Further characterization of Mitsunobu-type intermediates in the reaction of dialkyl azodicarboxylates with P(III) compounds

Kumara Swamy, K. C. ; Praveen Kumar, K. ; Bhuvan Kumar, N. N. (2006) Further characterization of Mitsunobu-type intermediates in the reaction of dialkyl azodicarboxylates with P(III) compounds Journal of Organic Chemistry, 71 (3). pp. 1002-1008. ISSN 0022-3263

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Official URL: http://pubs.acs.org/doi/abs/10.1021/jo051997x?prev...

Related URL: http://dx.doi.org/10.1021/jo051997x

Abstract

Structural characterization of compounds analogous to the proposed intermediates in the Mitsunobu esterification process is achieved by the combined use of NMR spectroscopy and X-ray diffractometric studies. The results show that compounds (t-BuNH)P(μ-N-t-Bu)2P[(N-t-Bu)(N-(CO2R)-N(H)(CO2R))] [R = Et (11), i-Pr (12)], obtained by treating [(t-Bu-NH)P-μ-N-t-Bu]2 (10) with diethylazodicarboxylate (DEAD) or diisopropylazodicarboxylate (DIAD), respectively, have a structure with the NH proton residing between the two nitrogen atoms ((P)N(t-Bu) and (P)N-N(CO2Et)); this is the tautomeric form of the expected betaine (t-BuNH)P(μ-N-t-Bu)2P+[(NH-t-Bu)(N-(CO2R)-N-(CO2R)]. Treatment of ClP(μ-N-t-Bu)2P[(N-t-Bu){N-(CO2-i-Pr)-N(H)(CO2-i-Pr)] (6) with 2,6-dicholorophenol affords (2,6-Cl2-C6H3-O)P(μ-N-t-Bu)2P+[(NH-t-Bu){N[(CO2i-Pr)(HNCO2i-Pr)]}](Cl-)(2,6-Cl2-C6H3-OH) (14) that has a structure similar to that of (CF3CH2O)P(μ-N-t-Bu)2P+[(NH-t-Bu){N[(CO2i-Pr)(HNCO2i-Pr)]}](Cl-) (13), but with an additional hydrogen bonded phenol. Both of these have the protonated betaine structure analogous to that of Ph3P+N(CO2R)NH(CO2R)(R'CO2)- (2) proposed in the Mitsunobu esterification. Two other compounds, (ArO)P(μ-N-t-Bu)2P+(NH-t-Bu){N(CO2i-Pr)(HNCO2i-Pr)}(Cl-) [Ar = 2,6-Me2C6H3O- (15) and 2-Me-6-t-Bu-C6H3-O- (16)], are also prepared by the same route. Although NMR tube reactions of 11 or 12 with tetrachlorocatechol, catechol, 2,2'-biphenol, and phenol revealed significant changes in the 31P NMR spectra, attempted isolation of these products was not successful. On the basis of 31P NMR spectra, the phosphonium salt structure (t-BuNH)P(μ-N-t-Bu)2P+[(HN-t-Bu){N-(CO2R)-N(H)(CO2R)](ArO-) is proposed for these. The weakly acidic propan-2-ol or water did not react with 11 or 12. Treatment of 12 with carboxylic acids/ p-toluenesulfonic acid gave the products (t-BuNH)P(μ-N-t-Bu)2P+[(HN-t-Bu){N-(CO2-i-Pr)-N(H)(CO2-i-Pr)](ArCO2-) [Ar = Ph (18), 4-Cl-C6H4CH2 (19), 4-Br-C6H4 (20), 4-NO2-C6H4 (21)] and (t-BuNH)P(μ-N-t-Bu)2P+[(HN-t-Bu){N-(CO2-i-Pr)-N(H)(CO2-i-Pr)](4-CH3-C6H4SO3-) (22) that have essentially the same structure as 2. Compound 18 has additional stabilization by hydrogen bonding, as revealed by X-ray structure determination. Finally it is shown that the in situ generated (t-BuNH)P(μ-N-t-Bu)2P+[(HN-t-Bu){N-(CO2Et)-N(H)(CO2Et)](4-NO2-C6H4CO2-) can also effect Mitsunobu esterification. A comparison of the Ph3P-DIAD system with the analogous synthetically useful Ph3P-dimethyl acetylenedicarboxylate (DMAD) system is made.

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Deposited On:23 Dec 2010 05:38
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