2H-Azaphosphirene complexes

H-Azaphosphirene Complexes, a NdfwraZ Starting Point for Nitrilium Phosphanylid Complexes . 

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 first route to 2H-azaphosphirene complexes XIII, published by Streubel et al. in 1994 [22], used a triethylamine-induced condensation-rearrangement cascade starting from amino(aryl)carbene metal complexes VIII and [bis-(trimethylsilyl)methylene]halophosphanes IX (X=Cl,Br) (Schemes) the yields are generally good (50 - 85%) [19,23]. Experimental details and scope of this method as well as progress in this field were summarized quite recently [24]. 

With respect to the following Sections, it is worthwhile to look at the currently discussed mechanism of 2H-azaphosphirene complex formation (Scheme 4). 

Scheme 3. First route to 2H-azaphosphirene complexes XIII [22]...

The following bonding picture for 2H-azaphosphirene complexes can be drawn based on the NMR, IR, UV/vis, MS, and X-ray diffraction data [24] The situation of this N,P-heterocyclic ligand should be described as a resonance hybrid of a three-membered ring system with a covalent bonding (XIII) and a... 

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 7. Example of a three-component reaction using a 2H-azaphosphirene complex, dimethylcyanamide and DMAD in toluene [42]...

So far, only very few reactions were reported on the use of 2H-azaphosphirene complex 11a in nitrilium phosphanylid complex chemistry, mainly because of the tendency of P-CjMes-substituted derivatives to undergo side reactions [47]. Nevertheless, it seems worthwhile to look at the reaction of 11a with DMAD, which finally led to the 1 -aza-7-phosphanorbornadiene complex 13 via 2H-l,2-azaphosphole complex 12 as reactive intermediate (Scheme 9) [48]. Later on, complex 12 was isolated and characterized using a different stoichiometry [49]. 

The first access to 2H-l,3,2-diazaphosphole complexes was reported in 1997 by Streubel et al. via reaction of 2H-azaphosphirene complex la and dimethyl-cyanamide in benzonitrile at 75°C, which led selectively to the 2H-l,3,2-diaza-phosphole complex 17 (Scheme 11) [50]. 

H-l,4,2-Diazaphosphole complexes were first discovered in thermolytic reactions of 2H-azaphosphirene complex la in benzonitrile, whereby both regioiso-mers, complexes 19a and 19b, were formed, the latter as major isomer (Scheme 13) [25]. 

Apart from photochemical reactions of 2H-azaphosphirene complex la with ethyl cyanoformate in -pentane/benzonitrile mixtures at low temperatures, which led selectively to a 2-ester-functionalized 2H-l,4,2-diazaphosphole complex [33], the most convenient and versatile routes to 2H-l,4,2-diazaphosphole complexes are the TCNE- [53] or ferrocenium salt-catalyzed [54] bond-selective ring-expansion reactions of 2H-azaphosphirene complexes at rt. 

In contrast to the reaction of the terminal phosphanediyl complex 3a with 1-piperidinocarbonitrile (cf. Scheme 7), the complex [(OC)5WPC5Me5] 22, thermally generated from complex 11a, did not furnish the 3-(l-piperidino)-2H-azaphosphirene complex 11b - instead, the polycyclic C,P,N-carbon cage compound 24 was obtained via intramolecular [3-1-2] cycloaddition of 23 (Scheme 15) [51]. 

The thermal reaction of 2H-azaphosphirene complex la with 1-piperidinocar-bonitrile and carbonyl derivatives led regioselectively to A -l,3,2-oxazaphos-pholene complexes, e.g., cyclohexanone furnished complex 28 (Scheme 18) [57]. Under the same reaction conditions with phenyl isocyanate selective formation of a A -l,3,2-oxazaphospholene complex having an exo-imino function was observed [58]. 

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