Carbenoids structure

These optimization studies are an important step in the study of Simmons-Smith cyciopropanations since they allowed for the development of a selective, catalytic method for introduction of a simple methylene unit. However, they also provide insights into the basic mechanism of this process. Together with earlier studies regarding carbenoid structure, the true nature of the reactive carbenoid, lCH2Znl, was confirmed. On the basis of these results, a revised transition structure was proposed. Although there is no direct evidence for such a transition... 

Some recent studies suggest that considerable improvement in the stereoselectivity of the cyclopropanation reaction may be possible by altering the carbenoid structure. For example, Doyle17 has shown that... 

The mechanism of cyclopropenations of alkynes with ethyl diazoacetate, catalysed by (AcO)4Rh2 and (DPTI)3Rh2(OAc), has been studied by a combination of kinetic isotope effects and theoretical calculations. With each catalyst, a significant normal 13C KIE was observed for the terminal acetylenic carbon, while a very small 13C KIE was detected at the internal acetylenic carbon. These isotope effects are consistent with the canonical variational transition structures for cyclopropenations with intact tetrabridged rhodium carbenoids but not with a 2 + 2-cycloaddition on a tribridged rhodium carbenoid structure.99... 

Studies of the reactions of chromenes with carbene or carbenoid structures have been described, including zinc enolates of dibromoalkanones <2005RJ0527>. Ring-opening reactions of naphthopyrans in the presence of ethyl-diazoacetate and Rh(OAc)4 have also been recorded (Equation 10) <2004TL6151>. 

H. M. L. Davies, S. A. Panaro, Effect of Rhodium Carbenoid Structure on Cyclopropanation Chemose-lectivity, Tetrahedron 2000, 56, 4871-4880. 

Isoxazoles are photolabile. The first step in the reaction is believed to be fission of the N—O bond with formation of a diradical which may recyclize. In the case of the isoxazolopyridine (4) the reaction is rationalized as taking place via a 2ZZ-azirine (5) and a carbenoid structure (6) before formation of an oxazole ring (7). A high pressure mercury lamp is used, and the reaction can be run on a preparative scale (79CB3282). Photolysis of the isoxazole (8), which is fused to an azine ring with a different order of its azole heteroatoms, also leads to oxazole formation (10) (78CPB2497), but in this instance another type of intermediate is postulated (9). 

Figure 27. Metal-a-carbenoid structures (a) alkynyl-carbene (b) alkynyl aUcyne-carbenoid.

There are also higher-energy CIs, which resemble the H migration carbenoid structures found for ethene. (See Figure 12.29) These structures have also been located... 

The metal-catalyzed decomposition of diazo compounds in the presence of carbonyl compounds is a well-established reaction to generate a carbonyl ylide intermediate. Several new developments have revolutionized this area of chemistry and are included in this review. Most notably, major advances have occurred in catalyst design, such that highly chemoselective, diastereoselect-ive and enantioselective carbenoid transformations can now be achieved. Furthermore, it has been recognized that a wide array of carbenoid structures can be utilized in this chemistry, leading to a broad range of synthetic applications. 

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