Carbenes solvent effects

Decomposition of more complex diaziriries follows first order kinetics also. Chlorophenyl-carbene adds to cyclohexene to give a norcarane derivative. Substituent effects of m-Cl, m-NOa or m-Me groups, as well as solvent effects, are small. Chlorotrichloromethyldiazirine yields tetrachloroethylene chlorocyclooctyldiazirine also leads to an alkene 74CJC246). 

As demonstrated in the two previous sections, TRIR spectroscopy can be used to provide direct structural information concerning organic reactive intermediates in solution as well as kinetic insight into mechanisms of prodnct formation. TRIR spectroscopy can also be used to examine solvent effects by revealing the inflnence of solvent on IR band positions and intensities. For example, TRIR spectroscopy has been used to examine the solvent dependence of some carbonylcarbene singlet-triplet energy gaps. Here, we will focns on TRIR stndies of specific solvation of carbenes. 

Gonzalez, C., Restrepo-Cossio, A., Marquez, M., Wiberg, K. B., De Rosa, M., 1998, Ab Initio Smdy of the Solvent Effects on the Singlet-Triplet Gap of Nitrenium Ions and Carbenes , J. Phys. Chem. A, 102, 2732. 

In contrast, 1,2-H shift to olefin 106 is the dominant reaction of carbene 104, and this process is slow enough to be measured by LFP r = 300 ns in cyclohexane and 560 ns in pentane at 25°C.117 There is a polar solvent effect the lifetime decreases to 52 ns in acetonitrile. However, at least in the case of cyclohexane, the lifetime is solvent limited, with a KIE of 1.5 on the lifetime in cyclohexane- (460 ns). Carbene 104 is much longer-lived than dimethylcarbene (r 21 ns in pentane) or methylcarbene (<1 ns).22,89... 

The choice of the solvent is critical, and both non-coordinating solvents or polar aprotic solvents such as DMF can lead to intractable product mixtures. Solvent effects and side-reactions in chromium carbene benzannulation reactions have been thoroughly investigated [207,333,334]. 

The reactions of carbenes are more varied than those of the species previously discussed in this chapter. Solvent effects have been observed in carbene reactions. The selectivity of certain carbenes is influenced by the nature of the solvent. the distribution of rearrangement products (see below) from tert-butylcarbene are... 

In recent years there have been many studies of organic reactions in cavities that exist in crystalline materials such as zeolites or in large macrocycles such as cyclodextrins. The relationship between the structure of these cavities, their microscopic environments, and the rates and products of organic reactions may be characterized in much the same way as solvent effects on organic reactivity. Murray Rosenberg and Udo Brinker summarize here what has been learned about the mechanism for formation and reaction of carbenes within cyclodextrins and zeolites. 

Todd et al. (23) have studied the NMR spectra of complexes of the type (CO)sMCRR, M = Cr, W R = CHs, Ph, m- andp-Ph R = NH2, OR (Tables XXXVI and XXXVII). They have also found (cf Ref. 123) that substitution of a methyl for a phenyl group (R) causes an 8 to 11 ppm downfield shift. A unique solvent effect was observed for this system the carbene carbon chemical shift was 6 to 7 ppm upfield in THF from the value in CHCI3. This effect was attributed to the formation of a solvent-solute complex in THF solution. 

In particular cases the presence of cosolvents seems to be somewhat useful. In the generation of dihalocarbenes, the addition of a small amount of ethanol to chloroform in some cases remarkably improves the yield of the corresponding carbene This is likely due, however, to the formation of a more lipophilic alkoxy anion, which participates in the reaction, rather than to a solvent effect. As a further example, the extraction of tetraalkylammonium hydroxides into aromatic hydrocarbons increases by 1 to 3 orders of magnitude when a small amoimt of alcohol is added however, the extracted species is mainly RO than OH ... 

DFT and CASSCF-PT2 computations have been carried out to smdy the oxygenation of phenylhalocarbenes in vacuum. The similar order of magnimde of rate constants between singlet and triplet carbenes is indicative of an efficient tunnelling rate associated to a large spin-orbit coupling. For reactions in solution, the solvent effect should also be considered to explain the observed rate smoothing. 

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