Reactions of Methylenephosphoranes

Reactions of Methylenephosphoranes - Aldehydes.—The use of the Wittig and Horner reactions in stilbene synthesis has been reviewed. ... 

Miscellaneous Reactions of Methylenephosphoranes.—The synthesis of three-membered rings by means of phosphorus ylides has been reviewed and there have been several new reports of this type of reaction. Optically active a,p-un-saturated esters have been converted into cyclopropanes highly stereoselectively by reaction with isopropylidenetriphenylphosphorane as the key step in syntheses of both (li ,3 )-chrysanthemic acid methyl ester (53) and the (IR) cis-gem dibromovinyl analogue (54). The addition of reactive phosphonium ylides to functionalized 1,3-butadienylphosphonates provides a new synthesis of 2-cyclopen ten-1 -ylphosphonates (5 5). ... 

Reactions of Methylenephosphoranes.- A recent review of dianion chemistry includes a number of references to P-ketophosphonate 1,3-dianions and their use in synthesis.20... 

The first silicon-organophosphorus betaine with a thiolate center (15a) was synthesized by the reaction of stable silanethione (14) with trimethyl-methylenephosphorane (Scheme 8) and characterized by multinuclear NMR spectroscopy.14 Compound 15a is formed under kinetic control and is transformed, under the thermodynamically controlled conditions, into the silaacenaphthene salt (16). The processes presented in this scheme reflect the competition of the basicity and nucleophilicity of phosphorus ylides. Betaine 15b prepared from less nucleophilic and less basic ylide with phenyl substituents at the phosphorus atom is much less resistant toward retro-decomposition compared to the alkyl analog. Its equilibrium concentration does not exceed 6%. 

Structure.—N.m.r. evidence has been presented which shows that the compounds generated by the reaction of butyl-lithium with methylphosphonium salts are not methylenephosphoranes, but lithium adducts of the ylides,14 and that they are planar18 (see Chapter 12). A carbonyl group separated by a benzene ring as in (12) has a weak stabilizing effect.16... 

The Wittig reaction of 6-methoxy-4-methylbenzofuran-2-carbaldehyde with methoxy-methylenephosphorane gave a 2 1 mixture of (E)- and (Z)-6-methoxy-2-(/3-methoxyvinyl)-4-methylbenzofuran, which was used directly in a Diels-Alder reaction with dimethyl acetylenedicarboxylate, a key step in the route to polyfunctional dibenzofurans (70AJC2119). The di-O-methyl derivative (397) of the lichen natural product strepsilin was synthesized by this route (Scheme 106) (72AJC545). 

From methylenephosphoranes (R = H) both mono- or bis-silylated products may result. Schmid-baur et al. have used a variety of compounds for the silylation of ylides. a-Silylated alkylidenephos-phoranes are also formed in the reaction of simple ylides widi 2-chloroethylsilane (equation 38), chloromethylsilane (equation 39), siletanes, 1,3-disiletanes (equation 40) and the highly strained hexa-methylsilirane (equation 41). ... 

Reaction of simple ylides with dialkyldihalosilanes leads in a twofold transylidation to the formation of Si-bridged bisphosphoranes (equation 42), which can undergo consecutive reactions with excess starting materials leading to cyclic products. Bis(chlorodimethylsilyl)methane also reacts with dilithio-methylenetrimethylphosphorane to give a methylenephosphorane with an exocyclic ylide function (equation 43). 

Preparation.—A historical account of the development of the synthesis of methylenephosphoranes has been given. Details have appeared - of the use of epoxides in the generation of ylides from phosphonium salts. Weakly acidic salts are deprotonated by the anions such as XCHaCH20 formed by attack of the phosphonium covmter-anion on epoxide, but strongly acidic phosphonium salts are deprotonated more rapidly than these species are produced. The base in these cases must be either epoxide or original anion. Side-reactions in olefin syntheses using epoxides as base include acetal formation from aldehyde and epoxide, cyclopropane formation from ylide and epoxide, and decomposition of quinquecovalent phosphoranes, e.g. (1), formed from phosphonium salt and... 

Reactions of Ketones with Methylenephosphoranes.—Intramolecular Wittig reactions have been used to prepare a variety of cyclic compounds including bicyclic alkenes (41) and cyclic imines. In a similar way both Wittig and phosphonate reactions can be used to synthesize many strained bicyclic alkenes (42), but fail with the most strained examples, e.g., (43). Intramolecular Wittig... 

However, the information on fluorine-containing derivatives of these materials is very limited. P-F Phosphoranes 193 and 194, resulted from the fluorination of diphosphine disulfide 192 or from the reaction of 1-methyl-methylenephosphorane 191 with hydrogen fluoride, were among the first representatives of the fluorine-containing phospholanes. ... 

Trimethylsilylmethylenephosphoranes have two different electrophilic sites (P, Si). Reaction of these species with methanol generates methoxytrimethyl-silane and methylenephosphoranes (51). Apparently silicon is a harder acceptor than tetrahedral phosphorus. 

A new ylide, tris(4-methylphenyl-)benzoylmethylenephosphorane, has been synthesized and its properties and reactivity studied.The metallation of the donor-functionalized ylide ligand (2-methoxyphenyl)methylene-triphenylphosphor-ane with conujlexes of yttrium and lutetium gave unprecedented phosphoranylidene complexes.The reaction of triphenyl-methylenephosphorane with Mo(NAr)-(CHCMe3)[OCMe(CF3)2l2> (Ar = 2,6- Pr2QH3), produced an anionic alkylidyne complex (Scheme 103). ... 

A few examples of the reactions of a (trimethylsilyl)methylenephosphorane with carbonyl compounds have been reported. Reactions of the silylated phosphorus ylide 106 with a,/ -unsaturated aldehydes lead to the 1,3-dienylphosphonium salts 107 (Scheme 2.64) [170). These reactions show that the silyl group is eliminated faster than the phosphorus group. 

This reaction may be visualized as proceeding by nucleophilic attack of tervalent phosphorus at the carbonyl group to give an intermediate such as (15). The structure of (16) was deduced from the fact that it was hydrolysed to the known phosphine oxide (17). Methylenephosphoranes (phosphorus ylides) may also be converted into monophosphazenes by reaction with benzonitrile ... 

A very stable type of organocopper compound, a metalated ylide, is obtained from the reaction between copper(I) chloride and trimethyl-methylenephosphorane in an aromatic solvent. The compound is soluble in aromatic and chlorocarbon solvents and can be sublimed 256a). 

Pretreatment of the monomethylenediphosphazene with sodium or n-butyl-lithium, followed by methyl iodide, resulted in the formation of a methylenephosphorane, (MePhN-PPh2=CH-PPhaMePh)+1. The same compound could also be converted into an imide by reaction with an aldehyde ... 

---