Zum Mechanismus der Cyclopropan‐„Walk”︁‐Umlagerung: Synthese und Eigenschaften von 2‐Diazopropan‐Aren‐Addukten; eine regiospezifische N₂‐Eliminierung

Abstract

The synthesis and the thermal and photochemical behavior of 2-diazopropane adducts of toluene 1 a, m-xylene 1 b, c, and Dewar benzene 2a. b are reported. At 60°C the thermal N2 elimination of the specifically deuterated compound 1 c proceeds regiospecifically leading to l,?,?-trimethy1-5-(trideuteriomethyl)-1,3,5-~ycloheptatriene (12c), whereas the photolysis of 1 c sensitized by benzophenone yields both regioisomeric cycloheptatrienes 12c and 13c in a 51:49 ratio. This finding excludes the symmetrical diradical 3c as an intermediate in the thermal Nz elimination of 1 c and suggests a novel pericyclic process for it. The formation of both regioisomeric cycloheptatrienes 25 b and 25c in a 74: 26 ratio in the thermolysis of the diazopropanetetradeuterio Dewar benzene adduct 2 b indicates that in this case a multi-step N2 elimination passing through the diradical intermediate 3 e competes with the concerted process. According to force-field calculations the course of the Nz elimination in la-e might be controlled by the conformational equilibrium A % B.Die Cyclopropan-,,Walk"-Umlagerung im Bicyclo[4.1.0]hepta-2,4-dien (Norcaradien) 4 + 5 verlauft mit Inversion am wandernden Kohlenstoff-Atom (si-ProzeB)") entgegen den Voraussagen von Woodward und Hoffmannlb). Derselbe stereochemische Verlauf wird auch bei den entsprechenden Umlagerungen der vinylogen Systeme Bicyclo[2.1. Olpent-Zen und Bicyclo[6.1.O]nona-2,4,6-trien beobachtet, der in diesen Fallen mit den Erwartungen fur einen Orbitalsymmetrie-kontrollierten ProzeB ubereinstimmt la! Ob diese Umlagerungen konzertiert oder zweistufig uber intermediare Diradikale ablaufen, 1a13t sich anhand der Ergebnisse stereochemischer sowie auch kinetischer Untersuchungen nicht ohne weiteres entscheiden l). Wir versuchen daher, die moglichen Diradikal-Zwischenstufen auf unabhiingigem Weg zu generieren'). Hier berichten wir uber die Synthese, Schema 1 ' -R 3 4 5 Thermolyse und Photolyse der 2-Diazopropan-Addukte von Toluol, m-Xylol und Dewar- Benzol 1 a -1 c und 2a, 2 b, die geeignete Vorstufen von Diradikalen des Typs 3 sindden potentiellen Intermediarprodukten der Norcaradien-"Walk"-Umlagerung 4 + 5. Die rnit Hilfe des spezifisch deuterierten m-Xylol -Diazopropan-Addukts 1 c entdeckte, uberraschende Regiospezifitat der thermischen N2-Eliminierung legt hierfiir einen neuartigen pericyclischen ProzeR nahe. Synthese der Diazopropan -Toluol-undrn-Xylol-Addukte 1 a -1 c Da Benzol und seine Alkyl-substituierten Derivate zu reaktionstrage sind und nicht rnit Diazoalkanen im Sinne einer 1,3-dipolaren Cycloaddition reagieren, schienen uns die Dihydrobenzoesiureester 8a3) und 8 b geeignete Ausgangsprodukte fur die Synthese von 1 a -1 c zu sein. Im Gegensatz zu 8a lie13 sich 8b nicht einfach durch Diels-Alder-Reaktion von Pentadien 6 mit Propiolsaure-methylester herstellen, da diese uberwiegend zu dem ortho-disubstituierten Addukt 9 fuhrt4"). Das gewiinschte Produkt 8b war auf dem von Danishefsky et al.4b) fur andere meta-disubstituierte Cyclohexadien-Derivate beschriebenen Weg uber die Diel...

Reactions of C2ν-symmetrical C60 pentakis-adducts with diazomethane: regioselective formation of hexakis- to octakis-adducts and mechanism of methanofullerene formation by addition of diazomethane followed by dinitrogen extrusion

“…The preferred formation of the thermodynamically less stable product suggested that thermolysis proceeded under kinetic control, and one of us (F. G. K.) recognised the close analogy of the N 2elimination of diazomethane-fullerene adducts to 6-5-open methanofullerenes to the previously reported N 2 -elimination from diazoalkane-toluene adducts (Scheme 2). 8 Starting from 7, thermolysis provides in an orbital symmetry controlled [p 2 s + p 2 s + s 2 s + s 2 a] rearrangement with high regioselectivity cycloheptatriene 8, resulting from ring-opening of the intermediate norcaradiene. In contrast, photolysis of 7 presumably proceeds via a diradical mechanism, leading to a mixture of 8 and isomeric 9.…”

Heavier Main Group Dimetallene Reactivity: Effects of Frontier Orbital Symmetry

“…One notable example is the cyclotrimerization reaction between 2 equiv of maleic anhydride and acetylene to give a product with a bicyclo[2.2.0]hexane structure (Scheme ). This reaction is induced by photochemical activation and occurs well below room temperature . Similar examples in the literature highlight that the reaction can proceed as described; however, the selectivity for this cyclization route is highly variable, as seen by the formation of five stereoisomeric products in the reaction of dimethyl acetylenedicarboxylate with dichloroethylene …”

“…This reaction is induced by photochemical activation and occurs well below room temperature. 21 Similar examples in the literature highlight that the reaction can proceed as described; however, the selectivity for this cyclization route is highly variable, as seen by the formation of five stereoisomeric products in the reaction of dimethyl acetylenedicarboxylate with dichloroethylene. 22 Notably, C−H activation and 1,2-addition reactions were observed between ArEEAr (E = Ge, Sn; Ar = terphenyl) and cyclopentene, a cyclic internal olefin (Scheme 9).…”

“…Scheme 8. [2 + 2 + 2] Cyclotrimerization of an Alkyne and 2 equiv of an Activated Alkene To Furnish a Product with a Bicyclo[2.2.0]hexane Structure21,22 …”