (use of ucpcl(dppm)) Wakatsuki et al. rg. Lett. 2001, 3, 735. proposed bifunctional role Grotjahn et al. Angew. Chem. Int. Ed. 2001, 40, JACS 2004, 126, Chem. Euro. J. 2005, 11, st attempt wet wet 21, 22 : The same ligand is coordinated in two different environments, one chelating, the other not. The reaction with alkyne didn't afford aldehyde, but 23 and 24. (2D M and X-ray) 7 / 13

8 * sterical environment tuning 5 eq. ucp(c 3 C)Tf δ = 9.1 ppm (2, brs) + To hinder the direct attack of nitrogen at vinylidene carbon, bulky substituent was introduced. + 2 was incorporated. + Two hydrogen bond * scope and limitations 1 entry 3, 4, 5 tert-butylacetylene and phenylacetylene were applicable although sluggish reaction entry 8 itrile didn't inhibit the reaction (Wakatsuki catalyst was inhibited by nitrile.) entry 11 and 12 Proper placement of imidazole is crucial for efficient catalysis. 8 / 13

9 * Postulated mechanism 2 has to be exchanged to alkyne substrate. (Two phosphine seemed to stay on the metal because added phosphine did not change the hydration rate) Imidazole + facilitate conversion from B to C? + activate 2? (C to D) * facilitate conversion from D to E via F (proton shuttle)? * stabilize the catalyst by hydrogen bonding? so far, not clear *next report * JACS 2004, 126, reduced hydrogen bonding to open catalytically active site for alkyne binding entry 3, 4, 5 enylacetylene and electron-rich arylacetylene were applicable. entry 6 itrile inhibited the reaction. entry 10 in situ deprotection 9 / 13

10 Breit et al. Angew. Chem. Int. Ed. 2006, 45, DPPAP concept adenine / thymine in DA mimic 3-DPICon aminopyridine / isoquinolone through hydrogen bonding * Search for appropriate catalyst Ligand P 2 2 P Piv P 2 6-DPPon 6-DPPAP 3-DPICon entry 6 and 7 eterodimer is responsible for the result. *characterization of catalyst P M : two doublet (around 50 z, 2 J(P-P) = 35.9 z) + 1 M : substantial shift of signals of two ligands to lower field + X-ray 10 / 13

11 * Scope and limitations catalyst 5, 2 (5 eq.) acetone, 120 o C The origin of the catalytic activity remains unknown. ydrogen bonding network incorporates 2 so that addition proceeds readily and regioselectively? 11 / 13

12 3. Control of π-alyne and vinylidene complex Iwasawa et al. JACS 2008, 130, 802. dynamic control π-alkyne metal complex vinylidene metal complex * condition optimization Addition of tert-amine controled the product distribution. The use of -Ms instead of -Ts improved the yield. * scope and limitations entry 5 and 6 In the case of alkyl substituted diene, small amounts of 2 were obtained even in the amine-promoted reaction. 12 / 13

13 * proposed mechanism * deuterium - and 13 C-labeling experiments to support the proposed mechanism without D 2 : 46% D incorporation A to C The presence of amine would facilitate the formation of vinylidene complex F E to F 1,2-alkyl migration aided by electron donation from nitrogen * Effect of amine Similar effect of amine has been suggested. W(C) 5 (thf) (3 eq.) TF, r.t., 24 h [W] [W] [W] In the presence of Et 3 : y. 72% In the absence of Et 3 : o xn. he and Uemura et al. JACS 2002, 124, 526. [h(c 2 2 ) 2 Cl] 2 (5 mol %) P(4-F-C 6 4 ) 3 (25 mol %) DMF, 24 h [h] [h] [h] In the presence of DABC : y. 95% In the absence of DABC : y. 30% Lee et al. JACS 2006, 128, / 13