Phospha-alkene

Gas phase pyrolysis of dimethylchlorophosphine leads to the formation of the reactive phospha-alkene (166), which has the... 

The unhindered phospha-alkene (175) has been characterised in solution as a complex involving ir-donation from the double bond to a tungsten carbonyl acceptor. This complex reacts with 1,3-dienes to give adducts e.g. (176), in which the phosphorus atom... 

Additions to the double bond of phospha-alkenes continue to be explored. The reactions of (177) with aliphatic diazo compounds... 

Reactions leading to substitution at the phosphorus atom of phospha-alkenes have also received further study. The chloro-phospha-alkene (182 X = Cl) undergoes halogen-exchange on... 

It has also been shown that electron-rich phospha-alkenes e.g. 

Turning from iminophosphanes to alkylidenophosphanes (phospha-alkenes), the orientation of the [2 + 2]-cycloaddition is inverted, as far as phosphorus is concerned only one example has been worked out (product VIII) 19). The phosphaalkyne iBuC=P does not react with the iminoborane BuB=NtBu, which instead trimerizes (IS). An exotic [2 + 2]-cycloaddition is observed when the very reactive titanaethene... 

The newest series of polymers based on a phosphorus-carbon backbone are poly(meth-ylenephosphines) of structure [-P(R)-CR2-] , produced by the polymerization of phospha-alkenes, P(R)=CR2.47a These polymers react with oxygen to form species of type [-P(R)(0)-CR2-] . A related polymer with phenyl rings in the backbone has also been described.47b This is a pi-conjugated polymer with P=C bonds in the main chain. 

The chemistry of trivalent phosphorus compounds in which phosphorus is one or two coordinate is rapidly developing. These systems contain sp or sp2 hybridized phosphorus and multiple TT-bonds between P and other elements and until 1964 it was believed that bond formation involving ptr-pir overlap was unfavourable. Subsequently certain -P=C systems resulted from the use of charged ( 1, 2, 3) and or delocalized systems (4J but it is only in the past few years that successful syntheses of phospha-alkenes, R2C=PR , and phospha-alkynes, RC=P, have been reported. These novel compounds are the subject of this paper. 

The P-C distances in simple phosphines, phospha-alkenes and phospha-alkynes are summarised in Figure 1 and the results contrasted with C-C, C=C and C=C bond lengths when a simple relationship is seen to exist. Additionally the data for the phosphabenzene systems shows excellent agreement with expectation. 

Compounds of the type R2C=PX were first postulated by Haszeldine and coworkers (24-27) as reaction intermediates in the reaction of certain perflrcroaLkyl phosphines with bases. These species were first isolated and characterised by spectroscopic techniques by Kroto and Nixon and coworkers (28). Microwave and/ or NMR data on CH2=PH, CH2=PC1 and CF2=PH were obtained on samples generated by the pyrolysis of suitable precursors, but subsequently a variety of alternative synthetic routes have been developed to obtain the following phospha-alkene molecules (28-34) ... 

Thermal stabilization of phospha-alkenes can be influenced by the attachment of a heteroatom to the P=C bond as evidenced by the elegant work of Becker (35-40, 43), Appel (41) and Issleib (42). 

In 1978 Bickelhaupt et al. 44) reported the first stable phospha-alkene with carbon substituents only.using the following synthetic approach. 

Kroto, Nixon et al. have reported several examples of complexes of the phospha-alkene, L, RP=CPh2 (R = mesityl) (45). In principle phospha-alkenes can coordinate to transition metals... 

The bond length between platinum and the phospha-alkene is only 2.193 8 which is much shorter than the Pt-PEt3 bond (2.255 ) and is only slightly longer than the shortest known Pt-P distance (to PF3 2.141 ) in cis-PtCl2(PF3)(PEt3). Clearly the s-character of the phospha-alkene lone pair is significantly enhanced. These results and a discussion of their relationship with Pt-Cl distances and jp will be the subject of future papers. 

Continued research on N-silyl-phospha-amidines and N,N-bis-silylated phosphaguanidines (5) led us to carbosilylated phos-phaalkenes. The various routes to trimethylsilylmethyl-chloro-phosphines and their conversion into carbosilylated phospha-alkenes is given in Scheme 4 (6). Interaction of these products with hydrogen chloride leads to Si2CH-PRCl and Si-CH2-PRC1, respectively. By HC1 abstraction from these products, the parent... 

Phosphaallenes exhibit three reactive centers, the PC double bond, the CC double bond, and the phosphorus atom. Here the PC double bond is significantly more reactive than the neighboring second double bond. The reactivity pattern is very similar to the phospha-alkenes, as far as the addition, cycloaddition, and coordination reactions are concerned. The cumulated CC double bond therefore has to be treated as an isolated group that is not greatly influenced by reactions at the PC n system. 

A strong shielding of the P atom is indicated by 31P NMR data on the butyl- ( — 180 ppm) and the tri-<-butylphenylphosphaketene ( —207.4ppm). According to this the direction of addition of hydrochloric acid to the phosphaketenes is opposite to that of the phospha-alkenes (Scheme 19). 

The reaction of cyanomethyllithium with 2,4,6-tri-t-butylphenyl-dichlorophosphine enables a one-step preparation of the functionalised phospha-alkene (144). A new route to phospha-alkenes is provided by nucleophilic attack at the vinylic CH2 of the halogenophosphine (145), with displacement of halogen from phosphorus. Thus, with DABCO as nucleophile, the novel system (146) is formed.Treatment of the phosphino-substituted ylides (147) with Lewis acids results in the 2-phosphonio-l-phospha-alkenes (148), reported to be sufficiently stable for X-ray analysis, but also undergoing various addition reactions. 

The diphosphetene-complex (154) undergoes a reversible, conrotatory ring-opening on treatment with maleic anhydride or N-phenylmaleimide, to form the complexed diphosphadiene (155). The reactions of P-halofunctional phospha-alkenes with N-trimethyl-silylimines and related compounds have given a series of azaphosphadienes (156). The first unsymmetrical 1,3-diphospha-... 

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