Azaphospholes functionalities

Density functional theory (DFT) calculations at the B3LYP/6-31H-G"" level were carried out on the 1,3-dipolar cycloadditions of various heterophospholes, including 1,3-azaphosphole, with diazo compounds across the P=N bond <2003JP0504>. In most cases, the dominant frontier orbital interaction is between HOMO(diazo) with LUMO(heterophosphole) however, 1,3-azaphosphole has a HOMO of high energy and for it, HOMO(heterophosphole)-LUMO(diazo) is also important (HOMO = highest occupied molecular orbital LUMO = lowest unoccupied molecular orbital). 

Gradient-corrected (BP86) density functional calculations have been carried out on various P-heterocyclic carbene metal complexes, including where the carbene was derived from a 1,3-azaphosphole <2005JOM(690)6068> and quantum-mechanical calculations at the DFT level were used to analyze metal-ligand interactions in mixed iron(ll) metallocenes where, in one instance, one of the ligands was a 1,3-diphospholide anion <20040M5308>. 

The azaphosphole ring incorporates several functionalities (Fig. 1) and hence exhibits interesting reactivities towards a variety of reagents. 

In view of this, we have undertaken a systematic investigation of the DA reactions of a variety of anellated azaphospholes whose syntheses have been reported by our group earlier. The results of some of these reactions are included in two recent reviews dealing primarily with cycloadditions of heterophospholes [84] and DA reactions with >C = P- functionality [85],... 

In the azaphosphole ring, both carbon and phosphorus atoms of the >C =P- functionality are prochiral and DA reaction with it leads to the generation of two stereogenic centres. The results, however, indicate that these reactions are accompanied by high diastereo- and regioselectivity that has been rationalized in many cases by theoretical calculations [84, 85],... 

A variety of anellated azaphospholes have become accessible through facile methods. These compounds have appreciable stability and incorporate several functionalities. In view of this, starting with these compounds, a variety of organophosphorus compounds including their transition metal complexes can be synthesized. Some promising results have already been obtained and it is hoped that this brief report will stimulate further work in this field. 

A real landmark in the generation of transient nitrilium phosphanylid complexes was the discovery that 7-phosphanorbornadiene complexes could be also employed, e.g., the complexes 14a,b in the synthesis of 2H-l,2-azaphosphole complexes 16a,b (Scheme 10), thus offering now the possibility to study the reactivity of sterically less encumbered 1,3-dipole species such as 15a,b [34]. Note worthy is the absence of any by-products such as A -l,3,2-oxazaphospho-lene complexes (cf. Scheme 7) in these reactions. Even more fascinating are perspectives to use 7-phosphanorbornadiene complexes with P-functional substituents in 2H-l,2-azaphosphole complex chemistry. 

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