Development of Catalytic Pauson-Khand Reaction Using Ruthenium Complexes

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Metal-mediated cocyclization of alkynes, alkenes and carbon monoxide, the Pauson-Khand reaction, becoming increasingly popular as a tool for selective organic synthesis. We previously reported the [2+cycloaddition of norbornenes with alkynes catalyzed by ruthenium complexes, in which we propose ruthenacyclopentene intermediate. If carbon monoxide is inserted into this ruthenacyclopentene, a cataly version of the Pauson-Khand reaction can be achieved. After many trials, we finally found the first example a ruthenium-catalyzed Pauson-Khand reaction of enynes with carbon monoxide. For example, treatment diethyl 8-nonen-3-yne-6,6-carboxylate with 2 mol% RuィイD23ィエD2(CO)ィイD212ィエD2 in N, N-dimethylacetamide under 15 atm carbon monoxide pressure at 140℃ for 8 h gave the corresponding bicyclic cyclopentenone, diethyl 2-eth3-oxobicyclo[3.3.0]oct-1-ene-7,7-dicarboxylate, in 89% isolated yield.On the other hand, during our effort extend the present reaction to intermolecular version of Pauso Khand reaction, we have found novel rethenium-catalyzed cross-carbonylation of alkynes and alkenes unsymmetrically substituted hydoroquinons, in which two molecules of carbon monoxide were incorporate into the products. For example, treatment of 4-octyne and 2-norbornene with 2 mol% RuィイD23ィエD2(CO)ィイD212ィエD2 in methylpiperidine under 60 atm of carbon monoxide at 140℃ for 20 h gave the corresponding hydroquinor 4,5-dipropyltricyclo[6.2.1.0ィイD12,7ィエD1]undeca-2(7),3,5-triene-3,6-diol, in 85% yield. This reaction apparently involve a maleoylruthenium intermediate which is generated by the reaction of an alkyne and two molecules of carbon monoxide on the rethenium.In coclusion, both reaction will open up new chemistry in the field of ruthenium-catalyzed carbonylation reactions, and offer the novel and practical methods for preparation of bicyclic cyclopentenon and unsymmetically substituted hydroquinones, respectively.