Benzazaphosphole

Miles, J.A., Grabiak, R.C., and Beeny, M.T., Synthesis of novel phosphorus heterocycles 2-aryl-l-methyl-2,3-dihydro-lH-2,l-benzazaphosphole 1-ox-ides,. Org. Chem., 46, 3486, 1981. 

A-Unsubstituted 1,2,3-benzazadiphospholes (179) (R, R = H, Me, Cl) are yellow solids and result from a synthesis analogous to that used for 1,3-benzazaphospholes (chapter 3.16) as yellow solids <90TL4463, 93PS(76)45>. They are converted to the anions by a strong base. 1,3-Dienes such as 2,3-dimethylbutadiene give a [2 -I- 4] cycloaddition to the P=P bond <90TL4463>. 

These compounds ( 1) are synthesized from primary o-amino-phenylphosphine and various cyclization reagents, from secondary o-aminophenylphosphines under elimination of an ether and by oxidation or thermal treatment of 1,3-benzazaphospholenes (Scheme 1). In these cyclizations the NH-benzazaphospholes are formed exclusively. This underlines the C=P double bond system in these special cases to be favored over the C=N bond. A PH isomer was never observed. 

The reactivity of the compounds is rather limited. For example, benzazaphospholes are not attacked by dilute aqueous acids and bases, oxygen, sulfur or alkyl halides. The coordinating abilities, metalation and interaction of the so formed ambi-dent anion with acyl halides is demonstrated in Scheme 3 (4). 

In the case of phenyl isocyanide, both the addition of f-BuLi and an ortho lithiation took place in the presence of TMEDA to provide the intermediate 53 and finally compounds 54 and 55 (Scheme 15)75. The corresponding 3-metalloindolines (metal = S, P, Si, Ge, Sn) were obtained by reaction of the dilithium intermediate 53 with metal dihalides. The obtained benzazaphospholes and benzazarsoles 54 (M = f-BuP, MeAs) can be converted into the aromatic III-1,3-benzazaphospholes and benzazarsoles 55 by flash vacuum pyrolysis76. 

Systems with a fused aromatic carbocyclic ring which have been studied, either theoretically or synthetically, in the period under review include l//-l,3-benzazaphospholes 14 and 15, [l,3]azaphospholo[l,5- ]pyridine 16, [l,3]azapho-spholo[l,5- ]isoquinoline 17, and [l,3]azaphospholo[l,5- 7]quinoline 18. 

The crystal structure of 2-lithiated l-methyl-l,3-benzazaphosphole 35 showed it to have a dimeric structure with two THF molecules associated with each lithium. The two lithium atoms and the two C-2 atoms form a four-membered ring <20020M912>. In contrast, the N-lithiated derivative of 2,5-dimethyl-l//-l,3-benzazaphosphole exists as a monomeric species 36 in the solid state <2002JOM(646)113>. The carbene-type complex 37 derived from the lithiated species 35 on reaction with tungsten hexacarbonyl was also studied by X-ray crystallography. 

A four-membered ring was also found in the complex 38 formed by the reaction of 2-/i r/-butyl-5-methyl-l//-l,3-benzazaphosphole with nickelocine <2002ZFA2869>. 

Complexes of 1,3-benzazaphosphole systems with other metal carbonyl compounds (Cr, Mo, as well as W) also follow the same pattern, reaction at phosphorus <1998EJI1079>, and exactly comparable behavior was seen in the formation of complexes 61 and 62 from dihydrothiazole-fused and dihydrooxazole-fused 1,3-azaphospholes, respectively <1999JOM(577)337>. 

C-Lithiation (to give 71) of the 1,3-benzazaphospholes 70 was achieved with /-BuLi and the C-lithiated reagents proved to be synthetically very useful, reacting with a variety of electrophiles to produce 2-substituted derivatives 72 as indicated in Scheme 8 < 20020M912>. 

Reduction of o/t/ ti-acylaminoaryl phosphonic acid esters 94 with LiAlH4 produced 1,3-benzazaphospholes 95 (Scheme 15) <2001T9963, 2002JOM(646)113>. 

Collins, D.J., Drygala, P.E, and Swan, J.M., Organophosphorus compounds. Part 20. Approaches to the synthesis of 2,3-dihydro-l/7-l,2-benzazaphospholes involving carbon-carbon and carbon-phosphorus ring closure, Aust. J. Chem., 37, 1009, 1984. 

---