solvent is then removed under vacuum and the residue taken up in benzene, filtered and crystallized from pentane/toluene. 0.68g ZnC12 affords 2.7g (69%) (3a) [m.p. 77"C], while 0.92g CdC12 affords 2.848 (65%) ( 3 b ) [m.p. 94"C], and 1.41 g (Me3P)2NiC12 affords 3.0g (78 %) ( 3 c ) [m.p. 90°C

Die in 1,2-Stellung mit den unterschiedlichsten Resten R substituierten Benzocyclobutadiene 3 a-e wurden aus den entsprechenden trans-konfigurierten Diacetylenen 1 a-e bei 110-190°C erzeugt. Wahrend die Derivate 3a, b, d bei diesen Temperaturen rasch dimerisierten, waren die Kohlenwasserstoffe 3 c, e stabil. Unter bestimmten Bedingungen konnten auch 3 b und 3d als Monomere gewonnen werden. Der Reaktionsmechanismus der Synthese von 3 aus 1 wurde geklart.Konstitution und Konfiguration der teilweise isolierbaren Neben-und Zwischenprodukte (4 b -e bzw. Stable Derivatives of 3,4,5,6-TetramethylbenzocyclobutadieneThe benzocyclobutadienes 3 a -e with different groups R at positions 1,2 are generated by heating the corresponding trans-configurated diacetylenes 1 a -e to 110-190 "C. Whereas the derivatives 3a, b, d dimerize rapidly at this temperatures, the hydrocarbons 3c, e are stable. 3b and 3d are obtained as monomers, too, under certain conditions. The mechanism of the synthesis of 3 from 1 is explained. The constitution and configuration of partly isolated side products 4 b -e and inter-

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).

No abstract

Die beiden konfigurationsisomeren Kohlenwasserstoffe 1 und 2 sind bei Raumtemperatur stabil. Beim mehrstiindigen Erhitzen in siedendem Cyclohexan erleidet das cis-Isomere 2 eine elektrocyclische, konrotatorische Ringoffnung und geht vollstandig in cis-truns-3,4,5,6-Tetramethyl-l &diphenyl-3,5-octadien-l,7-diin (3) uber. Das frunsIsomere 1 zeigt dagegen ein wesentlich komplizierteres thermisches Verhalten, das hier zusammenfassend behandelt werden soll.