Reactions of Monomeric Phosphanylboranes with Acyl Chlorides: 1‐Oxa‐3‐oxonia‐5‐phospha‐2‐borata‐3,5‐cyclohexadienes

Monomeric B-[bis(trimethylsilyl)phosphanyl]-9-borabicyclo-[3.3.1]nonane (1) reacts with acyl chlorides 2 in a 1:2 ratio to afford deep red derivatives of six-membered oxaoxonia-s2,λD3-phosphaboratacyclohexadienes 3. In contrast, the (di-tert-butylboryloxy)phosphaethene 5 is obtained from tBu2BP-(SiMe3)2 (4) and PhC(O)Cl. It is assumed that both reactions proceed via a common intermediate, an (acylphosphanyl)bo-rane 6. The color of compounds 3, which feature a flat boat conformation, is due to a π−π* transition.