Reaction mechanisms triplet carbenes

The four hitherto known routes of the C-H insertion are shown in Scheme 1. In general, the insertion by singlet carbenes proceeds via route a in one step, whereas the reaction by triplet carbenes proceeds sequentially via route b, i.e., hydrogen abstraction followed by recombination of the radical pairs.4 Other stepwise mechanisms are hydride abstraction (route c) and proton abstraction (route d), both being followed by the recombination of ion pairs. However, extended study on routes c and d for synthetic purposes had not been done before we started, except for a few earlier studies on carbanion-promoted P C-H insertion reactions.5,6 Recent advances in transition metal-catalyzed... 

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

The mechanism of the insertion is not clear, however, since both carbenes have triplet ground states, an abstraction-recombination mechanism with radical pairs as intermediates is most likely. The only other triplet carbene that has been reported to insert into CH4 in low temperature matrices is methylene.89,90 However, in this case it is not completely clear if the insertion is a thermal or photochemical reaction. 

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 ... 

In 1956, Doering et al. reported that methylene (CH2) inserted into the C H bonds of pentane, 2,3-dimethylbutane, and cyclohexene with no discrimination (other than statistical) between chemically different sites CH2 was classed as the most indiscriminate reagent known in organic chemistry. Doering and Kirmse also demonstrated that the C—H insertion reactions of CH2 in solution were direct, single barrier concerted processes with transition states that could be represented as 27 (Fig. 7.12). In particular, they did not proceed via initial H abstraction to give radical pair intermediates that subsequently recombined. (Triplet carbene C H insertions, however, do follow abstraction-recombination, radical pair mechanisms, as demonstrated in classic experiments of Closs and Closs and Roth (see Chapter 9 in this volume). 

The reaction of triplet diphenylcarbene with pyridine has been well studied, and a mechanism proposed from kinetic data (90TL953). The carbenes generated from laser flash photolysis of alkylbromo- and alkylfluoro-diazirines were trapped by pyridine to form the pyridinium ylides... 

The mechanism of this nonspecific reaction must be different. In fact, a concerted reaction is impossible for triplet carbenes because of the spins of the electrons involved. After the carbene adds to the alkene in a radical reaction, the diradical (triplet) intermediate must wait until one of the spins inverts so that the second C-C bond can be formed with paired electrons. This intermediate also lives long enough for C-C bond rotation and loss of stereochemistry. 

The mechanism of the aqueous photochemistry of 4-chlorophenol has been reviewed earlier [5,8]. Its basic features are the same as those of the carbene pathway described above for 2-bromophenol. The main differences are the fact that this is the only photolytic reaction of 4-chlorophenol and that its quantum yield is considerably higher than that of the 2-sub-stituted analogues

triplet carbene, 4-oxocyclohexa-2,5-dienylidene (A.max = 384 and 370 nm) from aqueous 4-chlorophenol (see Fig. 1) [20]. Photoproduct analysis yielded p-benzo-quinone (in the presence of O2), phenol (in the presence of an alcohol), hydroquinone and isomeric chlorodihydroxybiphenyls, which could all be accounted for by carbene reactions [20]. 

The mechanism of 4-chloroaniline photochemistry was independently studied by Guizzardi et al. in organic solvents they reached very similar conclusions [57]. These authors pointed out that the aminophenyl cation has a triplet-diradical character which fully explains its reactivity in organic solvents [57]. However, in aqueous solutions the cation reacted with hydroxyl ions with a rate constant of 3.1 x 1010 M s, which can only be interpreted as a deprotonation reaction [55]. The carbene 4-iminocyclohexa-2,5-dienylidene thus must exist in aqueous solutions, even though its properties have not yet been characterized. This is partly due to an expected low extinction coefficient, similar to the neutral anilino radical [55]. Following these arguments, the primary pathways of 4-chloroaniline photolysis in polar solvents may be pictured as shown in Scheme 7. 

The key steps in this mechanism are the initial attack of SiF2 [Eq. (63-1)] and the hydrogen abstraction of CH3NC by methyl radicals at low temperature [Eq. (63-2)]. The former resembles the reaction pattern of triplet carbenes the latter had been demonstrated experimentally to be an extremely facile process at low temperatures (97). 

There are three general classes of mechanism most often encountered in alkane reactions (i) radical (ii) electrophilic and (iii) carbenoid. The C—bond-lneaking st in (i) and (ii) are shown in equations (2) and (3). Carbenoid reactions can go either by direct insertion into the C—bond (equation 4), which tends to happen when the carbene in question has singlet character, or by a two-stqi process (equations 5 and 6), in which H-atom abstraction precedes collapse of the radical pair, a pathway which is characteristic of triplet carbenes. ... 

Like the normal mechanism for photochemical deazetization of 4,5-dihydro-3/f-pyrazoles, this reaction involves stepwise cleavage of the C N bonds. Unlike the 4,5-dihydro-3//-pyrazole case, however, the first intermediate formed (the vinyidiazo compound) is relatively stable. It can sometimes be isolated and it is often detected by spectroscopic methods (UV, IR, NMR). Its presence in the reaction mixture is often revealed by a red or pink coloration that develops during the photolysis. The second C-N bond cleavage leads to a diradical which can be formulated as a 1,3-diyl but which is more often thought of as a vinylcarbene. Evidence for this intermediate rests on trapping experiments using acetic acid or alkene" solvents and on the fact that the ESR spectrum of the triplet carbene can be obtained when the precursors are photolyzed at low temperatures in a frozen matrix. ... 

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