Mechanism of Molybdenum Pentacarbonyl-Catalyzed Cyclizations of Alkynols and Epoxyalkynes

Abstract

We recently reported that molybdenum pentacarbonyl will cyclize 1-alkyn-4-01s to the isomeric 2,3-dihydrofurans.2.3 Although the generation of Mo(C0)s from trimethylamine N-oxide and molybdenum hexacarbonyl produces a modestly catalytic reagent for this cycloisomerization, we have found that EtSN: Mo(C0)s is best prepared by photolysis Of Mo(CO)6 in a mixture of diethyl ether and triethylamine (Rayonet lamp, 350 nm, pyrex filter, eq l).* Subsequent addition of alkynol l5 (dark reaction) hv (Pyrex filter),and stirring for ca. 18 h gives dihydrofuran 26 (89% optimized yield from 26 mol % Mo(CO)~, eq 2). This reaction system is critically dependent on the presence of a tertiary amine, in that photogenerated molybdenum pentacarbonyl-diethyl ether leaves 1 unchanged. A practical advantage of supplanting trimethylamine N-oxide decarbonylation with the photochemical method is that cyclic products are more effectively removed from inorganic byproducts, thus resulting in higher isolated yields. Attempts to follow the mechanistic course of this reaction by deuterium-labeling of either the alcohol or alkynyl hydrogens of 1 were complicated by equilibration of the deuterium atom between these two moderately acidic functional groups upon standing in the absence of triethylamine or molybdenum carbonyls.' However, our current evidence for the intermediacy of molybdenum carbene anion 4 as a catalytic intermediate includes the isolation of the apparent aldol condensation-dehydration* products 5 and 6 (Scheme 1). Reaction of 1 with 1 equiv of

Gold Catalysis: Mild Conditions for the Transformation of Alkynyl Epoxides to Furans

“…[9] This reaction is initiated by an elimination to an cumulenyl alkoxide and thus depended on the propargylic position. Similar results with in situ formed alkynyloxiranes were obtained by Katrizky et al [10] A related class of starting materials, alkynyloxiranes with leaving groups in the propargylic position, furnished furans upon treatment with stoichiometric amounts of SmI 2 and catalytic amounts of Pd(0) or Pd(II) in the case of a sequential procedure developed by Aurrechoechea et al [11] Finally, in 1994 McDonald et al [12] reported a molybdenumcatalyzed and last year Liu et al [13] reported a ruthenium-catalyzed isomerization of alkynyloxiranes with a terminal alkyne to the corresponding furan. Another reaction that converts alkynyl epoxides to furans, not by isomerization but rather by the reaction with an external carbon nucleophile, is palladium-catalyzed and was published by Tsuji et al in 1987.…”

Metal Vinylidenes as Catalytic Species in Organic Reactions

“…[61] For example, treatment of 130 with a catalytic amount of a molybdenum pentacarbonyl catalyst led to 131 (equation 1, Scheme 40). Liu and coworkers subsequently disclosed that the same type of transformation could be catalyzed by a lower loading of a ruthenium complex (equation 2).…”

Metal Vinylidenes and Allenylidenes in Catalysis: Applications in Anti‐Markovnikov Additions to Terminal Alkynes and Alkene Metathesis

“…[Mo(CO) 5 (Et 3 N)] [108] (at room temperature) and [RuCl(Tp)-(PPh 3 )(CH 3 CN)] [109] are efficient catalysts for the preparation of these substituted furans from ethynyloxiranes, as they tolerate a variety of functional groups such as alcohols, ethers, esters, nitriles, and tosylamides.…”