Phosphaalkenes 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. 

Abstract Many similarities between the chemistry of carbon and phosphorus in low coordination numbers (i.e.,CN=l or 2) have been established. In particular, the parallel between the molecular chemistry of the P=C bond in phosphaalkenes and the C=C bond in olefins has attracted considerable attention. An emerging area in this field involves expanding the analogy between P=C and C=C bonds to polymer science. This review provides a background to this new area by describing the relevant synthetic methods for P=C bond formation and known phosphorus-carbon analogies in molecular chemistry. Recent advances in the addition polymerization of phosphaalkenes and the synthesis and properties of Tx-con-jugated poly(p-phenylenephosphaalkene)s will be described. 

Scheme 10 Synthesis of [l,2,4]diazaphosphole from transient phosphaalkene...

Phenylmethylsilanediol, synthesis, 42 155 Phenylsilanetriol, monosodium salts, 42 169 4 -Phenyl-2,2 6, 2 -terpyridine bis nickel complex, 30 74 molecular structure, 30 74 PhjtfluorenyllSiOH, 42 197 (Ph(Me2N)C-=Nli], 37 59-65 orientation of imino ligand, 37 61-63 (PhMe SiljCSiH OH, 42 244-245, 248 (PhMe SiljCsiMeHlOH), 42 191 Phosphaalkenes acyclic, 33 338-353 butadienes, 33 346-349 cumulenes, 33 352... 

Apparently independently, Markl et al. (139) and Regitz and co-workers (140-142) discovered that 1,3-dipolar cycloaddition reactions of mtinchnones and phosphaalkenes or phosphaalkynes provide a direct synthesis of 1,3-azaphospholes (240) (Table 10.7). The intermediate cycloadducts cannot be isolated. The various phosphaalkynes were generated from phosphaalkenes or, in the case of methyli-dynephosphane (239, R" =H), by flash vacuum pyrolysis of either 239 (R" = f-Bu) or dichloromethylphosphine. 

A new class of heteroaromatic compound was introduced by the synthesis of a diphosphathienoquinone (20). It can be reduced to a semiquinone radical anion and dianion at lower potentials than phosphaalkenes. The ESR spectrum indicates that the two P atoms are not equivalent213. 2,4>6-Tricyano- 1,3,5-triazine undergoes dimerization to yield 4,4, 6,6 -tetracyano-2,2 -bitriazine214. 

The synthesis and structural study of the stable P-heterocylic carbene 49 and related structures (e.g., structures 48 and 52 see Figure 3) have attracted some recent research activity <2005AGE1700, 2002JA2506, 2006AGE2598, 2006AGE7447>. The synthesis of the stable P-heterocylic carbene 49 was accomplished in two steps (1) a formal [3+2] cycloaddition of the readily available phosphaalkene 123 with acetonitrile in the presence of silver triflate afforded salt 124, and (2) the isolated and recrystallized salt 124 was deprotonated by lithium hexamethyldisilazide in tetrahydrofuran (THF) to afford carbene 49 as relatively stable light-yellow crystals (Scheme 10) <2005AGE1700>. 

The synthesis of alkali metal 1,4,2-diphosphastibolides parallels that of the 1,4,2-diphosphaarsolides 18 and 19. It is however regiospecific and no 1,2,4-isomer is formed. For the synthesis, a DME solution of lithium bis(trimethylsi-lyl)antimonide 31 (M = Li) is treated with 3equiv of the phosphaalkene 29. In the course of the reaction, the phosphaalkene 29 is converted to the phosphaalkyne 30 via the base-catalyzed elimination of hexamethyl disiloxane (Scheme 7). Alternatively, the phosphaalkyne 30 can be used directly in place of the phosphaalkene. After addition of TMEDA or 12-crown-4, the lithium 1,4,2-diphosphastibolide 22 (M = Li(TMEDA)2) or Li(12-crown-4)2 is isolated <1997JOM291>. 

Phosphaalkynes 9 are obtained almost exclusively by P-elimination of hexa-methyldisiloxane from appropriately substituted phosphaalkenes 8 (for their synthesis, see Protocol 3). The original elimination from 8 (R = f-Bu)31 performed in solution at room temperature in the presence of sodium hydroxide was optimized (solid NaOH, temperatures between 160 and 180°C, vacuum distillation techniques) and also generalized. In particular cases, aluminium trichloride in dichloromethane has proved to be a useful reagent for the elimination.32... 

Binuclear cumulenes, synthesis, 230 Bis(methylene)propenylphosphorane, synthesis with phosphaalkenes, 19 [ ( -BuSe)Fe2(CO)6h(/i4-Se)], synthesis and structure, 275, 276 Butene 1,4-diones, synthesis, 224... 

Diazaphospholes metal coordination, 17-18 synthesis with phosphaalkenes, 17 Dienediones, synthesis, 221 Dihydrofuran-2-ylidene complexes, synthesis, 212... 

Metallobis(methylene)phosphorane, synthesis with phosphaalkenes, 19-20... 

Phosphaalkenes transition-metal complexes, 2 phosphorous heterocyclic compound synthesis... 

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