The oxime ethers (7) and (8) (R = alkyl or aryl) undergo base-catalysed elimination to benzonitriles (3) in waterdioxan (4 : 1 ) at 25". The Z-isomer (82) reacts via hydroxide-catalysed antiperiplanar elimination 70-fold more rapidly than the corresponding €-isomer. Electronic effects on syn-elimination (from 7E) show that electronwithdrawing groups aid elimination in both Ar and in the leaving group -OR ; no intramolecular assistance is observed in these substrates. These results are interpreted in terms of a central €2 elimination, with appreciable C-H and N-Obond cleavage in the transition state. 2) is a reaction which permits the transformation of an aldehyde (1) to the corresponding nitrile (3) under mild conditions and in high overall yield.1*2 Aldoximes (2) themselves can rarely be dehydrated in the presence of basic reagents, being usually converted into their corresponding anionse3 Exceptions to this are the observed eliminations in dilute alkaline solution when the methine hydrogen in (2) is activated, e.g. Ar = O-N02C6H4.394 THE dehydration of aldoximes (The most widely used reagent for effecting the dehydration (2) + (3) is acetic a n h ~d r i d e . ~, ~

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.

Durch Trimerisierung der Acetylene (I) werden die Hexaalkyl‐benzole (II) hergestellt, durch Cotrimerisierung von (Ia) und (Ic) bzw. (Ib) und (Ic) werden die gemischt substituierten Derivate (III) und (IV) bzw. (V) und (VI), aus dem unsymmetrischen Acetylen (VII) das unsymmetrische (VIII) und das symmetrische Trimere (IX) erhalten.

No abstract

Die (E)‐Oximether (I), die aus den entsprechenden Aldehyden und Aryloxyaminen dargestellt werden, ergeben ebenso wie ihre (Z)‐Isomeren bei ihrer basenkatalysierten Eliminierung in wäßrigem Dioxan (7:3) mit Geschwindigkeitskonstanten von (1 .52‐61700)‐ 1O′5 l/mol‐s die Nitrile (II).