American Chemical Society
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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

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journal contribution
posted on 2004-03-19, 00:00 authored by N. Satish Kumar, K. Praveen Kumar, K. V. P. Pavan Kumar, Praveen Kommana, Jagadese J. Vittal, K. C. Kumara Swamy
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 2223 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.