¹³C Nuclear Magnetic Resonance Studies. 34. The ¹³C Spectra of Several Methylcyclopentanones and -cyclohexanones

Abstract

The I3C spectra of 16 methyl substituted cyclohexanones and 15 methylcyclopentanones have been determined. These series were chosen as model systems for the study of steric and conformation effects on I3C shieldings. Con~plete assignments of the individual signals were accomplished by intercomparison of the shielding data within each series and the trends observed are readily interpreted in conformational terms. Each of the cyclohexanones exists preferentially in chair conformations although there is evidence of ring distortion in the tetramethyl derivatives. The cyclopentanones apparently strongly favor half-chair forms with maximum puckering at C-3 and C-4. In general, shielding differences between cis and t~.ntts isomers are pronounced and the assignment of stereochemistry for some cis-trnrlsdimethyl derivatives is illustrated.Further examples of marked rlesl~ieldit~g trends associated with sterically crowded 6 nuclei in syrz-axial arrangements are described. IntroductionThe res~llts of carbon-13 n.ni.r. st~tdies of a variety of cyclic systems have established that 13C spectroscopy is a p o w e r f~~l method for stereochelnical assignments and confor~national analysis (1). Several six-membered ring systems, both mono-and polycyclic, have been examined in detail and in general it is found that axial substituents tend t o shield the neighboring carbons (C-1, -2, -3, -5, and -6 in a monosubstituted cyclohexane) relative to their positions in the spectrum of the equatorial analogs. The variation of carbon shieldings with substitution in simple five-membered ring derivatives has received much less attention. Apart from studies of a variety of norbornyl derivatives (2) the only detailed examinations of cyclopentane derivatives is that of Christ1 et al. (3) in which several methylcyclopentanes, cyclopentanols, and cyclopentyl acetates were studied. We wish t o report the results of a 13C n.m.r. investigation of

Estimation of the stereoselectivity of reduction of 3,3,5,5-tetramethylcyclohexanone by sodium borohydride

“…In the minor component C2.6 give signals at 6 52.4 and 6 54.7 (two peaks due to asymmetry of the hydroxyethyl group), very close to the signal position of C2.6 in 3,3,5,5-tetramethylcyclohexanone itself? 6 53.8 (9). In the major component C2,6 give signals at 6 48.6 and 6 50.2.…”

13C Nuclear Magnetic Resonance Studies. 47. 13C Spectra of some exo-Methylenecycloalkanes and -norbornanes

“…relative to those in the four-, fiveand six-ring systems. The series of methyl substituted methylenecyclohexanes was examined t o compare the effects of the methvl substituents with those found for cyclohexank (4) and cyclohexanone (6). From Table 2 it is apparent that an a-methyl group alters the shieldings of the olefinic carbons in a distinctive, and expected, fashion whereas monomethyl substitution at C-3 and C-4 has little effect.…”

“…Since the monomethyl derivatives exist predominantly in chair conformations with equatorial methyl groups, a set of substituent effects for the latter can be obtained by comparison of the observed shieldings for each carbon with those for the corresponding carbon in methylenecyclohexane. This sort of analysis is the same as that described for the methylcyclohexanones (6) and the data for the di-, tri-, and tetramethyl derivatives permit one to obtain substituent effects for axial methyl groups at C-2 and C-3 as described previously (6). To compare the results for cyclohexane (I), methylenecyclohexane (2), and cyclohexanone (3), the methyl substituent effects for the three systems are summarized in Table 4.…”

Part I. Conformational analysis of thiazolidine-2-thione

“…in the cis-Dimethylated Derivatives of TH-2-TH and in some Vicinal Din~ethylated Models Stothers and co-workers (13,14) showed the relationship between the spatial proximity and the observed shielding effects of the l3CH3 signal in vicinal methyl groups. Inspections of the computed electron density distributions and details of the geometrical dependences lead to the suggestion that the mecanism of y-effect may be considerably more complex than the commonly accepted C-H bond polarisation by nonbonded H .…”