New Lewis-Acidic Molybdenum(II) and Tungsten(II) Catalysts for Intramolecular Carbonyl Ene and Prins Reactions. Reversal of the Stereoselectivity of Cyclization of Citronellal

Abstract

New Mo(II) complexes BnEt(3)N(+)[Mo(CO)(4)ClBr(2)](-) (A) and Mo(CO)(5)(OTf)(2) (B) and their W(II) congeners D and E have been developed as catalysts for the title reactions. Unlike other Lewis acids, the latter catalysts exhibit cis-stereoselectivity in the cyclization of citronellal (1 --> 3 with A and 1 --> 5 with B). Isotopic labeling allowed formulation of the reaction mechanism, according to which these complexes act as bulky Lewis acids, eta(1)-coordinated to the carbonyl oxygen. The stereochemistry appears to be controlled by the protruding ligand L(p), which dictates the boatlike transition state III. The kinetically formed cis-alkenol 3 can be equilibrated by [Mo(CO)(4)Br(2)](2) (C) or ZnCl(2) to its trans-epimer 2 via a retro-ene reaction.

Highly selective synthesis of menthols from citral in a one-step process

“…For example, the selective hydrogenation of citral was widely investigated on different metalsupported catalysts for producing either nerol/geraniol or citronellal and citronelol [4,5]. The citronellal cyclization to isopulegol has been carried out by using liquid [6,7] and solid [8][9][10] acid catalysts, while the direct synthesis of menthols from citronellal was recently investigated on ruthenium-based catalysts [11]. In this note we report the development of bifunctional metal-acid catalysts that promote selectively the direct conversion of citral to menthols.…”

Intramolecular ene reactions catalyzed by NbCl5, TaCl5 and InCl3

“…Good selectivities favoring isopulegol can be achieved with zinc salts (ZnBr 2 and ZnI 2 ), 4 although in stoichiometric amounts, and catalytic scandium triflate, 6 whereas neoisopulegol is slightly favored using SbCl 5 4 or molybdenum(II) and tungsten(II) catalysts. 7 We wish to report that citronellal undergoes cyclization in the presence of catalytic amounts of NbCl 5 , TaCl 5 and InCl 3 . 8 NbCl 5 has been successfully used in our group as Lewis acid in a variety of reactions 9 and has gained increasing attention in recent years.…”

Molybdenum-Catalyzed Allylic Substitution in Glycals: A C-C Bond-Forming Ferrier-Type Reaction

“…We have also demonstrated that the Mo(II) and Mo(IV) complexes A-C act as very mild Lewis-acidic catalysts 11,12 , which is in sharp contrast to Mo(0)-and Pd(0)-complexes, known to react via η 3 -coordinated intermediates 13,14 . In view of the ready availability of the Mo(II) (refs 11e,11h,15 ) and Mo(IV) (refs 12,16 ) catalysts and their compatibility with various nucleophiles, it was of interest to extend this methodology to the carbohydrate realm with a view of developing a simple, non-expensive route to various C-glycosides, based on the Mo-catalyzed allylic substitution in the glycal molecule.…”