A novel procedure for a highly Z-selective isomerization (double-bond migration) of acyclic allyl acetals and allyl ethers based on NiCl 2 dppb catalysis by activation with SuperHydride (lithium triethylborohydride) is described. The stereochemical outcome is compared with the results obtained with Ru and Rh catalysts. The applicability of the new process is demonstrated in the stereoselective synthesis of Claisen substrates (allyl vinyl ethers) from diallyl ethers.The well-known double-bond migration (isomerization) of allyl ethers and allyl acetals to vinyl ethers (prop-1-enyl ethers) or vinyl acetals, respectively, has found many applications in organic chemistry. 1 For example, the allyl and the but-3-en-2-yl moieties have been introduced as stable protecting groups for carbohydrates, which are easily removed via isomerization and hydrolysis of the respective vinyl ether. 2,3 Vinyl glycosides were used as substrates in the Lewis-acid mediated oligosaccharide synthesis. 4The double-bond geometry resulting from the isomerization process is without interest as far as the allyl moiety is used only as a protecting group. However, the Z/E-selectivity of this process plays an important role especially when the vinylic products are used for stereoselective reactions. Thus, vinyl acetals are used for the diastereoselective construction of aldol ethers and b-(N-acylamino)aldehydes (Mannich products) by Lewis acid-induced rearrangement, 5,6 and allyl vinyl ethers prepared by transition metal catalyzed rearrangement of diallyl ethers are of interest in the stereoselective synthesis of aliphatic Claisen products. 7-10 Several catalysts have been reported in the literature for the isomerization of allyl ethers and acetals, e.g., potassium tert-butoxide in DMSO 11 and Rh, Ru, Ir, and Fe complexes, or palladium on active charcoal. 12 Among them, only the potassium tert-butoxide/DMSO method leads to high Z-selectivities. However, this method is hampered by the strong basic conditions, the tedious workup and only moderate yields. On the other hand, excellent yields are obtained with transition metal catalysts, but these procedures usually afford Z/E-mixtures, 12-14 and they are often complicated by the formation of reduced by-products (propyl ethers and acetals) due to the activation procedure of the catalyst precursor. 4,5 During the course of our investigations on the asymmetric isomerization of cyclic allyl acetals using NiCl 2 diop/ LiBHEt 3 [diop = 2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphino)butane] as a catalyst 15 we found, that NiCl 2 diop is also an effective catalyst precursor for the isomerization of acyclic allyl ethers and acetals. Activation of this complex with lithium triethylborohydride (Super-Hydride) probably affords a hydridonickel species as an intermediate, 16 which then initiates the catalytic cycle. 17As optical active ligands such as diop are not essential for the isomerization of acyclic substrates (Table), we explored the isomerization of allylic substrates with nickel complexes comp...

A Highly Z-Selective Isomerization (Double-Bond Migration) Procedure for Allyl Acetals and Allyl Ethers Mediated by Nickel Complexes.-Acyclic allylic substrates such as (I) and (III) undergo a Z-selective isomerization in the presence of LiBEt 3 H and catalytic NiCl 2 (dppb). Isomerization of the diallyl ether (V) provides the Claisen substrate (VI) with good stereoselectivity, whereas Ru-catalyzed isomerization is unselective. -(WILLE, A.; TOMM, S.; FRAUENRATH, H.; Synthesis (1998) 3, 305-308;