Aryl, rotations correlated

As expected, introduction of a second amino group on C in enamines with acceptors on lowers the barriers to C=C rotation. This can be ascribed to a better stabilization of the transition state, in which the carbocationic part assumes the character of an amidinium ion. Some typical barriers for l,l-bis(dimethylamino)ethylenes with acceptor groups in position 2 (24) are shown in Table 6. It is observed that, with a pair of good acceptors, the barriers become too low to be accessible with the NMR technique. With weaker acceptors, the order of the barriers follows the expected capacity of the acceptor groups to stabilize the carbanionic part of the transition state. Kessler has measured the C=C barriers in a number of p-substituted l,l-bis(dimethylamino)-2-cyano-2-phenylethenes (24a) and found that the logarithms of the estimated rate constants at 25 °C correlated well with the a values for the para substituents. Similar correlations with opposite signs for p and lower sensitivity were found for the C—N and C—aryl rotations. 

An elegant molecular-beam study of the photofragmentation of aryl halides and methyl iodide has permitted extraction of excited-state lifetimes from a measured anisotropy parameter which depends upon the lifetime of excited state, the rotational correlation time of the molecule, and the orientation of the electronic transition dipole with respect to the —X bond.38 The lifetimes obtained were methyl iodide 0.07 ps, iodobenzene 0.5 ps, a-iodonaphthalene 0.9 ps, and 4-iodobiphenyl 0.6 ps, from which it was concluded that, whereas methyl iodide dissociates directly, the aryl halides predissociate. A crossed-beam experiment using electron-beam excitation has yielded the results for the Si Tt intersystem-crossing relaxation time in benzene, [sHe]benzene, fluorobenzene, and... 

But we have a puzzle here since rotation around the aryl-CHP2 bond should be relatively unhindered, why does CP not couple to both aP and bP We will return to this question when we discuss the 2D phosphorus-phosphorus correlation experiment. 

Dynamic two-dimensional and NMR studies of the conformationally restricted amide iV-(l-naphthoyl)-d5-2,6-dimethylpiperidine reveal correlated rotation about the aryl-CO and amide bonds. 9-(l-Adamantylamino-methyl)-9,10-dihydroanthracene was prepared via condensation of 1-adaman-tylnitrile with 9-anthryllithium and its hydride reduction. An NMR study confirmed a boat conformation for the central ring, with the substituent of C9 in a pseudo-axial position. The structure of bis[(l,3)2-oxaadamantano]-18-crown-6 (I) and its potassium picrato complex were studied using NMR, X-ray structure analysis and molecular mechanics calculations. In the gas phase and in CDCI3 solution, in both I and the corresponding complex cation the most stable conformation of C2h symmetry is found. 

Obviously, in 2,4,6,8-substituted bispidine molecules there is a large steric hindrance for the rotation of the substituents. Cooperative effects in the restricted rotation of the aryl groups in 2,4,6,8-tetraphenyl-3,7-diazabicy-clo[3.3.1]nonanes have been described (165). The rotation of the 2,4-phenyl substituents seems to be correlated with that of the 6,8-phenyl groups. This emerges from large negative activation entropies for the rotation. 

The preference of this conformation (the barrier hindering the rotation of the p-XCgH — at X = Cl, CHj, CFj amounts to 8.5 kcal/mol., cf. also ° 0 is assumed to be due, at least partially, to the donor-acceptor interaction between the K-system of the aryl residue and the carbenium centre at Cjg in the electronic absorption spectra of the 9-p-X-phenyl-9,10-dimethylphenanthrenium ions the charge transfer bands have been revealed whose position correlates well with the ionization potentials of respective X-substituted benzenes. Judging by the NMR- C spectra, however, the extent of this interaction in the main state of ions is insignificant — the chemical shift of the Cjo atom nearly remains unchanged as the X substituent... 

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