Studies in nuclear magnetic resonance—XII: Complete analysis of the proton spectra of tert‐butylcyohexane‐3,3,4,4,5,5‐d₆ and ‐3,3,5,5‐d₄

Abstract

Abstract-Accurate parameters for the two title compounds have been obtained by the aid of a spectral subtraction technique in conjunction with analysis by LAOCN3. It is observed that Jleq,8eS and J1ax.zax are abnormally large and small, respectively, and the significance of these and other coupling constants are discussed in terms of the probable geometry of tert-butylcyclohexane. The tert-butyl group affects the chemical shifts of the 4-protons, indicating that tert-butylcyclohexyl derivatives are poor models for fixed conformers of monosubstituted cyclohexanes. The deuterium isotope effects on proton chemical shifts appear to be influenced by the same factors as proton-proton coupling constants.~-T~~~-BUTYLCYCLOHEXYL derivatives have played an important role in the development of the theory of conformational analysis of cyclohexane systems. A method frequently ~s e d l .~*~ for determining the equilibrium between the two rapidly exchanging chair conformers of simple cyclohexyl derivatives, utilizes chemical shift measurements on such averaged systems, together with values for the analogous protons in the epimeric 4-tert-butylcyclohexyl derivatives, it being assumed that the latter are satisfactory models for the fixed conformations. Low temperature NMR studies have, however, indicated that the chemical shifts for the 'pure' conformers are different from those of the model compound^.^*^*^ The origin of these discrepancies has not been determined, and several factors such as long-range shielding and geometric distortion might be implicated.There has been no direct determination of the geometry of tert-butylcyclohexane, although the electron diffraction study of cis-and trans-Ctert-butyl-1-chlorocyclohexane has been reported' and an X-ray diffraction structure determination has been carried out for cis-(pbromophenyl)-4-tert-butylcyclohexane.8 Several force field calculations of the structure of tert-butylcyclohexane have been published~~1° of which that by Altona and SundaralingamlO is the most complete.Recently, Remijnse, Van Bekkum and Wepster deduced all the chemical shifts of tert-butylcyclohexane, and four of the ten vicinal coupling constants, from the spectra of three tert-butylperdeuterocyclohexanes retaining cis vicinal pairs of protons, and from an incomplete analysis of the spectrum of tert-butylcyclohexane-3,3,5,5-~&.~l We now report complete analyses of this compound (2) and of the -3,3,4,4,5,5-d6 derivative (l), which confirm and augment the results of these workers. EXPERIMENTALThe mass spectral data were obtained using an AEL-MS 9 mass spectrometer at 70 eV. Vapour phase chromatography (GLC) data were obtained using a Perkin-Elmer 881 Gas Chromatograph.All solvents used in the experiments described were distilled. Ether for use in lithium aluminum hydride reductions was dried by distillation from calcium hydride and stored over sodium wire. 4-tert- tert-Butylcyclohexane-3,3,4,4,5,5-d, (1).A mixture of mossy zinc (1 g), mercuric chloride (0.3 g), DCl (0.1 ml, 38% in D,O, 99% D, Stohler Isotope Chem...

Vicinal Deuterium Perturbations on Hydrogen NMR Chemical Shifts in Cyclohexanes

“…Our values for ∆δ (eqH, eqH f D) and ∆δ (axH, eqH f D) agree well with the earlier parameters, but do not corroborate the larger ∆δ (axH, axH f D) effect reported. 13 The magnitudes of the deuterium-induced upfield shifts do not parallel stereochemically sensitive J coupling constants between analogously positioned vicinal protons in cyclohexanes.…”

“…12 The ∆δ shifts for cyclohexane were then estimated through regression analysis calculations utilizing 12 equations and selected data. 13 Values for three of the four possible deuterium isotope effects on a vicinal hydrogen were reported: ∆δ (eqH, eqH f D) ) 7 ( 2 ppb, ∆δ (axH, eqH f D) ) 7 ( 2, and ∆δ (axH, axH f D) ) 14 ( 1, while ∆δ (eqH, axH f D) was not noted. These indirectly derived values appeared to parallel trends in vicinal 1 H-1 H spin-spin coupling constants.…”

“…The ∆δ phenomenon, known for more than 40 years, [5][6][7] leads to perturbations demonstrated to be essentially linear with increasing deuterium substitution at equivalent positions. 8 The stereochemical aspects of such perturbations for 13 C chemical shifts in D-C-C- 13 C systems have received much attention, [9][10][11] but D-C-C-H fragments have been relatively neglected. Various deuterium-induced chemical shift perturbations for vicinal hydrogens in selectively labeled tert-butylcyclohexanes and cyclohexanes have been reported.…”

“…Various deuterium-induced chemical shift perturbations for vicinal hydrogens in selectively labeled tert -butylcyclohexanes and cyclohexanes have been reported . The Δδ shifts for cyclohexane were then estimated through regression analysis calculations utilizing 12 equations and selected data . Values for three of the four possible deuterium isotope effects on a vicinal hydrogen were reported: Δδ (eqH, eqH → D) = 7 ± 2 ppb, Δδ (axH, eqH → D) = 7 ± 2, and Δδ (axH, axH → D) = 14 ± 1, while Δδ (eqH, axH → D) was not noted.…”

On the 4JHH long‐range coupling in 2‐bromocyclohexanone: conformational insights

“…[1] The long-range 4 J H2,H6 coupling is expected to operate in the axial conformer, according to the Wcoupling pathway, [9] whilst the corresponding diaxial coupling in the equatorial conformer is usually smaller or even not observed. [10] Long-range 4 J H,H coupling constants were measured in rigid tert-butylcyclohexanes, [11] with W-type coupling constants being significantly larger than diaxial and equatorial -axial couplings. However, 4 J H2,H4 is not observed in 2-bromocyclohexanone, i.e.…”