Dual Role of Silanol Groups in Cyclopropanation and Hiyama-Denmark Cross-Coupling Reactions
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1 Dual Role of Silanol Groups in Cyclopropanation and Hiyama-Denmark Cross-Coupling Reactions L.-P. B. Beaulieu, L. B. Delvos, A. B. Charette* Département de Chimie, Université de Montréal, P.O. Box. 6138, Station Downtown Montréal, Québec, Canada H3C 3J7 Org. Lett. 2010, 12 (6),
2 The cyclopropane unit: - Present in several bioactive natural products as well as synthetic drugs. - The Simmons-Smith reaction is one of the most important preparative method. The Hiyama-Denmark cross-coupling reaction: - Pd-catalysed cross-coupling reaction involving organosilicon compounds. - Probably not the most famous cross-coupling reaction, but good alternative to Stille or Suzuki-Miyaura methods. 2
3 Introduction To couple both reaction by using Di-tert-butoxy(alkenyl)silanols as substrates: The silanol group will bear two distinct roles: - Directing group during the cyclopropanation. - Mediator for the transmetalation event during the cross-coupling. 3
4 Discovered in The original Simmons-Smith cyclopropanation involves CH 2 I 2 and Zn-Cu couple. Allows cyclopropanation of a wide range of alkenes: simple alkenes, α,β-unsaturated ketones and aldehydes, electron rich alkenes (enol ethers, enamines, etc.). - Stereospecific (stereochemical information in the alkene substrates translated to the products). - Highly diastereoselective (occurs from the less hindered face on chiral substrates). - Strongly directed by functional groups containing heteroatoms (OH, OAc, OMe, NHR, ) with delivery of the alkylidene from the face of the double bond having the closest proximity with the FG. - Tolerant with many sensitive functional groups. - Non-coordinating solvents used (DCM, DCE). Poceeding through a «butterfly-type» transition structure: H. Lebel, J-F. Marcoux, C. Molinaro, A. B. Charette, Chem. Rev. 2003, 103,
5 The Simmons-Smith cyclopropanation Zinc carbenoids: - Furukawa (1966): Et 2 Zn + CH 2 I 2 in DCM or DCE. Schlenk-type equilibria: Et 2 Zn + CH 2 I 2 EtZnCH 2 I + EtI EtZnCH 2 I + CH 2 I 2 Zn(CH 2 I) 2 + EtI 2 ICH 2 ZnI Zn(CH 2 I) 2 -ZnI 2 - Iodomethylzinc trifluoroacetate: CF 3 CO 2 ZnCH 2 I (more reactive than Furukawa s one towards unfunctionalised olefins). - Iodomethylzinc phenoxides: 2,4,6-Cl 3 C 6 H 2 OZnCH 2 I (more reactive as well). Other carbenoids: - Molander (1987): Sm(Hg) + CH 2 I 2 (chemoselective cyclopropanation of allylic alcohols in presence of other olefins). - Yamamoto (1985): i-bu 3 Al + CH 2 I 2 (cyclopropanation of unfunctionalised olefins in presence of allylic alcohols). H. Lebel, J-F. Marcoux, C. Molinaro, A. B. Charette, Chem. Rev. 2003, 103,
6 The Simmons-Smith cyclopropanation Cyclopropanation of allylic alcohols: Occurs via an oxygen group-assisted delivery of the reagent from a conformation in which minimisation of the A (1,3) strain is the predominant controlling element. Furukawa s conditions: unfavorable nonbonded interactions arising from the bulky zinc alkoxide substituent in B, C and D. H. Lebel, J-F. Marcoux, C. Molinaro, A. B. Charette, Chem. Rev. 2003, 103,
7 The Simmons-Smith cyclopropanation Only few examples in the literature. Reaction also enhanced by the presence of the hydroxy group on the silicon: K. Hirabayashi, A. Mori, T. Hiyama, Tetrahedron Lett. 1997, 38 (3), Chiral alkenylsilanols: chirality of the silicon efficiently transferred to the alkenyl carbons diastereoselective cyclopropanation (Y. Yamamura, F. Toriyama, T. Kondo, A. Mori, Tetrahedron: Asymmetry 2002, 13, 13-15). 7
8 Originally (1988): Pd-catalysed cross-coupling involving organosilicons with organic halides (or triflates) in presence of an activating agent as fluoride or hydroxide (Hiyama cross-coupling reaction). Highly chemo-, stereo- and regioselective reaction, compatible with a wide range of sensitive groups. C-Si bond much less polarised than C-Metal bond (M = Li, B, Al, Zn, Sn, Mg ) need of an activating agent to promote the transmetalation (rate-determining step) by generating a pentacoordinated silicate and then increasing the nucleophilicity of the organic group linked to the silicon. First examples: Y. Hatanaka, T. Hiyama, J. Org. Chem. 1988, 53, Tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF) better than TBAF, CsF or KF. - HMPA (with aryl halides) or THF + P(OEt) 3 (with vinyl halides). T. Hiyama, Y. Hatanaka, Pure & Appl. Chem. 1994, 66 (7),
9 The Hiyama-Denmark cross-coupling reaction R 3 Si-R used with R = F or OAlk to increase the reaction rate. 9
10 The Hiyama-Denmark cross-coupling reaction Cross-coupling of alkenyl fluorosilanes: - Complexation of the halogen of Pd to Si of the unsaturated silicate to give coordinately saturated silicate. - C-Pd bond formation producing β-cationic silicate. - Then path a: transmetalation favored by R 1 and/or R 2 cation-stabilising groups and negatively charged hexacoordinated silicate group (enhanced β-effect). Then reductive elimination to give the ipso-substitution. - Path b: 1,3-migration of R from Pd to β-carbon favored when nucleophilicity of R enhanced by EDG-substituent. Then β-h-elimination, hydropalladation and elimination to give the cine-substitution. T. Hiyama, Y. Hatanaka, Pure & Appl. Chem. 1994, 66 (7),
11 The Hiyama-Denmark cross-coupling reaction Biaryls and diaryl ketones synthesis: Trimethylsilylation: Cross-coupling with triflates: T. Hiyama, Y. Hatanaka, Pure & Appl. Chem. 1994, 66 (7),
12 The Hiyama-Denmark cross-coupling reaction Alkenylsilanols can also be used: offer many advantages - Easily available. - Stable and easy to handle. - Easy modulation of the substituents on the silicon. S. E. Denmark, R. F. Sweis, Acc. Chem. Res. 2002, 35, S. E. Denmark, D. Wehrli, Org. Lett. 2000, 2 (4),
13 The Hiyama-Denmark cross-coupling reaction 2001: fluoride-free cross-coupling of organosilanols to overcome the incompatibility of some functional groups with fluoride (Si-protecting groups for instance). Economical motivation to replace TBAF by another cheaper cross-coupling promoter (TBAF. 3H 2 O: 734 CHF/mol Aldrich). First mechanistic proposal: Based on the Hiyama mechanism: generation of the pentacoordinated silicon by attachement of a second molecule of silyloxide i (prerequisite for transmetalation). S. E. Denmark, R. F. Sweis, J. Am. Chem. Soc. 2001, 123,
14 The Hiyama-Denmark cross-coupling reaction S. E. Denmark, R. F. Sweis, J. Am. Chem. Soc. 2001, 123,
15 The Hiyama-Denmark cross-coupling reaction Mechanistic investigations through a full kinetic analysis provide revision of the first mechanistic proposal: Does not involve any pentacoordinated siliconate, unlike other nucleophilic-activated organosilicon crosscoupling reactions. Reaction is first-order in silanolate and Si-O-Pd linkage has here a critical importance for the transmetalation. Intermediate isolated and heated up to 100 C to provide the biaryl product in quantitative yield. neutral arylpalladium (II) silanolate complex can undergo direct transmetalation! S. E. Denmark, R. F. Sweis, J. Am. Chem. Soc. 2004, 126, S. E. Denmark, J. Org. Chem. 2009, 74,
16 The Hiyama-Denmark cross-coupling reaction Hiyama (1995) Denmark (2005) S. E. Denmark, J. H.-C. Liu, Angew. Chem. Int. Ed. Early View, March 24, 2010 DOI: /anie Denmark (2009) 16
17 First try: cross-coupling of a cyclopropylsilanol with bromobenzene using Hiyama s conditions. Modulation of the nature of the substituents on the silicon: introduction of more electron-withdrawing substituents (alkoxide groups, -I effect) to enhance the stabilisation of the pentacoordinated siliconate intermediate formed with TBAF and then facilitate the transmetalation. Preparation of Di-tert-butoxy(alkenyl)silanols: 17
18 Charette et al. results 18
19 Charette et al. results First attempt with (±)-Di-tert-butoxy((1S,2R)-2-phenylcyclopropyl)silanol: Insufficient reactivity of the organosilane in the cross-coupling solved by generating the trifluorosilane in situ: 19
20 Charette et al. results 20
21 Charette et al. results First cyclopropylsilanes to be reacted in the Hiyama-Denmark cross-coupling. Nature of the substituents on the silicon atom has a profound effect on reactivity in the cross-coupling. Dual role of the silanol: directing group during the cyclopropanation and mediator for the coupling transmetalation. Next steps: Development of fluoride-free conditions. Development of asymmetric cyclopropanation conditions. 21
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