Carbene-Metal Hydrides Can Be Much Less Acidic Than Phosphine-Metal Hydrides: Significance in Hydrogenations

摘要

Abstract: Acidities of iridium hydride intermediates were shown to be critical in some transformationsnmediated by the chiral analogues of Crabtree’s catalyst, 1 3. To do this, several experiments werenundertaken to investigate the acidities of hydrogenation mixtures formed using these iridium-oxazolinencomplexes. DFT calculations indicated that the acidity difference for Ir H intermediates in thesenhydrogenations were astounding; iridium hydride from the N-heterocyclic carbene catalyst 1 was calculatednto be around seven pKa units less acidic than those from the P-based complexes 2 and 3. Consistent withnthis, the carbene complex 1 was shown to be more effective for hydrogenations of acid-sensitive substrates.nIn deuteration experiments, less “abnormal” deuteration was observed, corresponding to fewer complicationsnfrom acid-mediated alkene isomerization preceding the D2-addition step. Finally, simple tests with pHnindicators provided visual evidence that phosphine-based catalyst precursors give significantly more acidicnreaction mixtures than the corresponding N-heterocyclic carbene ones. These observations indicate carbene-nfor-phosphine (and similar) ligand substitutions may impact the outcome of catalytic reactions by modifyingnthe acidities of the metal hydrides formed.