Phosphabenzene reactions

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. 

Benzene and all of the group 5 heterobenzenes except pyridine behave as dienes in Diels-Alder reactions n). Qualitatively reactivity increases with the atomic number of the heteroatom. For example, benzene reacts with hexafluorobutyne at 180° 80), while phosphabenzene gives adduct 44b at 100°. Arsabenzene reacts at 25° while stibabenzene and bismabenzene react at 0°. 

A striking difference to pyridine is the lack of basicity and nucleophilicity for phosphabenzene and arsabenzene, which are protonated at the a- and y-carbon atoms (this reaction is responsible for deuteration). Neither alkyl halides nor trialkyloxo-nium salts can alkylate phosphabenzene, therefore there will be no discussion of quaternary salts for these pnictogena-hetarenes. Whereas pyridine can be oxidized to the zwitterionic N-oxide, phosphabenzene affords non-aromatic -oxidation products 35 and 36 with tetracoordinated P(V) phosphorus atoms, similar to phosphin-oxides and phosphonic acids, respectively. 

Strong nucleophiles such as organolithium or organomagnesium derivatives do not react with substituted or unsubstituted phosphabenzene or arsabenzene (39, Y = P or As) by nucleophilic substitution as in the case of pyridines, but by addition to the heteroatom forming intermediate anions 40. These can then be converted into nonaromatic compounds by reaction with water to yield 1-alkyl-1,2-dihydro-derivatives 41, or they can be alkylated by an alkyl halide with the same or a different alkyl group, when two products may result a 1,2-dialkyl-1,2-dihydro 40-derivative 42, or a -derivative 43. The former products are kinetically controlled, whereas the latter compounds are thermodynamically controlled, so that one may favor the desired product by choosing the appropriate reaction conditions. 

The indole (78) and pyrrole (79) derivatives, containing phosphabenzene with a dicoordinated phosphorus atom as substituent, were obtained [76] in the reaction of indole- and pyrrole-substituted pyrrylium salts with trimethylolphosphine ... 

The hitherto unknown l-aza-3-phosphabenzenes are obtained via a regiospecific Diels-Alder reaction with inverse electron demand of 2-trifluormethyl-4-methyl-6//-l,3-oxazin-6-one with phosphaalkynes (81) or phosphaalkenes [Eq. (31)] (82). 

These molecules (10-XXXV) are known for all four elements. Older trivial names for these are phosphabenzene, arsabenzene, etc., but the current literature60 favors the names phosphinine, arsinine, stibinine, and bismuthinine. The bismuth compound, C5H5Sb, is extremely unstable and polymerizes at room temperature. Phosphinine and arsinine are thermally stable (distillable) and can be prepared by the reactions ... 

The reaction of stannoles 59 with two different phosphaalkynes has been examined <1996JOM(520)211>. In all cases, the formation of phosphabenzenes 62 and 63 is detected as the result of dialkyltin elimination from the primary cycloadducts 60 and 61. 

Figure 17 Reactions of phosphabenzene or arsabenzene initiated by RLi or RMgX.

Phosphabenzene, the phosphorus analogue of pyridine, is a species that, as with the methylenephosphines, formally contains a doubly coordinated phosphorus singly bound to one carbon and doubly bound to another. The first isolation of a phosphabenzene derivative was reported in 1966 using the reaction as shown below (equation 10). The parent phosphabenzene molecule was first isolated by the reaction (equation 11) of phosphorus tribromide with a staimane derivative, albeit in low yield.Further spectroscopic investigation of this parent molecule indicated it to be planar with indications of multiple bond character between phosphoms and carbon and aromatic delocalization. ... 

The 4-monosubstituted phosphabenzene (102) has been obtained from the corresponding 1,4-dihydrostannin by reaction with phosphorus tribromide. Phosphorins having one carbon- and one oxygen-linked substituent on phosphorus may be made directly by the reaction of phosphorins (103) with diazo-alkanes in the presence of alcohols or phenols. ... 

Tris(trimethylsilyl)phosphine (Me3Si)3P readily reacts with pyrylium salts, giving substituted phosphabenzenes12, and the reaction can be conveniently extended to include the preparation from 3-azapyrylium tetrafluoroborate of 1,3-azaphosphorines which, with acetylene carboxylates, gives the phosphabenzene with nitrile elimination (equation 6)13. Both 1,3,4-oxadiazolium and oxazolium salts react similarly to form the five-membered azaphospholes (equation 7)14. 

CB1454>. On the other hand, bis spirophosphazene (27) was made by the reaction of substituted phosphabenzenes with phenylazide <93TL3107>. Rhenium carbonyls have been used to dicyclize a silylated diphosphazene to produce the salt (17) <87CB1183>. 

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 ... 

Trisubstituted pyrylium salts yield 2,4,6-trisubstituted phosphabenzenes 16 (MArkl 1966) when treated with phosphanes such as tris(hydroxymethyl)phosphane, tris(trimethylsilyl)phosphane or phosphonium iodide (according to the reaction principle on p 225) ... 

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