Phosphirene derivatives

Recently, another approach to the problem of heterocyclopropene synthesis has resulted in a claim for the generation of a phosphirene derivative similar to that of the controversial silirene and germirene (I) from reaction of a dibromide precursor with l,5-diazabicyclo[+.3.0]non-5-ene (DBN) [189)-. 

Table 2 Structural parameters of some monocyclic phosphirene derivatives. ...

The phosphirene cation (5) (with two resonance structures 5a and 5b) has been predicted to possess a Zji aromatic system for quite some time." The aromaticity of the corresponding cyclopropenylium cation (C3H3+) is well-known.Substituted derivatives of 5, however, have only recently been observed in solution and in complexed form. 5 is the global... 

Phosphinidenes (R-P), short-lived, reactive intermediates that are isoelec-tronic with nitrenes and carbenes, are accessible by fragmentation of phos-phiranes, phosphol-3-enes, triazaphospholenes, and oxazaphospholenes and can be characterized by trapping reactions. The metal-complexed species [R-P-M, e.g., M = W(CO)5] are more stable (but also not isolable) and react more selectively. As a model example we describe the transformation of (3,4-dimethyl-l-phenylphosphole)(pentacarbonyl)tungsten 4 with dimethyl acetylenedicarboxylate to the 7-phosphanorbomadiene derivative 5.11 The (phosphinidene)(pentacarbonyl)tungsten complex 6 is generated from the latter by a [4+ 1]-cycloreversion and trapped with diphenylacetylene via a [2+1]-cycloaddition to furnish the triphenyl-2-phosphirene complex 7 (Scheme l).12... 

Typically the C-P-C intracyclic angle of phosphirane and its uncomplexed derivatives amounts mostly in the range of 46-47°. The C-P bond lengths range from 1.830 to 1.886 A whereas the C-C bonds vary from 1.488 to 1.466 A, reflecting the influence of the substitution pattern. Worthy of mention is that the complexed phosphirane has an explicitly lengthened C-C bond length in comparison to the uncomplexed phosphirane, which is also true in the case of the phosphirane oxide. The same tendency is determined in the phosphirene case. 

The chemistry of phosphiranes and phosphirenes and their derivatives comprehends theoretical questions, for example, antiaromaticity of phosphirenes, a -aromaticity, reaction behavior like ring opening of vinylphosphiranes, complex building ability, and synthesis strategies. The cycloaddition reactions of transient phosphinidene metal complexes play an outstanding role. 

The reactions of P-halophospha-alkenes with carbenes provide a new route to 1-chloro-lH-phosphirines (181). Such compounds are highly reactive, and a study of nucleophilic displacement reactions at phosphorus has been facilitated by preparation of the related P-W(CO)5 complexes.The first pentacoordinate systems derived from phosphirenes, (182), have been formed in the reactions of P-halo- or P-cyano-phosphirenes with tetrachloro-o-quinone. Evidence has been presented for the rearrangement of P-chlorophosphirenium ions (183) to the vinylphosphenium ions (184)... 

The phosphirene compound was prepared from the 7-phosphanorbornadiene complex as shown in Scheme 22.2 8 The complex converts to 2-keto-l,2-dihydrophosphete complex by insertion of CO into P—C bond via metallacycle intermediates.299 Analogous phosphirene,300 phosphirane,301 and the related alkenylidenephosphirane293 derivatives have been reported. 

As a general rule, tetra- and pentacoordinated derivatives of phosphirenes display a much higher stability than the corresponding derivatives of the phosphirane ring. Some kind of electronic stabilization is probably at work in phosphirenium salts (see the structural data of one such salt <88CC462 , which can either be synthesized directly (see below) or from tervalent phosphirenes (Equation (2)) <85JA4700>. Methyl triflate in chloroform at — 50°C has also been used successfully... 

Noteworthy, 2-bromophosphinines have also found an interesting application in the synthesis of a new class of phosphinine-based bidentate ligands featuring either a phosphine or a phosphorus heterocycle like phosphirene or 1,2-dihydrophosphete. Reaction of phosphorus tribromide with 2-bromophosphinine 25 afforded the 2-dibromophosphinophosphinine 27 which in turn could be converted to functional derivatives either through nucleophilic or electrophilic reaction at the P-Br bonds. The mechanism of this phosphination process was shown to involve the transient formation of the A5-di bromophosphinine 26 (Scheme 7) [26],... 

C-alkoxy derivatives of a phosphole have resulted from the insertion of ethoxyethyne into a phosphirene complex (Equation (67)) <87TL5021>. 

The aim of this chapter is to highlight important aspects concerning synthesis and physical properties of 2H-azaphosphirene complexes, especially,becanse their synthesis does not follow common coordination chemistry methodologies using, e.g., non-coordinated 2H-azaphosphirenes. It is interesting to learn that the knowledge about azaphosphirenes V-VII [19] is very scarce compared to azirenes I, II [5] and phosphirenes III, IV [18] (Scheme 2) and that derivatives of IH-azaphosphirenes V [20] and 2H-azaphosphirenes VI [21] have been claimed exclusively as reactive intermediates. 

The 1-H phosphirenes are a well-known class of compound, but relatively few derivatives of 2-H phosphirene have been prepared. Various derivatives of 2-phosphetenes are known as well as a few... 

Although parent phosphirenes (Table 6.21) are not known, a number of derivatives have been well characterised. Typical of these are 1,2,3 triphenylphosphirene (6.889a) and the tetraphenylphosphi-renium cation (6.889b) (Figure 6.23). Phosphirenium salts do not exhibit aromatic behaviour. 

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