Phosphorus ylides rearrangement

The first kinetically stable dibenzosilafulvene (7), whose structure and properties should more correctly be described by the resonance hybrid 7a 7b with a great contribution of the ylide form 7a, reacts with phosphorus ylide to form betaine (8), which is rearranged, under thermodynamically controlled conditions, into the salt (9) (Scheme 4).24,25... 

Addition of sodium tetrafluoroborate to 2d led to the carbodiphosphor-ane 77 possessing a P-H bond, which was isolated in 70% yield.31 Although 77 is thermally quite stable as a solid (mp 116°C), it slowly rearranges in solution at room temperature (1 week) into its isomeric phosphorus ylide 78. This last result was not surprising, since calculations predicted 78 to be 19 kcal/mol more stable than 77.16... 

Carbodiphosphoranes (R3P = C = PR3) are known,79 but ylides with a P-H bond are rare.80 Therefore, the spectroscopic characterization of 77 was unexpected. Even more surprising was the characterization of the carbodiphosphorane 79 featuring two P-H bonds.31 This compound, prepared by treatment of 2d with tert-butyllithium, rearranged in solution at room temperature over a period of 16 h to afford the phosphorus ylide 80 with one remaining P-H bond. This compound was also unstable and transformed completely into the diphosphinomethane 81 overnight. Note that calculations for the model compounds where R = NH2 predicted 79 to be 28 kcal/mol less stable than 80, which is also 34 kcal/mol above 81.16 The surprising stability of 79 and 80 is probably due to the presence of bulky substituents, since tetracoordinate phosphorus atoms can more readily accommodate the increased steric constraints than can their tricoordinate counterparts. 

Reaction of enol ether 1 a with sodium methoxidc or ethoxide. or 1 b with sodium methoxide, gives ketene acetals, which rearrange in the presence of triethylamine to give esters 3.3 The phosphorus ylide trimethoxy[2,2,2-trilluoro-l-(trifluoromethyl)cthylidene]-A ,-phosphane f(CF3)2C = P(OMc),] rearranges readily to dimethyl f2.2,2-trifluoro-1-mcthyl-l-(trifluoro-mcthyi)ethyl]phosphonate [(CF,)2C(Me)PO(OMe)2] in a similar 1.3-shift.5-0... 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

Preparation of phosphorus ylides Elimination of nitrous acid Steric control of elimination reactions. Mdol condensation Rearrangements Reactions with aryl isocyanates Preparation of carboxylic acid chlorides Synthesis of macromolecules... 

A review on the reaction of tervalent phosphorus acid derivatives with halogens has appeared. Alkylbis(diisopropyl-amino) phosphines (41) react with carbon tetrachloride or bromotrichloromethane to give the P-halogeno ylides (42) below 0 The ylides rearrange to the substituted phosphines (43)... 

CO)sCr=C(OR)R complexes allenes/ Photolysis of a mixture of the chromium carbenc I and the stabilized phosphorus ylide 2 under CO gives the allenc 3 in 60% > icld. Hydrolysis of 3 with HjO+ in ether provides the /3-substituted (Z)-enone 4. The jllcnc 3 rearranges to a (Z)-l,3-diene (5). Pyridinium p-toluenesulfonamidc is an effective catalyst for this rearrangement. 

Reaction of (5a) with phosphorus ylides affords free 4-alkyl-substituted 3-borahomoadamantanes (105) without contamination by the starting compound (5) (see Scheme 39). However, the parent compound (6) was not prepared by this method, because the corresponding adduct of (5) with methylenetriphenylphosphorane is exceptionally stable and rearranges only at temperatures... 

The synthesis of new types of trisubstituted oxazoles (6) has been accomplished via chemoselective O-acylations and intramolecular Wittig reactions with ester functionalities using in site-formed phosphorus ylides as key intermediates. A plausible reaction mechanism has been proposed on the basis of the existence of expected and rearranged isomeric oxazoles (6f). 

The next stage could involve the formation of the phosphino-substituted phosphorus ylide C. Whether or not this reaction actually occurs depends on the difference in reactivity of the CCl group in the original carbosilane A, which in turn depends on the extent of Si-chlorination, compared to the PMe2Cl group in the phosphorus ylide B. As far as the phosphino-substituted ylide C is concerned, it can react with excess starting material A to yield the phosphorus ylide B, or rearrange via a competitive reaction to produce the phosphane D. The extent to which these competitive... 

The formation of the phosphino-substituted ylide ( Si)2C=PMe2—PMe2 (C) is a kinetically controlled reaction in this case. Under thermodynamic control a competing reaction occurs in which the phosphorus ylide ( Si)2C=PMe2Cl (B) and, by thermal rearrangement, the phosphane ( Si)2C(H)PMe2 (D) are produced. 

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