Ylides, definition

Pyridine ylide/LFP studies of 83-85 in pentane or isooctane afforded carbene lifetimes of 21-24 ns (k 4 to 5 x 107 s 1), similar to the lifetime of dimethylcarbene under these conditions. Unfortunately, these lifetimes are limited by reactions with the hydrocarbon solvents the lifetime of 83 is 1.5 times longer in cyclohexane-d12 than in cyclohexane. The observation that the lifetimes of 55-CI ( 1000 ns) and 55-F (—7000 ns) are considerably longer than those of 83 and 84 could reflect the superior stabilization provided by the halogen spectator substituents of 55, but this conclusion is tentative in the absence of definitive intramolecularly controlled lifetimes for 83-85. 

Ylides, by definition, are nucleophiles. Probably the most complete definition has been given by AW Johnson [2], who stated that an ylide is a carbanion directly bonded to a heteroatom with a high degree of formal positive charge, this charge... 

Additional types of k C-k E chelating ylide complexes merit mentioning here, in addition to the species already presented, and some of them are shown in Scheme 24. The first is formed by ylides containing a pyridine functionality as substituent of the ylidic carbon. This type of ligand has allowed the S3mthesis of many loose clusters (82) of Cu, Ag, and Au which show weak metal( / °)-metal( / °) interactions. These facts have prompted the definition of a new phenomenon numismophilicity) to account for these weak interactions, uniquely shared by the three coinage metals [164]. Nitrogen and sulfur keto-stabilized ylides have been reacted with Pt and... 

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. 

Cyclopropanation reactions are one set in an array of C-C bond-forming transformations attributable to metal carbenes (Scheme 5.1) and are often mistakenly referred to by the nonspecific term carbenoid. Both cyclopropanation and cyclopropenation reactions, as well as the related aromatic cycloaddition process, occur by addition. Ylide formation is an association transformation, and insertion requires no further definition. All of these reactions occur with diazo compounds, preferably those with at least one attached carbonyl group. Several general reviews of diazo compounds and their reactions have been published recently and serve as valuable references to this rapidly expanding field [7-10]. The book by Doyle, McKervey, and Ye [7] provides an intensive and thorough overview of the field through 19% and part of 1997. 

The 13C NMR spectrum of the condensation product of thiazolium AT-ylide (194) with DMAD provided definitive evidence in support of the pyrrolo[2,l-c][l,4]thiazine (196) structure. Particularly important is the absence of an absorption that could be assigned to the tertiary C-5 in (195), this absorption being anticipated at 565.5 (76JOC187). 

To investigate the source of the unexpected cyclization, we combined a precursor 28 that did not have a 4-hydroxy group with the same stable ylide. This produced compound 29 with an oxabicyclo[2.2.1]heptane skeleton via the epoxide 30. Alternatively, the morphinan methyl ether 28 could also react with the stable sulfur ylide derived from trimethylsulfoxonium iodide at room temperature. The definitive intermediate, 6a-epoxide 30, was isolated, and then treated with NaH in DMF at 80 °C to produce the objective bicyclic compound 29 (Scheme 9). 

The latest proposal by Vedejs (3d) is that the Wittig reaction proceeds via a concerted but asynchronous puckered 4-center cycloaddition pathway in which the stereoselectivity is determined by multiple steric effects and varying degrees of rehybridization at the phosphorus atom in the transition state. At present, there is not definitive evidence to prove that the reaction must proceed in this manner. Recent MNDO-PM3 computations by us (7,8) and somewhat related MNDO computations by Yamataka et al, (9) do not support the puckered 4-center cycloaddition hypothesis for the reactions of unstabilized ylides with aldehydes (3d), Instead, the MNDO-PM3 computations indicate that such Wittig reactions proceed through an essentially planar transition state (TS) with respect to the four central atoms, P-C-C-O. This process is... 

A definitive study has been made of the photodecomposition of the ylide (5). It was shown that the relative yields of the two products (6) and (7) are... 

These experiments, while providing important new insights, cannot be considered definitive in answering the ylide question in view of their shortcomings, some of which are detailed above. Assuming nevertheless that oxonium ylides do exist and are implicated in C-C bond formation, there remains the question whether an acidic zeolite can function as a strong, hindered, non-nucleophilic base in ylide generation from alkyloxonium precursors. 

Even thiophene itself will react with carbenes, at sulfur, to produce isolable thiophenium ylides, and in these, the sulfur is definitely tetrahedral. The rearrangement of thiophenium bis(ethoxycarbonyl) methylide to the 2-substituted thiophene provides a rationalisation for the reaction of thiophene with ethyl diazoacetate, which produces what appears to be the product of carbene addition to the 2,3-double bond perhaps this proceeds via initial attack at sulfur followed by S C-2 rearrangement, then collapse to the cyclopropane. Acid catalyses conversion of the cyclopropanated compound into a thiophene-3-acetic ester. ° 2,5-Dichlorothiophenium bis(methoxycarbonyl)methylide has been used as an efficient source of the carbene simply heating it in an alkene results in the transfer of (Me02C)2C to the alkene. ... 

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