Isolation of 3,4,5,6‐Tetramethyl‐1,2‐diphenylbenzocyclobutadiene

Abstract

study ( < -30"C)1111. This is best evidenced by the upfield shift of protons H' (by 23Hz) and H5 (by 32Hz) on going from ( 3 ) to ( 5 ) , which simply reflects the increased availability of the lone pair for limited delocalization but is clearly inconsistent with the development of the diamagnetic ring current expected of this system. Obviously, the present findings indicate that neither 3-benzazonine ( 3 ) , at ambient temperature, nor the corresponding base ( 5 ) , generated at ca. -70 "C and observed at -35 'C[''], are capable of sustaining a ring current, ie., they are no more than benzopolyenes under the experimental conditions employed here.Interestingly the behavior of (3) and ( 5 ) sharply contrasts with that of their iso-x-electronic carbocycle, the benzocyclononatetraenyl anion (6) which was recently shown['21 to possess a well delocalized 14x frame exhibiting strong ring diamagnetism over a wide temperature range (-55 to + 35 "C).

Stereocontrolled preparation of chiral (E)-1-alkenyl sulfoxides. Efficient reduction of alkenyl sulfoxides to the corresponding alkenyl sulfides

“…Reaction of Grignard reagents with chiral sulfinate esters is one of the oldest and most often used procedures for preparation of chiral sulfoxides. 4 Harpp has recently summarized this area and has emphasized that a major byproduct in this type of reaction is often the sulfide derived from the initially formed sulfoxide. 5 Harpp recommends general use of organocopperlithium reagents for conversion of sulfinate esters into the corresponding sulfoxides with formation usually of only small amounts of sulfides.…”

Benzocyclobutadienes. 4. 1,2-Di-tert-butyl-3,4,5,6-tetramethylbenzocyclobutadiene

“…*Part 1: Winter & Straub (1978); part 2: Straub, D6ring & Winter (1979); part 3: Butters, Toda & Winter (1980). The compound (1) with R = tert-butyl is remarkably stable at room temperature in the crystalline state (Straub, 1976) and in a preliminary communication (Winter & Straub, 1978) we have reported our X-ray results on this derivative. The observed bond lengths in (1) rule out any noticeable participation of the resonance structure (1 C) and the benzocyclobutadiene ground state is accordingly best described in terms of the two resonance structures (1A) and (1B).…”

“…Although the parent cyclobutadiene molecule was isolated in 1965 (Emerson, Watts & Pettit, 1965), progress in benzocyclobutadiene chemistry was much slower and the first well characterized benzocyclobutadiene (1) (R = Ph) was reported by Straub (1974…”

Benzodisilacyclobutadienes: 8π‐Electron Systems with an Antiaromatic Silicon Ring

“…[1] Even though the parent benzocyclobutadiene A (Figure 1) readily dimerizes,e ven at temperatures as low as 75 K, [2] A can still be observed by fast-flow NMR spectroscopy techniques. [3] Thei ntroduction of sterically demanding substituents allowed the isolation of benzocyclobutadienes B-D. [4][5][6] According to these experimental and theoretical studies, [7] the diamagnetic ring current on the benzene rings of the benzocyclobutadienes is reduced owing to the paramagnetic ring current of the fused cyclobutadiene moieties. Progress in main-group-element chemistry has meanwhile enabled the incorporation of heavier Group 14 elements into neutral cyclobutadienes and their charged (dianionic and dicationic) derivatives.…”