Conformations cyclopropane

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... 

Disubstituted cyclopropanes exemplify one of the simplest cases involving stabil ity differences between stereoisomers A three membered ring has no conformational mobility so the ring cannot therefore reduce the van der Waals strain between cis sub stituents on adjacent carbons without introducing other strain The situation is different m disubstituted derivatives of cyclohexane... 

Cyclobutane has less angle strain than cyclopropane (only 19.5°). It is also believed to have some bent-bond character associated with the carbon-carbon bonds. The molecule exists in a nonplanar conformation in order to minimize hydrogen-hydrogen eclipsing strain. 

Cyclopent-2-en-l-one, 2-hydroxy-3-methyl-synthesis, 3, 693 Cyclopentenone, 4-methoxy-formation, 1, 423 Cyclopenthiazide as diuretic, 1, 174 Cyclopent[2,3-d]isoxazol-4-one structure, 6, 975 Cyclophane conformation, 2, 115 photoelectron spectroscopy, 2, 140 [2,2]Cyclophane conformation, 2, 115 Cyclophanes nomenclature, 1, 27 Cyclophosphamide as pharmaceutical, 1, 157 reviews, 1, 496 Cyclopiloselloidin synthesis, 3, 743 Cyclopolymerization heterocycle-forming, 1, 292-293 6H-Cyclopropa[5a,6a]pyrazolo[l,5-a]pyrimidine pyrazoles from, 5, 285 Cydopropabenzopyran synthesis, 3, 700 Cyclopropachromenes synthesis, 3, 671 Cyclopropa[c]dnnolines synthesis, 7, 597 Cyclopropanation by carbenes... 

Incorporation of stereogenic centers into cyclic structures produces special stereochemical circumstances. Except in the case of cyclopropane, the lowest-eneigy conformation of the tings is not planar. Most cyclohexane derivatives adopt a chair conformation. For example, the two conformers of cis-l,2-dimethylcyclohexane are both chiral. However, the two conformers are enantiomeric so the conformational change leads to racemization. Because the barrier to this conformational change is low (lOkcal/mol), the two enantiomers arc rapidly interconverted. 

The cyclopropane ring is necessarily planar, and the question of conformation does not arise. The C—C bond lengths are slightly shorter than normal at 1.5 A, and the H—C—H angle of 115° is opened somewhat from the tetrahedral angle. These structural... 

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planai geometry by adopting the nonplanai puckered conformation shown in Figure 3.11. 

Identify the lowest-energy conformer from among those provided cyclopropane, planar and puckered cyclobutane, planar and puckered cyclopentane and chair, half-chair, boat and twist-boat cyclohexane. (If... 

In conformity with our previous approach, we classify substituent effects on heterocyclopropane rings into four categories. They are substituted heterocyclopropyl sets, heterocyclopropylidene sets, heterocyclopropylene sets and hetero-cyclopropanes with heteroatom reaction sites. 

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. 

Cyclopropylchlorocarbene [20] has been generated by UV photolysis (A = 335 nm) of cyclopropylchlorodiazirine [21] frozen in a nitrogen matrix at 12 K (Ho et al., 1989). IR and UV spectra of [20] have been recorded. The reaction of [20] with HCl resulted in the formation of (dichloromethyl)-cyclopropane [22], and annealing of the matrix gave (dicyclopropyl)dichloro-ethene [23]. Subsequent irradiation (A = 450 nm) of the carbene [20] led to its isomerization to 1-chlorocyclobutene [24], which was partialy destroyed to give ethene and chloroacetylene. Ab initio calculations predict the existence of two carbene conformers, but attempts to distinguish them in IR or UV spectra were unsuccessful. 

These results can be interpreted in terms of competition between recombination of the diradical intermediate and conformational equilibration, which would destroy the stereochemical relationships present in the azo compound. The main synthetic application of azo compound decomposition is in the synthesis of cyclopropanes and other strained-ring systems. Some of the required azo compounds can be made by 1,3-dipolar cycloadditions of diazo compounds (see Section 6.2). 

In another reaction dendritic pyridine derivatives such as 82 or 83 were tested as co-catalysts for enantioselective cyclopropanation of styrene with ethyl diazoacetate [102]. Using catalyst 82, enantiomer ratios of up to 55 45 were obtained. However, with catalyst 83 bearing larger branches yields and selectivities did not increase. The relatively low selectivities were rationalized by the presence of a large number of different conformations that this non-rigid system may adopt. 

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. 

The molecular structure of the parent compound was investigated in the vapor and in the solid phase using X-ray, XN and GED methods. The reported data are shown in Table 16. In both phases a clear bond length separation could be detected with a localized three-membered ring and its three adjacent double bonds. The symmetry-equivalent cyclopropane bonds are rather long in C3v-symmetric BUL (1.533-1.542 A), which can be explained by the common electron-withdrawing effect of the 7r-systems in a. svM-ciinal conformation. For comparison, the unaffected bonds in unsubstituted cyclopropane are 1.499 A in the crystal and 1.510 A in the gas phase. Therefore, the bond lengths in BUL... 

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