PROPARGYLIC COUPLING REACTIONS VIA BIMETALLIC ALKYNE COMPLEXES: THE NICHOLAS REACTION

摘要

The discovery of ferrocene in the 1950s, with its unprecedented sandwich structure and aromatic reactivity, ushered in an era of intense research anddevelopment focused on the distinctive reactivity of organotransition-metal compounds that continues to this day. In the 1960s both ferrocenyl and (arene)Cr(CO)3 units were found to have remarkable stabilizing effects on α-carbocations 1 and 2, dramatically accelerating adjacent S_N1 substitution reactions (Scheme 1). These revelations stimulated early studies of the chemistry of (alkyne)dicobalt hexacarbonyl complexes 3, which, on one hand, displayed a suppressed reactivity of the triple bond upon coordination, and on the other, an activation of the propargylic position towards acid-promoted substitution and elimination, suggestive of an enhanced stability of the corresponding (propargylium)Co2(CO)6 cations 4 (Scheme 2).