Triplet diradicals, reaction with alkenes

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. 

Ketones and aldehydes can undergo photochemical [2-1-2] cycloaddition reactions with alkenes to give oxetanes. This is called the Paterno-Buchi reaction. For alkyl carbonyl compounds both singlet and triplet excited states seem to be involved, but for aromatic compounds the reaction occurs through the triplet state.The regiochemistry can usually be accounted for on the basis of formation of the most stable 2-oxa-1,4-diradical. For example, styrene and benzaldehyde give 2,3- not 2,4-diphenyloxetane. ... 

The reaction is stereospecific for at least some aliphatic ketones but not for aromatic carbonyls.197 This result suggests that the reactive excited state is a singlet for aliphatics and a triplets for aromatics. With aromatic ketones, the regioselectivity of addition can usually be predicted on the basis of formation of the more stable of the two possible diradical intermediates obtained by bond formation between oxygen and the alkene.198... 

Their kinetic and product analysis is consistent with the working hypothesis indicated previously for insertions the singlet process is stereospecific, while the triplet process is at best only selective. Here, as in the reaction of sulfur atoms with alkenes, (217), some progress has been made in separating the problem of the reacting species and their reaction rates from the observed SS in the product (Gunning and Strausz, 1966). In general, however, interconversion rates of triplet cisoid and transoid diradicals are uncertain, and deductions from overall stereoselectivity are also uncertain. 

The discussion of cis-trans photoisomerization of alkenes, styrene, stilbene, and dienes has served to introduce some important ideas about the interpretation of photochemical reactions. We see that thermal barriers are usually low, so that reactions are very fast. Because excited states are open-shell species, they present new kinds of structures, such as the twisted and pyramidalized CIs that are associated with both isomerization and rearrangement of alkenes. However, we will also see familiar structural units as we continue our discussion of photochemical reactions. Thus the triplet diradical involved in photosensitized isomerization of dienes is not an unanticipated species, given what we have learned about the stabilization of allylic radicals. 

Another class of molecules that are quite prone to undergo photochemical cycloadditions are a,/3-unsaturated carbonyl compounds. The reactive state in photochemical cycloadditions of or,-unsaturated ketones is believed to be the n-Tt triplet." Conservation of spin then implies that the initial intermediate is a triplet diradical, and the reaction need not be stereospecific with respect to the alkene component." The reaction has been most thoroughly studied in the case of cyclopen-... 

What do we expect of the triplet In particular, what will be the stereochemistry of the addition reaction The first step in the addition of a triplet carbene to an alkene is shown in Figure 10.47. In contrast to the singlet, the triplet cannot form two new bonds in a single step. A new species, called a diradical, is produced, with two nonbonding electrons on different carbons. Now what can happen Why not simply close up to the three-membered ring and be done with it Look closely in Figure 10.47 at the spins of the electrons involved in the reaction. The second bond cannot close The two electrons that would make up the new bond have the same spin and therefore cannot occupy the same orbital. 

FIGURE 10.47 Only one C—C bond can be formed in the reaction of a triplet carbene with an alkene. The first step in triplet addition is reaction with the Jt bond of the alkene to give a 1,3-diradical. 

A further restriction on the synthetic utility of the nitrene addition reaction is its unpredictable stereochemistry in reactions with disubstituted alkenes such as cis- and trans-but-2-ene. Nitrenes can exist in a singlet or triplet state. For most nitrenes the triplet (diradical) state is the ground state. Nitrenes that are generated thermally or by direct photolysis are initially in the singlet state and their (concerted) addition to alkenes is stereospeciflc. If the alkenes are relatively unreactive the nitrene can convert into its ground triplet state either partially or completely before addition. The resultant aziridines are produced with varying degrees of stereoselectivity because the addition of the triplet nitrene is a stepwise process. The triplet species can also be produced directly by photosensitized addition." " ... 

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