Cyclopropanation kinetic study

The stereochemistry of the resulting cyclopropane product (.s vn vs anti) was rationalized from a kinetic study which implicated an early transition state with no detectable intermediates. Approach of the alkene substrate perpendicular to the proposed carbene intermediate occurs with the largest alkene substituent opposite the carbene ester group. This is followed by rotation of the alkene as the new C—C bonds begin to form. The steric effect of the alkene substituent determines... 

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... 

The metal-catalysed hydrogenation of cyclopropane has been extensively studied. Although the reaction was first reported in 1907 [242], it was not until some 50 years later that the first kinetic studies were reported by Bond et al. [26,243—245] who used pumice-supported nickel, rhodium, palladium, iridium and platinum, by Hayes and Taylor [246] who used K20-promoted iron catalysts, and by Benson and Kwan [247] who used nickel on silica—alumina. From these studies, it was concluded that the behaviour of cyclopropane was intermediate between that of alkenes and alkanes. With iron and nickel catalysts, the initial rate law is... 

The glyoxime-Co(II)-catalyzed asymmetric cyclopropanation shown in Scheme 94 is noteworthy (226). The results of the detailed kinetic study are consistent with the mechanism of Scheme 92, however, the intermediary Co carbenoid species has substantial radicaloid properties, and only styrene and other conjugated olefins can be used as substrates. Simple alkenes are not cyclopropanated by diazo compounds. The reaction of deuterated styrene proceeds in non-stereospecific manner without retention of geometrical integrity. 

Kinetic studies on the decarbonylation of 113 and 114 are of interest in terms of the homoaromatic character of 105/106. It has been reported that the rate of decarbonylation of 113 was 1 x 10s times greater than that of 114247. It was suggested that this reactivity difference was due to partial opening of the cyclopropane in the transition state and overlap between the Walsh orbitals of the cyclopropane and the rehybridizing s-orbitals of the breaking bonds. Homoaromatic electron delocalization in a norcaradiene/cyclohepta-triene-like transition state is only possible with the anti-isomer 113. 

Early kinetic studies on the structural isomerization of cyclopropane to propene provided estimates of activation parameters73 75 and prompted speculation that the reaction might well involve a trimethylene diradical intermediate. This possibility seemed reinforced when the thermal interconversion of the els and trans isomers of l,2-d2-cyclo-propane at 414 to 474 °C (equation 1) was reported in 195876. This structurally degenerate isomerization was found to be substantially faster than conversion to deuterium-labeled propenes—about 24 times faster at the high pressure limit76 77. 

Kinetic studies by Doering and his collaborators at Harvard150-154 based on five sets of chiral 1,2-disubstituted cyclopropanes, with 1-cyano, 2-(phenyl or propen-2 -yl or -(E)-propenyl or phenylethynyl) (3) and 1 -phenyl-2-(propen-2 -yl) (4) substitution, established the ralative rotational propensities of these substituents and tested the proposition that they might be related to substituent moments of inertia. In all of these cases, the balance between one-center and two-center epimerizations from a trans isomer, reflected in (kt + k2) kl2, was fairly constant, ranging from 1.4 1 to 2.1 1. The kinetic advantages for one-center epimerizations at cyano-substituted carbons for the four cases studied were modest and not especially system-dependent the k, k2 ratios were 2.5,2.2,2.4 and 1.8, thus establishing that rotational propensities are not dictated by some simple function of the moments of inertia of substituents. 

An empirical correlation of enantiomerization rate constants ( 12) for chiral trans-l-R -2-R2-cyclopropanes was communicated in 1988177 when calculated values of AG1 (knj are plotted against the sum of radical stabilization energies (SE) of the two substituents, a linear correlation is evident. More recent work has provided a few more experimental points an updated version of this correlation is shown in Figure 2. Kinetic data from 20 kinetic studies reported by six different research groups are accommodated by this simple empirical AG (kn) versus SE(R CH2-) + SE(R2CH2 ) correlation. 

At any rate, by 1984 the kinetic study of l,2-d2-cyclopropanes reported in 1975276 278 became the only unchallenged claim of kinetically favored two-center epimerization and of a predominance of the theoretically predicted double rotation mechanism . 

Furue H, Pacey PD. Performance of a cylindrical flow reactor in a kinetic study of the isomerization of cyclopropane. J Phys Chem 1980 84 3139-3143. 

The experimental and theoretical work published by the early 1970s viewed the stereomutations of cyclopropanes as kinetically competitive one-center and two-center stereomutations some details, especially regarding relative rate constants for one-center epimerizations which defined relative rotational propensities, remained unclear, but all agreed that neither the Smith mechanism (one-center only) nor any two-center-only formulation for stereomutations could be sufficient. Thus when kinetic studies " on the isomerizations shown by chiral samples of l-phenyl-2-d-cyclopropane and 1,2-d2-cyclo-propane purported to show that, actually, two-center stereomutations were kinetically dominant, many were stimulated to fresh speculations and accommodations. Theoretical work at times hinted that the parent hydrocarbon might be an exceptional case and might... 

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. 

The pyrolysis of substituted cyclopropane leads to three types of unimolecular isomerizations (see Fig. 3). The first kinetic study of the conversion of cyclopropane into propylene (reaction a) was undertaken by Trautz and Winkler in 1922 ... 

The reversion reaction can be probed by isotope labelling experiments and kinetic studies, and rearrangement and nucleophilic attack by labelling studies or, for sufficiently dissymmetric cyclopropanes, by a study of product structure often combined with labelling studies. 

As far as the adsorption and skeletal isomerization of cyclopropane and the product propene are concerned, results mainly obtained by infrared spectroscopy, volumetric adsorption experiments and kinetic studies [1-4], revealed that (i) both cyclopropane and propene are adsorbed in front of the exchangeable cations of the zeolite (ii) adsorption of propene proved to be reversible accompanied by cation-dependent red shift of the C=C stretching frequency (iii) a "face-on" sorption complex between the cyclopropane and the cation is formed (iv) the rate of cyclopropane isomerization is affected by the cation type (v) a reactant shape selectivity is observed for the cyclopropane/NaA system (vi) a peculiar catalytic behaviour is found for LiA (vii) only Co ions located in the large cavity act as active sites in cyclopropane isomerization. On the other hand, only few theoretical investigations dealing with the quantitative description of adsorption process have been carried out. 

Alkyl-bridged ions may be generated as potential intermediates in 1,2-alkyl shifts ( a route ), by double bond participation ( tt route ), and by protonation of cyclopropanes. In the acylic series, kinetic methods are not very useful for detecting alkyl participation. The solvolyses of alkyl sulfonates proceed with 1,2-alkyl shifts only in the case of neopentyl-type substrates, (612) and (613). Isobutyl (610) and 3-methyl-2-butyl sulfonates (611) prefer solvolytic displacement and 1,2-H shifts. Alkyl shifts in (610) and (611) are induced only by deamination (X=Nj) and related reactions which are not amenable to kinetic studies. 

Modern carbene chemistry started, however, in a rather unusual way when Hine (1950) conducted a kinetic study of the alkaline hydrolysis of trichloromethane (chloroform). He proposed a two-step a-elimination involving CCI2 as an intermediate. The result of this mechanistic study was unusual because it opened a new field of synthetic organic chemistry, whereas, in general, such studies improve our understanding of fields already known from synthetic work. Mine s dichlorocarbene was subsequently applied to a cyclopropane synthesis by Doering and Hoffmann (1954, see later in this section). 

Fdrster H, Seebode J (1985) Spectroscopic and kinetic studies on the cyclopropane isomerization over mordenites of different acidity. Proceedings of the International Symposium on Zeolite Catalysis, Siofok. Acta Phys et Chem Szegediensis. p 413... 

The proposed mechanism involves a concerted 4-center cyclometala-tion with the hydrogen migrating as a proton. Hydride migration on [Cp IrH(i7 -C3H5)] gives [Cp Ir(PR3)(Pr)R] in the presence of PR3 and HR = arene or cyclopropane. Kinetics and labeling studies implicate [Cp Ir(i7 -propene)] as an intermediate. ... 

Monfort. J.P. Nzihou. A. Light scattering kinetics study of cyclopropane hydrate growth. J. Cryst. Growth 1993. 128, 1182-1186. 

This value, taken by Kerr and Trotman-Dickenson from Reference 65, comes ultimately from a kinetic study and was derived using an Evans-Polanyi activation energy-BDE relationship. In the same study, a value for the cyclopropane C—H bond of 100.7 was reported. The latter value is now known to be wrong, and we suspect the spiropentane value, which should not be very different from that for cyclopropane, is also a value near 106 2 seems more reasonable. 

Rearrangements.—Cyclepropyl-Allyl Rearrangement. Further kinetic studies of this isomerization in the gas phase have been reported, and analytical least-motion forms have been applied to the reaction. An orbital description of a concerted pathway for the conversion of cyclopropane into propene has been supplied by Baldwin and Grayston, who have noted that in dehydroadamantanes the reaction proceeds smoothly even though orthogonal trimethylene radical geometry cannot be achieved. 

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