Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millennium: A Review Edgars Abele Latvian Institute of Organic Synthesis, 21 Aizkraukles Street, Riga LV-1006, Latvia E-mail: [email protected] ABSTRACT Recent advances in the fluoride ion mediated reactions of Si-Η, Si-C, Si-O, Si-N, Si-P bonds containing silanes are described. Application of silicon bonds activation by fluoride ion in the syntheses of different types of organic compounds is discussed. A new mechanism, based on quantum chemical calculations, is presented. The literature data published from January 2001 to December 2004 are included in this review. CONTENTS Page 1. INTRODUCTION 45 2. HYDROSILANES 46 3. Si-C BOND 49 3.1. Vinyl and Allyl Silanes 49 3.2. Aryl Silanes 52 3.3. Subsituted Alkylsilanes 54 3.4. Fluoroalkyl Silanes 56 3.5. Other Silanes Containing Si-C Bond 58 4. Si-N BOND 58 5. Si-O BOND 60 6. Si-P BOND 66 7. CONCLUSIONS 66 8. REFERENCES 67 1. INTRODUCTION Reactions of organosilicon compounds catalyzed by nucleophiles have been under extensive study for more than twenty-five years. In this field two excellent reviews were published 11,21. Recently a monograph dedicated to hypervalent organosilicon compounds was also published /3/. There are also two reviews on 45 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review fluoride mediated reactions of fluorinated silanes /4/. Two recent reviews are dedicated to fluoride ion activation of silicon bonds in the presence of transition metal catalysts 151. Beside this, a review on recent advances in the synthesis and transformation of heterocycles mediated by fluoride ion activated organosilicon compounds was published in 2002 161. Fluoride ion as activator of silicon bonds is widely described in these works. The present work was carried out in continuation of the review published by Corriu et al. in 1993 111 and our review published in 2001 111. Our aim is to describe the modern methodologies in the synthesis of different classes of compounds (for example, alkenes, alkynes, and alcohols) mediated by silanes activated + by fluoride ion. The influence of different sources of fluoride ion (B114NF, Bu4N [Ph3SnF2]\ KF, CsF, tris(dimethylamino)sulfur (trimethylsilyl)difluoride (TASF), tetrabutylammonium triphenyldifluorosilicate (TBAT), diethylaminosulfur trifluoride (DAST), Ph4Bi-F, CdF2, CuF2) on the chemical process will be discussed. A new mechanism of fluoride ion action, based on quantum chemical calculations, is presented. The fluoride ion mediated reactions of organosilicon compounds containing Si —X bonds (X = H, C, N, Ο, P), and their use in organic synthesis published from January 2001 to December 2004, have been reviewed. The advances in the transformations of heterocycles mediated by fluoride ion activated organosilicon compounds are not included in this review and will be published separately. 2. HYDROSILANES Diastereoselectivity in the reduction of α-substituted ketones 1 to alcohols 2 with polymethylhydrosiloxane in the presence of Bu4NF was investigated /8/. The experiments showed high syn- selectivity for α-alkoxy-, α-acyloxy- and α-dialkylamino-substituted ketones (syn: anti = 73: 23 up to 100:0). Reduction of α-monoalkylamino ketone proceeded in anti-selective manner (Scheme 1). Me I Ο η / Bu,NF / THF / 0-5 " C OH OH Η R R' R' R' 1 syn-2 anti-2 R = alkyl, aryl; R' = NHR, NR 2, OCOR, OR Scheme 1 Nitroalkenes 3 undergo enantioselective conjugate reduction using poly(methylhydrosiloxane) (PMHS) in the AgF2 (1 mol.%)/ CH3N02 (10 mol.%)/ (tf)-(S>JOSIPHOS (l-[2-(diphenylphosphino)- ferrocenyl]ethyldicyclohexylphosphine) / toluene / H20 system. Nitroalkenes 4 were obtained in 52-88% yields with ee up to 96% (Scheme 2) 191. 46 Edgars Abele Main Group Metal Chemistry PhSiH, / CUF2 / (/?MS)-JOSIPHOS MeNO, / Η,Ο / PMHS / PhMe / rt NO2 _ ^ γ NO2 R· R' R = aryl; R' = alkyl Scheme 2 Synthesis of silyl ethers of oximes 6 was carried out using phase transfer catalytic (PTC) system oxime 5/ hydrosilane / CsF / 18-crown-6 / benzene at 50°C (Scheme 3). It has been found that the optimal amount of cesium fluoride is 5 mol% to oxime and hydrosilane. An increase in the amount of fluoride ion diminishes the yield of desired silylated oxime O-ethers. Silylated ketoximes were isolated up to 78 % yields as main products /10/. NOH , , „„ NOSiR2R3R4 ίΝ^π R2R3R4SiH ! ο,ρ ! !8<rown.6 / phH / MS 4A 20C 4 3 2 3 4 ivΛ R' R JL R' + R R R SiOSilf R R 5 6 R-R4 = alkyl, aryl Scheme 3 The mechanism of silylation of acetophenone oxime with dimethylphenylsilane was carried out using quantum chemical method AM-1 (Scheme 4). The first reaction step is the interaction of F-ion with the oxime group hydrogen atom. The calculations have shown that the reaction proceeds without any activation barrier and leads to formation of oxime anion and complex HF...Cs+...18-crown-6. The oxime anion and dimethylphenylsilane form complex 7. The positively charged complex [HF...Cs... 18-crown-6]+ serves as a source of proton. The next step of the reaction is interaction of complex 7 with [HF...Cs... 18-crown-6]+. After formation of the hydrogen molecule the approaching of oxime anion and silicon cation takes place. The reaction is completed with formation of the desired product 6. 47 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review Scheme 4 The catalytic system Me3SiN3 / solid CsF / 18-crown-6, as the most active, was used in the synthesis of unsymmetric siloxanes 9 from hydrosilanes 8 (molar ratio hydrosilane : Me3SiN3 : CsF : 18-crown-6 = 1:3: 0.1 : 0.1, Scheme 5). The siloxanes 9 were obtained in 45-70% yields. This method was also used in the synthesis of trisiloxane (yield 33%) from diphenylsilane and 6 equivalents of Me3SiN3 /ll/. DID2D3C- Me SiN / CsF / 18-crown-6 R'R2R3SiOSiMe, R R R SiH 3 3 9 8 H20 /PhMe / 8ffC R1 - R3 = alkyl, aryl Scheme 5 Quantum chemical calculations of unsymmetrical siloxanes 9 formation in the PTC system allowed to propose the following reaction mechanism (Scheme 6). The first step is the interaction of complex [HO*— 2(18-crown-6)Cs+-F"—H+] 10 with hydrosilanes 8. The next step is addition of hydroxyl anion to positively charged silicon and formation of silanol molecule. The following interaction between silanol and fluoride ion gives HF molecule and silanol anion 11. The next reaction step is interaction between anion 11 and azidotrimethylsilane, leading formation of the complex of azidotrimethylsilane with silanol anion. This complex reacts with the proton from 2(18-crown-6)Cs+-F"—H+ species. Addition of the proton to the nitrogen bonded to the silicon atom proceeds. The result of this process is the cleavage of Si-N bond, and formation of HN3 and unsymmetrical disiloxane 9. 48 Edgars Abele Main Group Metal Chemistry 3. Si-C BOND 3.1. Vinyl and Allyl Silanes ß-Silyl alkenals 12 in the presence of Bu4NF afforded 2-(arylmethyl)aldehydes 13 in 35-71% yields. The mechanism included formation of pentacoordinated silicon intermediate 14, aryl-l,2-anionotropic rearrangement to the adjacent carbon atom giving enolate 15, its possible cyclization and final removal of 49 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review silyl group from 16 in the presence of F" / H2O /12,13/ (Scheme 7). CHO H BU4NF/THF VK Ar OHC SiMe2Ar Z-12 13 Η F- \r Λ Ar Η Ar Z-12 ".Ph Me-—- OHC F—Si OHC Λ O" p Me Me Me Me 14 15 R Ar F-/H2O Η A χ 13 Ύ Η Η0-slM e OSiMe2F I Me 16 Scheme 7 Allylsilane 17 in the system B114NF / DMSO / H20 gives a mixture of silanol 18 and siloxane 19 observed by GC-MS (Scheme 8) /14/. Ph Ph^ .OH Ph^ BU4NF/H20 s /Si / i + /^i-o-si-Me Me Me Me Me Me Me Me 17 18 19 Scheme 8 Allylsilanes 20 undergo fluorodesilylation in the presence of Selectfluor™ 21 and afford allylic fluorides 22 in yields up to 100% (Scheme 9) /15/. Enantioselective fluorodesilylation of allyl silanes 23 in the presence of Cinchona alkaloid-derived catalyst 24 leads to fluorides 25 with ee up to 96% /16/. 50 Edgars Abele Main Group Metal Chemistry F + 21 CHjCN 20 MeO SiRRH" 24 21 / MeCN / -20°C 23 25 R-R"' = alkyl, Ph Scheme 9 Allylation of benzaldehyde was successfully carried out by allyldimethyl(2-pyridyl)silane 26 in the presence of Bu4NF or AgF (1.2 equivalents) in THF. The product of allylation 27 in the presence of these fluoride ion sources was isolated in 35 or 53% yields, correspondingly (Scheme 10) /17/. PhCHO/F- N ;sv Me Me 27 26 Scheme 10 Allyldifluorosilane (£)-28 bearing a 2-(phenylazo)phenyl group with excess of KF / 18-crown-6 in CDC13 at room temperature during 10 minutes, affords tetrafluorosilicate 29 in 82 % yield /18/. However, no changes were observed on treatment of (Z)-28 with this system in the dark. Photoirradiation (λ = 445 nm) of silane (Z)-28 with an excess of KF in CDClj for 30 minutes gave 91 % of the salt 29 (Scheme 11). 51 Vol. 28, No. 2, 2005 Activation of Silicon Bonds by Fluoride Ion in the Organic Synthesis in the New Millenium: A Review Ph Ph Ph—N, N=N KF 'N-N- KF / 18-crown-6 Ν 18-crown-6 hv x ' F CrFis CΓ^F ^ K+, 18-crown-6 (Z)-28 (£)-28 29 KF / 18-crown-6 no reaction Scheme 11 3.2.