Phospholes 1,3-diynes

Oxidative coupling of diynes with Ti(ll) complexes has been less investigated for the preparation of phospholes. Titanacyclopentadienes have been used as intermediates for the synthesis of a family of 2,5-difunctional phospholes 178 bearing ester groups (Scheme 56) <2006JOC5792>. Although the isolated yields are moderate (ca. 30-50%), this synthetic approach nevertheless offers a straightforward route to functionalized phospholes. 

These three approaches all start from a preformed phosphole ring. Recently, a fourth route has been reported in which the phospholide ring is built directly by combination of an anionic P-unit and a 1,3-diyne (Eq. 4) [11] ... 

However, most efforts focused on the elaboration of polydentate ligands using 1,4 and 1,6-diynes as substrates. Bi-, tri- and tetradentate ligands such as 45-48 were thus successively prepared and structurally characterized. This approach was also successfully extended to the preparation of the first mixed phosphinine ether macrocycles such as 49 [33], Mixed tridentate species featuring phosphole, phos-pholide or phosphaferrocene moieties were also prepared from the condensation of diazaphosphinine 35 with the corresponding bis(dimethylsilyl) alkynyl derivatives (Fig. 5) [34],... 

Construction of the phosphole ring, with one useful exception, is accomplished by methods that are quite different from those employed for the N, S, O ring systems. Because of fundamental differences in the chemistry of phosphines versus amines, none of the familiar carbonyl condensation processes are known to be applicable to phosphole synthesis. Thus an attempt to use the Paal-Knorr condensation of 1,4-dicarbonyl compounds with PhPH2 <65JCS2184> and with PH3 <88ZOB783> failed to give phospholes. The only successful method known to the present that is common to both the phosphole and pyrrole ring systems is the condensation of 1,3-diynes with primary phosphines... 

The first example of this method was discovered by G. Markl in 1967 <67AG(E)86>, and was one of the earliest that provided variety in the substitution pattern on the phosphole ring the others known at that time, to be discussed, were limited to the formation of C-phenyl derivatives. Markl employed diynes with both 1- and 4-substituents, and thus created phospholes with 1,2,5-trisubstitution (Equation (57)). 

Phospholes prepared in the original report as well as by later users of this method are shown in Table 12. In addition to bases, the reaction may be promoted by metal salts and by free radicals <7lT5523>. Markl had employed only symmetrical diynes in the reaction, but unsymmetrical diynes work quite well <71JA6205>. 

Table 12 Phospholes from phenylphosphine with diynes .

Tricyclic compounds containing two phosphole rings have been synthesized by the reaction of trishydroxymethylphosphine with certain 3-hydroxypenta-l,4-diynes (Equation (59)) <74PS(4)279>. 

A number of synthetic methods have developed for construction of the heterole skeletons. The most straightforward and atom-economical method for the synthesis of 2,5-disubstituted heteroles is the double addition of X—H bonds across 1,3-diynes, albeit it requires trans addition to the carbon-carbon triple bonds. Thiophenes [2], selenophenes [3], teUurophenes [4], phospholes [5], arsoles [6], and stannoles [7] have been synthesized by using this method without the aid of catalysts ... 

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