Lewis Acid-Promoted Oxidative Addition of Thioimidates to Pd(0)

2002-11-22T00:00:00Z (GMT) by Indranath Ghosh Peter A. Jacobi
The isomeric <i>S</i>-methyldihydropyrrins <b>9</b>-<i>Z</i> and <b>9</b>-<i>E</i> exhibit markedly different behavior in Pd(0)-catalyzed cross-coupling reactions. Thioimidates <b>9</b>-<i>Z</i> are readily converted to imines <b>10</b>-<i>Z</i> employing Pd(0)/AlkZnI. Under identical conditions <b>9</b>-<i>E</i> are inert. Oxidative addition to Pd(0) requires activation by Zn or other Lewis acids, which is sterically unfavorable with <b>9</b>-<i>E</i>. Analogous results were obtained with the related thioimidates <b>11</b>-<i>E,Z</i> as well as with methylthiopyridines <b>19</b>α−γ. In the case of both <b>11</b> and <b>19</b> oxidative addition to Pd(0) was greatly facilitated in the presence of BF<sub>3</sub>·Et<sub>2</sub>O. The importance of Lewis acid activation to Pd(0) oxidative addition in such substrates appears to be a general phenomenon not previously recognized.