Conversion of Allylic Alcohols to Carbonyl Compounds Catalyzed by Alkoxy-Bridged Dinuclear Areneruthenium Complexes

Visible light irradiation of an azobenzene-bridged dinuclear ruthenium complex ligated by a η⁶-arene group having a pendant alkoxy ligand, [Ru<INF>2</INF>{η⁶:η¹:μ-C<INF>6</INF>H<INF>5</INF>(CH<INF>2</INF>)<INF>3</INF>O}<INF>2</INF>(μ-C<INF>6</INF>H<INF>5</INF>N&dbd;NC<INF>6</INF>H<INF>4</INF>OMe)]BF<INF>4</INF>, led to generation of an active species capable of catalyzing isomerization of allyl alcohols to the corresponding carbonyl compounds under mild conditions. Treatment of the chloro-bridged dinuclear complex [Ru<INF>2</INF>{η⁶:η¹:μ-C<INF>6</INF>H<INF>5</INF>(CH<INF>2</INF>)<INF>3</INF>O}<INF>2</INF>(μ-Cl)]BF<INF>4</INF> with 1 equiv AgOTf or that of the methoxy-bridged complex [Ru<INF>2</INF>{η⁶:η¹:μ-C<INF>6</INF>H<INF>5</INF>(CH<INF>2</INF>)<INF>3</INF>O}<INF>2</INF>(μ-OMe)]BF<INF>4</INF> with HOTf also resulted in generation of catalyst for the isomerization, while these dinuclear complexes themselves were inactive as the isomerization catalyst. Reaction of the hydride-bridged dinuclear complex [Ru<INF>2</INF>{η⁶:η¹:μ-C<INF>6</INF>H<INF>5</INF>(CH<INF>2</INF>)<INF>3</INF>O}<INF>2</INF>(μ-H)]BF<INF>4</INF> with methyl vinyl ketone gave the allyloxy-bridged dinuclear complex [Ru<INF>2</INF>{η⁶:η¹:μ-C<INF>6</INF>H<INF>5</INF>(CH<INF>2</INF>)<INF>3</INF>O}<INF>2</INF>(μ-OCHMeCH&dbd;CH<INF>2</INF>)]BF<INF>4</INF> rather sluggishly, while the same reactants in the presence of an equimolar amount of HOTf rapidly gave CH<INF>3</INF>COCH<INF>2</INF>CH<INF>3</INF> and the triflate complex [Ru<INF>2</INF>{η⁶:η¹:μ-C<INF>6</INF>H<INF>5</INF>(CH<INF>2</INF>)<INF>3</INF>O}<INF>2</INF>(μ-OTf)]BF<INF>4</INF>. A mechanistic scheme for the isomerization of allyl alcohols involving the dinuclear acceptor [Ru<INF>2</INF>{η⁶:η¹:μ-C<INF>6</INF>H<INF>5</INF>(CH<INF>2</INF>)<INF>3</INF>O}<INF>2</INF>]²⁺ as a key species has been discussed.