Phosphabenzene derivatives

Tris(trimethylsilyl)phosphine l13 reacted with pyrylium salts in the presence of potassium fluoride to give thermodynamically stabilized phosphabenzene derivatives (X3-phosphinines) (equation 4)14. In addition, various heterophosphabenzenes and... 

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. ... 

Recent efforts in regard to phosphabenzene derivatives have particularly been concerned with metal complexes and theoretical considerations. These areas have been reviewed recently. ... 

Hydroformylation. Phosphabenzene derivatives are excellent ligands for high conversion of styrene to the homologated aldehydes, and the branched aldehyde is favored. ... 

Among the typically organic heterocycles of Fig. 1, the 1-methylphos-phole 12) 1 does not necessarily contribute dn orbitals to the rr-system besides its p lone pair13). The same holds for the phosphabenzene derivatives 2a 14> and 2b 1ST But in the phosphorine derivatives 3a 16>, 3b l7>, 3c 18> and 3d 19>, as well as in 1,1,3,3-tetraphenyl-l,3-diphosphabenzene 2°) 4 and 1-phenyl-1 H-phosphepine-l-oxide 21> 5 which contain tetrahedral phosphorous atoms, cyclic conjugation can only be achieved by the use of outer d orbitals. The resultant bonding situation has been treated theoretically by Mason 22> using HMO theory. 

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... 

Of course, with atoms from lower periods of the periodic system, such as P or S, it is possible to have aromatic systems that are not simply homologs of pyridine or pyrylium, (i.e., phosphinine and thiopyrylium, respectively) but also systems with P(V) and S(IV), i.e. phosphabenzene (/. -phosphinine) and thiabenzene, respectively. However, in some cases the aromaticity is strongly perturbed, as for instance in selenabenzene derivatives stabilized by electron-withdrawing groups, when these compounds are shown to be zwitterionic. 

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 ... 

Interestingly, the bond distances and angles of l,l-dimethyl-2,4,6-triphenyl-/l -phosphorin determined by X-ray diffraction are similar to those of /( -derivatives (Figure 18), yet the physical and chemical properties show that the 2 -phosphorins with electron-donor substituents (H, Me, SiHs) at the P heteroatom are not aromatic, and that the tetracoordinated phosphorus atom interrupts the ring current in such compounds. Flowever, when the P-substituents of /l -phosphorins have higher electronegativities (F, OFl, Cl, Br) the NlCS(l) values indicate aromaticity, but somewhat less than phosphabenzene (01HCA1578). 

The Group V analogs of pyridine show increasing sensitivity to oxygen and an increasing tendency toward polymerization from phosphabenzene to bismabenzene 11-13). The bismabenzene derivative could not be isolated, but its presence was inferred by trapping the product with CF3SCF3 ... 

The NMR spectra of pyridine, phosphabenzene, arsabenzene, and stibabenzene are quite similar in appearance (11,12). The y protons appear at lowest field partially as a result of remoteness from the heteroatom. The effect of the magnetic anisotropy of the heteroatom should decrease for the )3 and y protons. The chemical shift differences for the y protons in pyridine and arsabenzene are almost negligible. The low-field chemical shift value was interpreted in terms of the presence of ring current in the heteroatomic derivatives. 

The aromatic heterocycle phosphabenzene C5H5P (analogous to pyridine) was reported in 1971, some years after its triphenyl derivative... 

Some of the recent calculations on pyrimidine and purine bases can be listed here [134-144]. In the case of simple monocyclic compounds vith one heteroatom and their derivatives, ab initio approach has been used for pyrrole [145,146], furan [146-148], thiophene [146,147], pyridine [149-151], and phosphabenzene [149]. 

In contrast, two-coordinate derivatives are able to become involved in aromatic properties, and there are many examples of phosphazoles (9) and phosphabenzenes (10). 

R = R = Bu , R = COjMe) were formed in thermal rearrangements involving phosphabenzenes <87AG(E)85>. Dimerization of a dihydrazido derivative of triphosphine, YP=P—PCIY, gave the unexpected heterocyclic compound (96) <87AG(E)1256>. 

The A derivatives of phosphabenzene (e.g., sulfide (21)), also behave as dienophiles producing... 

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]. 

The unusual phosphole ring C FejP was synthesized by Huttner and co-workers (Scheme 12). In terms of Wade s rules (b) is an arachno system which can be converted into a nido cluster, (c), upon decarbonylation. The latter can form directly in the photochemical reaction of (a) with alkynes. The hve-membered organometallic heterocycle (c) is planar (deviation < 0.07 A) indicating delocalized bonding (structure 39). Signihcant developments may be expected from this new class of compounds. New types of heterocyclic phosphorus ligands are continuing to be synthesized, e.g. a 1,3,5-triphosphabenzene derivative, l,l,3,3-tetrakis(dimethylamino)-4-r-butyl-U 3x 5A -tri-phosphabenzene (40). 

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