Singlet state insertion reactions

Sensitizers have also been used to induce inters5retem crossing to the triplet The insertion reaction of methylene into C—H bonds in solution is widely acknowledged to be a concerted process resulting from the singlet state. Photolysis of... 

In contrast to 2-alkylarylcarbenes, triplet carbonyl carbenes do not abstract H from 5- or e-CH bonds. Photolysis of diazo compounds (7) in methanol gave products due to Wolff rearrangement (8) and 0-H insertion (9). Sensitized photolysis led, in addition, to the H-abstraction product (10). Analysis of the results indicated that a large proportion of the insertion product (9) arises from the excited diazo compound and that spin inversion of the triplet carbene is faster than H-abstraction from the solvent. Intersystem crossing to the singlet state is a major reaction of all triplet carbonyl carbenes that are not rapidly scavenged intramolecularly. 

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. ... 

All fluorocarbenes are ground state singlets. For laboratory use there are some precursors which thermally generate difluorocarbene.42 Its identification is usually made by a subsequent chemical insertion reaction. A few industrially important processes proceed via difluorocarbene. The thermal pyrolysis of chlorodifluoromethane (CHF2C1) for the production of tetrafluoroethene and hexafluoropropene gives the intermediate CF2 which dimerizes to the alkene. 

Luminescence is seldom observed from free radicals and radical ions because of the low energy of the lowest excited states of open-shell species, the benzophenone ketyl radical being however a noteworthy exception. There are few reports of actual photochemical reactions of free radicals, but the situation is different with biradicals such as carbenes. These have two unpaired electrons and can exist in singlet or triplet states and they take part in addition and insertion reactions (Figure 4.90). 

Analysis of the product distributions arising from both sensitized and non-sensitized irradiation of 2-allyloxyphenyldiazo species (8) showed that the C—H insertion product and much of the cyclopropanation arise from the triplet carbene.16 For the singlet carbene, intermolecular 0—H insertion with methanol is about 50 tunes faster than intramolecular addition to the double bond, hi this system, intramolecular reactions and intersystem crossing of the triplet carbene proceed at similar rates, hi the closely related indanyl system (9), the smaller RCR angle stabilizes the singlet state relative to the triplet and the intramolecular reactivity is dominated by the singlet state.17... 

Intramolecular C-H insertion reactions, N-H insertion reactions, and intermolecular C-H insertion reactions of aminocyanocarbenes (NCC NX2, X = H, CH3, CF3) have been studied using restricted and unrestricted CCD, CCSD, QCISD, B3LYP, and MP2 methods with the 6-31G(d), 6-311+G(d,p), cc-pVDZ, and cc-pVTZ basis sets. 66 HC NH2, NCC NH2, NCC N(CH3)2, and NCC N(CF3)2 have singlet ground states. 

FIGURE 32. (a) A schematic configuration mixing diagram for the insertion of MR2 into XH . A is the energy gap between the singlet and triplet configurations of the reactants. Adapted from Reference 440. (b) Dominant orbital interactions in the transition state of the insertion reactions of MR2 into a X—H bond. Based on the discussion in Reference 431... 

Figure 1 The minimum energy paths for the singlet and triplet symmetrical insertion reactions to produce the 1Ai, 3Bi, and 1Bi states o/HaO. The bracketed values are the optimum H H internuclear separations for each value of ro, the distance of the oxygen from the midpoint o/H-H axis (Reproduced by permission from/. Chem. Phys., 1971, 55, 5369)...

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