13 C Nuclear Magnetic Resonance Studies. 32. The 13 C Spectra of Several Norbornyl Derivatives

. Can. J . Chem. 58,599(1980 IntroductionIn this report, we will study the unsubstituted ring Conformational analysis of heterocyclic systems did not begin (1, 2) until well after the birth of classical conformational analysis (3). The uncertainty associated with the extension of well accepted tools of analysis for classical conformational systems, to unknown systems, is the main cause of this late development. For any new heterocyclic system under study, the obtained informations must be cross-checked by the use of different methods. The and in the next report (4), we will consider the effect of perturbations in this system due to the presence of substituents at positions 3, 4, and 5. This first paper tentatively answers the questions: is the heterocycle planar or puckered? If indeed the ring is puckered, to what extent is it, compared with its alicyclic homologs? Do the experimental results allow us to affirm that a pseudo-rotation process exists in this ring system? following reports illustrate this approach. The purResults and Discussion pose of this paper is to define the conformational properties of the nonsymmetric monounsaturated Obseruarion and Eualuation ofthe Degree ofpuckering heterocyclic system, namely thiazolidine-2-thione of the Thiazolidine-2-thione Ring (TH-2-33) 8.' H Nuclear Magnetic Resonance -JVi, -Application of the Simplified Karplus Equation 'Revision received November 12, 1979. he-analysis of the 'N nmr spectra (Table 1) torsional angle cp between the two corresponding vicinal carbon atoms.In the case of a planar ring in TH-2-TH: The abnormally large value for 180" (6-8) rules out the possibility of a planar conformation for the TH-ZTH ring.'N Nuclear Magnetic Resonance -JYi, -The Use of Lambert's R-ualue Method Assuming a puckered ring, thiazolidine-2-thione exists in a rapidly interconverting equilibrium mainly between two energetically equivalent conformations:The $I-CLCC5-S torsional angle cp, can be evaluated (Lambert's R-value method (9)) using the ratio of the experimental vicinal coupling constant values in the CH,-CH, fragment: i.e."tl~ere appears to be no theoretical reason to preclude applications to all common and medium rings" (9). In the TH-2-TH case, R = 0.84, i.e. cp, = 41"; therefore Lambert's R-value implies a significant puckering of the ring as in the cyclopentane (10) and dioxolane cases (1 1). 'H Nuclear Magnetic Resonance -2JHjH5, and J, , , , , Values In the series of 4-and 5-alkyl-substituted (Me, Et, iPr, tBu) TH-2-TH derivatives, the 2JH,,,r or 2J,j,j, values vary in a continuous and coherent way (4). However, substituent electronegativities vary only slightly along the series; thus the main factor which induces deviations in the J,,, values in the X-CH, fragment is the variation of the dihedral angle between the C-H bonds and the "lone-pairs orbitals" of the heteroatom x (12). The observed change in the J,,,, values in the alkylated series was therefore assigned to conformational modifications of the ring (4): that establishes the mobility of the thiazol...

Section: Mesupporting

confidence: 77%

Section: Introductionmentioning

confidence: 97%

Part I. Conformational analysis of thiazolidine-2-thione

Chanon¹,

Rajzmann²,

Chanon³

et al. 1980

“…3: 1. Since exo-deuteration is favored in the ring opening of 16 it follows, by the principle of microscopiq reversibility, that exo-proton abstraction from base to give the product derived from the more 20 21 stable carbanion (26) hence 21 + 15 is the favored path. The alternative ring opening of 21 would lead to an enolizable ketone which C-6 in l5 is the lower energy process.…”

Section: A Comparison Of the Results Inmentioning

confidence: 99%

Nuclear Magnetic Resonance Studies. XXVII. Homoenolization of Fenchone. A Mechanistic Study Utilizing C And H Nuclear Magnetic Resonance

Johnson¹,

Stothers²,

Tan³

1975

Self Cite

(1-BuO--1-BuOD, 18S0), C-6(exo and endo), C-8, C-9, and C-10. Both I3C and 2H n.m.r. methods were employed to monitor deuterium incorporation and the particular advantages ofthe latter are illustrated. The relative rates of the five exchange processes were determined directly by 2H n.m.r. in contrast to the earlier homoenolization studies for which the stereoselectivity of exchange was established indirectly. The intermediate homoenolate ion can lead to rearrangement and the rearrangement products have been characterized by I3C n.m.r. as the 3,3,6-trimethylnorcamphors. Homoenolization of this mixture gave further evidence on the stereochemistry of homoenolate formation.

“…The remaining 13 -a]cohols, that at lower field was assigned (10) while 26 was generated from norbornene as described by Baird (11).…”

Section: Resultsmentioning

confidence: 99%

“…I . 13 empirical analysis of the data on carefully selected systems, in conibination with the continuing development of shielding theory, may gain the ultimate goal of understanding. In any event, the results may be applied directly to problems involving the skeletons for which data are available.…”

mentioning

confidence: 99%

¹³C nuclear magnetic resonance studies. 64. The ¹³C spectra of a variety of bicyclo[3.2.1]octanols

Stothers¹,

Tan²

1977

Self Cite

The 13C nmr spectra of a series of 20 bicyclo[3.2.1]octanols and -0ctenols have been determined to examine the shielding effects of the hydroxyl group on the skeletal carbons in this ring system. For comparison, the previously unreported data for a few closely related alcohols in the [2. As part of our continuing examination of the effects of molecular geometry o n 13C chieldings we have obtained the spectra of a series of bicyclo[3.2.l]octanoIs and -octenols to compare the effects of the hydroxgl group on the skeletal shieldings in this rlng system with the results for other bicyclic systems (1). The prime reason for investigating these series ic that the well-defined carbon skeleton allows one to study the shielding variations as a function of niolecular geometry. The [3.2.1] saturated examples have the additional feature of a flexible six-menibered ring permittlng an examination of this factor on the -shielding trends. In general, the shifts produced by the substituent at carbons within four bonds are of particular interest as potential probes for stereoche~nical analysis. A deficiellcy in the use of bicyclic systems as models for these perturbations is the contribution of 'strain' to the observed shieldings, a factor which is difficult to assess. However, the influence of strain could be revealed in a compariso~l of the corresponding interactions in a kariety of systems differing in strain energy. For this reason, the bicyclo- empirical analysis of the data on carefully selected systems, in conibination with the continuing development of shielding theory, may gain the ultimate goal of understanding. In any event, the results may be applied directly to problems involving the skeletons for which data are available. A few of the compounds included in this study have been reported previously (7); these have been reexamined, however, to obtain data for the entire series under closely similar conditions. 5-1 5'7. solutions in deuteriochloroform, for internal consistencj~ and for meaningful comparison with previous results.