Phosphabenzene

Formal replacement of a CH group by trivalent phosphorus in benzene results in the formation of phosphinine 1, also known as phosphabenzene. Replacement by pentavalent phosphorus yields phosphinine 2. Saturated six-membered rings 3 with trivalent phosphorus are known as phosphinanes. 

Phosphabenzene has the geometry of an expanded planar hexagon, as shown by the X-ray structure of the 2,6-dimethyl-4-phenyl derivative (see Fig. 6.15). By its NMR data, phosphabenzene satisfies the criteria of a delocalized 6 i-heteroarene with a diamagnetic ring current. However, the electron distribution between P- and y-positions is the reverse of that of pyridine (see p 270)  

A comparison of the electronic spectra of their 2,4,6-tri- cr -butyl derivatives shows further differences between phosphabenzene and pyridine. The longest wave UV absorption of both systems is almost identical (262 nm), but phosphabenzene can be ionized (8.0 and 8.6 eV) more easily than pyridine (8.6 and 9.3 eV). Supported by MO calculations, it was concluded that the HOMO in phosphabenzene is a TT-orbital, whereas that in pyridine is the n-orbital of nitrogen [123]. Phosphabenzene can be regarded as a 7i-donor system and pyridine as a (ji-deficient) a-donor system. 

The reactions of the phosphabenzene system [124] confirm these conclusions. Phosphabenzenes have low basicity towards hard acids. They are not protonated by CF3CO2H nor alkylated by trialkyloxonium salts. However, soft acids attack at phosphorus. For instance, 2,4,6-triphenyl-phosphabenzene forms compounds 4 with the hexacarbonyl derivatives of Cr, W and Mo in which the phosphorus coordinates to the metal, possibly with metal-P back-donation. The complexes 4 rearrange photochemically or thermally affording the 67i-heteroarene complexes 5. Although 2,4,6-triphenyl-pyridine is protonated on nitrogen, it undergoes complex formation with chromium hexacarbonyl exclusively on the phenyl moieties yielding the ri -arene complexes 6 [125]. 

Almost all other reactions of phosphabenzene occur with involvement of phosphorus. For instance, hexafluorobut-2-yne undergoes a [4+2] cycloaddition furnishing the l-phosphabicyclo[2.2.2]octa-2,5,7-triene 7  

Nucleophilic and radical reagents transform A, -phosphinines into X -phosphinines. For instance, with alcohols or amines in the presence of Hg(II) acetate, 1,1-dialkoxy-, or 1,1,-diamino-X -phosphinines 8 are formed  

The adduct 9 is postulated as an intermediate, in which Hg(OAc)2 acts as a Lewis acid toward the heterocycle 9 reacts in two steps with the nucleophile, namely with reduction of Hg(II) to Hg(0) and conversion of 10 into the product 8. 

---

Replacement names are much less popular, and are not commonly used for the smaller monocycles, with the exception of analogs of pyridine having a heavier Group V element in place of nitrogen. Phosphabenzene occurs more frequently in primary publications than phosphinine , for example, probably because of the ambi uous state of Hantzsch-Widman... 

The aromatic heterocycle phosphabenzene C5H5P (analogous to pyridine) was reported in 1971, some years after its triphenyl derivative 2,4,6-Ph3C5H2P. See also HP=CH2 29) nd [P(CN)2] ° (p. 484). The burgeoning field of heterocyclic phosphorus compounds featuring... 

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. 

The reactions of benzyne and carbenes with phosphabenzenes and of carbenes with phospholes have been investigated. Whereas benzyne reacted in the anticipated manner with (27) to give (28), carbenes, or their precursors, reacted in a curious manner to give substituted benzenes (30). Good evidence for the intermediacy of the phosphepin (29) came from the finding that closely related compounds, such as (31), also decomposed to give substituted benzenes. The nature of the eliminated phosphorus entity has still to be determined. 

A structural study of a molybdenum carbonyl complex of phosphabenzene has shown that the phosphorus is a-bonded to the... 

From a library of mixtures of monodentate ligands an excellent catalyst for branched-selective hydroformylation of methacrylic esters was identified (Scheme 5) [45]. The best catalyst employs a 1 1 mixture of triphenylphos-phine and a phosphabenzene ligand 2 [32]. 

Complexes with diphosphenes, phosphabenzene, biphosphinine, and other unsaturated phosphorus donor ligands... 

The orange, air-stable, homoleptic tetrakis( 71-phosphabenzene)nickel (1046) is tetrahedral (point symmetry 54) and can be obtained from phosphabenzene and [Ni(cod)2].2 25 It features a short Ni—P bond length of 2.1274(5) A with considerable N i P 7r-backbonding and a i/(Ni—P) stretch at 168 cm-1. In solution, partial dissociation of one phosphabenzene ligand is observed. 2-Diphenylphosphino-3-methylphosphinine forms with [Ni(cod)2] in the presence of the CO the dinuclear complex (1047) with a W-frame structure.2526... 

Thiatriazole Pyridine Phosphabenzene 1,2,3-Triazine 1,2,4-Triazine s-Triazine... 

Phosphorus. Substituted phosphorus analogues of pyridine (phosphinines, A3-phosphabenzenes, also called phosphonins or phosphorins) were first prepared by Markl starting from pyrylium cations their chemical properties suggest that their aromaticity is lower than that of pyridine (e.g. phosphinine 186, Scheme 72).230-232 Molecular calculations for other six-membered jr-systems with planar tricoordinate phosphorus, such as phospininines 186 and 187, have evidenced their aromaticity (Scheme 72).156... 

The dipole moment of phosphabenzene is reinforced by the methyl group shown in (147) and increases it from 1.46 to 1.77 D thus it resembles pyridine, which has the heteroatom at the negative end of the dipole.181 The magnitudes and trends of the dipole moments of the methylphosphines have been investigated by MO studies and the dipoles partitioned into bond moments, bond polarization, and lone-pair moments.182 The reciprocal effects of the double bond and the phosphorus atom in... 

Phosphabenzene in Stability toward Dimerization (From Brown and Borden,... 

Brown, E. C. Borden, W. T. What Accounts for the Remarkable Difference between Silabenzene and Phosphabenzene in Stability toward Dimerization Organometallics 2000, 19, 2208-2214. 

The value of the C—P bond ellipticity e calculated for phosphabenzene with the 6-31G basis set to be 0.2558 (89MI4) exceeds that of the P—C bond in phosphine CH3PH2 (0.1457), which points to a somewhat doublebond character of the CP bond in (95). 

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. 

Nevertheless, the stabilization of the ylid by the cyclic delocalized dtr-electron system presents some interesting theoretical problems. In this connection Markl has coined the term non-classical phosphabenzene . We will return to this point (p.115). 

Markl, G., Matthes, D. 2.6-Diphenyl-l-aza-4-phosphabenzene. Angew. Chem. 84,... 

One of the most obvious examples is strong deshielding of the a-protons in the series pyridine (8 8.29 ppm), phosphabenzene (8.61), arsabenzene (9.68), stibabenzene (10.94), and bismabenzene (13.25), although other data unambiguously point to a falling off of the aromaticity in this sequence. Here the contribution by crAringcurr is mostly obscured by local effects connecting with nonuniform distribution of the electron density and by the anisotropy of the heavier heteroatoms. 

Another system, which is also allowed by IUPAC and is sometimes very convenient (but which should be avoided when official names are established by IUPAC and cited in Chemical Abstracts), is the so-called a -nomenclature, in which the term phospha indicates replacement of carbon by phosphorus, e.g. phosphacyclohexane = phosphorinane, and phosphabenzene = A3- phosphorin. Five-coordinate phosphorus compounds are called phos-phoranes. However, this name is also used for phosphorus ylides with a four-coordinate phosphorus and a more or less polar R3f—CX moiety or a d -p R3P=CX double bond, e.g. methylenetriphenylphosphorane = triphenylphosphorus methylene ylide or tri-phenylphosphonium methyl ylide, Ph3P=CH2 = Ph3P—CH2. 

The stability of phosphabenzene and of arsabenzene in the absence of air and the isolation of the silicon-carbon and silicon-silicon double bonds might suggest that silabenzene, appropriately substituted, could be stable enough to be isolable. Indeed, calculations suggest that it would have a it-resonance stabilization energy about two-thirds that of benzene (78JA6499). 

---