Cyclopropane interaction

Since they are equivalent to homobutadienes, cyclopropanes interacting directly with an olefin unit display a particularly rich chemistry. Pericyclic reactions come into play in this specific area. Similar to other cyclopropane derivatives, the reactivity of vinylcyclopro-panes with nucleophiles, electrophiles and radicals as well as their general chemical behaviour will be governed by substituents at the cyclopropane core and at the double bond. ... 

In 1963, Dauben and Berezin published the first systematic study of this syn directing effect (Scheme 3.15) [37]. They found that the cyclopropanation of 2-cyclohexen-l-ol 32 proceed in 63% yield to give the syn isomer 33 as the sole product. They observed the same high syn diastereoselectivity in a variety of cyclic allylic alcohols and methyl ethers. On the basis of these results, they reasonably conclude that there must be some type of coordinative interaction between the zinc carbenoid and the substrate. 

The discovery of viable substrate-direction represents a major turning point in the development of the Simmons-Smith cyclopropanation. This important phenomenon underlies all of the asymmetric variants developed for the cyclopropanation. However, more information regarding the consequences of this coordinative interaction would be required before the appearance of a catalytic, asymmetric method. The first steps in this direction are found in studies of chiral auxiliary-based methods. 

For a reaction as complex as catalytic enantioselective cyclopropanation with zinc carbenoids, there are many experimental variables that influence the rate, yield and selectivity of the process. From an empirical point of view, it is important to identify the optimal combination of variables that affords the best results. From a mechanistic point of view, a great deal of valuable information can be gleaned from the response of a complex reaction system to changes in, inter alia, stoichiometry, addition order, solvent, temperature etc. Each of these features provides some insight into how the reagents and substrates interact with the catalyst or even what is the true nature of the catalytic species. 

Each H <— H eclipsing interaction in ethane costs about 4.0 kj/mol. Wow many such interactions are present in cyclopropane What fraction of the overall 115 kj/mol (27.5 kcal/mol) strain energy of cyclopropane is due to torsional strain ... 

Cyclopropane (115 kj/mol strain) and cyclobutane (110.4 kj/mol strain) have both angle strain and torsional strain. Cyclopentane is free of angle strain but has a substantial torsional strain due to its large number of eclipsing interactions. Both cyclobutane and cyclopentane pucker slightly away from planarity to relieve torsional strain. 

In the interaction of the local 2pv orbitals, two more bonding molecular orbitals are formed against one less bonding. In all previous cases the opposite occurred. This is due to the negative overlap between adjacent 2py orbitals—whether, by convention, all positive lobes point in the clockwise direction, or whether all positive lobes point in the anticlockwise direction. The two bonding 2pv combinations in fact fall below the two antibonding (hybrid 2s, 2px) combinations. The former each have two electrons while the latter are empty. The six electrons of the three C—C bonds are nicely accounted for. The method creates simultaneously the acc and or c molecular orbitals of cyclopropane (note that the latter three lie relatively close in energy). 

Conjugated chains, 14, 46 Correlation diagrams, 44, 50 Cyclobutadiene, 171 Cyclobutane, 47, 222 orbital ordering, 26 through-space interactions, 26 Walsh orbitals, 27 Cyclobutene, 200 Cyclohexane, 278 Cyclohexene (half-boat), 274 Cyclopen tadiene, 225 Cvclopen tadienone, 269 Cyclopentadienyl anion, 237 Cyclopentane, 254 Cyclopen ten e, 241 Cyclopropane, 41, 47, 153 construction of orbitals, 19, 22 Walsh orbitals, 22, 36, 37 Cyclopropanone, 48, 197 bond lengths, 38 Cyclopropen e, 49, 132 reactivity, 40... 

This notion is also snpported by the following experimental observations. Because substitution of a cyano gronp on the cyclopropane ring lowers the energy of the Walsh orbital of the cyclopropyl group, the resultant attennation of the interaction of the olefin orbital with the Walsh orbital, if this interaction is indispensable, would reduce the facial selectivity. However, substitution of a cyano gronp on the cyclopropyl group, as in ejco-cyano 59c and endo-cymo 59d, essentially does not modify the syn-preference in dihydroxylation and epoxidation, but even increases the syn preference (59c (98 2) and 59d (>99 <1)) in the case of dihydroxylation. 

The ionization potentials of substituted cyclopropanes also show a significant correlation with eq. (2). The value of pr obtained is comparable to that observed for substituted ethylenes and 1-substituted propenes (section II.A.2.) and is considerably above that found for substituted benzenes (for which a value of Pr = 59 is obtained). This result confirms the existence of a large resonance interaction between the cyclopropane ring and substituents. The magnitude of a is considerably greater for substituted cyclopropanes than it is for substituted ethylenes or benzenes. 

Data are extant in the literature for four tra s-cyclopropylene sets. Of these, two are disubstituted (sets 39-9 and 39-11). Positions trans-2 and tranS 3 are completely equivalent in cyclopropanes bearing the reaction site at position 1. These sets have been correlated with eq. (30). The other two sets have been correlated with eq. (2). Three of the four sets studied gave significant correlations. The fourth set had only four points. The results obtained clearly show a significant resonance effect. They clearly demonstrate that the frans-cyclopropylene system does involve a resonance interaction between the substituent and the cyclopropane group. The rra s-cyclopropylene system again... 

Davies [30] studied the PyBOx-induced conformational effects by testing several ligands sterically hindered on the oxazoUne moieties (Scheme 11, structures 18 and 19). However, these new ligands gave poorer results in terms of yields and enantioselectivities than ligand 16 for the Ru-catalyzed cyclopropanation reaction, indicating unfavorable steric interactions between styrene and the carbene complex. 

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