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‐s̀ 2 ,λD 3 ‐phosphaboratacyclohexadienes 3 . In contrast, the (di‐ tert ‐butylboryloxy)phosphaethene 5 is obtained from t Bu 2 BP‐(SiMe 3 ) 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.