Triplet 1,3-diradical, double bond additions

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. 

Triplet oxygen is a stable and surprisingly unreactive electrophilic diradical. Its reactions with organic compounds (autoxidation) include abstraction of hydrogen atoms or single electrons and addition to reactive C-C double bonds (Scheme 3.14). 

The Woodward-Hoffmann rules predict high activation energies for the suprafacial-suprafacial addition of two carbon-carbon double bonds, which can be lowered, however, by polar effects. [2 + 2] Photocycloadditions are common and usually involve diradical intermediates e.g., photoexcited ketones react with a variety of unsaturated systems (Scheme 1). Both the singlet and triplet (n, 7t ) excited states of the ketones will form oxetanes with electron-rich alkenes. With electron-deficient alkenes only the singlet states give oxetanes. Diradicals are the immediate precursors to the oxetanes in all cases, but the diradicals are formed by different mechanisms, depending on the availability of electrons in the two components. 

The addition of a singlet nitrene to a double bond is supposed to occur concert-eddy, while the triplet nitrene is supposed to add in two discrete steps via a 1.3-di-radical intermediate. The rate of ring closure of the diradical is supposed to be smaller than that for the rotation around the C—C-bond, therefore the stereochemistry of the olefin will be lost. 

Electrophilic addition of carbenes to carbon-carbon double and triple bonds has been extremely useful synthetically. In many cases, the reaction goes with 100% stereospecificity, so that the stereochemistry about a double bond in the starting material is maintained in the product. Cases in which addition is not 100% stereospecific are rationalized on the basis of a triplet or diradical intermediate. If the triplet carbene is relatively unreactive, the formation of the two new carbon-carbon bonds may be a stepwise process that allows for rotation and, therefore, loss of stereochemistry in the intermediate. 

The spin state of the reacting acylcarbene has a decisive influence on the reactivity of this short-lived species (Houben-Weyl, Vol.4/5b, pp 1158-1257 Vol. E19b, pp 1052 and 1231 refs 67,100-103). As far as cyclopropanation is concerned, carbene addition to a(Z)- or ( )-alkene can occur with retention Or with loss of the stereochemical relationship present in the alkene (stereospecific and nonstereospecific cyclopropanation, respectively). Singlet carbenes add to C-C double bonds in a concerted manner, and therefore, stereospecifically. In contrast, a triplet carbene undergoes stepwise addition via an intermediate triplet 1,3-diradical. Since spin-inversion must occur before ring closure of the latter species, the extent to which stereospecificity... 

The reaction of carbene 2b with ethylene exclusively leads to the cycloaddition reaction (formation of a cyclopropane) and not to a C-H insertion. This is in accordance with DFT calculations which predict a small thermal activation barrier for the insertion of 2b into a C-H bond at an sp hybridized carbon atom, whereas there is no barrier for the addition of 2b to the double bond of ethylene to give a triplet diradical. The diradical is expected to have a very small singlet triplet splitting, and ISC to the singlet diradical should readily lead to ring-closure to the cyclopropane. 

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