Spin inversion

The electrons do not undergo spin inversion at the instant of excitation. Inversion is forbidden by quantum-mechanical selection rules, which require that there be conservation of spin during the excitation process. Although a subsequent spin-state change may occur, it is a separate step from excitation. 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

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. 

It is not unreasonable that the substitution of chlorines onto the acenaphthylene ring should have such a significant influence on the intersystem crossing processes, kd and lsc. It is somewhat surprising, however, that the other processes involving spin-inversion were affected to such a small extent. 

In the presence of benzophenone, (8) was again the major product (>95°/0) and only trace amounts of the cyclohexane products were produced. These results suggest the intermediacy of a singlet 1,6-hexylene biradical in the direct photolysis and a longer lived triplet 1,6-diradical in the sensitized photolysis. In the triplet biradical more time is available for 1,6-hydrogen transfer to occur prior to spin inversion and hence more olefin (8) is produced. Similar results were reported for the direct and photosensitized photolysis of the 3,8-dimethyl derivative of (7). 

Thus, the effect of the spin inversion would be to shift the balanced point by 8z = z o — z0. In order to estimate the shift 8z, we note that the field times the field gradient B(dB/dz) should shift by... 

By absorption of light a molecule is promoted to a higher electronic state. The monomolecular physical processes for the dissipation of the excess energy are outlined in Fig. 5 in a so called Jablonski diagramm. In principle one has to differentiate between radiative and non-radiative deactivation on the one side and on the other side one has to consider if the multiplicity of the system is conserved or not. Radiative deactivation, i.e. deactivation accompanied by emission of light, is termed fluorescence if the transition occurs with spin conservation and phosphorescence, if spin inversion occurs. 

Fig. 9. Jablonsky Diagram for energy conversion pathways of an excited molecule. While fluorescence occurs between states of the same spin, an ISC (inter system crossing) leads to spin inversion and a delay in emission (phosphorescence halftimes from 1CT4 s to minutes or even hours)...

In this state the addition of methylene occurs in the triplet state and starting from each a mixture of cis and trans products is obtained. This is because in the triplet state, the two electrons have parallel spins and carbene behaves as diradical. To explain non stereospecificity it is assumed that rotation about the single bond occurs more rapidly than spin inversion. The entire mechanism can be written in the following manner ... 

The [2 + 2] photocycloaddition reaction of enones with allenes was first reported in 1966. A diradical intermediate is formed from a triplet enone via an exciplex. The triplet diradical cyclizes to the product after spin inversion to the singlet state [31,32]. 

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... 

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. 

It seems unlikely that reaction following this pathway can proceed with simultaneous formation of both bonds. If the n,n triplet is involved, a spin inversion must occur before bonding. Another factor which might be important even with the n,it singlet is that the unshared (j) electron and the n orbited are orthogonal. This means that some bond rotation may be required before complete bonding. 

When benzophenone is irradiated in either cis- or trans-2-butene solution, the photocycloaddition yields essentially the same mixture of both isomeric oxetanes, 40 and 41, before significant isomerization of the starting olefin occurs.37 This result strongly supports a mechanism involving the diradical 39. Since 39 was undoubtedly formed initially in the triplet state (from the n,ir triplet) spin inversion was necessary before bonding. There is no evidence pertaining to the formation of this intermediate from the n,n singlet. 

Photochemical addition of acetone to cis and trans 1-methoxy-l-butene involves both acetone singlets and triplets.71 Since spin inversion in the 1,4-biradical is relatively slow, predominant loss of stereochemistry is observed in the oxetane derived from triplet acetone. On the other hand, stereochemistry is partially retained when acetone singlets attack the substituted olefin. 

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