Methylenephosphorane

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 ... 

The generally quite stable methylenephosphorane nevertheless resembles the short-lived highly reactive methyleneoxophosphorane. The oxaphosphetane intermediate 25 formed by -I- n2]-cycloaddition, which can only be isolated in exceptional cases21, is to be seen against the stable oxaphosphetanes of type 26, which can be photolyzed if suitably substituted or thermolyzed under drastic... 

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... 

As shown for the aziridines, BETMIP (68) has proved to be useful in the synthesis of azepines (Scheme 118). Treatment with methylenephosphorane leads to a phosphonium salt which in turn is deprotonated with BuLi and cyclized with benzene-l,2-dialdehyde in a Wittig and aza-Wittig step to form benzazepine 326 (93JOC1987). 

Methylenation using diazomethane, 2 78 Methylene, 29 303-304 Methylene blue, astatination, 31 77 Methylenephosphoranes, reaction with hexafluoroacetone, 30 258... 

These studies suggested a planar ylidic carbon and definite participation of the P d orbitals in the description of the HOMO. The earliest ab initio study of methylenephosphorane found a very small rotational barrier [0.003 kcal mol-1 (1 kcal = 4.184 kJ)] about the P=C double bond64. These three points, the degree of pyramidalization at the ylidic carbon, the role of the P d orbitals and the P=C rotational barrier, remain the focal points of all theoretical studies of the ylides. A summary of the theoretical structures determined using ab initio techniques is given in Table 7. 

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). 

A convenient, short-step synthesis of enantiomerically pure 2-methylenecyclopropyl-methanol has been achieved by treating optically pure epichlorohydrin with methylenephosphorane (equation 146)325. Vinyl ketones are cyclopropanated with ( )-1-... 

Leukotriene-B could be obtained starting with the aeetonide 88 of 2-deoxyribose 76). Wittig olefination of 88 with methoxycarbonyl-methylenephosphorane 67 and subsequent hydrogenation affords hydroxy ester 89. Tosylation of 89, cleavage of the protective group and treatment with K2C03 yields the epoxy ester 90. Benzoylation... 

Wittig reactions have been found in several cases when HFA reacts with substituted methylenephosphoranes 209,213,245,249), by analogy with the... 

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). 

Methylquinazolin-4(3//)-one is obtained m 75 % yield via an unusual methylenephosphorane extrusion process on heating salt 36 in acetonitrile under reflux with a catalytic amount of sodium hydrided ... 

Coupling of aromatic diazonium salts with stabilized methylenephosphoranes leads to arylazometh-ylenephosphoranes (equation 30). ... 

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). 

Analogously to the silyl group the Si—Si unit may be introduced into the a-position of methylenephosphoranes using chlorodisilanes (equations 44 and 45). 

Ylides with ambident character, like 1-formyl- or 1-acyl-methylenephosphoranes, may be attacked by alkylating reagents (the same is true for acylation) at the a-carbon or at the O atom. - Bromomethyl acetate C-alkylates acylmethylenetriphenylphosphoranes in a transylidation reaction (equation 55). Recently cyanomethylenetriphenylphosphorane has been successfully alkylated via its ylide anion, the substituted cyano ylides being accessible more readily in greater variety than previously (equation 56). ... 

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