Cyclopropane models

Another aspect of the geometry of the bound olefin that has been barely studied is the orientation of substituents capable of x interactions with the olefin. The most studied x substituent is the cyano group, but its linearity precludes discussion of the nature of the interaction. The structures of two complexes of diphenylethylenes (IV and XVIII, Table I) have been determined. On the basis of electronic effects one would expect a phenyl ring either to be coplanar with the olefin double bond for better conjugation or to be perpendicular to the metal-olefin plane for greatest x overlap (in the cyclopropane model). The limited evidence favors the second orientation. However, structural studies of olefins with substituent groups such as -COH, -COOR, or -N02 would be useful for the further definition of the orientation of x substituents. 

C. Cyclopropane Models for NADH Redox Transfer Mechanisms... 

Several hypotheses concerning the electronic structure of cyclopropane have been suggested. This topic has been interestingly pinpointed by Bemett (I). A cyclopropane model proposed by Walsh (2) indicates that C-C bonds of the rings are caused by an overlap of one of the sp- hybridized orbitals of each carbon atom and of each orbital (Walsh proposed, in fact, an sp2 hybridization state for each cyclopropane carbon). 

I 8.3. Consider a hypothetical cyclopropane model, Pd3Cl9 shown from the top and side view below. Using the 2a and 62 orbitals of PdCl3, take symmetry-adapted combinations to form the MOs. This is not a stable molecule. In Chapter 19 we see a M3CI9 molecule at the same electron count which is a known molecule. 

Conformational analysis is far simpler m cyclopropane than m any other cycloalkane Cyclopropane s three carbon atoms are of geometric necessity coplanar and rotation about Its carbon-carbon bonds is impossible You saw m Section 3 4 how angle strain m cyclopropane leads to an abnormally large heat of combustion Let s now look at cyclopropane m more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry... 

Wnte structural formulas or make molecular models for all the compounds that are tnchloro derivatives of cyclopropane (Don t forget to include stereoisomers ) Which are chiraL Which are achiral" ... 

Later there was an attempt by ab initio calculation to fit the electron structure of diazirine into the Walsh model of cyclopropane (69MI50800). According to these SCF-LCAO-MO calculations three MOs add to the description of the lone electron pairs, all of which also contribute to some extent to ring bonding. As to strain, 7r-character and conjugative effect, the term pseudo-rr-character was used. 

Solvolysis rate studies also indicate that there is greater stabilization by a cyclopropyl group in a bisected geomeby. In tosylate 1, the cyclopropane ring is locked into an orientation which affords a perpendicular arrangement. It reacts 300 times more slowly than the model compound 2. Tosylate 3, which corresponds to the bisected geomeby, undergoes acetolysis at least 10 times faster than the model 2-adamantyl tosylate 4. ... 

In keeping with the "bent-bond" description of Figure 3.10, the carbon-carbon bond distance in cyclopropane (151 pm) is slightly shorter than that of ethane (153 pm) and cyclohexane (154 pm). The calculated values from molecular models (see Learning By Modeling) reproduce these experimental values. 

In this model, the intermediacy of a monomeric zinc species is postulated. To support this assumption, an examination of the effect of stoichiometry and solvent in cyclopropanation involving the 2,4-pentanediol auxiliary was preformed [59]. In the initial reaction protocol, a large excess of both diethylzinc and diiodo-methane is employed. Such excessive conditions are justified on account of the instability of the zinc carbenoid under the reaction conditions. To minimize the un-... 

A third permutation group of the graph of cyclopropane obtains if the regular prism discussed as a model for (a) is subjected to rotations as well as to reflections which leave it invariant. The six vertices are thus subject to a permutation group of order 12. We call it the extended group of the stereoformula. Its cycle index is... 

Because of their cyclic structures, cycloalkanes have two faces as viewed edge-on, a "top" face and a "bottom" face. As a result, isomerism is possible in substituted cycloalkanes. For example, there are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyl groups on the same face of the ring and one with the methyls on opposite faces (Figure 4.2). Both isomers are stable compounds, and neither can be converted into the other without breaking and reforming chemical bonds. Make molecular models to prove this to yourself. 

Cyclopentenones. from 1.4-diketones. 886-887 Cyclopropane, angle strain in, 115 bent bonds in. 115 from alkenes. 227-229 molecular model of, 111. 115 strain energy of, 114 torsional strain in, 115 Cystathionine, cysteine from. 1177 Cysteine, biosynthesis of, 1177 disulfide bridges from, 1029 structure and properties of, 1018 Cytosine, electrostatic potential map of, 1104... 

It is thus anticipated that compressive stress inhibits while tensile stress promotes chemical processes which necessitate a rehybridization of the carbon atom from the sp3 to the sp2 state, regardless of the reaction mechanism. This tendency has been verified for model ring-compounds during the hydrogen abstraction reactions by ozone and methyl radicals the abstraction rate increases from cyclopropane (c3) to cyclononane (c9), then decreases afterwards in the order anticipated from Es [79]. The following relationship was derived for this type of reactions ... 

The cz5-aziridine substrate shows, as expected on the basis of this model, predominant formation of the trans-cyclopropane product. The starting materials for this MIRC reaction can readily be obtained from the aziridine esters by reduction to the corresponding aldehyde and a subsequent Knoevenagel reaction with malonate ester (Scheme 25) [34]. 

Chan et al. [38] prepared optically active atropoisomeric 2,2 -bipyridine by nickel(0)-catalyzed homo-couphng of 2-bromopyridylphenol derivatives (structure 28 in Scheme 16). Tested in the model test reaction, the copper catalyst led to frans-cyclopropanes as major products with up to 86% ee. 

Scott et al. [45] prepared diimine derivatives of 2,2 -diamino-6,6 -dimethyl-biphenyl (as structure 37 in Scheme 19) as copper chelates for the catalyzed cyclopropanation reaction. All catalysts were active in this reaction but enan-tioselectivities varied importantly according to the substitution pattern of the imine aryl group only ortho-substituted ligands (by chloride or methyl groups) led to products with measurable enantioselectivity for the model test reaction (up to 57% ee with 37). 

Cornejo et al. [65] reported the first immobihzation of pyridine-bis(oxa-zoline) chiral hgands and the use of the corresponding solid ruthenium complex in the model cyclopropanation test. They synthesized vinyl-PyBOx, the vinyl functionahty being introduced in the fourth position of the pyridine ring. This monomer was further homo- or copolymerized in the presence of styrene and divinylbenzene. The corresponding ruthenium catalysts proved... 

Fig. 12 The experimental geometries of cyclopropane- -HC1 and cyclopropane- -C1F (drawn to scale) and the Coulson-Moffitt pseudo-ir-electron model of cyclopropane. See Fig. 1 for key to the colour coding of atoms...

Fig. 16 The experimental geometries of methylenecyclopropane- HC1 and methylene-cyclopropane- -CIF, drawn to scale. The n-electron model for the Lewis base is also shown. The angles C- - H and C- Cl, respectively, where is the centre of the C - C double bond, are both close to 90°, as required by rule 2. The halogen bond again exhibits a smaller non-linearity 6 than the hydrogen bond. See Fig. 1 for key to the colour coding of atoms... 

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. 

Recently, several mechanistic studies have been performed by means of calculations based on density functional theory. - Pfaltz s model proposed for asymmetric cyclopropanation using copper-semicorrin or -bis(oxazolines) complex has been supported by calculation.295 Another calculation also supports the parallel approach.296... 

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