Silylation of substituted benzhydroxamic acids: NMR speetra (C-13, N-15 and Si-29) and structure of tert-butyldimethylsilyl derivatives

Twelve para- and meta-substituted benzhydroxamic acids were subjected to ex haustive silylation with N-(tert-butyldimethylsilyl)-N-methyltrifluoroaceta mide (with 1% of tert-butyldimethylsilyl chloride as a catalyst). In all ca ses only one product was isolated. According to the assigned NMR spectra (C -13, N-15, and Si-29), the product is the Z-O-1, O-4-bis(tert-butyldimethyl silyl) derivative of substituted benzhydroximic acid, independently of the nature of the para or meta substituent. For structure determination, the Si -29 shifts and (1)J(C-13, (CN)-C-13) coupling constants are decisive. The c hemical shifts (C-13, N-15, Si-29) show dependences on the substituent cons tants of various kinds: even the C-13 shift of the sixth atom from the benz ene ring varies with substitution. The C-13 and N-15 chemical shifts of the C=N moiety exhibit opposite dependences on the substituent; the Si-29 shif ts of the two silicon atoms are almost equally sensitive to the substituent effects despite their different distances from the substituent. Copyright (C) 1999 John Wiley & Sons, Ltd.