Influence of chelating silyl scavengers on the diastereoselectivity of chromium catalyzed pinacol cross couplings

The choice of the silyl scavenger shows a substantial influence on the diastereoselectivity of the chromium catalyzed pinacol cross coupling. In the case of the intramolecular cyclization, two competing effects are observed. If the silylation reagents contain chelating heteroatoms in the side chain, a remarkable trans-selectivity is observed. Conversely the cis-diols are preferred if bulky substituted scavengers are used. A suitable transition state model for this reaction is reported, based on formerly published results.