Phosphabenzene synthesis

A brief history of (3p-2p)7i bonds between phosphorus and carbon followed by an introduction to the methods of phosphaalkene synthesis that are pertinent to this review will be provided. The earliest stable compound exhibiting (3p-2p)7x bonding between phosphorus and carbon was the phosphamethine cyanine cation (1) [33]. An isolable substituted phosphabenzene (2) appeared just two years later [34]. The parent phosphabenzene (3) was later reported in 1971 [35]. These were remarkable achievements and, collectively, they played an important role in the downfall of the long held double bond rule . The electronic delocalization of the phosphorus-carbon multiple bond in 1-3, which gives rise to their stability, unfortunately prevented a thorough study of the chemistry and reactivity of the P=C bond. 

As early as 1963 Markl prepared the first representatives of this class of compounds via multi-step synthesis l,l-diphenyl-X -phosphorin 118 ( 1,1-diphenyl-phosphabenzene ) and 1,l-diphenyl-2,3-benzo-X -phosphorin i20 ( 1,1-diphenyl-phospha-naphthalene ). Neither compound could be obtained in crystalline form. Instead, treatment of the crystalline phosphonium salts iiP and 121 with aqueous alkali affords very reactive, air-sensitive yellow or orange powders 118 and 120). Acid treatment leads back to the phosphonium salts. 

Since 1965 arsenic chemistry and especially the chemistry of heterocyclic arsenic compounds has become revitalized because of the synthesis of the first six-membered, cyclic conjugated, Hiickel-aromatic phosphabenzenes (2) and arsabenzenes (3) <71JA3293>. Then, 50 years after the salvarsan period, a second boom in arsenic chemistry, especially in heterocyclic, unsaturated arsenic compounds began. 

Further products [e.g. (9) ] have been isolated from the oxidation of phosphabenzenes by mercuric acetate in the presence of aqueous alcohols. Replacement of the alcohols by amines has lead to a synthesis of several interesting quinquecovalent compounds [e.g. (10) ]. 

The synthesis of the parent compound is not possible by method 1. It is achieved by reaction of penta-l,4-diyne 17 with dibutyltin hydride forming stannacyclohexa-1,4-diene 18 by a cyclization/ addition sequence. Tin-phosphorus exchange by phosphorus tribromide converts 18 into dihydro-A -phosphinine 19 which, on dehydration with DBU (Ashe 1971), yields the phosphabenzene ... 

Ashe Sutherland Braye Woodward and Eschenmoser Ginsberg and Lindsell Franz Synthesis of phosphabenzene (phosphinine) C3H3P Importance of c-AMP established Phospholide anion characterised Synthesis of vitamin B12 Synthesis of first P4 complex with an M-P bond Development of the herbicide Glyphosate... 

Interest in the chemistry of the 6ji-phosphinine (phosphabenzene) system has continued, although considerably fewer papers have appeared compared to recent years. The synthesis, coordination chemistry and catalytic applications of phosphi-nines have been reviewed." New chiral bidentate phosphinine ligands (220) have been prepared and their coordination chemistry and applications in rhodium-catalysed asymmetric hydrogenations assessed." The diphosphinine (221) continues to find new applications as a ligand in homogeneous catalysis" and a new mode of coordination to a metal has been identified for the phosphinine (222), two such ligands simultaneously bridging a Mn-Mn bond." ... 

A similarly short synthesis for phosphabenzenes (phosphinines) has been suggested by Breit s group in cooperation with BASF (Scheme 2.9) [29-31]. Owing... 

Two approaches to the synthesis of 3-hydroxy-phosphabenzenes have been developed, and are illustrated in Scheme 42.The hydroxy-derivative (187) has phenolic properties, but is extremely sensitive to acid and base, which readily promote tautomerization to the unstable keto-tautomer (188). 

Muller C, Guarrotxena-Lopez L, Kooijman H, Spek AL, Vogt D (2006) Chiral bidentate phosphabenzene-based ligands synthesis, coordination chemistry, and application in Rh-catalyzed asymmetric hydrogenations. Tetrahedron Lett 47 2017-2020... 

Elschenbroich C, Nowotny M, Metz B, Graulich J, Massa W, Wocadlo S (1991) Bis(> -phosphabenzene)vanadium synthesis, structure, redox properties, and conformational flexibility. Angew Chem Int Ed 30 547-550... 

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