Rates of base-catalysed cleavage of pyridyl-, quinolyl-, picolyl- and (quinolylmethyl)-trimethylsilanes

Abstract

Rates of cleavage of some picoyl- and (quinolylmethyl)-trimethylsilanes (RSiMe 3, where R = PyCH 2 or QnCH 2SiMe 3) have been measured in “90%” aqueous methanolic sodium methoxide at 50°C. Relative reactivities are: 2-PyCH 2, 1.0; 3-PyCH 2, 0.030; 4-PyCH 2, 8.9; 2-QnCH 2, 41; 3-QnCH 2, 0.161; 4-QnCH 2, 37. The rates correlate well with those for base-catalysed hydrogen-exchange in the parent carbon acids RH. Approximate p K a's (based on the scale of ion-pair acidities in CsNHC 6H 11H 2NC 6H 11, with p K a of 9-phenylfluorene = 18.6) for the carbon acids, RH, can be derived as follows: 2-PyCH 3, 29.5; 3-PyCH 3, 34; 4-PyCH 3, 27; 2-QnCH 3, 25; 3-QnCH 3, 32; 4-QnCH 3, 25. Rates of cleavage of pyridyl- and quinolyl-trimethylsilanes (PySiMe 3 and QnSiMe 3) by sodium hydroxide in 4 : 1 v/v Me 2SO/H 2O at 50°C have also been measured; and the relative reactivities are: 2-Py, 1.0; 3-Py, 2.9; 4-Py, 8.4; 2-Qn, 15.9; 3-Qn, 12.7; 4-Qn, 184. The sequence of reactivity differes from that for base-catalysed hydrogen-exchange at the relevant positions of pyridine and quinoline, indicating that the reactivities are not determined in both cases (if in either) solely by the stabilities of the corresponding carbanions.