Unfunctionalised alkenes

The binaphthyl azepinium salt 59 (TT= tris(tetrachlorobenzenediolato)phosphate(V)) and corresponding azepine 60 were developed as effective catalysts for the enantioselective epoxidation of unfunctionalised alkenes, with enantiomeric excesses up to 87% <06TA2334>. 

This review focuses on the cross-metathesis reactions of functionalised alkenes catalysed by well-defined metal carbene complexes. The cross- and self-metath-esis reactions of unfunctionalised alkenes are of limited use to the synthetic organic chemist and therefore outside the scope of this review. Similarly, ill-defined multicomponent catalyst systems, which generally have very limited functional group tolerance, will only be included as a brief introduction to the subject area. 

In contrast, ruthenium catalysts gave the best results for the cross-metathesis reactions of vinylsilanes with a range of unfunctionalised alkenes [8] (a typical example is shown in Eq. 4). 

The first published report on the use of this catalyst for the cross-metathesis of functionalised acyclic alkenes was by Blechert and co-workers towards the end of 1996 [37]. This report was also noteworthy for its use of polymer-bound alkenes in the cross-metathesis reaction. Tritylpolystyrene-bound AT-Boc N-al-lylglycinol 18 was successfully cross-metathesised with both unfunctionalised alkenes and unsaturated esters (Eq. 17) (Table 1). 

It is noticeable that cross-metathesis with the unfunctionalised alkenes occurred in significantly higher yields over shorter reaction times and required a smaller excess of the soluble alkene. This was possibly due to the unfunctionalised alkenes, which are more nucleophilic than their ester containing counterparts, complementing the less nucleophilic/more carbon-metal bond stabilising allylglycinol 18. Comparable results were obtained from cross-metathesis reactions of the polymer-bound isomeric N-Boc C-allylglycinol with the same four alkenes. 

As complex/aq. H O /CH Cl they epoxidised unfunctionalised alkenes RCH=CHj to a mixture of the epoxide and the aldehyde RCHO with e.e. values from 4% to 41%. Thus (Z)-2-methylstyrene gave the cw-epoxide with only traces of the trans-isomer [928, 929]. The reagent [RuCl(PNNP)]Vaq. H O /CH Cl epoxidised cis-stilbene, Z-2-methylstyrene and 1,2-dihydro-napthalene [844]. 

The deprotonation and electrophilic quench of unfunctionalised allylic compounds provides a useful way of making functionalised Z-alkenes, since the intermediate allylmetal species prefers the endo configuration.281-19 For example, 1-dodecene 501 can be transformed in 57% yield to Z-dodecenol 502 on a 15 g scale.281... 

In contrast with unreactive, unfunctionalised terminal alkenes, allylic and homoallylic ethers (22, 24) and alcohols (20) from which the product organolithiums (21, 23, 25) can be chelated in a (preferably) five-membered, oxygen-containing ring, carbolithiate rapidly and cleanly.23 Coordination overrides any preference for the lithium to be bonded to the primary carbon, but cannot overcome the unfavourability of forming a tertiary organolithium - 26 gives 27, but 28 cannot be carbolithiated. Coordination to sulfur in similar thioethers 29 works too. 

The turn of the millenium will see the 20th anniversary of the seminal discovery of the asymmetric epoxidation [1, 2] of ally lie alcohols catalysed by titanium(IV) isopropoxide and tartrate esters. The utility of this transformation largely results from the regio- and stereocontrol possible in subsequent nucleophilic ring opening reactions of the derived epoxy alcohols. Thus, a sequence of asymmetric epoxidation, epoxide opening and further functionalisation leads to a diverse array of molecules in enantiomerically pure form. In comparision, asymmetric epoxidation of unfunctionalised alkenes [3] has yet to match the enantioselectivities which the Ti-tartrate system can deliver with allylic alcohols. The recent discovery of other asymmetric epoxidation reactions [4] suggests that a number of practical options may eventually become available. 

Metal complexes of enantiomericaUy pure N,N -ethylenebis(salicylideneaminato) (salen) complexes in combination with stoichiometric oxidants currently provide the most selective method for the catalytic asymmetric epoxidation of unfunctionalised alkenes. The use of C2-symmetric salen complexes of manganese(lll) were reported independently in 1990 by Jacobsen and coworkers and Katsuki and coworkers. The first generation catalysts are represented by the general structure (4.33). The complex with R = Bu is known as Jacobsen s catalyst. All of the first generation catalysts are composed of a enantiopure diamine core and possess large substituents at the 3/3 and 5/5 positions. Subsequently Katsuki and coworkers developed second generation catalysts such as (4.34) with axially chiral groups at the 3/3 positions. 

Titanoeene hydride 16 was also used for the hydrogenation of unfunctionalised trisubstituted alkenes " and enamines. Whereas for alkenes, severe eonditions were required (2000 psi, 9-184 h at 65 °C), enamines were converted into amines under lower hydrogen pressure (Seheme 7.12). Importantly, in contrast to many asymmetric reductions using other metals, no coordinating atom in the substrate was neeessary to get high enantioseleetivity with this catal) ic system. 

Increasing interest is being shown in hydrogenation by early transition metal ounplexes. Unfunctionalised alkenes are reduced (1 atm.) in the presence of terminal alkenes... 

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