Intersystem crossing triplet carbenes

The other carbene reactions shown in Scheme 8-15 will be discussed in Section 8.5. There again, we will see that violations of the Skell-Woodworth rule can be explained if the size of the singlet-triplet gap and the rates of forward and reverse intersystem crossing of carbenes are taken into account. In addition, it should be... 

The yields of cyclopropanes in this case are low in relation to the amount of acetophenone formed. However, similar cyclopropane product ratios are obtained when photolysis is carried out in the presence of Michler s ketone as sensitizer. Thus the carbene intermediate produced in the direct irradiation is thought to be a triplet, as suggested by the nonstereospecificity of its addition. Whether this intermediate arose from singlet diazoacetophenone (via singlet decomposition and intersystem crossing of the singlet carbene) or by decomposition of the triplet molecule was not determined. 

Generation of 78 by thermolysis or photolysis of a diazoalkane or diazirine precursor, however, affords the singlet carbene, whose 1,2-H shift to ethene is opposed by a barrier of only 0.678 to 1.298 kcal/mol. Consequently, even in cryogenic matrices, singlet 78 rearranges more rapidly than it intersystem crosses to the triplet, which has therefore not been detected by UV or ESR in either an Ar matrix at 8 K or a Xe matrix at 15 K." The lifetime of singlet 78 at ambient temperature has been estimated at <0.5 ns.89,98b (Note the enormous spectator substituent effect of Cl the lifetime of MeCCl is 740 ns,60 at least 1500 times longer than that of MeCH.)... 

The significance of a value of AG 5.2 kcal mol -1 is that most of the bimolecular reactions of the triplet carbene will be faster than the formation of the singlet from the triplet. Thus intersystem crossing from BA to 3BA is irreversible, and if 3BA is formed first (triplet sensitization), then there will be no reaction at all of BA. However, in the direct irradiation of DABA, some... 

The epr spectrum of AN clearly shows that it is a ground-state triplet carbene (Devolder et al., 1972). The optical absorptions of this species were assigned at low temperature (Bourlet et al., 1972) and confirmed recently by laser spectroscopy (Tables 3 and 4) (Field and Schuster, 1985). The chemical properties of AN are now readily recognized as those characteristic of a ground-state triplet carbene where intersystem crossing to the singlet is slow (Cauquis and Reverdy, 1975a,b). 

The singlet carbene can undergo intersystem crossing to the triplet carbene the latter on the other hand, can also be formed by direct decomposition of the excited triplet state of the diazo compound. These possibilities are shown in the foUowing scheme ... 

Skell s hypothesis proved to be extremely useful in carbene chemistry even though it was frequently opposed. The principal significance of these rules is represented in the scheme below. The singlet reaction occurs in a concerted step, the cis-addition product being formed in a stereospeciiic manner. In the triplet addition, which is a two-step reaction, rotation is thought to be faster than intersystem crossing (spin inversion) and ring closure, i.e., which would... 

The experimental ratio of ds- to trans-cyclopropane 43 46, i.e. the stereo-specifity of the reaction cannot be considered as a simple indication of singlet or triplet percentage of RaC , since the stereochemistry of the cyclo-addition depends on many factors. Photolysis produces the exdted 5i-state of the diazoalkane 41. This compound can lose nitrogen and form the singlet carbene 42 (So-state). 42 can add directly in a stereospecific manner if ki is large. If, however, intersystem crossing 42 45 (Aisc is large) competes favorably with... 

Dilution Experiments Resulting in Intersystem Crossing to Triplet Carbenes... 

As has already been mentioned (vide supra, p. 118) the population of the triplet state of a carbene depends on the effectiveness of the intersystem crossing steps kisc and k lse- These rates can be altered by dilution. Another technique which exclusively populates the triplet state of a carbene is sensitization, or energy transfer. A triplet sensitizer is required for this purpose — usually an aromatic ketone. In these ketones the intersystem crossing efficiency is almost 100%. Energy is then transferred from the sensitizer triplet to the diazoalkane, thus populating the triplet state of the latter. 

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. 

Photosensitized decomposition of 9b in substituted benzenes led to similar results [83DIS(B)(44)1113]. Therefore, either spin inversion from the triplet to the singlet form of 4-diazoimidazole is faster than the decomposition of the excited diazo compound, or intersystem crossing from the triplet to singlet carbene is easier and faster than the reaction of the triplet state with substrates. 

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

Assuming that singlet nitrene reacts with alkanes at near diffusion controlled rates allowed deduction of a rate constant of singlet-to-triplet nitrene intersystem crossing (ISC) of 2-8 X 10 s . This ISC rate is slower than in carbenes, but significantly faster than with arylnitrenes, which are discussed in a subsequent section. 

---