Cyclopropanes excited singlet states

There is a limitation to the use of stereospecificity of addition as a diagnostic test for singlet or triplet carbenes.When carbenes are generated by photolytic methods, they are often in a highly excited singlet state. When they add to the double bond, the addition is stereospecific but the cyclopropane formed carries excess... 

Cycloproparenes may be prepared by formation of one of the lateral cyclopropane o-bonds either via biradical closing, or via 1/3/elimination. The first reported synthesis of a benzocyclopropene derivative (see Section 1)" is an application of the former of these approaches. Upon irradiation, 3//-pyrazoles 70 loose Nj, and the intermediate biradical 71 cyclizes to 72. There is evidence that the intermediate biradical is in the triplet state, but an alternative interpretation in favor of an excited singlet state has also been presented. A variety of 1,1-disub-stituted benzocyclopropenes has been synthesized by the 3ff-indazole route, which is however limited. Cycloproparenes lacking substituents at Cl are not accessible in this way, because the required indazoles occur in the IH tautomeric form 73. 

On the basis of triplet sensitization experiments, this rearrangement was shown to occur from the excited singlet state and to involve the biradical intermediate 111. Interestingly, vinyl cyclopropane esters 113-114 underwent geometric isomerization at a much faster rate than rearrangement on direct excitation (Scheme 8.34) [57, 58]. However, the triplet-sensitized reaction of these esters give both cyclopentene and isomerization products. 

Thietanes are photochemically unstable and should be protected from light if they are to be stored for any length of time. Short-lived (hot) biradical intermediates, for example, 98a, appear to be formed and can undergo a variety of reactions, as shown for thietane. " Mercury-sensitized photolysis gives triplet biradicals that are longer lived than the singlet biradicals formed on direct excita-tion. " Some cyclopropane product is produced in the sensitized photolysis. The second excited singlet state of thietane decomposes either to ethylene and thio-formaldehyde via a 1,4-biradical or to elemental sulfur and cyclopropane. ... 

The electronic state responsible for reactions II and III is not so well known and it is almost certain that there are essential differences in this respect between the different cyclic ketones. On the basis of the different pressure dependence of propene and cyclopropane formation in the direct photolysis of cyclobutanone, McGee concluded that cyclopropane is formed from the excited singlet state, while propene is considered to be a triplet product. 

The effect of substituents on the temperature dependence of a-carbonyl-carbene reactivity has been examined using carbenes generated by low-temperature photolysis of methyl diazophenylacetate. A correction to the literature on the photoreaction of isopropylidene diazomalonate (98) with 1,3,3-trimethyl-cyclohexane (99) has been reported. The photoproduct, originally thought to be a cyclopropane derivative, has now been shown to be the cyclobutanone (100), the formation of which presumably involves a photo-Wolff rearrangement as illustrated in Scheme 11. Substituent effects observed in the product distribution of diazo-amide photochemistry have been ascribed to conformational factors the jS-lactam, oxindole, and Wolff rearrangement products appear to arise directly from the excited singlet state of the sym-Z form of the diazo-amide itself. 

The problem of the free-rotor effect and the efficient dissipation of triplet energy is still of interest. Hixson and Tausta42 have found that the excited singlet state of the naphthyl olefin (53a), a typical substrate for a di-7r-methane reaction, undergoes cw-Zra/w-isomerization to (53b) and conversion into the cyclopropane (54). Continued irradiation leads to the formation of cyclopropane at the expense of as-olefin (53b). Sensitized irradiation gave very efficient... 

Biradicals are frequently postulated to arise as intermediates in a number of chemical reactions and unimolecular isomerizations. Sometimes there are reasonable alternative concerted mechanisms in which the intermediate (or transition-state complex) is not a biradical. Such a case of much interest37,61 involves the reactions of singlet [5] and triplet [7] methylenes with olefins. We note that the permutational symmetry does not determine whether or not a reaction is concerted rather it is determined by the shapes of the intermolecular potential surfaces.78 The lowest 1Ai methylene is expected to react by a concerted mechanism, since it correlates with the ground state of the product cyclopropane higher excited singlets need not react via a concerted mechanism. 

The mechanism of the [3 + 2] cycloaddition is summarized in Scheme The first intermediate results from charge transfer interaction between the eli tronically excited aromatic compound at its singlet state S1 with the alkene w] leads to the formation of the exciplexes K. A more stable intermediate is generated by the formation of two C-C bonds, leading to the intermediates These intermediates have still singlet multiplicity and therefore possess zwii ionic mesomeric structures mainly of type M. In most cases and especially intramolecular reactions, chiral induction occurs during the formation of L. final products are then obtained by cyclopropane formation in the last step. 

A chemical test of a quite different nature is used for the identification of excited methylene, CH2. Methylene in the ground state is a triplet, but the photolysis of diazomethane or ketene (the standard methods for CH2 production) may lead to the formation of appreciable amounts of the excited singlet methylene. The fraction of singlet methylene has been determined by a study of the stereospecific addition of CH2 to double bonds eig.. the addition of methylene to tronj-butene-2 to yield dimethylcyclopropane ). It is assumed that spin will be conserved in the addition thus triplet methylene will form initially a triplet adduct (i.e. a biradical), and rotation may occur around the 2 3 C-C bond in the radical before ring closure. On the other hand, singlet methylene may form the cyclopropane directly, so... 

While direct irradiation of , E-2,4-hexadiene 5 gives only , Z-2,4-hexadiene from singlet excited state, triplet-sensitized reaction gives both E,Z- and Z,Z-2,4-hexadienes. The singlet state reaction proceeds with just one terminal double bond rotation involving allyUc methylene or cyclopropane methylene diradical with just one double bond rotation, whereas the triplet excited state reaction proceeds with double double bond rotation [11]. 

The photoelimination of nitrogen from 1-pyrazolines is one of the most thoroughly investigated photoreactions and it has been used extensively in the synthesis of cyclopropane derivatives.334 Both stereospecific and non-stereospedfic processes have been observed and these are believed, at least in simple 1-pyrazolines, to correspond to singlet and triplet excited states, respectively. Two reaction pathways have been proposed in the azoalkane 405335 direct excitation via a thermally activated S, state affords the C6H6 isomers 406 to 409, whereas triplet-sensitized excitation results in a tem-... 

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