Collect. Czech. Chem. Commun. 1990, 55, 2033-2037

29Si and 13C NMR spectra of some substituted bis(trimethylsiloxy)benzenes

Jan Schramla, Robert Brežnýb, Jan Čermáka and Václav Chvalovskýa

a Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, 165 02 Prague 6-Suchdol
b Faculty of Chemical Technology, Slovak Technical University, 812 37 Bratislava


NMR spectra of a few 4-substituted 1,2-bis(trimethylsiloxy)benzenes and positional isomers of bis(trimethylsiloxy)benzene were measured under standard conditions that approach those of infinite dilution. Under these conditions somewhat smaller values of steric or proximity deshielding effects on 29Si chemical shifts are found but the trends established earlier are confirmed; the deshielding effect of the ìsecondî ortho positioned trimethylsiloxy group in a tris(trimethylsiloxy)benzene derivative on 29Si chemical shifts is somewhat smaller. Also, overcrowding of three trimethylsiloxy groups probably drives the outer groups to assume, at least for a part of their time, coplanar conformation which is not encountered in bis(trimethylsiloxy) derivatives. The sensitivity of 29Si chemical shifts to para substitution is in 1,2-bis(trimethylsiloxy)benzenes equal to that in 2-methoxytrimethylsiloxybenzenes.