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Coupling reactions of alkynols

Transition metal catalyzed couplings of alkynes provide efficient routes to a large variety of unsaturated linear compounds such as butenynes or hexadienynes as well as aromatic and non-aromatic iso- and heterocycles. Alkynols R-C  C-CR’2OH have, however, only been used with very limited success in such reactions despite the great potential of any hydroxy-substituted products toward further derivatization. This is most probably due to the coordinating ability of the hydroxyl functional group which interferes with most cyclization catalysts used to date. We are presently investigating the potential of {(h6-arene)Ru}2+ templates in such reactions. This 12 valence electron entity is isoelectronic to the CpCo+ or CpRh+ fragments which are hallmarks in cyclotrimerization and co-cyclization chemistry. An advantage of the ruthenium systems is their lower price compared to rhodium and their high tolerance toward a broad variety of oxygen containing functional groups, including the hydroxyl one. During our studies we found evidence for the cyclotrimerization of disubstituted alkynols to isomeric 1,3,5- and 1,2,4-trisubstituted benzenes C6H3(CR’2OH)3. In the presence of the tetraphenylborate counter anion we observe the stoichiometric coupling of a phenylate and two equivalents of the alkynol to unprecedented h5-1-methylene-1,2-dihydronaphthalenide ligands, which remain coordinated to the ruthenium center (Figure 1). The structure of this novel ligand was unambiguously assigned by multinuclear NMR, including various correlation methods (cooperation with Prof. D. Gudat).
scheme
Figure 1. Novel 1-methylene-dihydronaphtalenide ligands by cocylization of a phenylate group and two equivalents of an alkynol.

Further studies into the reaction mechanism including utilization of perdeuterated BPh4- -d20 showed, that the phenylate unit originates from the tetraphenylborate ion. This provides a rare example of the BPh4- anion functioning as a phenylating agent. These studies also prove that the water molecule lost in the final dehydration step involves at least one D-atom from the phenylate label. A plausible reaction sequence is presented in Scheme 1.
scheme
Scheme 1. Proposed reaction sequence in the formation of the 1-methylene-2,3-dihydro naphthalenide ligands.

In the first step the arene ruthenium dichloro dimer is phenylated by the BPh4- counter ion. Addition of the alkyne, chloride loss and the insertion of the alkynol into the ruthenium-phenyl bond give intermediate B. Intermediate C is then formed by coordination of another equivalent of the alkynol and its insertion into the ruthenium vinyl bond. Intermediate C then reacts via electrocyclic ring closure to D. Allylyic 1,3 H shift and final dehydration complete the reaction. Interestingly, all addition/insertion steps occur with high regioselectivity. Computational studies to gain further insight into the reaction sequence, the intermediates and the energetics of this process are presently underway (cooperation with Prof. Martin Brészka). Preliminary studies show, that modification of the arene ligand provides different coupling products. This work is presently funded by the DFG. 

Reference:

J. Čubrilo, R. F. Winter, D. Gudat
Coupling of alkynols and a phenyl group to a novel h5-dihydronaphthalenide ligand on a ruthenium template
Chem. Comm., 2005, 510.




     
     
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