Phosphonium ylides conversions

With the fully functionalized heterocyclic core completed, synthetic attention next focused on introduction of the 3,5-dihydroxyheptanoic acid side-chain. This required initial conversion of the ethyl ester of 35 to the corresponding aldehyde through a two-step reduction/oxidation sequence. In that event, a low-temperature DIBAL reduction of 35 provided primary alcohol 36, which was then oxidized to aldehyde 37 with TRAP. Subsequent installation of the carbon backbone of the side-chain was accomplished using a Wittig olefination reaction with stabilized phosphonium ylide 38 resulting in exclusive formation of the desired -olefin 39. The synthesis of phosphonium ylide 38 will be examined in Scheme 12.5 (Konoike and Araki, 1994). 

Conversion of phosphines via phosphonium salts into phosphonium ylides induces a deshielding of the a alkyl and aryl carbons (Table 4.49). Considerable shielding of the ylide sp2 — C = P carbon nucleus reflects the presence of a large electron density, thus indicating a significant contribution of the ylide dipole to the actual state of the molecule. 

In order to study possible side reactions which may reduce the yield in the Woodward method of annulation of azetidin-2-ones (Scheme 31) similar reactions of ylides (214), which do not contain groups capable of conversion to carbonyl by DMSO-acetic anhydride treatment, have been investigated. 30 A range of products, e.g. (215) and (216), were obtained. A mild, four-carbon homologation of the 4-formyl-substituted azetidinone (217) involving reaction with the phosphonium ylide (218) has been used to synthesize (219), a useful intermediate in the synthesis of carbacephem antibiotics. 3 ... 

A number of triphenylbismuthonium diacylylides and also the diphenylsulphonylylide undergo transylidation when treated with dimethyl sulphide in the presence of copper(I) chloride in benzene at room temperature, giving the corresponding dimethylsulphonium ylides no such reaction took place with diphenyl sulphide Some of these bismuthonium ylides reacted with triphenylarsine or triphenylphosphine and were converted thereby into arsonium or phosphonium ylides they did not react with triphenylstibine L The conversion of the diacetylylide into the 4,4-dimethyl-2,6-dioxocyclohexylide as mentioned in Section IX.A should also be noted. 

A number of tricarbonylchromium-X -phosphorin complexes have been prepared and shown to possess a phosphonium ylide structure (Dimroth, Berger, and Kaletsch, Phosphorus Sulphur, 1981, 10, 295). For the preparation of tricarbonyIchromium-, tricarbonylmolybdenum-, and tricarbonyItungsten-X -phosphorin see Dimroth, Liickoff, and Kaletsch ihid., p.285) for the conversion of tricarbonylchromium-ri X -phosphorin complexes into tricarbonylchromium-r °X -phosphorin complexes, Dimroth and Kaletsch (J. organometallic. Chem.,... 

Treatment of iodonium tetrafluoroborates 792 with triethylamine in methanol in the presence of triph-enylphosphine and aldehydes results in Wittig oleflnation to give products 793 (Scheme 3.313), which involves the intermediacy of monocarbonyl iodonium ylides 788 and their subsequent conversion into the respective phosphonium ylides upon the in situ reaction with PhsP [1089]. 

Explain why (a) Cossee mechanism is not the only possible mechanism for chain propagation in ethylene oligomerization reaction (b) selectivity of the oligomerization of ethylene is expected to be a function of the electronic and steric properties of the P, O ligands (c) conversion of 6.46 to 6.47 is accompanied by the formation of styrene (d) the reaction of Ni(COD)j with appropriate phosphonium ylide and PPhj gives 6.46 (e) in ethylene oligomerization, product formation may take place by chain transfer (f) on replacement of PPhj in 6.46 by PMOj, there is an almost complete inhibition of the catalytic reaction. 

Unconventional transformations were also discovered. The reaction of a phosphonium ylide such as tributyl[(trimethylsilyl)methylene]phos-phorane with enolisable aldehydes did not afford the expected Wittig reaction product. Conversely, alkenylphosphonium salts were obtained in good yields (65-89%, Scheme 3). ... 

The formation of the heterocycle 1 from the xylylene-bis-phosphonium salt 2 and PCI3 proceeds via a detectable intermediate 3 in a cascade of condensation reactions that is terminated by spontaneous heterolysis of the last remaining P-Cl bond in a cyclic bis-ylide-substituted chlorophosphine formed (Scheme 1) [15]. The reaction scheme is applicable to an arsenic analogue of 1 [15] and to bis-phosphonio-benzophospholides with different triaryl-, aryl-alkyl- and aryl-vinyl-phosphonio groups [16, 18, 19], but failed for trialkylphosphonio-substituted cations here, insufficient acidity prohibited obviously quantitative deprotonation of the phosphonium salts, and only mixtures of products with unreacted starting materials were obtained [19]. The cations were isolated as chloride or bromide salts, but conversion of the anions by complexation with Lewis-acids or metathesis was easily feasible [16, 18, 19] and even salts with organometallic anions ([Co(CO)4] , [CpM(CO)3] (M=Mo, W) were accessible [20]. 

In the Wittig reaction an aldehyde or ketone is treated with a phosphorus ylid (also spelled ylide and called a phosphorane) to give an alkene. The conversion of a carbonyl compound to an alkene with a phosphorus ylid is called the Wittig reaction. Phosphorus ylids are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from a triaryl phosphine and an alkyl halide (10-31) ... 

The trivalent phosphorus atom bears a lone pair of electrons and therefore can be used as a nucleophilic reagent for substitution. Triphenylphosphine displacements on alkyl halides give phosphonium salts which, after the conversion into phosphorus ylides by strong bases. 

Conversion of phosphonium salts to salt-free solutions of ylides can also be effected with sodium bis(trimethylsilyl)amide. As it is soluble in many solvents, and easy to handle and to weigh out, sodium bis(trimethylsilyl)amide is preferred to sodamide in liquid ammonia in many cases (equation 11). The corresponding potassium and lithium compounds can also be used. ... 

Dimsylsodium (24) functions as a highly basic sulfur ylide. It can be used to convert phosphonium salts to phosphorus ylides for use in the Wittig reaction. Dimsylsodium also reacts with aldehydes and ketones by nucleophilic addition to form epoxides and with esters by nucleophilic substitution to yield p-ketosulfoxides (25) (Scheme 11). The p-ketosulfoxides (25) contain acidic a-hydrogens which can be readily removed to allow alkylation, and the products (26) suffer reductive desulfuration on treatment with aluminium amalgam to yield ketones (27) (Scheme 11) This procedure can, for instance, be applied to the conversion of ethyl benzoate to propiophenone (28) (Scheme 12). 

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