Conformations and internal mobility of side chains in heterosubstituted hexaalkylbenzenes. Isopropyl/ethyl and isopropyl/cyclopropyl systems

Abstract

Variable-temperature NMR spectra of hexaalkylbenzenes containing both isopropyl and ethyl or cyclopropyl groups, in conjunction with empirical force-field calculations, have shown that the alkyl groups in these heterosubstituted benzenes retain the conformation they individually adopt in the homosubstituted analogues: isopropyl groups maintain a bisected conformation, whereas ethyl or cyclopropyl groups adopt a perpendicular one. Two site-exchange processes have been observed in heterosubstituted benzenes containing vicinal isopropyl groups: a low-energy process, which leads to fast rotation of all alkyl groups except for the isopropyl groups, and a high-energy process that corresponds to the unfreezing of the gear-locked isopropyl groups. The magnitude of the barriers for both processes has been determined for 1,2-diethyl-and l,2-dicyclopropyl-3,4,5,6-tetraisopropylbenzene and for l,2-diisopropyl-3,4,5,6-tetracyclopropylbenzene.Homosubstituted hexaalkylbenzenes of the type C6-(Cab2)6 are found to follow two conformational patterns, conveniently characterized by the values of the a-C-C^-Ct orsion angles ( ) as perpendicular ( = 90 ± 30°) or eclipsed { -0 ± 30°); the latter is also called bisected with reference to the bCb angles (Figure l).2 In the perpen-(1) (a) Max Kade Postdoctoral Research Fellow, on leave from the Universitát Wien, (b) Government of India Scholar, on leave from Manipur University.

Influence of the spatial distribution of copper sites on the selectivity of the oxygen reduction reaction

“…We anticipate that the even more bulkier Cu-bmpa substituent will not lower this energy difference between the various conformers, and therefore, it is expected that the alternating up-down conformation is also predominantly adopted by Cu 3 L2 in solution. [59][60][61][62][63] For Cu 3 L1 the rotation of the Cu-bmpa substituents around the benzene node will be less prohibited due to the absence of the ethyl groups. Therefore, the distribution between the conformers is expected to be much more random for Cu 3 L1.…”

Three-Membered-Ring-Based Molecular Architectures

“…457 For comparison, in 1,2-diisopropyl-3,4,5,6-tetracyclopropylbenzene, which was prepared by cotrimerization of diisopropylacetylene and dicyclopropylacetylene (565) under catalysis with Hg[Co-(CO) 4 ] 2 , the four cyclopropyl groups adopt the same conformation with C ar -C ar -C cycl -H torsional angles close to 90°, whereas the two isopropyl groups maintain a bisected orientation. 458 Upon careful reinvestigation of the thermal reaction of 565 with Fe 3 (CO) 12 under almost the same conditions as previously reported 456 (4 h of heating), the known 455 tetracyclopropyl-p-benzoquinone (650) (3%) and the previously unknown tricarbonyl(tetracyclopropylcyclobutadiene)iron (651-H) (1%), along with the previously described products 648 (48%) and 649 (18%), were isolated (Scheme 113). 459 Under the same conditions, the reaction of bis(1-methylcyclopropyl)ethyne (621) with Fe 3 (CO) 12 yielded neither any of the peralkylated cyclopentadienone complex nor the corresponding benzene or benzoquinone derivatives.…”

Stereochemistry of Polyethylated Aromatic Systems

“…The substituents, however, in the latter case (phenyl groups) possess bilateral symmetry and, in contrast to an ethyl group, no isomerism re- Scheme 4 sults from a 180°rotation around the bond connecting the substituent to the central frame. [17] Although 1, or structurally related compounds [10,18] and their organometallic complexes, [19Ϫ22] have been studied extensively, when we started our work we were unaware of any stereochemical study of systems possessing two (or more) polyethylphenyl subunits.…”