Generation of Allenic/Propargylic Zirconium Complexes and Subsequent Cross-Coupling Reactions: A Facile Synthesis of Multisubstituted Allenes
作者:Hao Zhang、Xiaoping Fu、Jingjin Chen、Erjuan Wang、Yuanhong Liu、Yuxue Li
DOI:10.1021/jo9020419
日期:2009.12.18
The β-alkoxide elimination reaction of propargylic ether with Negishi reagent leads to allenes and/or alkynes after hydrolysis. The product distribution is highly dependent on the substitution pattern of starting propargylic ethers; that is, aryl- or alkyl-substituted propargylic ethers favor the allene products, whereas TMS-substituted propargylic ethers afford alkynes. DFT calculations revealed that
Partial reduction of alkynes to (Z)-alkenes is achieved with the low-valent group 5 metal reagents prepared from MtlCl5 (Mtl = Nb or Ta) and zinc in a mixed solvent of DME (or THF) and benzene.
Tantalum (or niobium)-alkyne complexes are produced by treatment of acetylenes with the low-valent tantalum (niobium) derived by zinc reduction of TaCl5 (NbCl5). Substituted 1-naphthols are prepared regioselectively by reaction of o-phthalaldehyde with the tantalum (or niobium)-alkyne complexes in good to excellent yields.
variety of 2,3,4-trisubstituted furans are prepared by treatment of tantalum-alkyne complexes with a aldehydes followed by addition of an isocyanide in DME-PhH-THF (1:1:1). The regioselectivity of the reaction depends on the bulkiness of substituents on a acetylenes and the bulkier substituents tend to possess 4-position of the produced furans. 4-Trialkylsilyl-substitued furans are produced exclusively when
Alkynes may be easily alkylated by sequential treatment with n-BuLi followed by an alkyl halide in THF. Primary iodides give excellent yields (75-99%) as do bromides in the presence of catalytic amounts of Bu4NI or NaI; in the absence of an iodide source, bromides react poorly. This method offers advantages over existing methods which use HMPA or NH3 as co-solvents. (C) 2001 Elsevier Science Ltd. All rights reserved.