Diradicals from cyclopropane

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 dominant pattern for the thermal fragmentation of thietane dioxides involves extrusion of sulfur dioxide leading to a 1,3-diradical (i.e. 242) which closes to final products, mainly cyclopropanes, accompanied by rearrangement products resulting from hydrogen migration within the diradical191,1930 230,256-258 (equation 92). 

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

From the point of view of both synthetic and mechanistic interest, much attention has been focused on the addition reaction between carbenes and alkenes to give cyclopropanes. Characterization of the reactivity of substituted carbenes in addition reactions has emphasized stereochemistry and selectivity. The reactivities of singlet and triplet states are expected to be different. The triplet state is a diradical, and would be expected to exhibit a selectivity similar to free radicals and other species with unpaired electrons. The singlet state, with its unfilled p orbital, should be electrophilic and exhibit reactivity patterns similar to other electrophiles. Moreover, a triplet addition... 

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

Concerning the structure, the cyclopropane derivatives 524—526 deviate from the generally observed cycloadducts of cyclic allenes with monoalkenes (see Scheme 6.97 and many examples in Section 6.3). The difference is caused by the different properties of the diradical intermediates that are most likely to result in the first reaction step. In most cases, the allene subunit is converted in that step into an allyl radical moiety that can cyclize only to give a methylenecyclobutane derivative. However, 5 is converted to a tropenyl-radical entity, which can collapse with the radical center of the side-chain to give a methylenecyclobutane or a cyclopropane derivative. Of these alternatives, the formation of the three-membered ring is kinetically favored and hence 524—526 are the products. The structural relationship between both possible product types is made clear in Scheme 6.107 by the example of the reaction between 5 and styrene. 

Calculations and Experiments on the Stereomutation of Cyclopropane. In 1965, Hoffmann published a seminal paper on trimethylene, another name for propane-1,3-diyl (8). He used extended hiickel (EH) calculations and an orbital interaction diagram to show that hyperconjugative electron donation from the central methylene group destabilizes the symmetric combination of 2p-n AOs on the terminal carbons in the (0,0) conformation of this diradical. Hoffmann s calculations predicted that the resulting occupancy of the antisymmetric combination of 2p-n AOs in 8 should favor conrotatory opening of cyclopropane (7), as depicted in Figure 22.8. 

Figure 22.10. Rearrangement products (13), predicted to be formed from 1,1-disilyl-cyclopropanes (11) by conrotatory ring opening, followed by a 1,2-sbift of a trimethylsilyl group in the 1,3-diradicals (12) generated. Pyrolysis of 11b should lead to a mixture of E/2) isomers (13b), but pyrolysis of 11c is predicted to form only the E stereoisomer (13c). ...

Cyclopropane derivatives, including spiropentanc, have proven to be virtually inert towards carbenes,1 For this reason, no literature report that describes cyclobutane synthesis from a C3 and a Cj building block by ring enlargement of cyclopropanes exists. However, due to the partial p character, as well as the increasing reactivity caused by its strain, the central bond of bicyclo[1.1.0]butane (l)2 has been found to react with carbenes.1 Photolysis of diazomethane in the presence of bicyclo[1.1.0]butane (1) at — 50 C provides a mixture of several compounds. The major fraction of the material (80%) was analyzed by means of NMR spectrometry and found to consist of penta-1,4-diene (2, 21%) and bicyclo[l.l.l]pentane (3, 1%), plus several other known compounds as well as some unidentified products.3 The mechanistic pathway for the formation of bicyclo[l.l.l]pentane (3) has not been addressed in detail, but it is believed that a diradical intermediate is involved, as shown below.3... 

The imine 26 shows a similar behavior (90CB1161). In this case, too, the reaction products (2,3-dimethyl-2-butene and methyl isocyanide) are the expected thermolysis products of a cyclopropane derivative, and it is therefore safe to postulate the latter as the ring contraction product that is formed by cyclization of an intermediate diradical evolving from 26. 

Thus a substantial body of experimental evidence shows that 1,2-disubstituted cyclopropanes, including vinylcyclopropanes, react thermally to give isomeric cyclopropanes through both one-center and two-center epimerizations, with (kt + k2) kl2 ratios from 1.4 to 4. Rate constants for both (, + k2) and kl2 events respond to the capacity of substituents to stabilize adjacent radicals in a regular fashion consistent with trimethylene diradical transition structures. Rate constants for vinylcyclopropane structural isomerizations do as well, thus reinforcing the notion that these reactions are nonconcerted diradical mediated reactions. 

In addition some p.opane and n-hexane are formed. These are also believed to originate from the trimethylene diradical. A direct comparison of these results with the results reported by other workers is not possible since the only other study in the presence of a foreign gas involved water vapor at a pressure of 70 mm. (5). In the latter instance the ketone pressure was 136 mm., which is 5 to 10 times greater than the pressure used in ref. (17). The presence of water vapor does not seem to have affected the yields of propylene and of cyclopropane. Since the diradical mecha-... 

Kinetic analysis of the rearrangement of benzylfluorocarbene, generated by laser flash photolysis of the corresponding diazirine, gave a rate constant of 9.2 x 106 s 1 at 26 °C with activation entropy —17.2 eu and activation energy 3.25 0.34 kcal mol-1, very similar to the values for the chlorocarbene.80 A product analysis study of the thermolysis and photolysis of the diazirine (73) in the presence of tetramethylethylene showed tiiat die ring-expanded cyclobutene and the cyclopropanation products do not arise via a common intermediate.81 The ring expansion was proposed to occur by loss of N2 from the diradical intermediate (74). 

For easier comparison the result of the thermal reaction is included for compounds 46 and 47. As can be seen in the reaction scheme above direct photolysis of the pyrazolines 46 and 47 proceeds mainly with retention of the original stereochemistry in the cyclopropanes isolated. 48,49 and 50 however lead mainly to the inverted stereochemistry in the cyclopropanes. The singlet biradical 51 formed from 46—49 is therefore clearly not on the same energy surface as a, . possible singlet diradical in the carbene cycloaddition. However one knows today that singlet carbene cycloaddition is a concerted process, so no such diradical should be detectable. 

In the triplet reaction however, the stereochemistry is lost and an almost similiar ratio of cis//raws-cyclopropanes is obtained from 46, 47 and 48, 49. The fm s/ciS-cyclopropane ratio for 46 of 2.77 and 47 of 2.63 is in good agreement with the ratio of 2.9 found for the triplet addition of CHT to cis- or irons- but-2-ene 66>. This shows that rotation around the C—C-bond in the diradical 51 is faster than spin inversion. 

After a brief historical recapitulation, the substantial body of experimental and theoretical work on these thermal epimerization reactions reported over the past 40 years is summarized. Of primary concern here are examples of stereomutations involving monocyclic, stereochemically unconstricted and minimally substituted molecules. Experimental studies of more heavily substituted cyclopropanes attempts to generate trimethylene diradical intermediates from pyrazolines " and the fascinating and still incompletely understood thermal chemistry of bicyclo[2.1.0]pentanes, 2-methylenebi-cyclop.l.OJpentanes", bicyclo[3.1.0]hex-2-enes and related reactions such as the pyrolysis of cyclopropane at 1200 °C to give products such as cyclopentadiene and toluene are neglected, in spite of obvious mechanistic interrelationships. 

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