Carbenes abstraction reactions

Correlated or geminate radical pairs are produced in unimolecular decomposition processes (e.g. peroxide decomposition) or bimolecular reactions of reactive precursors (e.g., carbene abstraction reactions). Radical pairs formed by the random encounter of freely diffusing radicals are referred to as uncorrelated or encounter (P) pairs. Once formed, the radical pairs can either collapse, to give combination or disproportionation products, or diffuse apart into free radicals (doublet states). The free radicals escaping may then either form new radical pairs with other radicals or react with some diamagnetic scavenger... 

Triplet Carbene Intermolecular Hydrogen Abstraction Reactions 434... 

This key paper was followed by a flurry of activity in this area, spanning several years." " "" A variety of workers reported attempts to deconvolute the temperature dependence of carbene singlet/triplet equilibria and relative reactivities from the influence of solid matrices. Invariably, in low-temperature solids, H-abstraction reactions were found to predominate over other processes. Somewhat similar results were obtained in studies of the temperature and phase dependency of the selectivity of C-H insertion reactions in alkanes. While, for example, primary versus tertiary C-H abstraction became increasingly selective as the temperature was lowered in solution, the reactions became dramatically less selective in the solid phase as temperatures were lowered further. Similar work of Tomioka and co-workers explored variations of OH (singlet reaction) versus C-H (triplet reaction) carbene insertions with alcohols as a function of temperature and medium. Numerous attempts were made in these reports to explain the results based on increases in triplet carbene population... 

EPR studies of diphenylmethylene and a number of other arylmethylenes have indicated that these carbenes have triplet ground states.<30) Photolysis of diphenyldiazomethane in olefin matrices results in the formation of triplet diphenylmethylene, which undergoes primarily abstraction reactions with the olefins. Cyclopropanes are produced as minor products. 

Since alkyllithium compounds and their carbanions have an isoelectronic structure with alkoxides, their reaction behavior with carbenes is expected to be similar to that of alkoxides, showing enhanced reactivity in both C-H insertion and hydride abstraction.35 In this reaction, the hydride abstraction cannot be followed by recombination and, therefore, can be differentiated from the insertion. Indeed, the reaction of alkyllithium compounds 70 or nitrile anions (see Section IV.B) with ethyl(phenylthio)carbenoid, which is generated by the reaction of 1-chloropropyl sulfide 69 with BuLi, takes place at the -position of 70 more or less in a similar manner giving both insertion product 71 and hydride abstraction products 72 and 73, respectively. This again supports a general rule C-H bonds at the vicinal position of a negatively charged atom are activated toward carbene insertion reactions (Scheme 22). 

A complicating factor associated with experimental application of the Skell Hypothesis is that triplet carbenes abstract hydrogen atoms from many olefins more rapidly than they add to them. Also, in general, the two cyclopropanes that can be formed are diastereomers, and thus there is no reason to expect that they will be formed from an intermediate with equal efficiency. To allay these problems, stereospecifically deuteriated a-methyl-styrene has been employed as a probe for the multiplicity of the reacting carbene. In this case, one bond formation from the triplet carbene is expected to be rapid since it generates a particularly well-stabilized 1,3-biradical. Also, the two cyclopropane isomers differ only in isotopic substitution and this is anticipated to have only a small effect on the efficiencies of their formation. The expected non-stereospecific reaction of the triplet carbene is shown in (15) and its stereospecific counterpart in (16). 

Nucleophilic substitution 313 Nucleophilic addition 337 Hydrogen-abstraction reactions 343 Carbene generation 355 Oxidation reactions 356 Reduction reactions 358 Miscellaneous reactions 360... 

The mechanism proposed for carbene-abstraction and carbene-insertion reactions is based on the calculations of Dewar (MINDO/2) and Hoffmann (extended Hiickel) Hoffmann dealt only with the concerted reactions of singlet carbenes, whereas Dewar discussed both singlet and triplet carbene reactions. The calculations of Dewar s ) for the reaction of triplet methylene with methane gave the following results ... 

It is not easy to explain why the triplet reactions that are energetically much less favored than those of the singlets become dominant at low temperature. Based on Ea and log A measured for triplet carbene abstraction (see Section 5.3), one can estimate the rate constant at 77 K to be <10 M s, suggesting that triplet carbene reactions in matrices at 77 K should not occur. Obviously, reactions of carbenes within matrices are controlled by factors that are not operating in solution phase, as one might expect from dramatic changes in reaction conditions. 

It would appear from the foregoing that the reactions of recombination and disproportionation of radicals fall into a reasonable relation with each other and with abstraction reactions if in the approach of the radical pair, either head-to-head as in recombination or head-to-tail as in disproportionation, there occurs an appreciable contribution of ionic structures to the attractive potential of the radical pair. These then also fall into a consistent relation with the insertion reactions of carbenes and the snap-out reactions of stable molecules from which latter, the present experimental evidence requires a large charge separation in the transition state. 

Finally, the metal-perfluoroalkyl linkage also appears to be less susceptible to facile decomposition by the a- or -elimination pathways that dominate much of the chemistry of hydrocarbon alkyls and lead to metal hydrides. The absence of these reaction pathways, at least for the later transition metals, may reflect the relative strength of the C—F bond versus the M—F bond compared to C—H/M—H analogues (32). However, a-fluoride abstraction reactions can be accomplished with exogenous fluoride acceptors to give fluorinated carbene complexes (see Section III,B,1). One example of an apparent -fluorine elimination reaction is shown in Eq. (2) (33) and presumably is driven by the stronger bond to fluorine formed by early transition... 

Another clear-cut example of abstraction was observed in the reactions of anthronyUdene (3f) S3, 34) th cyclohexane or toluene. AU possible dimerization products 30, 31 eind 32 can be isolated in this case. These abstraction reactions are attributed to triplet carbenes 2c and 3f. All other cycloalkenecarbenes show normal C—H insertion reactions (for the exceptional behaviour of di- and tribenzocycloheptatrienyhdene see p. 137). 

C-H carbene insertion reactions. Hydrogen abstraction products were formed at the expense of insertion products on irradiation of -nitrophenyldiazomethane under the same conditions. Possible explanations for this difference in reactivity have been offered. Photochemically generated aryl carbenes have been intercepted with carbon monoxide and with oxygen. Benzophenone 0-oxide has been detected spectroscopically on irradiation (515 nm) of diphenyl-... 

Fig. 24a and b. The approach of triplet carbene to methane a) direct insertion b) abstraction. Reaction coordinate for abstraction r — bi—b + 2... 

Carbanions can exist as metal salts, and are also important intermediates in reactions carried out under basic conditions. They react as strong bases and nucleophiles, undergoing hydrogen abstraction reactions, nucleophilic substitution and addition, fragmentation, and loss of halide ions to give carbenes, They can be identified spectroscopically or by trapping. 

As expected, the product ratios were quite different in acetonitrile and acetonitrile-ris. For example, the major products were diphenylmethane and benzophenone and only a small amount of benzophenone azine when the reaction was carried out in acetonitrile whereas the percentage of benzophenone azine was ten times larger, i.e. almost equal to the diphenylmethane when the solvent was acetonitrile-hydrogen-deuterium kinetic isotope effect in acetonitrile-ris slows the hydrogen abstraction reaction which leads to diphenylmethane. These product isotope effects are only consistent with the mechanism shown in equation 28 and clearly demonstrate that the radical anion does not decompose by losing nitrogen to form the carbene radical anion when diazoalkanes are reduced. 

The first absolute rate constant of a carbene reaction in solution was reported by Closs and Rabinow in 1976. [16] Upon photolysis of diphenyldiazomethane 1 in benzene with a conventional flash lamp, a transient spectrum with X 300 nm was detected. This absorption was assigned to triplet diphenylcarbene 3 because of the correspondence of this spectrum with that previously obtained for this carbene in a low temperature matrix." Upon flash photolysis of diphenyldiazomethane in cyclohexane the transient spectrum of the known spectrum of benzhydryl radical 9 334 nm) was produced. Carbene abstraction... 

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