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Biradical reactions

Particular interactions between energy levels could stabilize supra-facial-suprafacial [2+2] concerted reactions, [2.ns- - ig, 57) or supra-facial-antarafacial modes, [2Jts-hn a]- Other interactions might be favorable toward biradical reactions. These diagrams will be discussed in detail in Section VI. [Pg.149]

Eq. 17 is meant to represent the possibility for a concerted formation of oxetane product. A problem that always exist in cycloadditions is the question of whether the reaction takes place by a two-step biradical reaction pathway or through a concerted mechanism. Such questions have not even been resolved for purely thermal reactions. 4> A recent speculation on this point proposes almost universal concertedness for all cycloaddition reactions. 79> In that work, mixed stereochemistry in the products of [2+2] cycloaddition reactions is generally attributed to a mixture of two concerted reactions, suprafacial-suprafacial, and supra-facial-antarafacial. It will be seen later that the PMO calculations generally do not support this idea. A mixture of biradical and concerted reactions is in better agreement with experimental facts. [Pg.152]

Qualitatively, the interaction diagram would closely resemble that in Fig. 3, since electron-donating substituents in both addends would raise the molecular levels of both the carbonyl compound and the olefin. Only the energy gap, E(n)-> F(n), would increase, the net result being that the calculated ratio of concerted to biradical reaction, Eqs. 40 and 41, should be even closer to unity than in the formaldehyde-ethylene case. Detailed calculations 38> support this conclusion, so PMO theory predicts that the overall stereochemical results are due to a combination of concerted (singlet) and biradical (triplet) mechanisms. This explanation agrees with the experimental facts, and it bypasses the necessity to postulate differential rates of rotation and closure for different kinds of biradical intermediates. [Pg.162]

The first-order interaction of the two bonding levels should be the controlling interaction. The calculated PMO energies for concerted reactions are 0.61/ for the observed orientation and 0.53y for the other orientation. Calculated energies for biradical reactions are much smaller. [Pg.170]

Give examples of simple radical and biradical reactions - combination, disproportionation, hydrogen abstraction and fragmentation. [Pg.161]

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. " ... [Pg.191]

Electron transfer catalyzed cycloadditions via radical cations show remarkable selectivity that could be exploited for expanded synthetic methodology. As a complement to the neutral Diels-Alder reaction, ET catalysis hlls the void of the electron-rich diene/electron-rich dienophile cyclizations. In attempt to understand the intricate details of the reaction, experimentalists and theorists have uncovered a range of novel factors to control and manipulate these high-energy reactive intermediates. As exemplihed by the cases discussed in this contribution, the charged character of the intermediates and the presence of back electron transfer leading to the biradical reaction manifold opens new pathways to control the chemo-, peri-, and stereochemical patterns in these dynamic species. [Pg.79]

In addition to the practical interest, the process presents challenges encouraging further fundamental exploration. A thorough study not reported here, has been performed on the mechanism and kinetics of the polymerization of acrylamide in AOT/water/toluene microemulsions (Carver, M.T.r Dreyer, U. Knoesel, R. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press. Carver, M.T. Candau, F. Fitch, R.M. J. Polym. Sci. Polym. Chem. Ed., in press). The termination reaction of the polymerization was found to be first order in radical concentration, i.e. a monoradical reaction instead of the classical biradical reaction. Another major conclusion was that the nucleation of particles is continuous all throughout the polymerization in contrast to conventional emulsion polymerization where particle nucleation only occurs in the very early stages of polymerization. These studies deserve further investigations and should be extended to other systems in order to confirm the unique character of the process. [Pg.59]

Johnston, L.J., Scaiano, J.C. (1989), Time-Resolved Studies of Biradical Reactions in Solution, Chem. Rev. 89, 521. [Pg.242]

Cyclic enones, such as substituted cyclohex-2-enones or cyclohexa-2,5-diones, also undergo sigmatropic photorearrangement to form bicyclo[3.1.0]hexanones (lumiketones) or bicyclo[3.1.0]hex-3-en-2-ones, respectively, for which both concerted and stepwise (biradical) reaction mechanisms have been proposed.640,641,770 For example, a [l,2]-shift concurrently with the ring contraction (termed the type A reaction) is observed upon irradiation of the methylphenyl derivative 159 in polar solvents, whereas phenyl migration (termed the type B reaction) predominates in nonpolar solvents (Scheme 6.70).771,772 The reactions are believed to proceed via both the n,n and n,Tt triplet ketone states. In the presence of alkenes, cyclic enones may readily undergo a competitive photocycloaddition reaction (Section 6.1.5). [Pg.271]

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... [Pg.477]

The photolyses of H2Se,118 dimethyl sulphide, dibutyl sulphide, and methyl vinyl sulphide,119 and dimethyl sulphoxide,120 the role of concerted and hot biradical reactions in the photolysis of thietan and thietan-cyclopentadiene mixtures,121 and the photolysis of aromatic sulphur compounds122 have been reported. [Pg.121]

Johnston LI, Caiano JC (1989) Time-resolved studies of biradical reactions in solution. Chem Rev 89 521, and references cited therein... [Pg.281]

The hypothesis of dynamical control has been pursued in biradical reactions, vide infra, but the notion that temperature independence of reaction pathways is a criterion for such behavior seems inappropriate since the parent bicyclo[2.I.I]-hexene does not show such behavior despite the fact that it, having many fewer vibrational modes, should be most likely to do so. The question of whether or not the stereochemistry of the rearrangement of the parent compound is under orbital symmetry control requires that the reaction be concerted. However, the heat of formation of the parent material and the heat of formation of the methyl cyclopentene diyl suggests a BDE of less than roughly 22 kcal/mol for the non-concerted cleavage of the C1-C5 bond which is 12 kcal/mol less than that observed. However, these types of BDE estimates are invariably 8-10 kcal/mol less. So, it is unclear whether or not the reaction in the parent case is perhaps partially concerted. [Pg.102]

The formation of a retained, label-scrambled material, SA, is suggestive of a significant contribution from a concerted 3,3-shift, but the formation of some inverted scrambled product, RA, requires the intermediacy of a cis,cis-hisa y ic biradical for its formation. This latter intermediate then is probably responsible for ca. 3% of the 3,3-shift product as well as 3% of inverted, unscrambled material, RV. However, most of the inverted, unscrambled material probably arises from a cis,trans-hisai y ic biradical which cannot give any other 4-vinylcyclohexene except starting material. Thus, biradical reactions appear to compete well with the 3,3-shift in this case, most likely because the 3,3-shift transition state must not only be boat-like, but suffer from steric interactions not unlike those in bicyclo[2.2.2]octane. [Pg.255]

Steric effects can influence the paths of 1,4-biradical reactions. Moorthy and co-workers reported a kinetic study of the lifetimes of the triplet 1,4-biradicals formed in Norrish type II reactions of diastereomeric 2,3-dimethyl-l,4-diphenylbutan-l-ones. " Irradiation of the diastereomers with "syn" methyl groups (82) on C2 and C3 gave a 2 1 ratio of cyclization products to fragmentation products, while irradiation of the diastereomers with those two methyl groups anti (83) gave those products in a 3 17 ratio. Combined with quantum yield data and kinetic data from the decay of transient absorptions attributed to the 1,4-biradical intermediates, these investigators were able to calculate rate constants for cyclization kc), fragmentation k ). [Pg.837]

Careful application of the three simple postulates listed above can yield insight into the mechanism and stereochemistry of biradical reactions as complex as the thermal dimerization of cis, irons-1,5-cyclooctadiene [26] or the isomerization of allyl-substituted cyclopropanes via internal [2 + 2]-cycloaddition [27]. An attempt to do so here would take us too far afield, in view of the ease with which biradical intermediates interconvert. Instead let us move on to the considerably more stereoselective cycloaddition of reactant pairs with complementary polarity, that proceeds stepwise along a zwitterionic pathway. [23]... [Pg.147]


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See also in sourсe #XX -- [ Pg.15 ]




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Biradical

Biradical intermediates, Patemo-Buchi reaction

Biradical reactions stereoselectivity

Biradicals

Biradicals as reaction intermediates

Competing biradical reactions

Cycloaddition reactions of biradicals

In biradical reaction

Quinone reactions with biradicals

Reactions Involving Biradical Intermediates (ER)

Reactions with Formation of Biradicals

Singlet oxygen reaction intermediates 1,4-biradicals

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