Functionalization of terminal olefins via H migratory insertion /reductive elimination sequence Hydrogenation

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1 M.C. White, Chem 153 verview Week of ovember 11, 2002 Functionalization of terminal olefins via migratory insertion /reductive elimination sequence ydrogenation ML n E ydrosilylation Si 3 Si 3 ML n Si 3 E Si 3 ydrocyanation C C ML n C E C ydroacylation masked ML n E ydroformylation C C ML n C ML n E

2 M.C. White, Chem 153 verview Week of ovember 11, 2002 Wilkinson s Catalyst ydrogenations: A of 2 PPh 3 h (I) cat. 2 (1 atm), benzene, rt quantitative PPh 3 h (III) ydrosilylations: A of 3 Si- PPh 3 h (I) cat. 3 Si-, benzene, rt Si 3 PPh 3 h (III) Si 3 ydroacylation: A of C("")- PPh 3 h (I) cat. h (III) Ph Ph Ph ydroformylation PPh 3 h (I) cat. 2 :C (1:1, 100 atm) Et 3 PPh 3 h (I) C C apid P 3 /C ligand exchange occurs with Wilkinson's catalyst under a C atmosphere (h has a high affinity for C). C's are strong π-acids and may serve to disfavor A of 2 to generate a h(iii) dihydride.

3 M.C. White, Chem 153 verview Week of ovember 11, 2002 ydrogenation via σ-bond metatheses: u(ii) catalysts Wilkinson's u catalyst Monohydride identified as active catalyst: Formation of u(iv) dihydride is prohibitively high in energy. u 2 (PPh 3 ) 2 cat. 2 (1 atm), benzene:ethanol, rt quantitative u 2 (PPh 3 ) 3 2 (1 eq) Et 3 (1 eq), benzene u (II) PPh 3 PPh 3 - Et 3 oyori's u catalyst 3 C Ac CC 3 Ac Ph 2 P P Ph 2 (II) u ()-1(0.5-1 mol%) 3 C Ac CC 3 Ac Et:C 2 2 (5:1), 2 (4 atm), 23 o C C 3 92% yield 95% ee C 3 Possible monohydride hydrogenation mechanisms (ydrogenations, pg. 156). X X 2 2 n X σ-bond metathesis B B no oxidation state change σ-bond metathesis B base-promoted heterolytic cleavage protonolysis

4 M.C. White, Chem 153 verview Week of ovember 11, 2002 ate of migratory insertion into olefin: M->>M-C h III h III h III (Me) 3 P (Me) 3 P (Me) 3 P G = 12.0 kcal/mol h III h III h III (Me) 3 P (Me) 3 P (Me) 3 P G = 22.3 kcal/mol The difference of 10.3 kcal/mol in the free energy of activation of ethyl vs hydride migratory insertion into ethylene in these systems corresponds to a rate ratio of k /k Et of The effect is thought to be kinetic in nature. Brookhart JACS 1985 (107) ationalization: Better overlap is possible because of non-directionality of the orbital. π* π π* π σ M- σ M- σ M-C σ M-C Trigonal-bipyramidal structure ote: at the TS of olefin insertion, both σm- to π*c=c back-donation and πc=c to σ* M- donation are necessary for bond exchange. Morokuma JACS 1988 (110) Morokuma M 1997 (16) 1065.

5 M.C. White, Chem 153 verview Week of ovember 11, 2002 egioselectivity of hydrometallation Aliphatic terminal olefins: For aliphatic terminal olefins, there is both a strong thermodynamic preference to form the sterically less hindered M-C bond. Structure & Bonding, pg. 32: As seen for C- σ bonding, there is a general trend towards weaker M-C σ bonds with increased substitution. L n Ir (III) C 4 13 L n Ir (III) L n Ir (III) D Ir-C : 58 kcal/mol D Ir-C : 52 kcal/mol D Ir-C : 48 kcal/mol ydrosilylation, pg. 183 If an equilibration mechanism exists, M-alkyls will isomerize to the least sterically hindered 1 o product. For more examples of this see: ydroformylation, pg.205 and 206. olefin isomerization to terminal olefin leads to 1 o metal alkyl that undergoes E to product. h(pph 3 ) 3 3 Si- L n (Si 3 )h h(si 3 )L n L n (Si 3 )h 3 Si hl n C Si C 4 13 Conjugated terminal olefins: For conjugated olefins, hydride insertion results in formation of the 2 o M-C which can be stabilized as a delocalized η 3 -intermediate. ydroformylation, pg. 192, 194, 195; ydrocyanation, pg h(c)(pph 3 ) 2 :C (1:1) h(c)l n h(c)l n branched: linear (11:0) Ligand Effects? ayashi's observed hydrosilylation regioselectivities with aliphatic terminal olefins cannot be rationalized using our models (ydrosilylation, pg. 189). C 4 9 Pd Pd Si 3 (1.2 eq) 0.1 mol% MP (0.002 mol%) C 4 9 Si 3 regioselectivity (87:13) MP Me PPh 2

6 M.C. White, Chem 153 verview Week of ovember 11, 2002 Unsolved problem: intermolecular silylformylation of terminal olefins Desired silylformylation product: catalyst* C, 3 Si- Si 3 * Tamao oxidation * acetate aldol equivalent polyacetate polyol bserved silylformylation product: SiEt 2 Me C 4 9 Co 2 (C) 8 (0.7 mol%) SiEt 2 Me (1 eq), C C 4 9 benzene, 140 o C, 20h mj. product linear oute of the problem? ne hypothesis is that hydrometalation of terminal olefins with Co- occurs at a faster rate than silylmetalation with Co-Si 3. The overall result is the hydroformylated aldehyde product which can further react with Co-Si 3 (recall Si is oxophilic) to give the silylenol ether. See Silylformylation, pg (C) 4 Co Co(C) 4 Co(C) 4 3 SiCo(C) 4 Si 3 σ-bond metathesis SiEt 2 Me Silylenol ether formation regenerates the Co- species. Si 3 Co(C) 4 Co (I) (C) 4 Co(C) 4 Si 3 Co(C) 4 3 Si Co(C) 4 Si 3 Co(C) 3 Co (III) Si 3 (C) 3

7 M.C. White, Chem 153 verview Week of ovember 11, 2002 Unsolved problem: direct hydroacylation of terminal olefins Desired hydroacylation product ' catalyst direct route to di-substituted ketones from unactivated precursors ' bserved intermolecularly: decarbonylation C 4 9 saturated solution h PPh 3 PPh 3 stoichiometric C 3 C h C5 12 PPh 3 oute of the problem? ne hypothesis is as follows: The catalyst must have three "open" and adjacent coordination sites for the acyl, hydride, and olefin moieties to effect successful hydroacylation. Upon oxidative addition of the aldehyde to give the hydridoacyl metal species, intramolecular binding of the acyl alkyl substituent to the syn open site competes effectively with intermolecular olefin binding and results in decarbonylation. PPh h 3 PPh 3 ' h desired cycle ' ' h h PPh 3 ' ' ' h h C C h PPh 3 h

8 M.C. White, Chem 153 verview Week of ovember 11, 2002 Functionalization of terminal alkynes via migratory insertion /reductive elimination sequence? ydrogenation ML n E E Silylformylation Si 3 C C Si 3 C Si 3 L n (Si 3 )M Si 3 E Si 3 ydroacylation ' masked ' ML n ' E '

9 M.C. White, Chem 153 verview Week of ovember 11, 2002 Unsolved problem:hydroformylation of terminal alkynes Desired hydroformylation product: catalyst* C, 2 (E)-α,β-unsaturated aldehydes oute of the problem: acetylenes form stable metallocyclopropane/propene complexes that resist hydroformylation under mild conditions. ' 2 C ' (C) 4 Co Co(C) 4 (C) 3 Co Co(C) 3 Under forcing conditions of high C/ 2 pressure and high temperatures, terminal acetylenes are converted to fully saturated linear aldehydes. h PPh (I) 3 PPh 3 L C (0.01 mol%) 2 /C (1:1, 100 atm), 80 o C, 24 hrs, 73 % conversion C possible intermediate (never observed) C

10 M.C. White, Chem 153 verview Week of ovember 11, 2002 terminal alkynes: Silylformylation of alkynes Me terminal sp C is selectively silylated h 4 (C) 12 (1 mol%) Me 2 PhSi (1 eq) Et 3 (1 eq), C (29 atm) 2h, 100 o C Z-isomer is the kinetic product. E-isomer arises from isomerization under the carbonylation conditions. Me C 99% yield Z:E (80:20) SiMe 2 Ph ' (C) 4 h (I) h 4 (C) 12 3 Si (C) 3 h (I ) ' ' (C) 3 h (I ) ' (C) 4 h (I) Si 3 (C) 4 h (I) C Si 3 ' 3 Si (C) 3 (Si 3 )h (III) ' (C) 4 h (I ) Si 3 ' C (C) 3 (Si 3 )h (III) 3 Si 3 Si 3 Si oxidative addition 3 Si (C) 3 h (I) ' migratory insertion (C) 4 h (I) (C) 3 h (I ) Si 3 3 Si ' C (C) 4 h ' (C) 4 h There is no mechanism to regenerate h- once it has been consumed in a cycle. Unlike the silylformylation of alkenes, the product of aldehyde silylmetalation has no β-hydrogen available for elimination. '