η⁶‐(1‐oxobenzocyclobutene)tricarbonylchromium(0): Preparation, reduction, and ring‐opening/cycloaddition chemistry of the 1‐hydroxybenzocyclobutene complex

Abstract

The syntheses of the tricarbonylchromium complexes 2 and 3 of 1‐oxobenzocyclobutene and 1‐hydroxybenzocyclobutene are described. Alcoholate 6 of 3 undergoes a ring opening to the corresponding ortho‐quinodimethane intermediate between −30 and −20°C, which is trapped by dienophiles with high diastereoselectivity. While with most dienophiles products with a 1,2‐cis configuration (7, 8, 10) are formed, vinyl sulfones react in an exo‐selective cycloaddition with formation of 1,2‐trans‐configurated cycloadducts (12, 13). The configuration of the cycloadduct with phenyl vinyl sulfone is confirmed by an X‐ray crystal structure analysis. As one of the important aspects of the chemistry of benzocyclobutene complexes is the chirality of compounds like 2 and 3, the enantiomeric resolution has been achieved via the semioxamazones of ketone 2.

The First Benzo[1,2:4,5]dicyclobutenones and Their Tricarbonylchromium Complexes

“…The configurations were assigned with regard to the corresponding reactions of (benzocyclobutenone)tricarbonylchromium and related compounds. [31,32,35] …”

“…[31,32] Because the issue of diastereoselectivity is even more relevant for the double reductions of the dione complexes 18 and rac-20 we chose the less reactive, usually more selective, diisobutylaluminum hydride (DIBAL-H) as the reducing agent. Treatment at -78°C followed by acidic workup afforded the syn diols 21 and rac-22 in 90 % and 96 % yield, respectively.…”

“…Treatment at -78°C followed by acidic workup afforded the syn diols 21 and rac-22 in 90 % and 96 % yield, respectively. The relative configurations were assigned with regard to that of the reduction of (benzocyclobutenone)tricarbonylchromium [31,32] and the stereochemical outcomes of nucleophilic attack at various (arene)tricarbonylchromium complexes with anellated cyclopentanone or cyclohexanone moieties [33] and related substrates. [34] The higher reactivity of lithium aluminum hydride as the reducing agent results in decreased selectivity, with formation of unseparated diastereoisomeric mixtures of 21 (69 %) and other diols (20 %) from 18 and of rac-22 (68 %) and other diols (24 %) from rac-20 as indicated by the NMR spectra, which, however, showed substantial signal overlap.…”

Addition and Ring Expansion Reactions of Tricarbonyl(1,2-dioxobenzocyclobutene)chromium(0) with Carbon Nucleophiles – Unexpected Formation of Benzocycloheptene Derivatives and the First Head-to-Head Coupling of Two Methoxyallene Molecules

“…[10] [11] [15] [16] [17] However, in addition to this dismate [6] in 1965 and in enantiopure form ten years later. [7] tal ring opening a proximal ring opening is frequently obThe transfer of the planar chirality of the complex to the served, which, less interestingly, usually leads to phennovel asymmetric centre by diastereoselective alkylation of ylacetic acid derivatives.…”

“…[7] tal ring opening a proximal ring opening is frequently obThe transfer of the planar chirality of the complex to the served, which, less interestingly, usually leads to phennovel asymmetric centre by diastereoselective alkylation of ylacetic acid derivatives. [11] [18] The most important reaction the enolate from the anti face represents now a textbook of the 1,2-dioxobenzocyclobutene complex 4 is the double example for the use of tricarbonylchromium complexes in nucleophilic addition to both keto groups which usually stereoselective organic synthesis. [8] [9] leads to decomposition products when uncoordinated 1,2-dioxobenzocyclobutene is used.…”