Singlet addition

It is apparent that two collisional deactivation processes must be working in competition—one a very efficient removal of some intermediate which yields nonstereospecific products, and the other a relatively inefficient production of some other intermediate which yields nonstereospecific products. The very efficient process, which produces a sharp drop in the amount of tripletlike products at low pressures of inert gas, undoubtedly involves collisional stabilization of the hot cyclopropane formed by singlet addition, while the less efficient process would seem to involve intersystem crossing to triplet methylene, just as Anet and Frey proposed. 

An added complication in the interpretation of long-wavelength ketene photolysis is demonstrated by Cundall s discovery of ketene sensitized cis-trans isomerization of the 2-butenes.33 As the pressure of olefin increases, the rate of ketene decomposition decreases and the rate of olefin isomerization increases. At high olefin concentrations part of the apparent nonstereospecificity of cyclopropane formation can thus result from stereospecific singlet addition to already isomerized olefin. 

As pointed out by Gaspar and Hammond (1964), the logic of this widely-held and much applied interpretation, despite its plausibility, does not bear close scrutiny. Thus, singlet addition does not have to be concerted merely because it covld be without breaking the spin conservation rule. Moreover, there is little independent evidence concerning the relative rates of spin inversion and rotation about single bonds in diradicals. 

Presumably the adducts are formed by singlet addition yielding (257) followed by a second addition. [2 + 4]Addition is also encountered yielding the photo-chemically labile 2,3-diphenylnorbornadiene. The irradiation (206 nm, pentane solution, degassed) of cyclo-octa-l,5-diyne at room temperature rapidly gave butatriene as the only product.176... 

Before we close the discussion of carbene reactions with alkenes we mention tetrahedral boron hydride-substituted diazomethanes (8.11-8.13) which were obtained by the group of Jones (Li and Jones, 1992 Li et al., 1993) by manipulating substituted o-carboranes, as shown in (8-19). Reaction of 8.13 with ( )-but-2-ene yielded the pure (jE )-derivative, i.e., the product of singlet addition. With (Z)-but-2-ene 3 % triplet reaction product was observed. The percentage of triplet products was higher with 8.13 (22 and 18 7o with (E)- and (Z)-but-2-ene, respectively) (Huang et al., 1992). 

Classical chemiluminescence from lucigenin (20) is obtained from its reaction with hydrogen peroxide in water at a pH of about 10 Qc is reported to be about 0.5% based on lucigenin, but 1.6% based on the product A/-methylacridone which is formed in low yield (46). Lucigenin dioxetane (17) has been prepared by singlet oxygen addition to an electron-rich olefin (16) at low temperature (47). Thermal decomposition of (17) gives of 1.6% (47). 

Subsequent studies (63,64) suggested that the nature of the chemical activation process was a one-electron oxidation of the fluorescer by (27) followed by decomposition of the dioxetanedione radical anion to a carbon dioxide radical anion. Back electron transfer to the radical cation of the fluorescer produced the excited state which emitted the luminescence characteristic of the fluorescent state of the emitter. The chemical activation mechanism was patterned after the CIEEL mechanism proposed for dioxetanones and dioxetanes discussed earher (65). Additional support for the CIEEL mechanism, was furnished by demonstration (66) that a linear correlation existed between the singlet excitation energy of the fluorescer and the chemiluminescence intensity which had been shown earher with dimethyl dioxetanone (67). 

Reaction between oxygen and butadiene in the Hquid phase produces polymeric peroxides that can be explosive and shock-sensitive when concentrated. Ir(I) and Rh(I) complexes have been shown to cataly2e this polymerisation at 55°C (92). These peroxides, which are formed via 1,2- and 1,4-addition, can be hydrogenated to produce the corresponding 1,2- or 1,4-butanediol [110-63-4] (93). Butadiene can also react with singlet oxygen in a Diels-Alder type reaction to produce a cycHc peroxide that can be hydrogenated to 1,4-butanediol. 

Characteristic reactions of singlet oxygen lead to 1,2-dioxetane (addition to olefins), hydroperoxides (reaction with aHyhc hydrogen atom), and endoperoxides (Diels-Alder "4 -H 2" cycloaddition). Many specific examples of these spectrally sensitized reactions are found iu reviews (45—48), earlier texts (15), and elsewhere iu the Engchpedia. 

Another interesting cycloaddition, the detailed mechanism of which is still under investigation, is the addition of singlet oxygen to alkenes producing 1,2-dioxetanes (Section 5.15.3.3.2). 

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