Synthesis and Lewis acidity of triorganosilyl and triorganostannyl esters of orthoboric, metaboric, and arylboronic acids

The synthesis of 30 new triorganosilyl and triorganostannyl esters of orthoboric, metaboric, and arylboronic acids are described: (R3SiO)(3)B {R-3 = (BuMe2)-Bu-n, PhMe2, [2-(3-cyclohexenyl)ethyl]Me-2, ((n)Octadecyl)Me-2, (n)Hex(3)}; (R3MO)(3)B3O3 {M = Si: R-3 = Pr-n(3), Bu-n(3), (n)Hex(3), EtMe2, (BuMe2)-Bu-n, [2-(3-cyclohexenyl)ethyl]Me-2, [(Me2HC)(Me)(2)C]Me-2, ((n)Octadecyl)Me-2, PhMe2, Ph2Me, Ph-3. M = Sn, R = Ph, Me}; (R3MO)(2)BAr {M = Si: Ar = Ph, R-3 = Pr-n(3), Bu-n(3), Ph2Me; Ar = C6H4Br-4, R = Ph, Pr-n; Ar = C6H4Me-2, R = Ph, Pr-n. M = Sn: R = Ph, Ar = Ph, C6H4Br-4, C6H4Me-2; R = Bu-n, Ar = Ph, C6H4Br-4}. The esters were characterised by elemental analysis, m.p./b.p., H-1- C-13-, and B-11-NMR and IR spectroscopy; solids were characterised by solid-state B-11 MAS NMR spectroscopy. The Lewis acidity of liquid compounds was determined by Gutmann's acceptor number (AN) method and well-defined ranges were observed for the compound classes {(R3MO)(3)B, 23-36; (R3MO)(2)BAr, 23-31; (R3MO)(3)B3O3, 44-81}. The effect of the R3Si or R3Sn group was to enhance the Lewis acidity at boron relative to organic esters. Increasing the steric bulk of the R3M moiety significantly reduced AN values and there was little differentiation in AN values between R3Si and R3Sn derivatives. (C) 1999 Elsevier Science S.A. All rights reserved.