D. A. Evans Bonding considerations: Carbon vs Silicon Chem 206 Bonding Considerations: Carbon vs Silicon Hypervalent 5-Coordinate Silicon Compounds Average Bond dissociation energies(Kcal/mol) Akiba, " Chemistry of hypervalent Compounds"Wiley-VCH, Chapters 4-5, 1999 C-c C-Si Si-si C-F Si-F Penta-coordinate silicates are commonly observed MesiA nEta Ph3 SiF2 verage Bond Lengths(A) C-H Si-H C-c C-Si C-o Si-c Nucleophilic substitution at Silicon 1541.871431.66 10 F-SiMe ○→ better thai Me3 SHOCMe3 C-SP Duhamel et al. J. Org. Chem. 1996, 61, 2232 gC-c H3C-CH3 BDE= 83 kca/mol H3C-SiH3 BDE -76 kcal/mol OSiMe3 Bond length=1.534A Bond length =1.87A HF This trend is even more dramatic with pi-bonds Stork et al. JACS. 1968. 90. 4462 4464 TC-C= 65 kcal/mol C-Si= 36 kcal/mol I Si-Si= 23 kcal/mol i Thermal Rearrangements One may readily access divalent intermediates Group /V Electronegativities(Pauling CH2 H,C=Cl Carbon Silicon Germanium Tin Lead Me 255 190 201 196233 +2 Oxidation state becones increasingly more stableD. A. Evans Bonding Considerations: Carbon vs Silicon Chem 206 Bonding Considerations: Carbon vs Silicon C–C C–Si 83 76 Si–Si 53 C–O Si–O 86 108 C–H Si–H 83 76 C–F Si–F 116 135 Average Bond dissociation eneregies (Kcal/mol) C–C C–Si 1.54 1.87 C–O Si–O 1.43 1.66 Average Bond Lengths (Å) p C–C = 65 kcal/mol p C–Si = 36 kcal/mol p Si–Si = 23 kcal/mol This trend is even more dramatic with pi-bonds: s* C–Si s* C–C s C–Si s C–C Bond length = 1.87 Å Bond length = 1.534 Å H3C–SiH3 BDE ~ 76 kcal/mol H3C–CH3 BDE = 83 kcal/mol better than Group IV Electronegativities (Pauling) Carbon Silicon Germanium Tin Lead 2.55 1.90 2.01 1.96 2.33 +2 Oxidation state becones increasingly more stable C Si d– d+ Hypervalent 5-Coordinate Silicon Compounds Akiba, "Chemistry of hypervalent Compounds" Wiley-VCH, Chapters 4-5, 1999 Penta-coordinate silicates are commonly observed Nucleophilic substitution at Silicon F F–SiMe3 + OSiMe3 KOCMe3 O K Me3Si–OCMe3 Duhamel et al. J. Org. Chem. 1996, 61, 2232 OSiMe3 MeLi O Li Me3Si–Me Stork et al. JACS. 1968, 90, 4462, 4464 THF THF –20 ° 2h Thermal Rearrangements One may readily access divalent intermediates Si Me Me Si CH2 Me Me H2C CH2 thermolysis H Si Me Me thermolysis Si Me Me Si Me Me H Colvin, pp 7-9 C-SP3 Si-SP3 C-SP3 C-SP3 C C C C C Si C Si MeSiF4 NEt4 Ph3SiF2 NR4 RO–SiMe3 RO