Acylation Reactions of Dibenzo-7-phosphanorbornadiene: DFT Mechanistic Insights.

Extensive DFT calculations provide deep mechanistic insights into the acylation reactions of -butyl dibenzo-7-phosphanobornadiene with PhCOX (X=Cl, Br, I, OTf) in CHCl solution. Such reactions are initialized by the nucleophilic P⋅⋅⋅C attack to the carbonyl group to form the acylphosphonium intermediate together with X anion, followed either by nucleophilic X⋅⋅⋅P attack (X=Cl, Br, and I) toward to eliminate anthracene or by slow rearrangement or decomposition of (X=OTf). In contrast to the first case (X=Cl) that is rate-limited by the initial P⋅⋅⋅C attack, other reactions are rate-limited by the second X⋅⋅⋅P attack for X=Br and I and even thermodynamically prevented for X=OTf, leading to isolable phosphonium salts. The rearrangement of phosphonium is initiated by a P-C bond cleavage, followed either by sequential proton-shifts to form anthracenyl acylphosphonium or by deprotonation with additional base EtN to form neutral anthracenyl acylphosphine. Our DFT results strongly support the separated acylphosphonium as the key reaction intermediate that may be useful for the transfer of acylphosphenium in general.