Triplet carbenes polarization

It is easy to see how difficult it is to stabilize triplet carbene. It is not stabilized by a polar effect, either internally or externally, since it is electronically neutral... 

WORKED PROBLEM 11.1 Find an exception to the generalization stated in this chapter s first paragraph. What reaction have we studied that involves no polar intermediates and doesn t have a highly polar transition state Hint Think recent. ANSWER One example is the reaction studied in Section 10.4e (p. 433), the addition of a triplet carbene to an alkene. Only neutral intermediates are involved in this reaction. Another example is hydrogenation, the conversion of an alkene and hydrogen into an alkane (p. 411). 

Chemically induced dynamic nuclear polarization (CIDNP) is a very powerful tool for establishing the existence of radical pair intermediates and their spin. CIDNP has reinforced the view that singlet carbene undergoes direct insertion into C—H bonds and that the triplet abstracts hydrogen. 

Eq. 15). On the other hand, thermolysis of azo compound (45) is known to generate the same radical pair in the singlet state and gives the same products in a different ratio (Eq. 16). The NMR spectrum of the coupling product 31 shows that the sense (AE) of polarization observed for the carbene reaction is opposite to that observed (EA) for the thermolysis of azo compound. This result implies that the coupling product 31 in the photolysis is formed by recombination of benzyl and diphenylmethyl generated from a precursor believed to be triplet DPC. 

The carbene mechanism of heterolytic dehalogenation of 4-chlorophenol was subsequently confirmed by studies using flash photolysis [21] and FT-EPR [22], A detailed account of the EPR measurements was published later [23], in which it was shown that the spin polarization of the phenoxyl-propanoyl radical pair produced in the photolysis of 4-chlorophenol in 2-propanol is consistent with a triplet state precursor. The proposition that this precursor is the above-mentioned carbene was proved by generating the same radical pair, with identical spin polarization, by photolysis of p-benzo-quinone diazide [22,23]. 

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. 

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