Azaphospholes anellated

Monocyclic 2H-[ 1,2,3 Idiazaphospholes (B) are easily accessible from the condensation of the four-membered chain incorporated in hydrazones or azoalkanes with phosphorus trichloride making available a large number of representatives that have been intensively studied [2, 4, 7], In contrast, their 1//-isomers (A) are less known and are obtained only as second minor product during the synthesis of 2//-[l,2,3]diazaphospholes in some cases. A facile synthesis for pyrido-anellated azaphospholes has been developed in our group by making use of 1,2-disubstituted pyridinium salts for condensation with phosphorus trichloride [8, 13-15], Accordingly, cyclocondensation of 2-alkyl-1-aminopyridinium iodides (1) with phosphorus trichloride in the presence of triethylamine affords pyrido-anellated l//-[l,2,3]diazaphospholes, i.e. l,2,3]diazaphospholo 1,5-a] pyridines (2) (Scheme 1) [16],... 

The first article on anellated azaphospholes presents a brief account of then-synthesis, structures, and reactions. 

Pilgram and Korte [1] can be credited with perceiving for the first time the possibility of the existence of an anellated azaphosphole when in 1963 they reported the formation of a product that was insoluble in common organic solvents and did not melt up to 350 °C, from the condensation of /V-phenyl-1,2-diaminobenzene with triphe-nylphosphite at 150-180 °C. They assigned the structure 1, namely 1,3,2-benzodia-zaphosphole to the product obtained. 

The earlier reviews on heterophospholes in general include anellated azaphospholes as well, though in a limited manner [3-6], A comprehensive review on anellated heterophospholes covers the literature up to 1994 and includes a detailed list of the compounds and their physical data reported till then [12], In another review, analogy between the synthetic methods and reactions of anellated heterophospholes and their non-phosphorus analogues has been highlighted [13]. A review is limited to the analogy between the synthesis of indolizines and phosphaindolizines [14]. The synthesis of anellated heterophospholes from 2-substituted cycloiminium salts forms the subject matter of two reviews [15,16]. Some recent advances in the chemistry of anellated azaphospholes have been published in a recent review [7],... 

The reviews dealing primarily with the synthetic applications of phosphaalkynes include the synthesis of anellated azaphospholes as well through [3 + 2]cycloaddi-tion [17-19], The 31P NMR chemical shifts of a large number of anellated azaphospholes find place in two reviews [12, 20],... 

It was, however, impossible to obtain /V-alkyl or N-aryl substituted 1,3-benzaza-phospholes or pyrido-anellated 1,3-azaphospholes through the above route. However, palladium-catalyzed cross coupling, followed by reduction of the resulting o-ani-lino- or o-arninopyridophosphonates and condensation with dimethylformamide dimethylacetal led to the desired TV-substituted benzazaphospholes (36, X = CH) or pyrido-anellated azaphospholes (36, X = N) (Scheme 11) [43 47],... 

Cycloadditions of azomethine ylides and -imines with appropriate 1,3-dipolarophiles have been employed for the preparation of anellated pyrroles and pyrazoles, respectively [49, 55-57], In analogy, phosphaalkynes have been used as 1,3-dipolarophiles for introducing a two-coordinate phosphorus leading to a variety of anellated azaphospholes. Many such examples are included in earlier reviews [17-19], In order to illustrate the synthetic strategy, two examples are cited here. 

NMR spectroscopy (31P, H, 13C) has been extensively employed for establishing structures of the anellated azaphospholes. Mass spectral fragmentation has been studied in a few cases. Single crystal X-ray diffraction studies have also been reported for a few representatives. 

H and 13C NMR data of a large number of anellated azaphospholes have been reported [12], The 13C NMR spectra are characterized by characteristic PC couplings. 

Mass spectral fragmentation of a few anellated azaphospholes has been reported [23, 25, 34, 66], In general, molecular ion peak constitutes the base peak and the basic fragmentation resembles that of the analogous non-phosphorus heterocycle. 

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],... 

The coordinating behaviour of several anellated azaphospholes has been investigated. These compounds are found to be weak o-donor and more efficient 71-acceptor ligands. 

Mononuclear complexes of the type [Cp RhCl2(L)]H20 and [Ru(cymene) C12(L)] H20 have been obtained from the reaction of anellated azaphospholes (L = 2-phosphaindolizine and l,3-azaphospholo[5,l-a]isoquinoline) with [r 5-Cp RhCl2]2 (Cp = pentamethylcyclopentadienyl) and [Ru[r 6-cymene)Cl2]2 respectively. NMR studies reveal a dynamic equilibrium between covalent and ionic forms of the complexes in solution [101]. The zwitterionic pyridinodithio-phosphinates (103), obtained from the reaction of 2-phosphaindolizines with H2S/S8, have been used as chelating ligands to obtain [PdCl(L)]+ Cl" type complexes (Scheme 35) [102],... 

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. 

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