Singlet state intermolecular reactions

Analysis of the product distributions arising from both sensitized and non-sensitized irradiation of 2-allyloxyphenyldiazo species (8) showed that the C—H insertion product and much of the cyclopropanation arise from the triplet carbene.16 For the singlet carbene, intermolecular 0—H insertion with methanol is about 50 tunes faster than intramolecular addition to the double bond, hi this system, intramolecular reactions and intersystem crossing of the triplet carbene proceed at similar rates, hi the closely related indanyl system (9), the smaller RCR angle stabilizes the singlet state relative to the triplet and the intramolecular reactivity is dominated by the singlet state.17... 

The irradiation of azides mainly results in the formation of the singlet nitrene and the reactions of the singlet nitrene have to compete with the intersystem crossing process leading to the production of the triplet nitrene. Therefore intermolecular reactions with singlet state nitrenes may be possible at high nitrene quencher concentrations. 

C S, So-H heat (10 M0 s) ISC inter system crossing T, ->So -t heat (10" -10 s) ISC S, T, -H heat (10-"-10" s). P, Reaction product from the singlet state (intramolecular) P2 (intermolecular) reaction product from the triplet state. The reactions from Sj and T] may also include electron or energy transfer reactions. The arrows in the boxes represent the spin orientation of the electrons in the participating MOs. 

The transient ketocarbene is also able to effect both intramolecular and intermolecular H-abstraction. The fact that a triplet state of the carbene is often implicated follows from observations that sensitized photolyses frequently enhance the abstraction pathway. Abstraction and radical combination results in the formal insertion of the carbene into the C—H (or S—H, N—H, etc.) bond. Insertions may also occur into C—C, C—O, C—S, C—Hal, etc. bonds the mechanisms are often not known, but a singlet state of the ketocarbene is probably involved in many cases. Reactions at soft basic centers (e.g. —S or —Br ) generally proceed by the preliminary formation of an ylide. The multiplicity of the possible pathways is illustrated in Scheme 7, equation (14) (photolysis in dioxane affords only 27, 57% and 28, 43%), and Scheme 8. ... 

There are only a limited number of intermolecular [2 + 2] photocycloaddition reactions known to occur in solution because alkene excited singlet state lifetimes are very short (on the order of 10 ns).708 Direct irradiation of neat but-2-ene, for example, yields tetramethylcyclobutane with stereochemistry suggesting the concerted mechanism (Scheme 6.45).709 Inefficient dimerization (

with efficient E Z isomerization (

[Pg.257]

The earliest studies of excited-state proton reactions were concerned with the measurement of excited-state pK values both in the singlet and the triplet manifold. These studies are thoroughly reviewed in Refs. 13 and 18. Since the excited singlet state of most aromatic molecules lasts only a few nanoseconds, it was difficult to measure directly the dynamics of intermolecular proton transfer. Instead, as outlined in the introduction, steady-state measurements were used. Picosecond spectroscopy has made it possible to probe in real time the intermolecular transfer process. 

In addition, Tsuda and Oikawa carried out molecular orbital calculations of the electronic structures in the excited states of poly(vinyl cinnamate) [131, 132], They based their calculations on the reaction of intermolecular concerted cycloaddition that take place according to the Woodward-Hoffmann s rule. This means that the cyclobutane ring formation takes place if a nodal plane exists at the central double bond in the lowest unoccupied MO(LLUMO) and not in the highest occupied MO (HOMO) of the grotmd state cinnamoyloxy group. This is within the picture of Huckel MO or Extended Huckel MO theory. The conclusion is that the cmicerted cycloadditions occur favorably in the lowest triplet state Ti and in the second excited singlet state S2 [132]. 

Th e photochemistry of alkenes attract a great deal of attention and it startes a new era in organic synthesis. Electronically excited olefinic bond gives a number of reactions like isomerisation, dimerisation, intermolecular addition and many more. The energy of triplet excited state T, of an alkene is lower than that of excited singlet state Si. However intermolecular system crossing [ISC] is inefficient and in order to examine reactions of the triplet excited state, population of this state must be achieved by sensitisation. 

Many different solvents can be used, although nonpolar benzene is recommended for most purposes. Generally speaking, simple distillation is adequate for the purification of solvents when singlet excited states are involved. Metal ion templates for the synthesis of crown-type phanes or crownophanes help to increase product stabihties but do not necessarily increase the efficiency of the cychzation. The concentration of substrates should be in the 10" to 10" M range. The intermolecular reaction needs more concentrated solutions to raise the yield of adduct, but the intramolecular reaction occurs efficiently in more dilute conditions. 

Thiocarbonyl compounds have a large energy gap between S, and Sj (> 30 kcal mol ) and have lower and upper excited states with different electronic configuration (ntt and nn ) Because of these, the Sj state of thiones has a long lifetime (10" sec in hydrocarbon solvents and lO " sec in perfluoro hydrocarbon solvents), allowing them to react from the upper singlet states. They undergo both intra- and intermolecular reactions from S. ... 

UV photolysis (Chapman et al., 1976 Chedekel et al., 1976) and vacuum pyrolysis (Mal tsev et al., 1980) of trimethylsilyldiazomethane [122]. The silene formation occurred as a result of fast isomerization of the primary reaction product, excited singlet trimethylsilylcarbene [123] (the ground state of this carbene is triplet). When the gas-phase reaction mixture was diluted with inert gas (helium) singlet-triplet conversion took place due to intermolecular collisions and loss of excitation. As a result the final products [124] of formal dimerization of the triplet carbene [123] were obtained. 

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