Preparation and Reactions of 1,3‐Diphosphacyclobutane‐2,4‐diyls That Feature an Amino Substituent and/or a Carbonyl Group

The preparation and properties of a 1‐amino‐1,3‐diphosphacyclobutane‐2,4‐diyl and a 1‐benzoyl‐1,3‐diphosphacyclobutane‐2,4‐diyl, which can be regarded as functionalized cyclic biradical derivatives, were investigated. Hydrolysis of 1‐diisopropylamino‐3‐methyl‐2,4‐bis(2,4,6‐tri‐ tert ‐butylphenyl)‐1,3‐diphosphacyclobutane‐2,4‐diyl ( 7 ), which is formed by reaction of Mes*CP ( 4 ; Mes*=2,4,6‐ t Bu 3 C 6 H 2 ) with lithium diisopropylamide and iodomethane, resulted in ring‐opening of the 1,3‐diphosphacyclobutane‐2,4‐diyl skeleton, as well as de‐aromatization of one of the Mes* rings. 3‐Oxo‐1,3‐diphosphapropene 8 and 7‐phosphabicyclo[4.2.0]octa‐1(8),2,4‐triene 9 were the resultant products, and these were subsequently characterized. Isomerization and oxidation of 7 occurred in the presence of TEMPO (2,2,6,6‐tetramethyl‐1‐piperidinoxy) to give the first example of a cyclic dimethylenephosphorane derivative, namely 3‐oxo‐1,3‐diphospha‐1,4‐diene 10 . 1‐Benzoyl‐3‐ tert ‐butyl‐2,4‐bis(2,4,6‐tri ‐tert ‐butylphenyl)‐1,3‐diphosphacyclobutane‐2,4‐diyl ( 12 ) was isolated and characterized from the reaction of 4 with tert ‐butyllithium and benzoyl chloride. Compound 12 was subsequently heated and underwent rearrangement of the benzoyl group and ring‐expansion to afford 1‐oxo‐1 H ‐[1,3]diphosphole 13 . Reaction of 4 with lithium diisopropylamide and benzoyl chloride afforded the 2 H ‐[1,2,4]oxadiphosphinine 15 , which was probably formed through the 1,3‐diphosphacyclobutane‐2,4‐diyl intermediate 14 . Thermolysis of 15 afforded 1‐oxo‐1 H ‐[1,3]diphosphole 16 in an Arbuzov‐type rearrangement.