Singlet adduct

Recoil UC atoms have been produced by nuclear transformations and allowed to react with ethylene.15 Both Q1/)) and C(3P) atoms are formed, and both add to the double bond and insert into the vinylic C—H bond. The resulting hot singlet adducts relax primarily to allene and methylacetylene, whereas the hot triplet adducts decompose to acetylene or are stabilized as carbenes, which mainly add to more ethylene to yield various C5 products. 

Decomposition of adducts such as triphenyl phosphite—ozone provides a convenient method for accomplishing chemical oxidations involving singlet oxygen and making it a useful oxygenating agent for synthetic and mechanistic appHcations. 

Dialkoxypyrazines and 2,5-dihydroxypyrazines have been shown to add to singlet oxygen to yield peroxy adducts such as (75) in high yields (76CC417, 79JCS(P1)1885 and the reaction is believed to be important in the biosynthesis of the antibiotic bicyclomycin (76)... 

The thermally formed difluorocarbene adds stereospecifically to either (Z)- or ( )-butene to form the corresponding dimethyldifluorocyclopropanes, which points to a singlet difluorocarbene. With butadiene mono- (220) and bis-cyclopropanated product (221) are formed, and with perfluorobutadiene the mono-adduct (222) results 65JA758). 

The [S3N3]" cation is of interest as an example of an antiaromatic eight TT-electron system (Section 4.4). Ab initio molecular orbital calculations indicate that a triplet cation, with a planar ring, is more stable than the singlet cation. The [S3N3]" cation has been obtained as the norbomene adduct 5.16, but salts of the free cation have not been isolated. ... 

In the H NMR spectrum of the adduct 363, the acetyl is presented by a singlet in the 2.00 ppm region, while the ethene moiety appears at 4.75-4.99 ppm. The amino group gives a broadened signal in the 3.65 ppm region. 

The Methylene Blue sensitized photoaddition of singlet oxygen to ethyl l//-azepine-l-car-boxylate was reported originally to yield only a C2-C5 adduct 269 however, a reinvestigation with the methyl ester, and using tetraphenylporphine as a sensitizer, revealed that in addition to adduct 46 the [6 + 2] cycloadduct 47 is also produced.270... 

Ethyl l//-l,2-diazepine-l-carboxylate and ethenetetracarbonitrile give the Diels-Alder adduct 11 97 an analogous cycloadduct 12 is produced by the action of singlet oxygen.98... 

The pure adduct had the following proton magnetic resonance spectrum (chloroform-d) <5, multiplicity, number of protons, assignment 6.75 (singlet, 2, cyclohexene vinyl protons), 6.20 (multiplet, 2, cyclobutene vinyl protons), 3.5 (broad multiplet, 4, cyolobutane protons). 

The primary interaction of singlet oxygen, produced by energy transfer from the excited sensitizer, with the diene can give rise to an exciplet that then collapses to peroxide, to a 1,4-biradical or to a 1,4-zwitterion alternatively, the adduct is the result of a concerted action without the involvement of an intermediate. Detailed kinetic Diels-Alder investigations of singlet oxygen and furans indicate that the reactions proceed concertedly but are asynchronous with the involvement of an exciplex as the primary reaction intermediate [63]. 

The bicyclic phosphite (11) with ozone gave a remarkably stable adduct (12) which decomposed to form singlet oxygen only above 0 °C. 

There is a continuing interest in the use of phosphite-ozone adducts as sources of singlet oxygen and as reagents for mimicking the reactions of this species. The commercially available phosphite (54) forms an ozone adduct of striking stability. Decomposition of the adduct only becomes appreciable at temperatures > 0 °C the decomposition exhibits first-order kinetics, so that at 10 °C= 9.10 x 10 min and= 76.2 min. These... 

Scheme 12.17 gives some examples of oxidations by singlet oxygen. The reaction in Entry 1 was used to demonstrate that 102 can be generated from H202 and CIO-. Similarly, the reaction in Entry 2 was used to verify that the phosphite-ozone adducts... 

The formation of the benzocyclo-octatetraene (26) undoubtedly involves a singlet species which undergoes benzo-vinyl bridging rather than the vinyl-vinyl bridging of the triplet which leads, eventually to compound (27) 51>52>. In accord with this the photo-isomerisation of the adduct (28) formed by the reaction of tetrafluorobenzyne with cyclohexa-1,3-diene, results in the formation of tetrafluorobenzo-dihydrosemibull-valene (29) 53>. 

Triplet photoaddition of simple non-cyclic monoolefins is unknown. The sensitized dimerization of ethyl vinyl ether gives exclusively head-to-head adducts, Eq. 21, and probably should not be classed as an example of simple acyclic olefin. Usually the triplets have high energies and are severly twisted. 55> Some cyclic rigid molecules, Eq. 20, that do dimerize 63> do not incorporate substituents that allow regioselectivity to be determined. Butadiene gives principally head-to-head dimerization, Eq. 19, concordant with the PMO prediction, and so does indene, Eq. 22. The anti dimer that is formed would not be expected from a singlet excimer reaction. 

Based on this tentative assignment of Fe(CO)3(C2H4) as having a singlet ground state, addition of ethylene to Fe(CO)3 now becomes a spin-forbidden process, which might seem inconsistent with the observed rapid addition. However, as mentioned above, addition on the triplet surface should be barrierless, and hence rapid. Conversion of the initially formed triplet adduct to the... 

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