Carbene from ylides

In either neat dioxane or THF, carbene-ether ylides are observed as a broad IR absorption band between 1560 and 1610 cm , distinct from the IR bands of the free carbenes. With discrete spectroscopic signatures for the free carbene and its corresponding ether ylides, TRIR spectroscopy was used to confirm that the effects described above with dilute ether in Freon-113 were due to specific solvation of the carbene (Scheme 4.6, Reaction 2) rather than a pre-equilibration with the coordinating solvent (Scheme 4.6, Reaction 3) or reactivity of the ylide itself (Scheme 6, Reaction 4). In Freon-113 containing 0.095M THF simultaneous TRIR observation of both the free carbene (x = ca. 500 ns) and the carbene-THF ylide (x = ca. 5ps) was possible7 The observation that lifetimes of these species were observed to be so different conclusively demonstrates that the free carbene and the carbene-THF ylide are not in rapid equilibrium and that Reaction 3 of Scheme 4.6 is not operative. By examining the kinetics of the carbene 34 at 1635 cm directly in Freon-113 with small amounts of added dioxane, it was observed that the rate of reaction with TME was reduced, consistent with Reaction 2 (and not Reaction 4) of Scheme 4.6. 

Although we are not specifically concerned here with kpp and the kinedcs of carbene-pyridine ylide formation, we note that the magnitude of is directly related to the structure and reactivity of the carbene. fcpyr ranges from 105 M s-1 for ambiphilic alkoxycarbenes to 109-10I° M-1 s 1 for electrophilic halocarbenes or alkylcarbenes. Very nucleophilic carbenes (MeOCOMe) do not react with pyridine.13... 

Ylides from R N2. This reagent is more effective than bis(acetylacetonate)-copper(II) (5, 244) for generation of carbenes from diazo compounds.2 The decomposition proceeds at a lower temperature, even at room temperature. The mild conditions are particularly useful in the preparation of heat-sensitive ylides, such as those of antimony, bismuth, and tellurium. 

Miscellaneous. An interesting synthesis of 1,1-difluoro-l-alkenes from ylides and chlorodifluoromethane has been described.47 The ylide acts both as a carbene generator and trapping agent (Scheme 11). 

Fig. 4.1. Generation of acceptor-substituted carbene complexes from ylides. X N2, SR2, S(0)Me2, Arl Z COR, CO2R, CONR2, SO2R, CN, P(0)(0R)2.

Decarboxylation of 1,3-dimethylorotic acid in the presence of benzyl bromide yields 6-benzyl-1,3-dimethyluracil and presumably involves a C(6) centered nucleophilic intermediate which could nonetheless have either a carbene or ylide structure. Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been used to explore the gas-phase reactions of methyl nitrate with anions from active methylene compounds anions of aliphatic ketones and nitriles react by the 5n2 mechanism and Fco reactions yielding N02 ions are also observed nitronate ions are formed on reaction with the carbanions derived from toluenes and methylpyridines. 

Epoxide 96 was prepared such that photolytic conversion to the carbonyl ylide could be followed by an intramolecular cycloaddition with the tethered pendant olefin. However, photolysis of epoxide 96 led only to the formation of the regio-isomer 97 and the aldehyde 98 with no evidence of the corresponding cycloadduct. It was presumed that 97 arose from the ylide by thermal recyclization to the epoxide while 98 could form through the loss of a carbene from the ylide. The failure of the tethered alkene to undergo cycloaddition may have resulted from a poor trajectory for the cycloaddition. An extended analogue (99) allowed greater flexibility for the dipolarophile to adopt any number of conformations. Photolysis of epoxide 99 did lead to formation of the macrocyclic adduct 100, albeit in modest yields. 

Although not as common as the ylide derived from metal carbenes, the ylide-like species generated from metal nitrene or free nitrene has been attracting increasing attention in recent years. The overall transformation is parallel to that of metal carbene reactions. Progress in this direction is also covered in this chapter. 

Epoxide Formation from Ylides Prepared by Metal-catalyzed Carbene Formation... 

Generation of triplet carbenes from photolysis of iodonium ylides has been discussed in a short review.9... 

The development of the two-color and laser jet approaches has also allowed the study of the photochemical behavior of excited states of reaction intermediates, i.e., transient species that are chemically distinct from the original ground or excited state, such as neutral and ion radicals, biradicals, carbenes, and ylides. In fact, the study of excited reaction intermediates has been more comprehensive than the study of upper states. Originally, the short-lived nature of the ground-state transient itself led to the incorrect assumption that the excited transient would be too short-lived to participate in any chemical or photophysical processes other than deactivation to the ground state. However, this is now known not to be the case and some surprising differences between the ground- and excited-state behavior of reaction intermediates have been observed. 

An extremely versatile approach to ylides involves the light-promoted or metal-catalyzed generation of carbenes from diazonium salts. In most cases, metal-catalyzed reactions give better yields and cleaner reaction mixtures than do corresponding photolytic techniques (Scheme 16, equation a). A classical example is to be found in penicillin chemistry (Scheme 16, equation b). However, copper catalysts have their limitations in the generation of carbenoid intermediates. These limitations can be largely over-... 

Virtually every possible reactive C intermediate has been invoked to explain the crucial step of initial C-C bond formation from methanol/DME. Proposed mechanisms can be broadly classified as carbenic, carbocationic, ylide, and free radical. In some proposals several of these categories are combined. 

As is the case for the [2,3] Wittig rearrangement, the stereochemical consequences of the [2,3] ylide rearrangement are sensitive to perturbation by external steric and stereoelectronic factors, presenting a useful opportunity for both substrate- and reagent-based asymmetric induction. Rearrangements of carbene-derived ylides of allylic sulfide 1 provide a simple example of substrate-directed diastereosclcction, in which diastereoface selectivity results from attack on the exocyclic olefin via the less-hindered equatorial approach vector112. 

The formation of ylides by use of the carbenes from chloroform249 or methylene dichloride109,250 and a base require only a mention here since such ylides are always used for Wittig olefinations without isolation. 

The formation of metal carbenes from sulfonium ylides has been also recently disclosed. Treatment of the sulfonium salt with KHMDS followed by the addition of tris(triphenylphosphine) ruthenium dichloride and phosphine exchange with tricyclohexyl-phosphine led to the formation of the Grubb s metathesis catalyst... 

The mechanism of 0-H bond insertion by carbenes remains an intense held of investigation. In the case of ether formation, three distinct pathways can be proposed (i) abstraction of protons from the alcohol forming an intermediate ion pair, (ii) reaction with the oxygen atom of an alcohol forming an intermediate ylide, and (hi) direct (concerted) insertion into the O-H bond. In that context, the carbene - alcohol ylide resulting from the reaction between carboethoxycarbene and MeOD has been experimentally detected for the first time, thus corroborating the viability of the ylide pathway. ... 

Aziridinium ylides appear to be the initial products from the reaction of aziridines with carbenes. - The ylides (531) derived from N-substituted... 

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