Isomerization of Propargylic Alcohols into α,β‐Unsaturated Carbonyl Compounds Catalyzed by the Sixteen‐Electron Allyl‐Ruthenium(II) Complex [Ru(η³‐2‐C₃H₄Me)(CO)(dppf)][SbF₆]

Abstract

) under mild conditions. This complex has been also used as catalyst for the preparation of conjugated 1,3-enynes via dehydration of propargylic alcohols.

Efficient Access to Conjugated Dienones and Diene‐diones from Propargylic Alcohols and Enolizable Ketones: A Tandem Isomerization/Condensation Process Catalyzed by the Sixteen‐Electron Allyl‐Ruthenium(II) Complex [Ru(η3‐2‐C3H4Me)(CO)(dppf)] [SbF6]

“…[9] Apparently, all these limitations seem to arise from the low activity of the catalysts in the initial isomerization step, i.e., using 5 mol 2 (10 mol % in Ru) HC CCHPh(OH) is isomerized into PhHC=CHCHO in only 15 % yield after 20 h (1,2-dichloroethane, 60 8C). [9] This result contrasts with the behaviour shown by the 16e 6 ] reported by us, which catalyzes the quantitative isomerization of HC CCHPh(OH) into PhHC=CHCHO after only 3.5 h in refluxing THF. [6] With all these precedents in mind, and as part of our ongoing interest in the catalytic applications of the allyl-ruthenium(II) complex 6 ], we have investigated the potential of our catalytic system for the tandem isomerization/aldol condensation coupling processes.…”

“…[9] This result contrasts with the behaviour shown by the 16e 6 ] reported by us, which catalyzes the quantitative isomerization of HC CCHPh(OH) into PhHC=CHCHO after only 3.5 h in refluxing THF. [6] With all these precedents in mind, and as part of our ongoing interest in the catalytic applications of the allyl-ruthenium(II) complex 6 ], we have investigated the potential of our catalytic system for the tandem isomerization/aldol condensation coupling processes. Thus, herein we describe a general and highly efficient synthetic approach to conjugated dienones R [6] we have studied the reaction in the presence of 10 equivalents of acetone.…”

“…[6] With all these precedents in mind, and as part of our ongoing interest in the catalytic applications of the allyl-ruthenium(II) complex 6 ], we have investigated the potential of our catalytic system for the tandem isomerization/aldol condensation coupling processes. Thus, herein we describe a general and highly efficient synthetic approach to conjugated dienones R [6] we have studied the reaction in the presence of 10 equivalents of acetone. Thus, in a preliminary experiment it was found that 1,1-diphenyl-2-propyn-1-ol (2a) is converted into (E)-6,6-diphenyl-3,5-hexadien-2-one (3a) in ca.…”

“…[6] Moreover, this catalytic system has proven to be highly chemoselective since it leads to the exclusive formation of enals A (Meyer-Schustertype rearrangement) or enones B (Rupe-type rearrangement) depending on the nature of the propargylic alcohol substituents. [7] Quite recently, Nishibayashi and co-workers have found that the treatment of terminal propargylic alcohols with acetone in the presence of a catalytic amount of the dinuclear species [Cp*RuA 2 (X = S, Se, Te) does not result in the expected propargylic substitution reaction, [8] leading instead to the formation of conjugated dienones C (see Scheme 2).…”

“…The stereoselectivity of this coupling process is in complete accord with the E-stereochemistry previously reported by Nishibayashi, [9] and with the stereoselective MeyerSchuster rearrangement of monosubstituted propargylic alcohols observed by us. [6] The following features are also worth noting: (i) As inferred by monitoring the reactions by GC, the ratelimiting step of this tandem process is in all cases the aldol condensation between the intermediate enal R 1 R 2 C=CHCHO and acetone (the initial isomerization step proceeds quantitatively in 2-3 h).…”