Strain Effects in Protonated Carbonyl Compounds. An Experimental and ab Initio Treatment of Acyclic Carboxamides and Ketones

Abstract

Strain effects have been quantitatively evaluated for a set of 22 compounds including ketones (R(2)CO), carboxamides (RCONH(2)), and N,N-dimethylcarboxamides (RCONMe(2)), where R = Me, Et, i-Pr, t-Bu, 1-adamantyl (1-Ad), in their neutral and protonated forms. To this end, use was made of the gas-phase proton affinities and standard enthalpies of formation of these compounds in the gas phase, as determined by Fourier transform ion cyclotron resonance mass spectrometry (FT ICR) and thermochemical techniques, respectively. The structures of 1-AdCOMe and (1-Ad)(2)CO were determined by X-ray crystallography. Quantum-mechanical calculations, at levels ranging from AM1 to MP2/6-311+G(d,p)//6-31G(d), were performed on the various neutral and protonated species. Constrained space orbital variation (CSOV) calculations were carried out on selected protonated species to further assess the contributions of the various stabilizing factors. Taking neutral and protonated methyl ketones as references, we constructed isodesmic reactions that provided, seemingly for the first time, quantitative measures of strain in the protonated species. A combination of these data with the results of theoretical calculations (which also included several "computational experiments") lead to a unified, conceptually satisfactory, quantitative description of these effects and their physical link to structural properties of the neutral and protonated species.

Gas-phase basicities of polyfunctional molecules. Part 4: Carbonyl groups as basic sites

“…This effect is obviously enhanced by the replacement of a hydrogen by the highly polarizable t ‐butyl group: according to the data reported in Scheme , the increase in PA attains 94 and 73 kJ/mol. Note that the second t ‐butyl group seems to be less efficient since the increase in proton affinity attains only 55 kJ/mol, this may be related to a larger steric repulsion between the two t‐butyl groups in the protonated form (Homan et al, ).…”

“…It will be shown that the basicities of cyclic carbonyl bases increase with the size of the ring. This phenomenon has been interpreted by the combination of two effects (Abboud et al, ; Bordeje et al, ; Homan et al, ; Alcami et al, ). First, the net positive charge located on the carbon atom in canonical structure 1H + b (Scheme ) is extending at the carbonyl substituents (particularly the hydrogen atoms).…”

Thermochemistry of neutral and protonated 1,7,7-trimethylbicyclo[2.2.1]heptan-2-thione (thiocamphor)

“…(5,30) Strictly speaking, this technique provides standard Gibbs energy changes for reaction (7), rather than the corresponding enthalpies. Fortunately, however, reliable and simple methods exist enabling the correction of the entropy effects.…”

“…Over the last few years one of the aims of our work on thermochemistry has been the quantitative assessment of the structural effects in the gas-phase of acyclic (1)(2)(3)(4)(5) and cyclic carbonyl compounds. (6)(7)(8)(9) The last decade has witnessed important developments in the field of thiocarbonyl chemistry, (10) but as a consequence of the high reactivity of thiocarbonyl compounds, [XC(S)Y], experimentally determined standard enthalpies of formation, f H o m for these compounds are extremely scarce.…”

Synthesis and structural studies of symmetric and unsymmetric adamantylmethyleneazines

“…This work presents some novelties which can be summarized as follows: (i) for the first time adamantylmethyleneazines have been synthesized; these azines can be used as starting materials for preparing diazapentalenes (criss-cross cycloaddition reaction), 35 2-pyrazolines 36 and many other heterocyclic compounds; 37 (ii) the first x-ray structures of azines bearing adamantyl groups have been reported; recently, there has been much interest in the structure of molecules containing an 1-adamantyl residue, such as Ad-CO-Ad 38 and Ad-NH-COCH 3 , 39 related to the plasticity of adamantane and its derivatives; 40 (iii) A set of novel …”