Carbenes insertion-abstraction

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... 

Singlet Carbene C-H Insertions Although [1,2]-H shifts are formally carbene C-H insertions, these rearrangements have different orbital symmetry aspects than those of intramolecular insertions. As described above, overwhelming evidence exists that triplet carbenes undergo abstraction-recombination reactions to... 

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). 

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 ... 

Many of the limitations of C—C bond formation by C —H insertion outlined for intermolecular reactions (Section 1.2.1.) can be overcome by making the reaction intramolecular. Thus, hydrogen atom abstraction followed by intramolecular radical-radical coupling or radical addition to an alkene are increasingly popular processes. Two-electron carbene insertions, either thermal or transition metal catalyzed, have also been used extensively. In either case, ring construction involves net C—C bond formation at a previously unactivated C-H site. 

The quenching of a triplet carbene reaction with methanol is frequently used as the standard means of probing the singlet-triplet gap. It is widely believed that singlet carbenes insert readily into the O—H bonds of methanol, while the triplet states undergo hydrogen abstraction from the C—H bonds." The behavior of diarylcar-... 

Other differences between singlet (concerted) insertion and triplet (abstraction-recombination) carbene insertion are seen in selectivity, stereochemistry, and the kinetic deuterium isotope effect. The triplet states are more selective in C—H insertion than the singlets. For example, the triplet shows higher tertiary to primary selectivity than the singlet in the insertion reaction with 2,3-dimethylbutane. Singlet carbene is shown to insert into C—H bond with retention of configuration, while racemization is expected for triplet insertion reaction from the abstraction-recombination mechanism. For example, the ratios of diastereomeric insertion product in the reaction of phenylcarbene with roc- and mcTO-2,3-dimethylbutanes are 98.5 1.5 and 3.5 96.5, respectively. ... 

The easiest reactions are those in which the nucleophile is the gold-activated species. Examples of this are Au(I)-catalyzed carbene and nitrene transfers (equations 142 and 143) that convert olefins into cyclopropanes or aziridines, respectively. In the carbene transfer, ethyl diazoacetate is the source of carbene and the active NHC-gold cationic catalyst is generated by chloride abstraction with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate NaBAT4. The cyclopropanation is competitive with other carbene insertions with active C H or N H bonds present in the substrate. For the aziridinations of olefins, nitrene formation is accomplished by the oxidation of sulfonamides with PhI(OAc)2 and the catalyst of choice is a gold-(I) triflate with a terpyridine ligand. 

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-... 

D. Triplet carbene insertions go via radical abstraction followed by recombination. H- D., 5 hVr... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Abstract The dirhodium(II) core is a template onto which both achiral and chiral ligands are placed so that four exist in a paddle wheel fashion around the core. The resulting structures are effective electrophilic catalysts for diazo decomposition in reactions that involve metal carbene intermediates. High selectivities are achieved in transformations ranging from addition to insertion and association. The syntheses of natural products and compounds of biological interest have employed these catalysts and methods with increasing frequency. 

That this mechanism can take place under suitable conditions has been demonstrated by isotopic labeling and by other means. However, the formation of disproportionation and dimerization products does not always mean that the free-radical abstraction process takes place. In some cases these products arise in a different manner.We have seen that the product of the reaction between a carbene and a molecule may have excess energy (p. 247). Therefore it is possible for the substrate and the carbene to react by mechanism 1 (the direct-insertion process) and for the excess energy to cause the compound thus formed to cleave to free radicals. When this pathway is in operation, the free radicals are formed after the actual insertion reaction. 

The olefinic products which formally correspond to C—H insertion reactions are thought to arise by stepwise abstraction of hydrogen by triplet carbene and subsequent recombination ... 

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