Phosphole oxides

Re3Cl9(l,2,5-triphenyl phosphole oxide)2,4, 162 Re3Cs4H4SBr9P 3 Re3Br9(PPh3)3,1,143 RejC9 H88Cl9P6... 

I-Phenyl-A -phospholen-l-oxide (126) and /rani -l,4-diacetoxybutadiene gave (127), which could be converted to the phosphole oxide. Reduction... 

It is known that the phosphole oxides (80), obtained either by the oxidation of phos-pholes (2) or by the dehydrobromination of dibromophospholane oxides (79) undergo spontaneous dimerization to furnish cycloadducts 81 (Scheme 21) [4, 65],... 

Scheme 21 Regio- and stereospecific cyclodimerisation of phosphole oxides...

Generating the phosphole oxides (82) in the presence of trapping agents, such as maleic acid derivatives, phosphanorbomenes of type 83 were obtained (Scheme 22) [4, 65],... 

Oxidation of arylphospholes (17) by peroxides led to phosphole oxides (84) that dimerized to the corresponding phosphanorbomene derivatives (85) (Scheme 23) [36, 38, 66], As in earlier cases, the cyclodimerization took place in a regio- and stereospecific manner. The interesting observation was that, due to the bulky P-substituent, oxidation was slower and the phosphole oxides (84) became relatively stable hence, they could be characterized by NMR. 

Mixed phosphole oxide dimers (98 and 99) were prepared by the possible combinations of two different phosphole oxides (96 and 84d), generated simultaneously in the same flask. In the reaction shown, homo dimers 97 and 85d were also formed (Scheme 25) [68],... 

The instability of phosphole oxides is the consequence of their antiaromaticity [69],... 

Scheme 25 Dimerisation of two different phosphole oxides leading to homo and heterodimers... 

Comparison of the computed electrostatic potentials (Fig. 4) of phosphole (41 E lone pair) and phosphole oxide (41 E 0) (Holloczki and Nyulaszi, unpublished result) shows that this is indeed the case. The positive charge (shown in red in... 

Fig. 4) becomes more pronounced at the phosphorus atom in the hypervalent compound. Calculation of the aromaticity% [127] revealed similar tendencies. While phosphole itself exhibits 18% aromatic character, for phosphole oxide -13% has been reported [176], It is noteworthy that for 1-methoxyephosphole and 1-fluoro-phosphole small negative values (-4.9% and -9.2%) were also obtained, in agreement with the conclusions of Mathey and coworkers [169]. Also, the phosphole sulfide and selenide exhibited smaller antiaromaticity (-11.6% and - 9.7%, respectively) [176] than the oxide, in agreement with the data in Table 2 [61]. It is also worth noting that the decrease of the NICS value and the conjugation upon sulfur addition has been noted in a phosphole-thienyl and a phosphole-pyridyl oligomer by Delare et al. [177],... 

Fig. 4 B3LYP/6-311+G electrostatic potentials calculated for phosphole (left) and phosphole oxide (right). Red and blue colors denote positive and negative charge, respectively (Holloczki and Nyulaszi, unpublished result)...

Derivatives with CN = 4 include phosphole oxides and sulfides, phospholium salts, imine and ylide derivatives. Their reactivity is completely different to that of phospholes having CN = 3 since the lone pair of the P-atom is not available. In most cases, the CN = 4 compounds behave as functionalized cyclic dienes. However, phosphole oxides and sulfides can also be employed as protected CN = 3 phospholes for synthetic purposes. 

Finally, three studies devoted to phosphole derivatives are noteworthy, but beyond the scope of this chapter. The first concerns the stereospecific cyclodimerization of phosphole oxide (PM3 semi-empirical and ab initio 3-31G calculations) <1999JOM166>. The second is a DFT study of the reduction products of phospholium cations <2006PCP862>. The third is a structure and bonding study in the isolectronic series C H Ps <2004JCD2080>. 

The general methods of preparation of phosphole oxides, sulfides, and selenides have been described in Section 3.15.5.1.3. A tentative resolution of chiral phosphole 76 under kinetic dynamic resolution conditions is noteworthy, despite only low enantioselectivities (10-20%) having been obtained (Scheme 20) <2004TA3519>. 

A previously unknown series of 4,5,8,7-tetraflourobenzo[4/phosphole oxides 78 has been obtained by reacting diethylpentafluorophenylphosphinites and ethyl bis(pentafluorophenyl)phosphinites with activated alkynes (Scheme 21) <2004RJC189>. The CN 5 derivatives 77 are stable enough to be characterized by NMR spectroscopy, and one derivative 78 (R =C6F5, R = R = C02Me) has been studied by X-ray diffraction. 

Phosphole oxides are generally unstable since they dimerize rapidly via Diels-Alder reactions. Sterically demanding aryl substituents at the P-atom can provide some stabilization of the phosphole oxide, but not enough to allow for their isolation <1996JOM7801, 1997JOM109>. Note that such sterically hindered phosphole oxides can also be trapped by A-phenylmaleimide to give [4-F2] cycloadducts (Scheme 23) <1997JOM109>. 

The dehydrogenation step can also be conducted on phosphole oxides and sulfides as illustrated with the synthesis of a series of previously unknown 1,3,4-triphenylphospholes. The starting material is l,3,4-triphenyl-3-phospholene-... 

If similar Diels-Alder reactions are undertaken with equimolar mixtures of 1-arylphosphole oxides, four different [4+2] cycloadditions take place <2001HAC633>. Both the expected symmetric phosphole oxide dimers as well as crossed cycloadducts are obtained. 

II.6. b. Phosphole Oxide, Sulfide, Selenide, and Phospholium Salt n-Complexes... 

The first fully characterized / -coordinated phosphole oxides have been described by Barrow et al. ( -C5H5)Co(PF3)2 reacts with hexafluorobut-2-yne to give (C5H5)Co(C4Ffi)2PF3 which undergoes hydrolysis to XXVIII and XXIX. 

More recently, Yasufuku et al. obtained >/ -coordinated phosphole oxides by reaction of a cobaltacyclopentadiene ring with phosphites ... 

In contrast to the other r-complexed phosphole oxides described above, the crystal structure showed that one heterocycle is an -diene and the other a 5-electron donor. One oxygen atom of the - P(0)0 group is bonded to iron. 

Photolysis of (244) in the presence of alcohols likewise results in the formation of esters of methyl(phenyl)phosphinic acid. However, O-isotopic labelling studies provide evidence of, at least, the partial involvement of a pentacovalent phosphorus intermediate in this reaction, and so the previous assumption of the intermediacy of (245) may not be the whole story. The phosphole oxide dimers (246) have been shown to undergo regioselective reduction and complexation on treatment with the dimethylsulfide-borane adduct, to form the bicyclic system (247). Treatment of the propadienylphosphine oxide... 

Ring Expansion of 2,5-Dihydro-lH-Phosphole Oxides to 1,2-Dihydro- and 1,2,3,6-Tetrahydrophosphinine Oxides... 

Easily available 2,5-dihydro-l//-phosphole oxides (1) served as excellent starting materials for six-membered P-heterocycles when subjected to ring enlargement. In the first step, dichlorocarbene was added on their double-bond to afford the phos-phabicyclohexanes (2), in most cases, as a mixture of two diastereomers (2l and 22) (Scheme 1)[10, 11, 12, 13,14, 15,16, 17],... 

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