I-Alkenes

Silver(I)-alkene complexation is implicated in the silver-catalysed isomerization of alkenes [152] an example is shown in Figure 4.32. 

I. Alkenes. 2. Ring-opening polymerization. 3. Metathesis (Chemistry)... 

The asymmetric epoxidation of /i-alkenes and terminal alkenes proved to be more difficult, though a recent finding, describing the use of a modified salen complex to epoxidize ( )-0-methylstyrene to form the corresponding epoxide in 83% ee, represents another important step forward. Alternatively, chiral (D2-symmetric) porphyrins have been used, in conjunction with ruthenium or iron, for efficient asymmetric oxidation of trans- and terminal alkenes[92]. 

In contrast to the normal-scXcctwc hydroformylation mainly developed in industry, asymmetric hydroformylation, which requires /i o-aldehydes ( branched aldehydes) to be formed from I-alkenes, was first examined in the early 1970s by four groups independently, using Rh(i) complexes of chiral phosphines as catalysts. " Since then, a number of chiral ligands have been employed for asymmetric hydroformylation and used in combination with transition metal ions, especially Pt(ii) and Rh(i). Asymmetric hydroformylation of I-alkenes is most extensively studied. 

Long-chain alkyl iodides. F.vcn-numbcred alkyl compounds of chain lengths up to 42 carbon atoms can be prepared by metathesis of I-alkenes with WC14-Sn(OH i)4, originally used for metathesis of unsaluratcd fatty esters to alkenes and... 

Figure 8 The preparations and crystal structures of copper(I) alkene and alkyne complexes... 

These carbene (or alkylidene) complexes are used as either stoichiometric reagents or catalysts for various transformations which are different from those of free carbenes. Reactions involving the carbene complexes of W, Mo, Cr, Re, Ru, Rh, Pd, Ti and Zr are known. Carbene complexes undergo the following transformations (i) alkene metathesis (ii) alkene cyclopropanation (iii) carbonyl alkenation (iv) insertion to C—H, N—H and O—H bonds (v) ylide formation and (vi) dimerization. Their chemoselectivity depends mainly on the metal species and ligands, as discussed in the following sections. 

The ACMP ligand can be used in situ with a soluble Rh(I)-alkene complex to produce a catalytically active system, but normally it is converted to a stable crystalline complex of the type [(ACMP)2Rh (diene)] +X, where the diene is, for example, 1,5-cyclooctadiene and X- is BF4, PF6", or B(Ph)4 . 

Dehydration. Although early studi< shown to involve an anft-elimination. lit applied to preparation of (E)- or (Z)-alke of (E)- and (Z)- A22- or Ar -sterols, and i alkenes for isomerization of trisubstitutc... 

Reagents (i) alkenic acetate, dilauroyl peroxide, 1,2-DCE, reflux (ii) EtOC(S)SK, Me2CO, 0 °C (iii) alkenic acetate, dilauroyl peroxide, 1,2-DCE, reflux (iv) /7-Bu3SnH, AIBN, heptane, reflux ... 

---