Carbenes dialkoxy

Z,Z)-l,4-Dialkoxy-l,3-dienes can be readily prepared from propargyl ethers and molybdenum carbene complexes (equation 185)307. High stereoselectivity in this reaction may be due to the formation of stable vinyl hydride complex with the enol ether. 

The absolute rate constants for oxygen and sulfur transfer to a range of carbenes (dialkyl, cycloalkylidene, alkylchloro, diaryl, arylchloro, arylalkoxy, and dialkoxy), generated by laser flash photolysis of diazirine or oxadiazoline precursors, were determined. No evidence was seen for ylide formation and a concerted mechanism via an ylide-like transition state was proposed. 

Thermolysis of the oxadiazoline (123) gives rise to the corresponding dialkoxy-carbene, which can be trapped by reaction with f-butanol to form orthoesters. The formation of a regioisomeric mixture of esters was explained by fragmentation of the carbene to radicals (124) which recombine at either end of the allyl system. 

Carbenes (Continued) substituents on, 115 Carbocations, 105-108 acids and electrophiles, 97 dialkoxy, 105 ethyl, 84 fluoro, 105 norbornyl, 84 reactions, 107 substituents on, 106... 

In this regard, it is noteworthy that while surface bound hydroxycarbenes are postulated species, discrete complexes containing hydroxy- and alkoxy-carbenes have been known since E. O. Fischer s studies beginning in 1964 (56, 57). These complexes are possibly analogous to proposed surface intermediates, and their chemistry may model some of the heterogeneously catalyzed transformations. Coupling of alkoxy carbenes, for example, gives dialkoxy olefins as observed in (20). 

Carbenoids present a full range of reactivities some are extremely reactive species (e.g., copper carbenoids have never been observed, even by spectroscopy) others arc very stable compounds. Such a dramatic stabilization, discovered by Fischer [13), occurs when some soft transition metals complexes (Mo, W, Pd) are associated with a very stabilized (soft) carbene (e.g., alkoxy or dialkoxy-carbenes, heterocyclic carbenes, etc.). 

Another plausible pathway, not excluding the preceding one, is the coupling of carbenoid intermediates as observed by Wojcicki in the reaction of alkoxy-carbenes into dialkoxy-olefins. 

It is a useful reagent for orthoester homologation via dialkoxy-carbenium ions and for oxazole formation by reaction of keto-carbenes (via diazo esters/Cu(OTf)2) with nitriles (eq 10). With unsaturated nitriles, the nitrile group is selectively attacked. Kinetic and ESR evidence shows that Cu Cu reduction is the key step. ... 

Intramolecular addition processes involving latent carbenes have been reported twice. On one hand, the thermolysis of 2,2-dialkoxy-5,5-dimethyl-A -l,3,4-oxadiazolines (38) as latent carbenes formed bicyclic compounds (39) via an efficient and highly stereoselective formal 4- -1-cycloaddition process, carbene (40) being postulated as the key intermediate. On the other hand, the deprotonated carbenic forms of alkyne-tethered imidazolium (41) and 1,2,4-triazolium salts (42) have been found to undergo 6-exo-dig intramolecular addition, leading to zwitterionic intermediates (43) and (44) that rearrange to give 2-substituted imidazole (45) and bicyclic 1,2,4-triazole (46), respectively. ... 

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