Conformational studies by dynamic nuclear magnetic resonance. Part III. Rotamers arising from restricted motion in substituted benzaldehydes

Abstract

Rotational isomers due to the restricted motion of the formyl group have been unambiguously detected in orthosubstituted benzaldehydes. The conformers with the ortho-group and formyl oxygen anti have been shown, through l H and 13C n.m.r. spectroscopy, to be the more stable species, the relative abundance depending on the bulkiness of the substituents. Thermodynamic activation parameters have been also obtained by computer simulation of the spectral lines, and a rotational model is proposed to rationalize the dependence of AG3 on the steric hindrance of o-alkyl groups. Proof of the existence of cis-and trans-isomers in terephthalaldehyde has been provided by means of liquid crystal n.m.r. spectra as well as by low temperatures n.m.r. measurements. Comparison has been made between this result and that of an e.s.r. conformational investigation on the corresponding radical anion obtained by photolytic as well as by chemical reduction.

Theoretical and experimental conformational preferences of the aldehyde and fluoroaldehyde groups in 2-methylbenzaldehyde, 2-trifluoromethylbenzaldehyde, and 4-chloro-2-methylbenzoyl fluoride. Proximate couplings

“…The latter are not always in number of their derivatives by a variety of tech-agreement with those found at room temperature, a niques (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18). In solution at least (1,14), the fact perhaps attributable to varying solvent effects twofold barrier to rotation about the exocyclic car- (20) in the experiments; particularly if the conforbon-carbon bond is 31.4 kJImol for benzaldehyde mational isomers have rather different dipole m0-and 29.3 kJImol for benzoyl fluoride, so that at am-merits (1 1).…”

“…From the enthalpy and entropy data obtained in that work (2), it is clear that the 0-syn population can become greater than 50% at 305 K (formally 72 + 20%). In another investigation (3) at 128K in a CCl,F,ICHFCl, solution, the ratio was 50150 as obtained from 13C nmr peak areas of the two con- We take these comparisons to mean that SJmH,cHu is a reliable indicator of the conformer preferences in 1.…”

“…Substitution at both ortho positions by methods should agree. For example, the 0-anti chlorine atoms or methyl groups causes a twist form, l a , of Zmethylbenzaldehyde, cannot have a about the exocyclic bond (12,19), as shown by the dipole moment very differentfromthat of the O -s~n appearance of spin-spin couplings over six bonds, form, lb (structure Because 'H and 13C nmr 6 J HLHO and 6 J,H,CFO, involving the para ring pro-data at low temperatures are available (2,3), it is of tons in such derivatives. interest to see whether the use of stereospecific coupling constants at ambient temperatures will yield compatible population ratios or whether there exists a fundamental flaw in the assumptions (1 1) about such couplings.…”

“…The conformational behaviour of the aldehyde low temperatures, I3C Or '~n m r spectra of both and the fluoroaldehyde groups has been investi-iS0merS are detectable and their peak areas give the gated for benzaldehyde and benzoyl fluoride and a ~o~u l a t i o n ratios (2,3). The latter are not always in number of their derivatives by a variety of tech-agreement with those found at room temperature, a niques (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18).…”

The conformational distribution of 2-cyanobenzaldehyde and 3-cyanobenzaldehyde in solution and in the vapor

“…The only internal bamers known for 2-X-benzaldehydes are those measured for X = alkyl, by dynamic nmr (22,23). For X = CH3, the barrier is 3.6 kJ/mol (22) lower than for benzaldehyde in the same solvent system.…”