Silicon–oxygen and acyl–oxygen bond cleavage in reactions of sterically hindered organosilicon carboxylates

Abstract

Because of steric hindrance to attack at the functional silicon centre in VsiSiMe₂OCOR [where Vsi denotes (Me₃Si)₂(CH₂CHMe₂Si)C] with R = CF₃(1a) or Me (1b), nucleophiles in MeOH can preferentially attack at the carbonyl group to give VsiSiMe₂OH, rather than at Si as would be usual for less hindered silicon carboxylates. With CsF or KSCN in MeOH the products are exclusively VsiSiMe₂F and VsiSiMe₂NCS, respectively, whereas KOCN gives exclusively VsiSiMe₂OH, and NaN₃ gives very predominantly VsiSiMe₂OH along with a little VsiSiMe₂N₃. The overall effectiveness of the salts is KOCN > CsF, NaN₃ > KSCN; the sequence of the case of attack of the nucleophiles at Si is F^– > SCN^–(>N₃^–?), and that for attack at carbon is OCN^– > N₃^– > F^– > SCN^–. The sequence of ability of various nucleophiles to catalyse cleavage of the SiO–COR bond of (1a) in MeOH is RNH₂(R = H₂N, Me, Et, HOCH₂CH₂, or H₂NCH₂CH₂), OCN^–, HO₂^– > MeO^– > N₃^– > Me₂NH > Me₃N > 4-Me₂NC₅H₄N C₅H₅N > PhNH₂, and the sequence for reactions of the less reactive (1b) is HO₂^– > H₂NNH₂ > MeO^– > H₂NCH₂CH₂NH₂ > MeNH₂ > HOCH₂CH₂NH₂ > EtNH₂. Except for the unexpected effectiveness of OCN^–, the sequences are reasonably consistent with those established, for example, for reactions of aryl acetates in water (and also with Ritchie's nucleophilic parameters N₊); it is suggested that, as in the latter reactions, the cleavage of the acyl–oxygen bond is rate-determining. Reactions of (1a) with salts MX, namely NaN₃, CsF, KSCN, and KOCN in MeCN, give exclusively the corresponding species VsiSiMe₂X. In the representative reactions examined the reactivities of (1a) and (1b) were very similar to those of the corresponding compounds (Me₃Si)₃CSiMe₂OCOR.