1.3- cyclopentane biradical

Steady-state kinetics. The cycloaddition reaction between the singlet ground state of 2-isopropylidene cyclopentane-1,3-diyl ( = S ) with acrylonitrile (A) is believed to occur by way of a biradical intermediate (BR),17... 

Biradical I would yield cyclopentene plus ethylene, biradical II the hepta-1,6-diene. Process I may have a lower energy of activation because of the stabilization of the free electron by the secondary carbon atom and also because less energy is required to compress the appropriate carbon-carbon bond, in the cyclopentane ring to yield the cyclopentene, than to rupture the ring to give the diene. 

Similarly, ab initio calculations on the thermal reaction of propene forming methyl-cyclopentane suggested a three-step biradical reaction with 1,4-biradical and 1,5-biradical as intermediates. Quantum-chemical calculations have been carried out for the cyclization of the neocarzinostatin chromophore cyclonona-l,2,3,5-tetraen-7-yne to 1,5-didehydroindene biradical. The degree of stereoselectivity of the Diels-Alder reaction of 2-methylfuran and maleic acid in water has been found to reduce significantly in the presence of heavy atoms. Taking into account the relatively low concentration (3.5-7 m) of heavy-atoms, and the rapid fall off of the heavy-atom effect with distance, these results show that a large portion of the Diels-Alder reaction occurs via diradical intermediates. " ... 

The photochemically generated cyclopentane-1,3-diyl diradials (87) were part of a study of spin delocalization through the EPR Z)-paramctcr. These biradicals were a model system for cumyl and benzyl radicals and experimental data were combined with MO calculations to map the electronic effects on D by varying the aromatic substituent (Ar = heterocycle).218 This parameter was also measured for a related series of... 

The photochemical fragmentation of 7-methyl-2,2,5-triphenyl-l-oxa-5,6-diaza-spiro-[2,4]-hept-6-en-4-one has been studied." Interest in solid state photochemistry continues to burgeon. The present paper" discusses the problems associated with the proximity of the components of radical pairs or biradicals within the constrictions of the crystalline environment. This problem has been addressed by examining the photochemical reactivity of a series of cyclohexanone derivatives (119) whose solution-phase photochemistry is well known. The irradiations, using X = 350 nm, were carried out on microcrystals dispersed in potassium bromide. The influence of the conformations within the crystals and substitution were studied. The relative yields of the product, the corresponding cyclopentane, are shown beside the appropriate structure." ... 

Photo-decarbonylation of the cyclohexanone (32a) is efficient with a quantum yield of 0.9. The reaction yields the two products (33) and (34) in a ratio of 1 2. The cyclopentanone (32b) also decarbonylates photochemically but is less efilcient with a quantum yield of 0.5. A laser flash study has been carried out on these systems and has identified the biradicals produced by the Norrish type I process. The lifetime of the biradicals (35a) and (35b) are O.Ojxs and 0.5 xs respectively. In a related study the photodecarbonylation of cIs- and frans-2,6-diphenylcyclo-hexanone has been shown to yield a mixture of cis- and 2-diphenyl-cyclopentane and cIs- and... 

Case Study 5.2 Mechanistic photochemistry - cyclopentane-1,3-diyl biradicals... 

Similarly, ab initio calculations on the thermal reaction of propene forming methyl-cyclopentane suggested a three-step biradical reaction with 1,4-biradical and 1,5-biradical as intermediates. Quantum-chemical calculations have been carried out for the cyclization of the neocarzinostatin chromophore cyclonona-l,2,3,5-tetraen-7-yne to... 

The stereochemical integrity of the alkene is usually lost during the reaction. Sequential formation of the new a bonds makes possible a rotation about single bonds within the biradical before its closure. Then the same mixture of stereoisomers can be obtained by photoaddition on either (Z)-or ( )-alkenes. With cycloalkenes such as cyclopentane, isomerization cannot occur and the cis-anti-cis adduct is the main product of 2 + 2 photocycloaddition. 

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nji triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5). ... 

Interest in the cyclopentane 1,3-biradical stems not only from its ease of formation from bicyclo[2.1.0]pentane, but also its formation from 2,3-diazabicyclo[2.2.1]hept-2-ene by either thermolysis or photolysis. Remarkably, the gas phase thermolysis of xo-deuterated material gave mostly inverted bicyclo[2.1.0]pentane (Scheme 6.24). ... 

When cis- and tra 5-2-(hex-4-enyl)cyclopent-2-enones were irradiated separately, a different ratio between the stereoisomeric adducts was obtained. This observation was taken as evidence that biradicals A and B, resulting from the initial formation of either a cyclopentane or a cycloheptane ring on the alkene unit, respectively, were required to explain the results. When a large t-butyl substituent was present, only the less hindered stereoisomer could be observed. 

The double inversion of the molecular skeleton upon thermal and photochemical denitrogenation, first observed for the parent 2,3-diazabicyclo[2.2.1]hept-2-ene, DBH (1), constitutes a unique stereoselective process, much under mechanistic dispute for almost four decades. The mechanistic alternatives for the denitrogenation process pertain to whether the diazenyl ( DZ) or the nitrogen-free cyclopentane-1,3-diyl ( DR) biradical intervenes in the product-branching point to afford the diastereomeric housane... 

This unprecedented inversion for the triplet pathway is rationalized in terms of the unsymmetric nature of the n-propyl and acetoxymethyl substituents in regard to rotation about the bridgehead position of the planar cyclopentane-1,3-diyl triplet biradicals DR(d,e) cf. Scheme 9. For the lower-energy conformation of the DR(d,e) biradical, the X fragments of the bridgehead substituents points, for steric reasons, away from the annellated cyclopentene ring and are preferably located on the upper side of the... 

The early studies of the photochemistry of cyclohexanone have been reviewed by Srinivasan (1964), Pitts and Wan (1966), Cundall and Davies (1967), and Wagner and Hammond (1968). The major products are identified as CO, cyclopentane, 1-pentene, and 5-hexenal, but the mechanisms of their formation remain ill-defined. Two alternative mechanisms have been discussed through the years (Calvert et al., 2008). The first invokes intramolecular rearrangements of the original excited ketone (e.g., see Shortridge and Lee, 1970), while the second implies formation of biradicals that are intermediates to the final products (e.g., Benson and Kistiakowski, 1942 Blacet and Miller, 1957). 

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