Transition state cyclopropanes

These results indicate an energy profile for the 3-methyl-2-butyl cation to 2-methyl-2-butyl cation rearrangement in which the open secondary cations are transition states, rather than intermediates, with the secondary cations represented as methyl-bridged species (comer-protonated cyclopropanes) (Fig. 5.10). 

Fig. 3.9 Proposed transition state for cyclopropanation with dioxahorolane 108. [Charette, A.B. juteau, H. Lebel, H. Molinaro, C.J. Am. Chem. Soc. 1998, 120, 11943. Reprinted with permission from The American Chemical Society]...

The next step in the calculations involves consideration of the allylic alcohol-carbe-noid complexes (Fig. 3.28). The simple alkoxide is represented by RT3. Coordination of this zinc alkoxide with any number of other molecules can be envisioned. The complexation of ZnCl2 to the oxygen of the alkoxide yields RT4. Due to the Lewis acidic nature of the zinc atom, dimerization of the zinc alkoxide cannot be ruled out. Hence, a simplified dimeric structure is represented in RTS. The remaining structures, RT6 and RT7 (Fig. 3.29), represent alternative zinc chloride complexes of RT3 differing from RT4. Analysis of the energetics of the cyclopropanation from each of these encounter complexes should yield information regarding the structure of the methylene transfer transition state. 

This study suggests a radically new explanation for the nature of Lewis acid activation in the Simmons-Smith cyclopropanation. The five-centered migration of the halide ion from the chloromethylzinc group to zinc chloride as shown in TS2 and TS4 has never been considered in the discussion of a mechanism for this reaction. It remains to be seen if some experimental support can be found for this unconventional hypothesis. The small energy differences between all these competing transition states demand caution in declaring any concrete conclusions. 

The activation energy for the favored transition state TS4 (22.8 kcal mol ) is still somewhat high. Still, the qualitative predictions of enhanced reactivity of the zinc alkoxide-zinc chloride complexes are in full agreement with contemporary ideas about this reaction and represent a major advance in the theoretical understanding of the cyclopropanation process. 

It is likely that protonated cyclopropane transition states or intermediates are also responsible for certain non-1,2 rearrangements. For example, in superacid solution, the ions 14 and 16 are in equilibrium. It is not possible for these to interconvert solely by 1,2 alkyl or hydride shifts unless primary carbocations (which are highly unlikely) are intermediates. However, the reaction can be explained " by postulating that (in the forward reaction) it is the 1,2 bond of the intermediate or transition state 15 that opens up rather than the 2,3 bond, which is the one that would open if the reaction were a normal 1,2 shift of a methyl group. In this case, opening of the 1,2 bond produces a tertiary cation, while opening of the 2,3 bond would give a secondary cation. (In the reaction 16 14, it is of course the 1,3 bond that opens). 

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... 

It is concluded from these results that with this kind of non-C2 symmetric ligand (that led necessarily to poor enantioselectivities in homogeneous phase), it is possible to exploit support effects to change the trans/cis selectivity and to improve the enantioselectivity. This is demonstrated for the trans-cyclopropanes obtained with ligand 10a in styrene. Due to the relative disposition of the ester and phenyl groups in the transition state, support ef-... 

Sketch plausible transition states for (a) the dissociation of a molecule in the gas phase (b) the reaction of cyclopropane to give propene (c) the isomerization of CH3CN to CH3NC (d) the desorption of an atom from a surface (e) the dissociation of an adsorbed molecule such as CO on a metal surface. 

As an explanation, if was suggested that the degree of charge development in the transition state determines the preferred site of cyclopropanation A transition state with little charge development should prefer the endocyclic double bond (Pd catalysis), whereas one with much charge development should favor the exocyclic bond (Rh catalysis). 

Intramolecular cyclopropanation is a useful method for construction of [n.l.0]-bicyclic compounds.17-21 225 275 As a matter of course, alkenyl and diazo groups of the substrate are connected by a linker and the transition-state conformation of intramolecular cyclization is influenced by the length and the shape of the linker. Thus, the enantioselectivity of the reaction often depends upon the substrates used. Use of a catalyst suitably designed for each reaction is essential for achieving high enantioselectivity. 

Based on these mechanisms and ligand structures, various transition-state models to explain the stereochemistry of asymmetric cyclopropanation reactions have been proposed. For details, see the reviews17- 1 and the references cited for Figure 12. 

However, the generation of cyclopropane derivatives was also shown to implicate an alternate route not requiring a metallocyclobutane transition state (14). In addition, a metathesis catalyst successfully converted certain cyclopropanes to metathesis-related olefins by way of a carbene elimination process (15-17), according to Eq. (3). 

It remains unanswered whether observed cyclopropanations and carbene retroadditions [Eq. (3)] always share a common transition state with olefin metathesis. 

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