Molybdenum metathesis catalysts

Metathesis with alkynes is also quite useful in synthesis,particularly for internal alkynes although terminal alkynes are not good partners in this reaction.358 internal metathesis reactions with alkynes are known,including the conversion of 439 to 440 (in 73% yield) in FUrstner s synthesis of prostaglandin E2-1,15-lactone.360 Note the use of a molybdenum metathesis catalyst for this reaction. Diynes also react with alkynes in an intermolecular reaction to form aromatic rings. An example is the conversion of 441 to a 6 1 mixture of 442/443, in 82% yield.36la a similar, palladium-catalyzed cycloaromatization is also known.362 The metathesis disconnections are... 

Acyclic Diene Metathesis (ADMET) Polymerization represents a versatile route towards the synthesis of linear well-defined unsaturated polymers [1,2]. The reaction developed by Wagener et al. occurs in the presence of highly active tungsten and molybdenum metathesis catalysts such as Schrock alkylidenes, e.g. 

Ring-Opening Polymerization. Ring-opening polymerization of cycloolefins in the presence of tungsten- or molybdenum-based catalysts proceeds by a metathesis mechanism (67,68). 

Two significant items were confirmed in this work (a) Molybdenum, and most probably tungsten, can expand its sphere of coordination beyond 6 and (b) hydride shifts transforming olefins to allyls or 7r-allyls, via v -> (t and 77 - Tj3 processes, respectively, are feasible in metals that are known to produce active metathesis catalysts. 

Molybdenum dinitrosyl complexes with the general formula Mo(NO)2(CHR) (0R )2(A1C12)2 have been found to be active in a variety of metathesis reactions [110]. New alkylidenes could be identified. Variations such as Mo(NO)2(CHMe) (RC02)2 also are known [111]. Complexes of this type are believed to be more reduced than typical d° species discussed here, although they appear to be much more active as metathesis catalysts than typical Fischer-type carbene complexes. 

Ruthenium-based metathesis catalysts versus molybdenum-based metathesis catalysts, 26 936-937 Ruthenium catalysts... 

Table 3.19 lists examples of the preparation of nitrogen-containing heterocycles by RCM. As mentioned in Section 3.2.5.3, free amines can partly deactivate metathesis catalysts. With the highly reactive molybdenum catalyst 1 it is, however, possible to cyclize dienes containing a basic amino group. If the less reactive catalysts 2 or 3 are to be used, protonation or acylation of the amine can be used to reduce their nucleophilicity. This will generally lead to higher yields with smaller amounts of catalyst. 

An area, in which catalytic olefin metathesis could have a significant impact on future natural product directed work, would be the desymmetrization of achiral molecules through asymmetric RCM (ARCM) with chiral molybde-num-(for a chiral molybdenum-based catalyst available in situ from commercial components, see Refs 156 and 156a-156c) or ruthenium-based catalysts. 

---