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1 2001

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Alternatywne mediatory reakcji Mitsunobu

Falkiewicz B., Wiśniewski K., Kołodziejczyk A.S.

Wiadomości Chemiczne

2001

[Z] 55, 9-10

849-858

The Mitsunobu reaction is a versatile method for the alkylation of various Bronsted-Lowry acids (HA) by alcohols, proceeding in neutral media in a presence of the redox system which traditionally consists of diethyl azodicarboxylate and triphenylphosphine. The key step of the process proceeds according to the SN2 mechanism and results in one of the most useful attributes of the reaction, namely complete configurational inversion at the carbinol carbon. The reaction, however, has a serious limitation - the acidic component has to have pKa smaller than 13 for the reaction to proceed smoothly. Moreover, the classical methodology of the reaction suffers from low yields when applied to secondary alcohols. In recent years, in order to overcome these drawbacks and expand the versatility of the original combination of the Mitsunobu mediators, significant progress in the reaction methodology has been made, mainly due to the work of Tsunoda and Itô. Two types of new mediators have been developed to replace the azodicarboxylate-TPP system. The first one is an N,N,N',N'-tetrasubstituted azodicarboxamide - tributyl phosphine system. All azodicarboxamide derivatives were found to be more efficient than traditional DEAD in the Mitsunobu reaction, especially for less acidic HX. N,N,N',N'-Tetramethylazodicarboxamide, TMAD, gives the best overall results among acyclic amides, whereas 4,7-dimethyl-3,5,7-hexahydro-1,2,4,7-tetraazocin-3,8-dion, DHTD, in combination with TBP was found to be unique in mediating the formation of the C-C bond with sec-alcohols at room temperature. The other type of new mediators in the Mitsunobu reaction, structurally based on betaine, is cyanomethylenetrialkylphosphorane. The phosphorane reagents are generally less active at room temperature, but in higher temperatures they are, esp. CMMP, better than DHTD-TBP and afford satisfactory alkylation of secondary alcohols. Furthermore, the phosphorane reagents mediate the reaction of acids of pKa up to 23.5 [33]. Comparative studies of the C-alkylation revealed the general reactivity of new mediators as TMAD-TBP ( DHTD-TBP ( CMBP ( CMMP.