Phospholes General

This discussion will deal mainly with phosphorus heterocycles, but the points will generally be applicable to the emerging chemistry of the lower group members (As, Sb and Bi). Phosphorus heterocycles such as phosphabenzene or the phospholes can form bonds with transition metals in a variety of modes. Because the chemistry of the phospholes is more fully developed, this series will be used to exemplify the area generally. Phospholes are readily synthesized in a one pot process (equation 69).270 The yield is variable depending on the nature of X and the nitrogen base used, but typically is between 60 and 85%. 

In an attempt to generalize this original bridging phosphane coordination mode to other metal ions, we have investigated the coordination chemistry of ligand 3b with Cu(I) salts [29]. Thus, reaction of di(2-pyridyl)phosphole 3b with Cu(CH3CN)4PF(l in a 1 2 ratio afforded quantitatively the dimetallated complex 15 (Scheme 12.9). 

Dihydrophospholes have been found to react readily with dichloro carbenes with ring expansion to form six-membered rings with phosphorus at various substitutions and oxidation levels both at carbenes and phospholes. The general procedure is as follows dichlorocarbene is added to the double bond of phosphole 191 to afford phosphabicyclohexene 192 which on thermolysis leads to dihydrophosphine derivative 193 <1996JCM528> (Scheme 8). Thermolysis of phosphole thione 194 afforded phosphothione 195 <1996PS(109)457> (Equation 39). 

In conclusion, for the most part, polymers incorporating phosphole units are still rare. However, the work described above demonstrates that they are accessible via a number of diverse synthetic routes including organometallic coupling or electropolymerization processes. The general property-structure relationships established with well-defined small oligomers have been shown to extend to the corresponding polymeric materials. 

As an example we describe here the synthesis of the l//-l,2,4-diazaphosphole 22 that, on account of its easy accessibility and as parent compound of the class, is of general interest. The synthesis proceeds through condensation of the cation 21. The reaction of 21 with hydrazine involves cleavage of an ammonium salt to furnish the phosphole 23.46 The cation 21 is obtained via a methanaminium chloride—generated as an intermediate from N, A-dimethylformamide and oxalyl chloride—by condensation with tris(trimethylsilyl)phosphine. 

The ligand capacity of phosphabenzene has already been noted (Section 10-14). The phospholes (10-LXXXI) can also behave simply as -donors, or also engage their -electron density, whence they generally become bridging ligands, as in... 

The use of H NMR spectroscopy in the study of phospholes with protons attached directly to the heterocyclic ring skeleton has been extensively described in CHEC(1984) and CHEC-II(1996). The assignment of spectroscopic data for compounds that have been reported over the last decade can generally be interpreted in terms of these previous observations. 

The most commonly used methods for isolation and purification of phospholes are crystallization, generally performed at low temperature, and column chromatography on alumina or silica gel. A wide variety of solvents have been used as eluents, including protic solvents such as methanol. 

It is noteworthy that phosphole derivatives with CN 4 (oxo-, thiooxo-phospholes, phospholium salts, etc.) as well as transition metal complexes are generally air stable and can be purified by crystallization or column chromatography. 

As discussed in Section 3.15.2.1 (Scheme 3), lf/-phospholes can be transformed into 2/f-phosphoies upon heating. To date, very few stable 2f/-phospholes are known <2004ACR954>. However, a great variety of 2f/-phospholes are accessible using the lf/-phosphole/2/f-phosphole equilibrium, and their heterodiene behavior makes them powerful intermediates for the synthesis of P-heterocycles. Only the results that have appeared since 1996 are presented in this section for a general overview of the chemistry of 27f-phospholes, an excellent review by Mathey is available <2004ACR954>. 

The general methods of preparation of phosphole oxides, sulfides, and selenides have been described in Section 3.15.5.1.3. A tentative resolution of chiral phosphole 76 under kinetic dynamic resolution conditions is noteworthy, despite only low enantioselectivities (10-20%) having been obtained (Scheme 20) <2004TA3519>. 

Phosphole oxides are generally unstable since they dimerize rapidly via Diels-Alder reactions. Sterically demanding aryl substituents at the P-atom can provide some stabilization of the phosphole oxide, but not enough to allow for their isolation <1996JOM7801, 1997JOM109>. Note that such sterically hindered phosphole oxides can also be trapped by A-phenylmaleimide to give [4-F2] cycloadducts (Scheme 23) <1997JOM109>. 

The chemistry of reduced phospholes has been extensively developed since the two dihydro isomers, namely 2- and 3-phospholene, together with phospholanes are easily accessible and generally stable derivatives. 

The first synthesis is the simplest and most general route to these systems and, since its recent optimization it allows the preparation of phospholes on a large scale. 

The coordination chemistry of arsoles and stiboles is much less developed than that of phospholes. This is partly due to their reduced availability. Indeed, the simplest and most general method for preparing phospholes, starting from conjugated diene-dihalophos-phine cycloadducts, cannot be transposed for synthesizing arsoles or stiboles. Scheme 3 summarizes the various routes to arsoles and stiboles which have been described in the literature. 

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