Azaphospholes synthesis

The use of 2-substituted cycloiminium salts in azaphosphole synthesis <95S36l> and the use of phosphaalkynes as building blocks for heterophosphole synthesis <90CRV191, B-90MI 316-02,... 

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],... 

A potentially useful synthesis of 177-1,2-azaphospholes by the reaction of alkynes with l-aza-2-phospha-4-vanada-2-cyclobutenes generated from R1N=VC13 and phosphaalkynes may be considered as an example of cyclic carbovanadation 4 (Scheme 50). 

Apparently independently, Markl et al. (139) and Regitz and co-workers (140-142) discovered that 1,3-dipolar cycloaddition reactions of mtinchnones and phosphaalkenes or phosphaalkynes provide a direct synthesis of 1,3-azaphospholes (240) (Table 10.7). The intermediate cycloadducts cannot be isolated. The various phosphaalkynes were generated from phosphaalkenes or, in the case of methyli-dynephosphane (239, R" =H), by flash vacuum pyrolysis of either 239 (R" = f-Bu) or dichloromethylphosphine. 

Several of the systems reviewed in this chapter have been the subject of chapters in The Science of Synthesis series which deal in detail with the synthesis of those systems 1,3-azaphospholes and 1,3-azaarsoles <2002SOS(12)679>,... 

Phospholes bearing additional heteroatoms other than phosphorus have also been the subject of further study. Cycloaddition reactions of heterophospholes have received a theoretical treatment and these reactions have also been reviewed. Improvements in the synthesis of fused 1,3-azaphospholes via the reactions of N-alkyl-isoquinolinium salts with phosphorus trichloride have been described and their reactivity towards cycloaddition studied. New fused... 

This procedure was first developed for the synthesis of 1,5-annellated 1,3-azaphospholes like 2-phosphaindolizines <91CB475>. It needs a four-membered chain with reactive methylene groups at both ends for condensation with phosphorus trichloride to form the ring. For 1,5-anellated 1,3-... 

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

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],... 

Scheme 11 Synthesis of benzo- and pyrido- anellated [l,3]azaphospholes...

The electronic structure and degree of aromaticity of 1,3-azaphospholes has received attention from the theoreticians. An alternative synthesis of the phospha-indolizine system (201), from the reaction of pyridinium salts with phosphorus trichloride, has undergone further development " and has also been applied in the synthesis of fused 1,4,2-diazaphospholes, e.g., (234) ", 2 (235). The 1,2,4,3-triazaphosphole system (236) has been isolated as an... 

Scheme 11.24. One-pot synthesis of pyrimidine systems [75, 76], thiazolo-s-triazine C-nucleosides [77], and 1,3-azaphospholes [78].

As shortly described in the preceding section, the synthesis of 2H-l,2-azaphos-phole complexes relies (mainly) on three-component reactions. A good illustration of this methodology, which was first described in 1998 [42], is given by the reaction of 2H-azaphosphirene complex la with dimethylcyanamide and DMAD in toluene, which led to the 2H-l,2-azaphosphole complex 5b and the diastereomeric A -l,3,2-oxazaphospholene complexes 6a,b (1 1 1 ratio) (Scheme 7). Formation of either the IH-phosphirene complex 4 or the 2H-l,2-azaphosphole complex 5a was not observed [42]. This competing reaction path-... 

Scheme 9. Synthesis of l-aza-7-phosphanorbornadiene complex 13 via 2H-l,2-azaphosphole complex 12 [48]...

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. 

Bansal et al. (2003) reported the use of microwave technique for the synthesis of azaphospholes by the Diels-Alder reaction of l,3-azaphospholo(5,l-a)isoquino-lines. 

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