Cyclohexane diyl

For example, EPR evidence showed that cyclohexane-1,4-diyl, generated by radiolysis of hexadiene, rearranged to cyclohexene radical cation. Similarly, ant/-5-methylbicyclo[2.1.0]-pentane radical cation (33) rearranged to 1-methylcy-clopentene radical cation (34) via a 1,2-shift of the syn-5-hydrogen. ... 

D. Unsaturation is senior to saturation. The more the unsaturation, the greater the seniority, with all other items being equal. Thus, 1,4-phenylene is senior to 2,5-cyclohexadiene-l,4-diyl, which is senior to 2-cyclohexene-1,4-diyl, which in turn is senior to cyclohexane-1,4-diyl. For linear chains-CH=CH—CH=CfU is senior to-CH=CH—CH2-CH2-which is in turn senior to the totally saturated chain segment. 

Dimethylhexane-2,5-diyl dication 231 Bicyclo[2.2.2]octane-l,4-diyl dications 231 Cyclohexane-1,4-diyl dications 232... 

The third radical cation structure type is the cyclohexane-1,4-diyl radical cation (22 +) derived from 1,5-hexadiene. The free electron spin is shared between two carbons, which may explain the blue color of the species ( charge resonance). Four axial p and two a protons are strongly coupled (a = 1.19 mT, 6H). + ... 

The third radical cation structure type for hexadiene systems is formed by radical cation addition without fragmentation. Two hexadiene derivatives were mentioned earlier in this review, allylcyclopropene (Sect. 4.4) [245] and dicyclopropenyl (Sect. 5.3) [369], The products formed upon electron transfer from either substrate can be rationalized via an intramolecular cycloaddition reaction which is arrested after the first step (e.g. -> 133). Recent ESR observations on the parent hexadiene system indicated the formation of a cyclohexane-1,4-diyl radical cation (141). The spectrum shows six nuclei with identical couplings of 11.9G, assigned to four axial p- and two a-protons (Fig. 29) [397-399]. The free electron spin is shared between two carbons, which may explain the blue color of the sample ( charge resonance). At temperatures above 90 K, cyclohexane-1,4-diyl radical cation is converted to that of cyclohexene thus, the ESR results do not support a radical cation Cope rearrangement. 

Indirect (chemical) evidence for the cyclohexane-1,4-diyl structure type was provided by an elegant study of the electron transfer initiated rearrangement... 

One significant aspect of the cyclohexane-1,4-diyl radical cation has been a point of contention the question whether it undergoes cleavage to the hexa-1,5-diene radical cation, i.e. whether it completes the Cope rearrangement. Results obtained in the laboratory of Miyashi, particularly the exchange of a deuterium label between the terminal olefinic and allylic positions, seem to suggest such a... 

To account for the possible contribution of cyclohexane-1,4-diyl, we must include the configuration arising from the excitation of the electrons from the HOMO to the LUMO. Thus, the diyl Slater determinant T djyi is... 

The 1,4-diyl species, 73 +, is obtained also upon electron transfer from 1,5-cyclohexadiene, 77, both in cryogenic matrices [143] and in solution [201]. Solution experiments provided chemical evidence for the cyclohexane-1,4-diyl structure type in an elegant study of the electron-transfer-initiated photochemistry of 2,5-diphenylhexa-1,5-diene, 78, and derivatives in the presence of molecular oxygen [201-203]. The intermediate 1,4-cyclohexanediyls, 79 , were intercepted by O2 the stereochemistry of the en /o-peroxide products, 80, showed that the initial cycloaddition occurred in the same stereospecific manner established for the thermal rearrangement of the neutral parent [204]. 

Many studies used radiation chemistry to produce the radical and radical cations and anions of various dienes in order to measure their properties. Extensive work was devoted to the radical cation of norbomadiene in order to solve the question whether it is identical with the cation radical of quadricyclane . Desrosiers and Trifunac produced radical cations of 1,4-cyclohexadiene by pulse radiolysis in several solvents and measured by time-resolved fluorescence-detected magnetic resonance the ESR spectra of the cation radical. The cation radical of 1,4-cyclohexadiene was produced by charge transfer from saturated hydrocarbon cations formed by radiolysis of the solvent. In a similar system, the radical cations of 1,3- and 1,4-cyclohexadiene were studied in a zeolite matrix and their isomerization reactions were studied. Dienyl radicals similar to many other kinds of radicals were formed by radiolysis inside an admantane matrix. Korth and coworkers used this method to create cyclooctatrienyl radicals by radiolysis of bicyclo[5.1.0]octa-2,5-diene in admantane-Di6 matrix, or of bromocyclooctatriene in the same matrix. Williams and coworkers irradiated 1,5-hexadiene in CFCI3 matrix to obtain the radical cation which was found to undergo cyclization to the cyclohexene radical cation through the intermediate cyclohexane-1,4-diyl radical cation. 

A correlation diagram shows that the filled MOs in two weakly interacting allyl radicals and in cyclohexane-1,4-diyl are correlated with each other [4], Therefore, these two structures may both be regarded as resonance contributors to the TS for the Cope rearrangement. From this fact it is possible to make two qualitative predictions. 

By combining the heat of formation of cyclohexane with the then-current value of a secondary C-H bond dissociation enthalpy (BDE), in 1971 Doering obtained an estimated heat of formation for cyclohexane-1,4-diyl (A) [6]. The heat of formation of this fictional diradical, in which the two radical centers interact neither through bonds nor through space, was estimated by Doering to be far lower than that of two allyl radicals... 

On the basis of MlNDO/3 calculations, Dewar argued for a non-concerted mechanism for the parent Cope rearrangement, involving formation of cyclohexane-1,4-diyl (A) as an intermediate [12]. Several years later, Dewar published another paper in which he boldly claimed that multi-bond reactions, such as the Cope rearrangement, cannot, in general, involve synchronous bond making and bond breaking [13]. 

The results of this study again call attention to the chameleonic nature of the C2 wave function for the Cope rearrangement. At small values of R the CASSCF wave function is essentially that for cyclohexane-1,4-diyl (structure A in Fig. 30.1) whereas, at large values of R the CASSCF wave function approaches that for two allyl radicals (structure... 

A, found by B3LYP/6-31G calculations [22,23] are in reasonable agreement. Both are on the short side of the range of C-C bond lengths commonly computed for the TSs in concerted pericyclic reactions [Ib-d]. The reason for the slightly shorter than usual bond lengths in the Cope TS is almost certainly that cyclohexane-1,4-diyl (A) [6,7,27] is considerably lower in enthalpy than two allyl radicals (C) [9]. Therefore, the electronic structure of the TS resembles A more than C. 

Thus, the case for a non-concerted 3,3-shift via a cyclohexane-1,4-diyl is weak. Nonetheless, substituent effects on the rate of the 3,3-shift were intially interpreted in terms of the diyl species. In particular, Dewar found the 2-phenyl and 2,5,-diphenyl-l,5-hexadiene rearrange 40 and 1600 times, respectively, more rapidly than that of the parent diene. Further, semi-empirical MINDO/3 calculations supported the proposition that even the parent species proceeded via the chair-like cyclohexane-1,4-diyl. These observations and calculations provided stimulus for a substantial effort in the subsequent years to address the question of transition state structure in and the energy surface for the 3,3-sigmatropic shift of 1,5-hexadiene. 

Some perspective is necessary here. As indicated in Chapter 7, Section 4.1 on the Cope rearrangement, the free energy for formation of a cyclohexane-1,4-diyl is 50-53 kcal/mol and that for formation of two allyl radicals is roughly 57 kcal/mol. However, in the current system, the diyl is destabilized by roughly 20 kcal/mol due to the bicyclo[2.2.1]ring system that must be generated. Such a species is kinetically inaccessible due, in part, to a substantial negative entropy despite the fact that the activation enthalpy for its formation would appear to be 50-55 kcal/mol. 

The 3,3-shift of 1,2,6-heptatriene was found to occur at 300°C with log A = 9.97 - 30 470/2.While a concerted, cyclic transition state was proposed for the reaction, the possibility that a cyclohexane-1,4-diyl intermediate stabilized by the additional double bond was examined. Extensive calorimetric and kinetic studies as well as oxygen trapping experiments led to the enthalpy surface of Scheme 8.49, which also includes data on the conversion of 2-methylenebicyclo[2.2.0]hexane to 3-methylene-l,5-hexadiene, back to itself with bridgehead double inversion, and to 1,2,6-heptatriene. " ... 

Finally, it is worthy of note that 3-methylene-1,5-hexadiene is also the product of pyrolysis of tricyclo[4.1.0.0 ]heptane with log = 14.21 - 36 500/2.3i T. A small amount of 2-methylenebicyclo[2.2.0]hexane was also formed with log k = 14.07 — 37 300/2.3/ r (Scheme 8.52). It would appear as if the 2-methylene-cyclohexane-1,4-diyl species were an intermediate in this process formed by initial cleavage of a spiropentane radial bond as opposed to a peripheral bond which is the lower energy pathway in the parent compound (see Chapter 6, Section 2). 

Clearly, this indicates that the 3,3-shift transition state has little cyclohexane-1,4-diyl character. This is not unreasonable considering that bond formation in this case must occur with generation of a strained cyclopropane bond so the transition state, no doubt, has much more bisallyl radical character (see Chapter 7, Section 4.1). Indeed, radical stabilizing substituents on the cyclopropane ring bond being broken dramatically increase the rate of the 3,3-shift to the point where the transition state is nearly equi-energetic with starting materials (Scheme 10.35). ... 

For comparison, the 3,3-shift in the triene was 91 times faster than that of 2-isobutenylmethylenecyclobutane indicating substantial stabilization of a cyclohexane-1,4-diyl transition state by a radical-stabilizing group at the 2-position of this species (see Chapter 7, Section 4.1). 

The mechanism of the Cope rearrangement has been the subject of numerous experimental studies. Three different pathways are possible a priori (i) a concerted pericyclic pathway (ii) a non-concerted pathway involving cr-bond formation to give cyclohexane-1,4-diyl and (iii) a non-concerted pathway involving ff-bond cleavage to afford two allyl radicals. The latter possibility is ruled out by labelling studies. ... 

Tsuji, T, Miura, T, Sugiura, K., Matsumoto, M., and Nishida, S., Photoinduced electron transfer reaction of some l,4-dialkylbicyclo[2.2.0]hexanes. Generation of cyclohexane-1,4-diyl radical cations in boat form and their stereospecific [o s + O s] cleavage,/. Am. Chem. Soc., 115,482, 1993. 

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