A Rh(I)-Catalyzed Cycloisomerization of Homo- and Bis-homopropargylic Alcohols

Abstract

The ability to form rhodium-vinylidene complexes in situ from terminal alkynes has led to the development of a catalytic process, the cycloisomerization of homopropargylic and bis-homopropargylic alcohols to dihydrofurans and dihydropyrans. Among the transition metals that perform similar reactions, rhodium catalysts demonstrate the best chemoselectivity and turnover numbers to date. Both secondary and tertiary alcohols participate equally well. The presence of proparylic oxygen and nitrogen functionality, which potentially can be induced to ionize via formation of allenylidene metal complexes, is compatible with this catalyst. The formation of a 5-amino-dihydropyran which is not compatible with some of the previous catalysts proceeds in good yield with the rhodium catalysts. A substrate bearing a benzylic hydroxyl group adjacent to an electron-rich aromatic ring also participates without complications of ionization. The method provides access to useful aminosugars. A mechanism to account for the different selectivity of this catalyst as compared to others is proposed.

Osmium‐Catalyzed 7‐endo Heterocyclization of Aromatic Alkynols into Benzoxepines

“…[2] The formation of dihydrofurans and dihydropyrans by catalytic heterocyclization with molybdenum-and tungsten vinylidenes has been pioneered by McDonald et al, [3] and later by Trost and Rhee using cationic ruthenium and rhodium complexes. [4] More recently, the efficient preparation of indoles by the rhodiumcatalyzed cycloisomerization of 2-(ethynyl)anilines, [5] and the smooth preparation of benzofurans and benzopyrans by the ruthenium-catalyzed 5-endo and 6-endo heterocyclization reactions of substituted (2-ethynyl)phenols and benzylic alcohols, respectively, have been also described. [6,7] On the other hand, the heterocyclization of alkynols into the sevenmembered oxepines, a framework commonly found in complicated polycyclic marine natural products, [8] has only been achieved from specific acetonide-protected alkynol substrates via tungsten-vinylidene complexes.…”

Metal Vinylidenes as Catalytic Species in Organic Reactions

“…[51] Although relatively large amounts of ligand were still required, the reaction required less additives and showed higher catalyst turnover numbers (lower catalyst loadings) than the ruthenium-based system. Importantly, propargyl ethers were resistant to elimination (equation 2).…”

“…[51] Thus N-tosyl propargyl enamine 193 underwent cyclization to provide 1,3-diene product 194 (equation 1), while N-benzoyl propargyl enamine 195 cyclized to form 1,4-diene 196 in excellent yield (equation 2), both products containing the indolizidine skeleton (Scheme 64). [81] The different products obtained may be rationalized according to Scheme 65.…”

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

“…[104] Similar cycloisomerizations of homo-and bis-homopropargylic alcohols into five-and six-membered oxygen-containing heterocycles have been performed with Rh I catalyst precursors such as [{Rh(cod)Cl} 2 ], in the presence of an excess of an electron-poor phosphine such as a tris(monofluoro-or difluorophenyl)phosphine, or [RhCl(PAr 3 ) 3 ] without any further additive. [105] The electron-poor tris(p-fluorophenyl)phosphine (in the complex [RuCl(Cp){(p-FC 6 H 4 ) 3 P)} 2 ]) in the presence of Bu 4 NPF 6 and N-hydroxysuccinimide sodium salt selectively provided dihydropyrans from pent-4-yn-1-ols. [106] Transient metal oxacarbenes can be converted into cyclic esters by selective oxidation by adding N-hydroxysuccinimide as a mild oxidant to a catalytic system based on [RuCl(Cp)(cod)], trisfurylphosphine in the presence of Bu 4 NBr, or Bu 4 NPF 6 in a DMF/water mixture.…”