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

Extensive DFT calculations provide deep mechanistic insights into the acylation reactions of tert-butyl dibenzo-7-phosphanobornadiene with PhCOX (X=Cl, Br, I, OTf) in CH₂Cl₂ solution. Such reactions are initialized by the nucleophilic P⋅⋅⋅C attack to the carbonyl group to form the acylphosphonium intermediate A⁺ together with X⁻ anion, followed either by nucleophilic X⁻⋅⋅⋅P attack (X=Cl, Br, and I) toward A⁺ to eliminate anthracene or by slow rearrangement or decomposition of A⁺ (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 A⁺ is initiated by a P-C bond cleavage, followed either by sequential proton-shifts to form anthracenyl acylphosphonium or by deprotonation with additional base Et₃N to form neutral anthracenyl acylphosphine. Our DFT results strongly support the separated acylphosphonium A⁺ as the key reaction intermediate that may be useful for the transfer of acylphosphenium in general.