Cyclopropane structure-sensitivity

One of the most extensively used addition reactions of cyclobutanes is hydrogenolysis.36 With regard to the mechanistic aspect, evidence has been provided that hydrogenolysis of cyclobutane is structure sensitive to the particle size of the platinum on alumina catalysts.37 Moreover, a kinetic study has also revealed that the mechanism for the hydrogenolysis of cyclobutanes is likely to be different from that for cyclopropanes.37... 

Structure-Sensitive Reactions of Cyclopropane with Cobalt and Iron Zeolite Catalysts... 

The ODPM reactivity of (B.y-unsaturated aldehydes is not restricted to y-phenyl-substituted compounds but can also be extended to systems in which the intermediate biradicals are stabilized by conjugation with a vinyl group. Thus, m-methoxyacetophenone-sensitized irradiation of 40 (Structures 40-49) for 20 min, affords the cyclopropane derivative 41 (47%) as a 1 8 mixture of cisitrans isomers. Similarly, irradiation of 42, for 15 min, under the same conditions, yields 43 (52%) as the trans isomer exclusively [51]. 

Photostationary cis/trans ratios vary with sensitizer structure in a manner not yet perfectly understood, and both the time required for attainment of a photostationary state and actual flash spectroscopic quenching rates indicate that energy transfer to diphenylcyclopropane is quite inefficient. Such should be the case if nonvertical energy transfer with the production of a biradical is occurring. When care is taken so that only the sensitizer absorbs light, only cis-trans isomerization is observed. Direct excitation of the cyclopropane produces 1,3-diphenyl-propene and 1-phenylindane as well.298... 

Calculations based on the steady-state treatment of trimethylene intermediates, and one-center epimerization as well by way of EF(ts) structures, provided kn kD values of 1.293 and 1.324 for k] and kn for 1,2-drcyclopropane, and of 1.328 and 1.368 for 1,2,3-d3-cyclopropane the balance given by the ratio kx kn, according to these calculations, is not very sensitive to deuterium labeling, for the ratios are 1.06,1.08 and 1.09 for unlabeled, 1,2-d2- and l,2,3-d3-cyclopropanes. Thus the theoretically based kt kl2 ratios for cyclopropanes and these deuterium-labeled cyclopropanes agree closely with the experimentally secured ratio k] kn = 1.0 0.2 found for the 1- C-l, 2, 3-d3-cyclopropanes166. 

The rhodium(ii)-catalyzed intramolecular reaction between linked vinyldiazomethanes and pyrroles leads to a novel synthesis of fused tropanes <1996JOC2305>. The reaction occurs by a stepwise [3- -4]-annulation mechanism between a rhodium-stabilized vinylcarbenoid intermediate and the pyrrole rather than by the typical tandem cyclopropanation/Cope rearrangement sequence. The outcome of the reaction is very sensitive to the vinylcarbenoid structure. In particular, the presence of a siloxy substituent on the vinylcarbenoid strongly favors the formation of fused tropanes 1063 or 1064 (Scheme 206) <1996JOC2305>. 

Enough substituted cyclopropanes have now been subjected to careful kinetic studies so that a characteristic pattern of reactivity and stereochemical preferences has emerged. Substituents facilitate stereomutations in proportion to their ability to stabilize 1,3-trimethylene diradical structures. The values for both k 2 and (k + k2) stereomutation rate constants relate linearly with consistent measures of substituent radical stabilization energies with equal sensitivities. Experimentally determined (A , + / 2)- i2 ratios do not vary widely they range from 1.4 to 2.5 over a fair diversity of substituents. Neither do kf.kj ratios vary widely. The majority fall between 1 1 and 2.5 1 the largest yet reported gives 2(CHD) a symmetry corrected kinetic advantage over A i(CDPh) in 1-phenyl-1,2,3-d3-cyclo-propanes of 5 1. 

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