Diverse modes of reactivity of dialkyl azodicarboxylates with p(III) compounds: synthesis, structure, and reactivity of products other than the Morrison-Brunn-Huisgen intermediate in a Mitsunobu-type reaction

Abstract

The reactivity of diethyl azodicarboxylate (DEAD)/diisopropyl azodicarboxylate (DIAD) with P(III) compounds bearing oxygen or nitrogen substituents is explored. Compounds with structures quite different from that of Morrison-Brunn-Huisgen intermediate R'₃P+N(CO₂R)N-(CO₂R) (1), observed in the Mitsunobu reaction, have been established by using X-ray crystallography and NMR spectroscopy. Thus reactions with X(6-t-Bu-4-Me-C₆H₂O)₂P-NH-t-Bu [X = S (8), CH₂ (9)] or XP(μ-N-t-Bu)₂P-NH-t-Bu [X = Cl (14) or NH-t-Bu (15)] and DEAD/DIAD lead to phosphinimine-carbamate-type of products X{6-t-Bu-4-Me-C₆H₂O}₂P{N-t-Bu}{N(CO₂R)NH(CO₂R)} [X = S, R = Et (16); X = CH₂, R = Et (17); X = CH₂, R = i-Pr (18)] or XP(μ-N-t-Bu)₂P(N-t-Bu){N-(CO₂-i-Pr)-N(H)(CO₂-i-Pr) [X = Cl (19), NH-t-Bu (20)]. Treatment of 19 with 2,2,2-trifluoroethanol afforded the product [(CF₃CH₂O)P(μ-N-t-Bu)₂P⁺(NH-t-Bu){N(CO₂-i-Pr)(HNCO₂-i-Pr)}][Cl^-] (21) whose structure is close to one of the intermediates proposed in the Mitsunobu reaction. The isocyanate CH₂(6-t-Bu-4-Me-C₆H₂O)₂P-NCO (10) underwent 1,3-(P,C) cycloaddition with DEAD/DIAD to lead to CH₂(6-t-Bu-4-Me-C₆H₂O)₂P{N(CO₂R)N(CO₂R)-C(O)-N} [R = Et (22), i-Pr (23)]. Reaction of 22-23 with 1,1'-bi-2-naphthol or catechol leads to novel tetracoordinate CH₂(6-t-Bu-4-Me-C₆H₂O)₂P(2,2'-OC₁₀H₆-C₁₀H₆-OH){NC(O)-(CO₂R)NH(CO₂R)} [R = Et (24), i-Pr (25)] or pentacoordinate CH₂(6-t-Bu-4-Me-C₆H₂O)₂P(1,2-O₂C₆H₄){NHC(O)-N(CO₂R)NH(CO₂R)} [R = Et (26), i-Pr (27)] compounds in which the original NCO residue is retained; this mode of reactivity is quite different from that observed for the MBH betaine 1. In 27, the nitrogen, rather than the oxygen, occupies an apical position of the trigonal bipyramidal phosphorus violating the commonly assumed preference rules for apicophilicity. It is shown that the previously reported azide derivative 3, obtained from the reaction of 11 with DIAD, undergoes a Curtius-type rearrangement to lead to the fused cyclodiphosphazane [{CH₂(6-t-Bu-4-Me-C₆H₂O)₂}P{OC(O-i-Pr)NN(CO₂-i-Pr)N}]₂ (28); this compound is in equilibrium with its monomeric form in solution at >300 K. Finally, reaction of S(6-t-Bu-4-Me-C₆H₂O)₂P(OPh) (13) with DIAD gave the hexacoordinate compound S{6-t-Bu-4-Me-C₆H₂O}₂P(OPh){N(CO₂-i-Pr)NC(O-i-Pr)O} (30) with an intramolecular S→P bond. X-ray crystallographic evidence for compounds 16, 19, 21, 22, 25, 27, 28, and 30 has been provided.