Phosphorus-carbon double bonds

Carbon-phosphorus double bonds are also formed in addition reactions of tris(trimethylsilyl)phosphine 1692 (which can be readily prepared from white phosphorus, sodium, and TCS 14 [13a,b,c]) to give oxazohum fluorides 1691 which then give the azaphospholes 1694, via 1693 [3, 14]. On addition of 1692 to 1695, the diazaphosphole 1696 [3, 15] is prepared, whereas l,3-azaphospholo[l,2a]pyridines 1698 [16] are formed from 1692 and 1697, and 1,3-thiaphospholes 1700 are formed from the dithiohum fluorides 1699 [17]. l,3-Benzodiphospholyl anions 1703 are generated by reaction of acid chlorides with the dihthium salts 1701, via 1702 [18] (Scheme 11.3). 

This extremely air-sensitive compound, which is valence isoelectronic to an olefin, has been structurally characterized by X-ray diffraction. It has a short carbon-phosphorus double bond (1.62 A) the phosphorus and carbon atoms adopt a trigonal planar geometry with a dihedral angle of 60° (Fig. 3). This value is significantly larger than that reported for the most crowded olefin.61 Formally, this compound can be viewed as the product of a car-bene-carbenoid coupling between bis(trimethylsilyl)carbene and bis(diiso-propylamino)phosphenium triflate. Note that another route to methylene-phosphonium salt has been reported by Griitzmacher et al.62... 

Diphosphaallene derivatives ArP=C=PAr are peculiar compounds because of the presence of the two orthogonal carbon-phosphorus double bonds. The compounds were transformed into cation-radicals on electrochemical or chemical one-electron oxidation. As found, the unpaired electron is located on an MO constituted mainly by a p orbital of each phosphorus atom and a p orbital of the carbon atom (Chentit et al. 1997, Alberti et al. 1999). 

Before 1964 no stable compound with a localized or delocalized carbon-phosphorus double bond was known. Indeed, it was generally assumed that the atomic radius of phosphorus, being larger than that of carbon or nitrogen, would not provide sufficient 2pn—3pir overlap for such a ir system to be stable Our first communication, written jointly with Peter Hoffmann which described the synthesis of a stable phosphamethin-cyanine 1 with a delocalized P—C double bond was therefore received with skepticism However, after Allmann confirmed the structure by X-ray analysis the existence of a new type of phosphorus bond in a cationic delocalized tt system was unambigously established... 

Terminal phosphinidene-metal complexes, L TM=PR, readily add to double bonds (equation 17). Both metal-stabilized and phosphine-stabilized phosphinidenes, R P= PR3, can act as phospha-Wittig reagents, which undergo metathesis reactions with carbonyl-containing compounds to generate carbon phosphorus double bonds (equations 18 and 19). Pnictinidenes and their complexes have been extensively reviewed in terms of both their syntheses and reactivities. ... 

Carbon-phosphorus double bonds also participate in the Diels-Alder reaction and are a source of a variety of tetrahydrophosphinine derivatives. The C=P unit has a relatively short history the first compounds were not isolated until 1976. Special characteristics must be present to give stable compounds that resist dimerization or polymerization. Strongly electron-attracting groups or large bulky groups that provide steric retardation of the dimerization or polymerization are required. The former type are the more useful in the Diels-Alder reaction, because steric effects from the bulky substituents can also reduce the reactivity of the C=P unit to additions. An example of a successful cycloaddition is given in Scheme 5.17 (where TMS is MesSi). The literature on Diels-Alder reactions of C=P compounds is the subject of a 2008 review. ... 

Notice that we may formulate a second resonance structure for the ylide by delocalizing the negative charge onto the phosphorus. In this form, the valence shell on phosphorus has been expanded and a carbon-phosphorus double bond is present. 

Stable compounds with carbon-sulfur multiple bonds are somewhat more common than compounds with either carbon-silicon or carbon-phosphorus double bonds. 

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