Conformation nonplanar

A second but much less stable nonplanar conformation called the boat is shown in Eigure 3 14 Like the chair the boat conformation has bond angles that are approximately tetrahedral and is relatively free of angle strain It is however destabi hzed by the torsional strain associated with eclipsed bonds on four of its carbons The... 

Cyclopentane has two nonplanar conformations that are of similar sta bility the envelope and the half chair... 

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. 

FIGURE 3.11 Nonpl anar ("puckered") conformation of cyclobutane. The nonplanar conformation avoids the eclipsing of bonds on adjacent carbons that characterizes the planar conformation. 

H321). Tliese radical cations exist in a nonplanar conformation. With fra s-3,4-dimethyl substituents, the barrier to ring flip was estimated to be >5 kcal/mol. In connection with reactions of thiirane with the thiirane radical cation, ab initio calculations were reported on the 1,2-dithietane radical cation (93JA12510). 

We JI look in the next chapter at cycloalkanes—saturated cyclic hydrocarbons— and we ll see that the molecules generally adopt puckered, nonplanar conformations. Cyclohexane, for instance, has a puckered shape like a lounge chair rather than a flat shape. Why ... 

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

Besides the parent system oxonin (l),5,6 which is thermally unstable and undergoes a valence isomerization to 3a,7a-dihydrobenzofuran (2) above 35 C, some annulated derivatives are known. All reported systems are nonaromatic and exist in nonplanar conformations. 

On page 132, atropisomerism was possible when ortho substituents on biphenyl derivatives and certain other aromatic compounds prevented rotation about the bond. The presence of ortho-substituents can also influence the conformation of certain groups. In 88, R= alkyl, the carbonyl unit is planar with the trans C=0 - F conformer more stable when X=F. When X=CF3, the cis and trans are planar and the trans predominates. When R = alkyl there is one orthogonal conformation but there are two interconverting nonplanar conformations when R=0-alkyl. In 1,2-diacylbenzenes, the carbonyl units tend to adopt a twisted conformation to minimize steric interactions. " ... 

Tricyclic dithiine derivatives of tetrathiafulvalene (TTF) 32 have been prepared for their increased electropolymerization potential <2000CC1005>. The effect of different tricyclic heterocycles upon the redox properties of TTF analogues was explored, and showed that the furan derivative 33 had an unusual nonplanar conformation that allowed for the attainment of higher oxidation states at relatively low oxidation potentials <2004JMC2822>. 

Cycloheptane, cyclooctane, and cyclononane and other higher cycloalkanes exist in nonplanar conformations. 

Since the solvent properties of dimethyl sulfoxide are widely different from those of hydrocarbons and halogenated hydrocarbons, it may be difficult to compare the kinetic and thermodynamic data for the C02H group (Table 16) directly with others. However, heating the carboxylic acid (68, X = OH) in toluene affords the sp isomer almost exclusively. Probably, the observed results with the carboxylic acid derive from difficulty in the formation of a hydrogen bond owing to a steric effect, in addition to the nonplanar conformation of the carboxyl group relative to the naphthalene. 

The torsion angle co, which is common to peptides and nonpeptidic amides, always prefers a planar over a nonplanar conformation due to the partial double-bond character of the amide bond (Fig. 6.3, right). Thus, a peptide bond resembles an amide bond in conformational and electronic terms [2] [3], However, peptides differ from amides in that both the carbonyl C-atom and the amido N-atom are nearly always bound to an sp3-hybridized C-atom. As a result, the trans-conformer (a>=180°) is consistently preferred over the cw-conformer, the energy difference usually being ca. 90 kJ mol 1 (Fig. 6.3). The relationship between the partial double-bond character of the amide bond (Fig. 6.3, right) and hydrolysis will be considered in Sect. 6.3. 

Snyder, R.G, Maroncelli, M., Qi, S.R, and Strauss, H.L., Phase-transitions and nonplanar conformers in crystalline normal-alkanes, Science, 214, 188, 1981. 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

The porphyrin ligand is sufficiently flexible to adopt nonplanar conformations in response to steric or electronic effects induced by the central metal, the axial ligands, or substitution at the porphyrin periphery. Coordination of the small, nonmetallic Si(IV), Ge(IV), P(V), and As(V) ions to porphyrins offer an ideal opportunity to study not only the direct effect on this phenomenon of... 

No enetetramine formation has been observed with NHCs derived from seven-membered heterocycles. The C2 resonance in these derivatives is shifted further downfield (5 250-260 ppm) in the NMR spectra. The most striking difference in comparison to all NHCs discussed previously is the twisted, nonplanar conformation of the seven-membered heterocycle both in the azolium precursor and in the carbene ligand. The twisted conformation appears to be essential to accommodate the strain within the seven-membered heterocycle. 

Table 5. Relative Energies (Kcal/Mol) of the Planar Versus Nonplanar Conformers of 25 and 26...

The gas-phase dipole moment of 1,2,3-trioxolane was determined to be 3.43 D from microwave spectroscopic studies <88JA799l>. This compares with calculated values of 3.90 D and 4.03 D using SCF and MP2 methods, respectively <89JA2497>. The dipole moment of 1,3,2-dioxathiolane 2-oxide has been measured as 3.65 D <49DOK(69)4i> and 3.74 D <6IJA2105> (both in benzene), and that of 1,3,2-dioxathiolane 2,2-dioxide has been determined to be 5.64 D (in dioxane) <68JHC289>. These results are consistent with the compounds adopting nonplanar conformations. 

Dioxathiolane 2,2-dioxide (21) adopts a puckered conformation in the solid state, as shown by x-ray diffraction (Section 4.15.3.1). In solution, the H NMR spectrum of (21) indicates that the compound undergoes a rapid pseudorotation between twist-envelope forms (Section 4.15.3.3.1) the O NMR spectra of simple derivatives also indicate a rapid conformational equilibrium by pscudorotation, although substitution may act as a barrier to complete ring inversion (Section 4.15.3.3.3). The solution-phase dipole moment of 1,3,2-dioxathiolane 2,2-dioxide is consistent with a nonplanar conformation (Section 4.15.3.8). 

Example 14.4 4,5-Dimethylphenanthrene. Using the C NMR spectrum measured by Stothers et al. [275], we deduce A/// = 36.8 kcal/mol assuming planarity. Closely neighboring methyl groups that are separated by five bonds in the molecular skeleton, however, result in chiral nonplanar conformations [276]. Modeling, where appropriate, the CC bonds on those described for biphenyl and m-stHbene, one predicts A/// = 47.8 for the nonplanar form, in acceptable accord with the reported value [248], 46.26 + 1.46 kcal/mol, a result that is self-explanatory. 

Aryl derivatives of thieno[3,2-f]pyridines, 36 and 37, have been the subject of two-dimensional (2-D) NMR studies. Phase-sensitive nuclear Overhauser enhancement spectroscopy (NOESY) and correlation spectroscopy (COSY) experiments confirm the nonplanar conformation of the two aromatic ring systems <1999SAA1035>. 

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