An Unusual Ruthenium‐Catalyzed Cycloisomerization of Alkynes and Propargyl Alcohols.
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Cycloisomerization
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Propargyl alcohol
[reaction: see text] 1,6-Enynes and allyl propargyl ethers undergo a cobalt-catalyzed formal 5-endo-dig cyclization to form vinyl cyclopentenes and dihydrofurans, respectively. The use of equimolar amounts of dicobalt octacarbonyl and trimethyl phosphite affords optimum yields and improved selectivity for cycloisomerization vs alkene isomerization.
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Allyl propargyl ethers undergo cobalt-mediated cycloisomerization reactions to form dihydrofurans in good yield and with excellent diastereoselectivity. The reaction works with a range of substrates, and its utility in synthesis is exemplified by the preparation of a bistetrahydrofuran unit.
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Abstract Ruthenium‐catalyzed cycloisomerization of 2‐ethynylbiaryls was investigated to identify an optimal ruthenium catalyst system. A combination of [η 6 ‐( p ‐cymene)RuCl 2 (PR 3 )] and two equivalents of AgPF 6 effectively converted 2‐ethynylbiphenyls into phenanthrenes in chlorobenzene at 120 °C over 20 h. Moreover, 2‐ethynylheterobiaryls were found to be favorable substrates for this ruthenium catalysis, thus achieving the cycloisomerization of previously unused heterocyclic substrates. Moreover, several control experiments and DFT calculations of model complexes were performed to propose a plausible reaction mechanism.
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Abstract For Abstract see ChemInform Abstract in Full Text.
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Catalytic hydrophosphination of propargyl alcohols by ruthenium complexes RuCl(cod)(C5Me5) and RuCl(PPh3)2(C5Me5) leads to the formation of functionalized vinylphosphines, with linkage of the phosphorus atom to the terminal alkyne carbon, via a ruthenium vinylidene intermediate.
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In our efforts to probe the reaction mechanism of the endo cycloisomerization of alkynols catalyzed by the complex [Ru(N3P)(OAc)](BPh4) (1; NP3 = N,N-bis[(pyridin-2-yl)methyl][2-(diphenylphosphino)phenyl]methanamine), the reactions of 1 with alkynes were investigated. Several complexes related to intermediates in the catalytic reactions were isolated and characterized. In the presence of DIPEA (N,N-diisopropylethanamine), complex 1 reacted with 3-butyn-1-ol to afford the Ru−oxocyclocarbene complex {Ru(N3P)[═C(CH2)3O](OAc)}(BPh4) (4). Under anhydrous conditions, the reaction of complex 1 with tert-butylacetylene afforded [Ru(N3P)(η3-tBuC≡CC═CHBut)](BPh4) (8). In the presence of water, the reaction of complex 1 with tert-butylacetylene afforded [Ru(N3P)(CO)(η1-CH2But)](BPh4) (13). All the reactions are likely to proceed through ruthenium vinylidene intermediates. These results support that ruthenium vinylidene complexes are involved as the key intermediates in the cycloisomerization of alkynols catalyzed by complex 1.
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The synthesis of substituted dihydrooxazoles by the CuI-catalyzed cycloisomerization of terminal propargyl amides is reported. The reaction has been shown to have good substrate scope and experiments to delineate the mechanism have been performed. Substrates containing a benzylic methylene were oxidized to the ketone under the reaction conditions.
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