Abstract:Synthese und thermisches Verhalten der diastereomeren 9‐Methyl‐ und 4,9‐Dimethyl‐cis‐bicyclo‐[6.1.0]nona‐2,4,6‐trien‐9‐carbonitrile 1a,b und 10a,b werden beschrieben. Die in den Systemen 1a und 1b entartete Bicyclo[6.1.0]nonatrien ⇌ Bicyclo[6.1.0]nonatrien‐Umlagerung konnte am Beispiel der Dimethylderivate 10a und 10b experimentell nachgewiesen werden. Die wechselseitige Umlagerung 10a ⇌ 11a ⇌ 12a ⇌ 13a bzw. 10b ⇌ 11b ⇌ 12b ⇌ 13b erfolgt bei 102.5°C ([D6]Benzol) stereospezifisch mit der von Woodward und Hoffma… Show more
“…The competing 1,3/1,5 rearrangement occurs with lower stereoselectivity and favors the stereochemistry that would be retention for a 1,3 migration or inversion for a 1,5 migration (see Scheme )i.e., the “forbidden” stereochemistry from an orbital symmetry conservation standpoint. The 1,3/1,5 shift also appears to have a higher barrier than the ring-walk since in similarly substituted molecules it occurs at a reasonable rate only at about 180 °C, whereas the ring-walk occurs at about 100 °C. , The computational and experimental results reported in the present paper are designed to distinguish among several mechanisms that could be invoked to explain these facts. The mechanisms under consideration are as follows: 1 Summary of Possible Rearrangements of Bicyclo[6.1.0]nonatriene, with the Products Selected Being Those Corresponding to the Observed Stereochemical Preference…”
Section: Introductionmentioning
confidence: 85%
“…The second potential exception could occur for the “ring-walk” rearrangements of those bicyclo[ n .1.0]polyenes for which the thermally allowed process should occur with inversion at the migrating carbon. The first two members of the series are the [1,3] shift in bicyclo[2.1.0]pentene 8 and the [1,7] shift in bicyclo[6.1.0]nona-2,4,6-triene . In this class of reaction, ready access to a pericyclic transition state seems possible because the cyclopropane ring opening is expected to be accompanied by a rehybridization at the migrating carbon and a more-or-less “automatic” completion of the pericyclic array.…”
CASSCF and CASPT2 calculations have been carried out on some of the thermal rearrangements of bicyclo[2.1.0]pentene (BCP), bicyclo[4.1.0]hepta-2,4-diene (BCH), bicyclo[6.1.0]nona-2,4,6-triene (BCN), and 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene (DCBCN). In addition, experiments have been conducted to determine the stereoselectivity and temperature dependence of the nondegenerate rearrangement of 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene-exo-15N. The calculations and experiments allow a consistent picture to be drawn for these reactions. The principal conclusions are as follows. (1) The ring-walk rearrangements of BCP, BCN, and DCBCN are pericyclic reactions occurring with a strong preference for inversion of configuration at the migrating carbon. However, the ring-walk rearrangement of BCH is a nonpericyclic reaction. (2) The rearrangement of DCBCN to 9,9-dicyanobicyclo[4.2.1]nona-2,4,7-triene occurs with a preferred stereochemistry corresponding to a 1,3 migration with retention. However, this reaction is not a pericyclic process; the stereoselectivity is probably of dynamic origin. (3) Cyano substituents can significantly reduce the activation energy for a reaction occurring via a singlet biradical, but they do not necessarily cause the intermediate to sit in a deeper local minimum on the potential energy surface.
“…The competing 1,3/1,5 rearrangement occurs with lower stereoselectivity and favors the stereochemistry that would be retention for a 1,3 migration or inversion for a 1,5 migration (see Scheme )i.e., the “forbidden” stereochemistry from an orbital symmetry conservation standpoint. The 1,3/1,5 shift also appears to have a higher barrier than the ring-walk since in similarly substituted molecules it occurs at a reasonable rate only at about 180 °C, whereas the ring-walk occurs at about 100 °C. , The computational and experimental results reported in the present paper are designed to distinguish among several mechanisms that could be invoked to explain these facts. The mechanisms under consideration are as follows: 1 Summary of Possible Rearrangements of Bicyclo[6.1.0]nonatriene, with the Products Selected Being Those Corresponding to the Observed Stereochemical Preference…”
Section: Introductionmentioning
confidence: 85%
“…The second potential exception could occur for the “ring-walk” rearrangements of those bicyclo[ n .1.0]polyenes for which the thermally allowed process should occur with inversion at the migrating carbon. The first two members of the series are the [1,3] shift in bicyclo[2.1.0]pentene 8 and the [1,7] shift in bicyclo[6.1.0]nona-2,4,6-triene . In this class of reaction, ready access to a pericyclic transition state seems possible because the cyclopropane ring opening is expected to be accompanied by a rehybridization at the migrating carbon and a more-or-less “automatic” completion of the pericyclic array.…”
CASSCF and CASPT2 calculations have been carried out on some of the thermal rearrangements of bicyclo[2.1.0]pentene (BCP), bicyclo[4.1.0]hepta-2,4-diene (BCH), bicyclo[6.1.0]nona-2,4,6-triene (BCN), and 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene (DCBCN). In addition, experiments have been conducted to determine the stereoselectivity and temperature dependence of the nondegenerate rearrangement of 9,9-dicyanobicyclo[6.1.0]nona-2,4,6-triene-exo-15N. The calculations and experiments allow a consistent picture to be drawn for these reactions. The principal conclusions are as follows. (1) The ring-walk rearrangements of BCP, BCN, and DCBCN are pericyclic reactions occurring with a strong preference for inversion of configuration at the migrating carbon. However, the ring-walk rearrangement of BCH is a nonpericyclic reaction. (2) The rearrangement of DCBCN to 9,9-dicyanobicyclo[4.2.1]nona-2,4,7-triene occurs with a preferred stereochemistry corresponding to a 1,3 migration with retention. However, this reaction is not a pericyclic process; the stereoselectivity is probably of dynamic origin. (3) Cyano substituents can significantly reduce the activation energy for a reaction occurring via a singlet biradical, but they do not necessarily cause the intermediate to sit in a deeper local minimum on the potential energy surface.
“…interconvert via a sulphur 'walk'. In some cases it has been shown that these intermediates can be trapped as their 2 + 4 furan adducts or, as in the case of compound (33), isolated from the reaction mixture so that the sulphur 'walk' can be studied in i~olation.~' An interesting, related carbon 'walk ' [bicyclopentene (35) to its isomer (36)] has also been reported.31 This thermal 1,3-shift involves inversion at the migrating centre, as expected for an orbital-symmetry-controlled process. However, the 1,3-shift in compound ( 3 9 , the related 1,5-shifts in compounds ( 37) and ( 38) 32 and the 1,7-shift in compound (39)33 all apparently involve inversion at the migrating centre, whether this is 'allowed' or not.…”
The pericyclic reactions of singlet oxygen' and of the photoexcited states of organic anions2 have been reviewed.The effect of substituents on pericyclic reactions has long remained a source of difficulty. This year two interesting approaches to the problem have been described. Carpenter and Wilcox3 have used a simple HMO model. They have shown that for electrocyclic reactions, 1,5-hydrogen shifts, cyclopropane stereomutations, and reactions involving carbon-carbon bond homolysis, there is a linear relationship (1) between AAH* (the measured effect of the substituent on the enthalpy of activation in kcal mol-l) and AAE, (the calculated difference between the effect of the substituent on the HMO .rr-energy of the substrate and that of a suitable model for the transition state, both in units of p. Mobius cyclobutadiene is taken as a model for the transition state in the conrotatory opening of cyclobutene).
“…Sigmatropic [1, n ] carbon migrations (Scheme , I, R = alkyl) on the other hand, do not generally involve concerted transition states because the overlap of the orbitals in the transition structure is usually too weak but are believed to occur via intermediate singlet-state biradicals. , Exceptions, involving pericyclic transition states with good overlap, are the [1,5] sigmatropic migration in 1,3-cyclopentadienes , and the so-called “walk rearrangements” − of bicyclo[ n .1.0]polyenes for which the thermally allowed process should occur with inversion of configuration at the migrating carbon atom. ,, Walk rearrangements are [1,5] sigmatropic shifts which involve the migration of a divalent group (O, S, NR, or CR 2 ) that is part of a three-membered ring in a bicyclic system (Scheme , II, for CH 2 ). These thermally induced processes have been demonstrated in various bicyclo[ n .1.0]polyene structures.…”
The thermal equilibration of the methyl esters of endiandric acids D and E was subject to a computational study. An electrocyclic pathway via an electrocyclic ring opening followed by a [1,5] sigmatropic alkyl group shift were found to be higher in energy. However, the stepwise pathway is suggested to only be feasible for appropriately substituted compounds.
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