Carbene, 28. 2,3,4‐Tri‐tert‐butylcyclobutadien‐1‐carbonsäure‐tert‐butylester

Abstract

Der Cyclopropenyl-diazoessigester 3 a -zuganglich aus Tri-tert-butylcyclopropenylium-bromid (1) und dem Quecksilber-bis(diaz0essigester) 2a -ist Ausgangsprodukt zur Herstellung der Titelverbindung 5a. Das photolytisch aus 3a erzeugte Carben 4a ist sowohl fur die Bildung des Cyclobutadiens 5a verantwortlich als auch fur das Entstehen der Acetylene 6 und 7a. Acetylenbildung wird auch bei der Blitzpyrolyse von 3a (Bildung von 6, 7a, 8 und 9) sowie 5a (Bildung von 6, 8 und 9) beobachtet. Das extrem sauerstoffempfindliche Cyclobutadien 5a (-16) zeigt ausgepragtes Cycloadditionsverhalten gegeniiber Dienophilen. Mit Acetylendicarbonsaure-dimethylester, Maleinsaureanhydrid, Cyclopentadien, Azodicarbonsaure-diethylester und 4-Phenyl-l,2,4-triazolin-3,5-dion werden Cycloaddukte isoliert (17, 18, 19, 20, 21). In die Reihe der DiazadihydroDewar-Benzole (23) gelangt man ebenfalls bei der Umsetzung der Cyclopropenyl-diazoester 3 mit dem Triazolindion; die Betaine 22 und 24 werden als Zwischenstufen dieser Reaktion angesehen. Carbenes, 28l) tert-Butyl 2,3,4-Tri-tert-butylcyclobutadiene-l-carboxylateThe cyclopropenyldiazoacetate 3a -available from tri-tert-butylcyclopropenylium bromide (1) and the mercury bis(diazoacetate) 2a -is the starting material for the synthesis of the title compound 5a. The carbene 4a, generated photolytically from 3a, is responsible for the formation of both the cyclobutadiene 5a and the acetylenes 6 and 7a. Acetylene formation is also observed in the flash pyrolysis of 3a (formation of 6 , 7 a , 8, and 9) and 5a (formation of 6 , 8 , and 9). The extremely air sensitive cyclobutadiene 5a (-16) shows a pronounced cycloaddition behaviour towards dienophiles. With dimethyl acetylenedicarboxylate, maleic anhydride, cyclopentadiene, diethyl azodicarboxylate and 4-phenyl-I ,2,4-triazoline-3,5-dione cycloadducts were isolated (17, 18, 19, 20, 21). The series of the diazadihydro Dewar benzenes is also entered by the reaction of the cyclopropenyldiazoacetates 3 with the triazolinedione; the betaines 22 and 24 arc regarded to be intermediates of this reaction.Kiirzlich haben wir gezeigt, da13 Cyclopropenylium-Kationen geeignete Elektrophile zur Substitution von Diazomethylverbindungen sind'); auf diesem Wege sind substituierte Diazomethylcyclopropene gut zuganglich 3,4), deren Chemie weitestgehend unerschlossen ist. In der vorliegenden Arbeit gehen wir hauptsachlich am Beispiel von 3a Teilaspekten der Frage nach, ob und inwieweit solche Diazoalkane einen Zugang in die Cyclobutadien-bzw. Tetrahedranchemie ermoglichen.

Rhombic Cyclobutadiene with a Boryl/Amino-Substitution Pattern: Boryl Group Migration Induced by Reaction with Water

“…The resonances of the ring carbon atoms were observed at 76 and 185 ppm for the boryl- and amino-substituted carbon atoms, respectively. At room temperature, these values are significantly shifted relative to those of the reported rectangular CBDs in solution (173.4–132.1 ppm). − The calculated negative charges on the boryl-substituted carbon atoms (−0.687/-0.688) may contribute to the high-field shifted resonance of them. The gauge-independent atomic orbital (GIAO) calculation at the M06-2 X /6-311+G­(2d,p) level of theory predicted chemical shifts for rhombic 1 at 87 ppm (B–C) and 211 ppm (N–C), which is in conformity with the experimental results.…”

“…A large number of theoretical and experimental studies on this fundamental class of molecules have revealed that CBDs do not adopt a square but rectangular structure in order to avoid an unstable antiaromatic electronic configuration with four π-electrons (Figure ) . Despite the high reactivity of most CBDs, some examples could be isolated after kinetic stabilization with suitable protecting groups. − It is even possible to replace one of the four bulky substituents in these CBDs by smaller substituents, e.g., ester, aryl, or sulfonyl groups . In contrast to the rectangular structure of CBDs protected by bulky substituents, the captodative (push–pull) effect of a combination of donor and acceptor substituents in an alternating order leads to the formation of a rhombic CBD structure .…”

4-π-Photocyclization of 1,2-Dihydropyridazines: An Approach to Bicyclic 1,2-Diazetidines with Rich Synthetic Potential

“…8 Nevertheless, two further reports describe the use of tetrasubstituted cyclobutadienes to give highly substituted versions of bicycles A. 9,10…”

“…Despite its efficiency, this approach is impractical on larger scales, since the required cyclobutadiene precursor (the corresponding iron tricarbonyl complex) is not commercially available, and its preparation is expensive and not atom-economical . Nevertheless, two further reports describe the use of tetrasubstituted cyclobutadienes to give highly substituted versions of bicycles A . , Alternatively, Feng and co-workers described two examples of the titanium-mediated conversion of tetrasubstituted 1,3-butadienes into tetrasubstituted versions of A in good yields, although the preparation of A itself was not reported. Finally, Altman and co-workers reported a single example of the 4-π-photocyclization of a 1,2-dihydropyridazine (Scheme , Method 2; PG = CO 2 Me) to give A in 61% yield, accompanied by a pyrrole side product (14% yield; vide infra) …”

Synthesen mit Cyclobutadienen, 13. Azapentafulvene aus 2,3,4‐Tri‐tert‐butylcyclobutadien‐1‐carbonsäureestern und Isonitrilen

“…Gleichzeitig bietet sich hier die Chance, die 1-Oxocyclopentadiencarbonsaureester 20a und/oder 21 unmittelbar aus 6a zuganglich zu machen. Ausgangsverbindungen: Die Cyclobutadiene 6a und b wurden aus Diazo(l,2,3-tri-tertbutyl-2-cyclopropen-l-yl)essigsaure-tert-butylester 31) bzw. -rnethylester3') im Gegensatz zu bisherigen Gepflogenheiten nicht photolytisch, sondern thermisch nach der in Lit?3) angegebenen Vorschrift hergestellt.…”