Carbonyl compounds, rotational barriers

Due to a partial 77-character, aromatic carbonyl compounds have an activation energy barrier for rotation around the phenyl-carbonyl bond, the value of which is substantially increased upon protonation.44 In para-anisaldehyde a second protonation of the methoxy group will drastically decrease their barrier. The temperature-dependent NMR spectrum will reflect both exchange processes, intra- and intermolecular, as shown in Scheme 1.1. 

Nitrogen - carbonyl rotational barrier in (1) is greater than 20 kcal/mole (2 ). the corresponding barrier in compounds... 

H3SiOCSN(CH3)2 the free rotation barrier differs only slightly from that of H3SiOCON(CH3)2 and no magnetic equivalence is observed even at +60 °C. Therefore, an intramolecular interaction between the silicon atom and the carbonyl oxygen in the first compound has been su estai The fact that no broadened signals... 

Enones. Simple a,/3-unsaturated carbonyl compounds also show thermodynamically a preference for the s-trans conformation. Acrolein has a smaller difference in energy than butadiene between the s-trans 2.134a and s-cis 2.134b conformations of 7 kJ mol-1 (1.7 kcal mol ), but a similar barrier to rotation of... 

The preferred conformations of carbonyl compounds, like 1-alkenes, are eclipsed rather than bisected, as shown below for ethanal and propanal. The barrier for methyl group rotation in ethanal is 1.17kcal/mol. Detailed analysis has indicated that small adjustments in molecular geometry, including a-bond lengthening, must be taken into account to quantitatively analyze the barrier. The total barrier can be dissected into nuclear-nuclear, electron-electron, nuclear-electron, and kinetic energy (At), as described in Topic 1.3 for ethane. MP2/6-311+G (Mf,2p) calculations lead to the contributions tabulated below. The total barrier found by this computational approach is very close to the experimental value. Contributions to the ethanal energy barrier in kcal/mol are shown below. 

General.— The heats of formation of aliphatic ketones have been further studied, and an improved force-field method for the calculation of the structures and energies of carbonyl compounds has been described. Ab initio calculations on acetone show that the methyl groups have hydrogen eclipsing oxygen in the ground state, and that the rotational barrier is 0.99 kcal mol . ... 

Coldham and coworkers have shown that the asymmetric deprotonation protocol can be used to regioselectively alkylate the 5-position of imidazolidines (Scheme 35). The process is used as part of a sequence that results in asymmetric alkylation of 1,2-diamines with high stereoselectivity. The yields are limited, in this case, by the barrier to rotation around the carbamate C—N bond. Thus, only the amide rotamer having the carbonyl group syn to C-5 of the heterocycle is deprotonated. There are several examples in this review where this limitation is possible whether it is a factor or not may depend on the temperature at which amide bond rotation occurs versus the stability of the organolithium compound. In this case, the barrier to amide bond rotation was determined as 16.6 kcalmoD at 60 °C. 

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