Lanthanide catalysts regioselectivity

As for the regioselectivity, it is important that in the presence of lanthanide catalysts the dienes can enter insertion, upon isomerisation from the transoid to cisoid conformation, only via the double bond containing no methyl group. The reactions for this phenomenon are different for piperylene and isoprene. 

Alkene hydroamination has been known for many years, but has been little used as a method in organic synthesis. Tobin Marks of Northwestern recently published a series of three papers that will make this transformation much mote readily accessible. In the first (J. Am. Chem. Soc. 125 12584,2003) he describes the use of a family of lanthanide-derived catalysts for intermolecular hydroamination of alkynes (to make imines, not illustrated) and alkenes. With aliphatic amines, the branched (Markownikov) product is observed, 1 — 2. With styrenes, the linear product is formed. When two alkenes are present, the reaction can proceed (3 —> 4) to form a ring, with impressive regioselectivity. 

A mechanistic study of the role of the lanthanide cations suggests that they catalyze decomposition of borohydride by the hydroxylic solvent to afford alkoxyborohydrides, which may responsible for the observed regioselectivity. The stereoselectivity of the process is also modified by the presence of Ln " ions, in that axial attack of cyclohexenone systems is enhanced. a,p-Unsaturated aldehydes are regio-selectively reduced to allylic alcohols by bis(triphenylphosphine)copper(I) tetrahydroborate in the presence of Lewis acid catalyst. ... 

A series of lanthanide metallocene catalysts are active in the regioselective ring-opening polymerization of strained f .vo-rnethylcnecycloalkanes to yield f . v>-mcthylene-functionalizcd polyethylenes.9 a Methylenecyclobutane affords the polymer [-CH2CH2C112C( =CH2)-] under the catalytic action of [ 1,2-... 

The demand for environmentally friendly chemistry and its widespread applicability have made water an increasingly popnlar solvent for organic transformations. Mixtures of water and other solvents snch as tetrahydrofnran are now commonly anployed for a number of organic transformations. For instance, the Lewis acid catalysed aldol reaction of silyl enol ethers, commonly known as the Mnkaiyama aldol reaction, which was firstly reported in the early seventies, can be carried ont in snch media. With titanium tetrachloride as the catalyst this reaction proceeds regioselectively in high yields, but the reaction has to be carried ont strictly nnder non-aqneons conditions in order to prevent decomposition of the catalyst and hydrolysis of the sUyl enol ethCTS. In the absence of the catalyst it was observed that water had a beneficial influence on this process (Table 4, entry D) . Nevertheless, the yields in the nncatalysed version WCTe still unsatisfactory. Improved results were obtained with water-tolerant Lewis acids. The first reported example for Lewis acid catalysis in aqueous media is the hydroxymethylation of silyl enol ethers with commercial formaldehyde solution using lanthanide trillates. In the meantime, the influence of several lanthanide triflates in cross-aldol reactions of various aldehydes was examined " " ". The reactions were most effectively carried out in 1 9 mixtures of water and tetrahydrofnran with 5-10% Yb(OTf)3, which can be reused after completion of the reaction (Table 19, entry A). Although the realization of this reaction is quite simple, the choice of the solvent is crucial (Table 20). 

The Lewis acid and coordinating properties of trivalent lanthanides were used very early. In 1922, lanthanide trichlorides were compared to other metal halides as catalysts for acetalization (38). The first detailed study of the application of lanthanide derivatives to problems of organic synthesis seems to be the report of Pratt in 1962 (39) who showed that CeCl is a superior catalysts for the regioselective addition of p-toluidine on 5,8-quinoline-quinone (followed by reoxidation) (eq.j 26j). 

The thiol-epoxy reaction has proved to be an important synthetic tool in the preparation of a variety of pharmaceutical and natural products [55-64]. As mentioned above, this reaction can be performed using an acid or a base as a catalyst. The acid catalysts include boron trifluoride etherate [57], lanthanide chlorides [65], lithium perchlorate [60], cobalt chloride [66], and neutral alumina [67]. The basic catalysts can be organic or inorganic and include, among others, triethylamine, l,8-diazabicyclo[5.4.0]undec-7-ene (DBU), tetra-n-butylammonium fluoride (TBAF), and LiOH [61-64, 68, 69]. In the literature on small molecules, it is important to note that this reaction is complete in a few minutes to a few hours of reaction time, with quantitative yields and with high regioselectivity. In the case of LiOH and TBAF, quantitative conversion of the epoxide to a desired thio-ether compound is observed within minutes and with 100% regioselectivity (isomer I) [68, 69]. 

---