Wittig reaction catalysts

Wilkinson s catalyst 117,345,777 Wittig condensation 73 Wittig homologation 503 Wittig methylenation 649 Wittig reaction 17, 68 f., 86 f.,... 

Whittlesey, Williams and co-workers fnrther developed the catalytic indirect Wittig reaction and fonnd that the more electron-rich NHC present in complex 18 provided a more reactive catalyst [8]. Catalyst 18 was used to convert benzyl alcohol 8 and phosphoninm ylide 19 into the product 20 under slightly milder reaction conditions and in a shorter time than in previous work (Scheme 11.4). Other C-C bond-forming reactions from alcohols using a borrowing hydrogen approach have been reported, with Peris and co-workers using Ir-NHC complexes for the C-3 alkylation of indoles with alcohols [9]. 

Scheme 11.4 An improved catalyst for indirect Wittig reactions on an alcohol...

The Horner-Wittig reaction of a-phosphoryl sulphoxides 442, which are chemically stable, results in the formation of a, -unsaturated sulphoxides 443 in high yields (equation 264). The reaction has been found to be non-stereoselective, mixtures of E and Z isomers being formed from aldehydes and unsymmetrical ketones . In the case of aromatic aldehydes this reaction can also be advantageously performed in a two-phase catalytic system even without the usual PTC catalysts (Table 24). Intramolecular Horner-Wittig reaction of a-phosphoryl-5-oxosulphoxides 444 leads to a, -unsaturated cyclic sulphoxides 445 (equation 265). Starting from optically active 0,0-... 

The use of water-soluble reagents and catalysts allows reactions to be performed in aqueous buffered solutions. PEG-supported triarylphos-phine has been used in a Wittig reaction under mildly basic aqueous conditions (Eq. 8.115). The PEG-supported phosphine oxide byproduct can be easily recovered and reduced by alane to regenerate the starting reagent for reuse.312 The aqueous Wittig reaction has also been used in... 

Direct hydrogenation of key intermediate 248 over the Adams catalyst and subsequent lithium aluminum hydride reduction yielded the two stereoisomeric alcohols 256 and 257, which were separately transformed to ( )-corynantheal (258) and ( )-3-epicorynantheal (259), respectively, by Moffatt oxidation, followed by Wittig reaction with methyltriphenylphosphonium bromide and, finally, by demasking the aldehyde function (151, 152). 

Catalyzed Wittig reactions. Wittig reactions of cyclopropylidenetriphenyl-phosphorane with carbonyl compounds proceed in low yield under standard conditions. Reactions conducted at 62° in THF with 1 (10 mole %) as the phase-transfer catalyst result in alkylidenecyclopropanes in 60-95% yield. ... 

The effect of catalysts on the stereochemistry of the Horner-Wittig reaction under liquid-liquid conditions has been studied by Mikolajczyk et al. (1975). The E/Z isomer ratios in the product [162] proved to depend on the catalyst... 

Another reported example of the carbon-carbon bond-forming reaction from alcohols is that of the indirect Wittig reaction, which utilizes an [IrCl(cod)]2/dppp/ CsCOs catalyst system and leads to the production of alkanes (Equation 10.24) [47]. 

An interesting l,3-oxazol-2-one synthesis of 126 starts from propargyl alcohol 125, CO2, and primary amines with n-butylphosphane as a catalyst (Scheme 50). It is not yet clear if the phosphane reacts by formation of an iminophosphorane followed by an aza-Wittig reaction with CO2 (90TL1721). 

In traditional synthetic organic chemistry, the Wittig reaction plays an important role in carbon-carbon bond extension from the carbonyl group. CM is an attractive alternative for carbon-carbon extension from a terminal alkene. In fact, a pyrroh-dine ring of anthramycin derivative 55 has been constructed by RCM of 52, and the sidechain has been extended by CM of terminal alkene of 54 with ethyl acrylate. " In the CM, ruthenium carbene complex Ij, reported by Blechert, gives a good result since the ligand of the catalyst easily dissociated from the ruthenium metal at room temperature ... 

Ford, W. T., Wittig Reactions on Polymer Supports, Chap. 8 in Polymeric Reagents and Catalysts, W. T. Ford, ed., Am. Chem. Soc. Symp. Ser., Vol. 308, American Chemical Society, Washington, DC, 1986b. 

Improved synthesis by this method has been achieved in order to obtain solenopsin A (Id) more stereoselectively 388). A cis and trans mixture of pyridine derivative 158, obtained from 156 by the Wittig reaction, was reduced by Raney nickel catalyst in the presence of PtOj to yield the m-piperidine (Ic). The Af-nitroso derivative 159, obtained from Ic by treatment with isoamyl nitrite, was treated with potassium rert-butoxide and then subjected to hydrogenolysis over Raney nickel to give a mixture of ( )-solenopsin A (Id, 1 part) and its isomer (Ic, 1 part) (Scheme 3). 

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