Bildung eines metallorganischen Ladderan‐Derivats durch eine dynamische topochemische Reaktion

Abstract

Professor Manfred Christl zum 65. Geburtstag gewidmetTopochemische Reaktionen eröffnen einzigartige Möglich-keiten für die Kohlenstoff-Kohlenstoff-Verknüpfung.[1] Typische Beispiele sind die Bildung von Cyclobutanen und Ladderanen [2, 3] (anellierten Oligocyclobutanen) durch photochemisch induzierte [2+2]-Cycloadditionen zwischen substituierten Alkenen, die in einem Kristallgitter, [4] an starren organischen Gerüsten [5] oder durch Assoziation über Wasserstoffbrücken [6] in einer geeigneten räumlichen Orientierung vorliegen. Die Bildung von Cyclobutanringen durch photochemische [2+2]-Cycloaddition wird oft dann beobachtet, wenn sich die beteiligten p-Systeme nahe genug sind ( 3.6 ) -bei größerem Abstand der einzelnen Oligo-enKetten im Kristall oder in Lösung findet diese Reaktion nicht mehr statt.[5] Eine Alternative könnte sich dadurch eröffnen, dass man Butadienylsubstituenten an den Cyclopentadienyl(Cp)-Ringen eines gewinkelten Metallocens anbringt (V < 1808, siehe Schema 1). In solchen konformativ flexiblen Systemen [7] könnten die Dienylsubstituentenpaare einander nahe genug kommen, um Cyclobutane oder gar Ladderane durch photochemische [2+2]-Cycloadditionen zu bilden.Ausgehend von Arbeiten zu den verwandten ansa-Metallocenen [8] gelang uns jetzt die Bildung eines metallorganiSchema 1.

Zirconocene Functional Group Chemistry: Photochemical [4+4] Cycloaddition of Isoprenyl Sidechains to the Bent Metallocene Framework

“…2:1) plus some unidentified decomposition products. The structure of 11a was tentatively assigned to the ladderane structure in comparison with the previously described analogous product obtained by low-temperature photolysis of the parent compound 1 [13] Complex 11a features a typical 1 H NMR ABCD pattern of the Cp-ligands and 1 H/ 13 C NMR signals of the newly formed tricyclic ansa-bridge at δ = 4.06 (6-H), 3.09 (7-H), 2.14/2.12 (9-H) and 1.08 (8-CH 3 ) [ 13 C: δ = 46.9 (C7), 45.0 (C8), 44.5 (C6), 32.1 (C9), 16.5 (8-CH 3 )].…”

“…Elemental analyses: Foss-Heraeus CHNORapid. NMR: Varian UNITY plus NMR spectrometer ( 1 H: 600 MHz, 13 C: 151 MHz). Assignments of the resonances were supported by 2D NMR experiments.…”

“…This reaction type proved to be somewhat substituent-sensitive but use of a proper substitution pattern at the alkenyl groups allowed very efficient ansa-metallocene formation by the photochemical [2+2] cycloaddition route. [7][8][9][10][11][12] Photolysis of bis(butadienyl-Cp)ZrCl 2 systems takes a similar route. At ambient temperature this reaction rapidly and selectively leads to the formation of the isomeric cyclooctadienylene-bridged ansa-metallocenes by formal [4+4] cycloaddition.…”

“…At ambient temperature this reaction rapidly and selectively leads to the formation of the isomeric cyclooctadienylene-bridged ansa-metallocenes by formal [4+4] cycloaddition. A detailed mechanistic study [13,14] had revealed that this reaction proceeds stepwise; the initial step is a [2+2] cycloaddition leading to the intermediate 2a. Sub-sequent conformational equilibration -monitored by dynamic NMR spectroscopy -eventually provides a suitable situation for the final thermal Cope rearrangement of the cis-1,2-divinylcyclobutane substructure [15] , which yields the final product (see Scheme 1).…”

Functional Group Chemistry at the Zirconocene Backbone: Addition Reactions to Pendant Alkynyl Substituents

“…An increasing number of examples are now known where pre-attached groups allow functional-group interconversions at the bent metallocene complex stage. This development includes addition reactions [6] but has also allowed a variety of important and useful carbon-carbon coupling reactions to be performed at the metallocene backbone such as the formation of novel ansa-metallocenes by intramolecular olefin metathesis (RCM), [7,8] photochemical [2+2] cycloaddition reactions, [9,10] and even by a variant of the Mannich reaction. [11] We have now extended this work to the chemistry of alkynyl-substituted zirconocenes and report here the syn-coupling reaction went smoothly, and product 2a was isolated in 77 % yield.…”