Synthesis of Phosphaalkenes

Nb and Ta derivatives are hard acids and then-complexes with P- or As-donors are limited. Tertiary phosphines, especially PMes, have been widely used to stabilize low-valent derivatives. C-H activation reactions, promoted by the formation of thermodynamically stable Ta-H, Ta-C, and Ta=C bonds have resulted in metallacycles based on unusual anionic phosphorus donors. Nucleophilic Ta phosphinidene complexes could be stabilized by a tripodal tetradentate [NN3] amido ligand. The terminal PR ligand reacts smoothly with aldehydes, providing a general synthesis of phosphaalkenes RP=C(H)R and act thus as a phospha-Wittig reactant see Phosphorus Organophosphorus Chemistry). 

The scope and limitations of the base-catalysed Phospha-Peterson synthesis of phosphaalkenes of the type MesP=CRR (R,R = aryl), involving the reaction of MesP(SiMc3)2 and a carbonyl compound in the presence of a trace of KOH or NaOH, has been investigated and shown to provide a convenient and general route to these compounds in 40-70% yield, usually as a 1 1 mixture of E- and Z-isomers. A simple access to a series of l,l -ferrocenylenebis(dihalophosphines) has facilitated the synthesis of the first metaUocene-bridged bis(phosphaalkene)... 

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. 

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. 

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

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>. 

In certain cases the process analogous to the isonitrile synthesis for the preparation of phosphaalkenes, showing proton- and halogene-substituted C-bridged atoms, is a successful one. 2,4,6-tri-i-butylphenyl-phosphane can be transferred to the phosphaalkene using a strong alkaline solution of chloroform [Eq. (7)] or methylene chloride [Eq. (8)]. A carbene addition mechanism is involved in this reaction (36, 37). 

The availability of phosphaalkenes and phosphaalkynes has led to a further route for the synthesis of phosphiranes and phosphirenes by the formal addition of carbenes or carbenoides to P-C multiple bonds. An example already depicted in Scheme 6 involved in the [2+1] cycloaddition reaction of a stable phosphinotrimethylsilylcarbene to tert-butylphosphaalkyne <1995JA10785, 1999CEJ274>. A carbenoid was also used in the synthesis of an unusual phosphirene from a siloxy-substituted phosphaalkene (Equation 30) <1997JOM(529)127>. 

As already described as a crucial step in the synthesis of diphosphirene 93 from diphosphirenium salt 91 ring opening of the latter species by LiAlH4 gave phosphaalkene H-P=C(NR2)P(NR2)2 (R = Pr ) in 80% yield <1997CC2399, 1999EJI1479>. 

Another reaction for the synthesis of diphosphirane complexes by thermal rearrangement of metallo-l,2-diphosphapropenes was described by the same authors <93CB1963>. Finally, the synthesis of the W(CO)5—diphosphirane complex was realized by the ene reaction of phosphaalkyne with the corresponding phosphaalkene complex <93T10279>. 

During the synthesis of diphosphiranes from the symmetrical diphosphene by cyclopropanation, whatever the nature of the carbenoids used (diazo derivatives, carbenes or halogenocarbenes), in all cases the formation as byproducts of the phosphaalkenes and the unstable phosphinidene was observed (5-20%). So, for isopropylidene carbene, the formation of the phosphacumulene, more stable than the parent diphosphirane can reach 30% <91TL3687>. 

Little attention has been paid over the last decadeto the synthesis of fused phosphinines. However, in 2008, the preparation of a dithienophosphinine was reported. Compound 71 was obtained in three steps from dithienophosphole 70 by reaction successively with acetyl chloride, triethylamine and water [43], This transformation involves the transient formation of the oxide 69 (Scheme 17). DFT calculations reveal that the fused derivative is less aromatic than the parent compound C5H5P and suggest that a substantial electronic delocalization takes place within the three rings. Both the HOMO and the LUMO are localized on the P = C-Ph double bond and thus resemble those of a phosphaalkene derivative. 

Scheme 6-2 Synthesis of phosphaalkynes 9 by 3-elimination from phosphaalkenes. Ad, adamantyl.

Soluble phosphine support 57 was prepared starting from mesityl-substituted phosphaalkene 56 (Scheme 37). This was co-polymerized with various equivalents of styrene to give a series of materials, soluble in dichloromethane, thf, and toluene, with phosphine loadings ranging from 5-39 mol%. This methodology in essence shows that P=C bonds can mimic C=C bonds in their polymerization chemistry and opens avenues for the synthesis of other phosphine-substituted supports since a wide range of phosphaalkenes are known. 

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