Alkynyldicobalt Derivatives of Dibenzosuberenol and Dibenzocyclooctatrien‐5‐ol: Ring Conformations, Ease of Carbonyl Elimination and Relevance to Pauson–Khand Cyclization
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abstract
Under very mild conditions, (5‐alkynyl‐5H‐dibenzo[a,d]cycloheptatrien‐5‐ol)Co2(CO)6 clusters 1 lose a carbon monoxide ligand and form the corresponding η2‐alkene‐µ‐alkyne‐Co2(CO)5 complexes 2 (the first isolated examples of the initial phase of the Pauson–Khand reaction). This behaviour is not paralleled by the homologue in which an additional methylene group has been inserted within the seven‐membered ring, as in the [5‐alkynyl‐6H‐dibenzo[a,e]cyclooctatrienol]Co2(CO)6 system 8, for which several X‐ray crystal structures are reported. The apparent additional flexibility of this molecular framework does not translate into conformations appropriate for the formation of η2‐alkene‐µ‐alkyne‐Co2(CO)5 complexes. The relative inertness of the ethynyl complex 1a, compared with the ease of carbonyl loss in the trimethylsilylethynyl‐ and the tri(isopropyl)silylethynyl‐substituted hexacarbonyldicobalt clusters, 1b and 11, respectively, is discussed in terms of out‐of‐plane angle strain within the dibenzo[a,d]cycloheptatrienyl moiety. Comparison of the changing conformations of the seven‐membered ring in a series of free and complexed molecules reveals a marked flattening of the organic framework, as bulkier substituents are incorporated at C5. The possibility of η2‐alkene‐µ‐alkyne‐Co2(CO)5 clusters functioning as hemilabile systems was explored by monitoring the ease of carbonyl substitution by triphenylphosphine in 2b (alkynyl substituent C≡CSiMe3).
