Alkenyl ligands

An important contribution of the resonance form b requires the donation of electron density form the metal to the dienyl ligand [M(dM) -> C(pn-) contribution], The presence of a carbonyl group (a strong TT-acceptor ligand) trans to the dienyl reduces the M(dM) - C(ptt) contribution and, therefore, the nucleo-philicity of the unsaturated ii -carbon ligand. Then the nucleophilic center of the molecule is not the alkenyl ligand but the metallic center, and the protonation at the metal leads to the olefin via reductive elimination from a hydride-dienyl intermediate.24... 

The cis-trans isomerization of alkenyl ligands in transition metal alkenyl compounds is proposed to occur via zwitterionic carbene intermediates.46 According to this, the low contribution of the form b to the metal-dienyl bond in Os (Z)-CH=CHC(Me)=CH2 K1-0C(0)Me (C0)2(P,Pr3)2 could explain why this compound does not evolve into its (if )-isomer. 

The palladium/copper-cocatalyzed coupling of the readily available trisubsti-tuted alkenyl(phenyl)iodonium triflates 77 with alkynyl- and alkenylstannanes proceeds under exceedingly mild conditions with retention of geometry of the alkenyl ligand of the iodonium salt (Scheme 35) [60]. 

Vinyliodonium ions, 35 and 36, are hypervalent iodine species in which one or two alkenyl ligands are bound to a positively charged iodine(III) atom. Although they are reactive with nucleophilic reagents, they are less labile than alkynyliodonium ions, and stable halide salts of vinyliodonium ions can be prepared. The first vinyliodonium compounds [i.e. (a, / -dichlorovinyl)iodonium salts] were synthesized by the treatment of silver acetylide-silver chloride complexes with (dichloroiodo)arenes or l-(dichloroiodo)-2-chloroethene in the presence of water (equation 152). The early work was summarized by Willgerodt in 1914115. This is, of course, a limited and rather impractical synthetic method, and some time elapsed before the chemistry of vinyliodonium salts was developed. Contemporary synthetic approaches to vinyliodonium compounds include the treatment of (1) vinylsilanes and vinylstannanes with 23-iodanes, (2) terminal alkynes with x3-iodanes, (3) alkynyliodonium salts with nucleophilic reagents and (4) alkynyliodonium salts with dienes. 

Electrophilic substitutions of alkenyl-, aryl-, and alkynylsilanes with heteroatom-stabilized cationic carbon species generated by the action of a Lewis or Brpnsted acid (acyl cation, oxocarbenium ion, etc.) provide powerful methods for carbon-carbon bond formation. Particularly, intramolecular reactions of alkenylsilanes with oxocarbenium and iminium ions are very valuable for stereoselective construction of cyclic ether and amine units.21-23 For example, the BFj OEt -promoted reaction of (E)- and (Z)-alkenylsilanes bearing an acetal moiety in the alkenyl ligand gives 2,6-disubstituted dihydropyrans in a stereospecific manner (Scheme l).23 Arylsilanes also can be utilized for a similar cyclization.24... 

K. F. Bernady and co-workers6 3 from the Lederle Laboratories have described a useful procedure for the synthesis of dl-1 l-deoxy-3-thiaprostanoids based upon the conjugate addition of E-l-alkenyl ligands from lithium E-l-alkenyltrialkyl-alanate reagent 3 to the sulfur containing cyclopentenone (2)64 ... 

It was found that the ate -complex formed by treatment of alkenyl-lithium with trialkylaluminum conjugatively transfers the alkenyl ligand to cyclopentenone in relatively good yield. It is noteworthy that the total yield of the expected product depends on the solvents used. The results — obtained in an analogous reaction — indicate that the addition in hydrocarbons gives as by-products more 1,4-reduction product than cyclopentenone polymer. On the other hand the cyclopentenone derived polymer becomes significant in the more basic THF. The michael addition... 

One significant feature of alkenyl ligands is that in addition to simple... 

Figure 4.12 Synthesis of o-alkenyl ligands via alkyne modification...

Considerable information about the course of aldehyde decarbonylations has been gleaned from the decarbonylations of alk-4-enals. Pent-4-enals form cyclopentanones in high yield in decarbonylations catalyzed by [RhCl(PPh3)3], The major product from the decarbonylation of hex-4-enal is 2-methylcyclopentanone. As shown in Scheme 5, the cyclization reaction requires a vacant site on rhodium. The other products result from decarbonylation of the unsaturated acyl before cyclization can take place. In these cases, there is competition between addition of deuterium to C-1 of the alkenyl ligand or its addition to the alkene bond and the formation of an unstable metallocycle. ... 

Other complexes of great interest are the / -conjugated diene metallocenes. They are conveniently prepared by the one-pot reaction of the photochemically induced metallocene with the diene at low temperature. Alternative routes include the treatment of the metallocene dichloride with a conjugated diene dianion equivalent or the coupling of alkenyl ligands in the coordination sphere of the metal. The resulting systems show unusual behavior. For example, buta-1,3-diene directly leads to the s-trans complex (8), which can be thermally equilibrated into the s-cis isomer (9). This form exhibits a substantial metallacyclopentene character and is best described as a d metal (cr, 7r)-type complex (10), as shown by X-ray crystal analysis. On the other hand, the s-trans isomer should be formally regarded as a real d metal -alkene 7T-complex. 

Erker and co-workers (41, 41a) have studied the photochemistry of several zirconium alkenyl complexes and observed both isomerization of the alkenyl ligand and reductive coupling of the alkenyl ligand with X. 

Hegedus and Tamura (66) postulated that reactions of acyliron, acyl-nickel, and cobalt carbonyl anions with ij -allyl complexes of palladium proceed via unstable bimetallic intermediates with bridging o-,7r-alkenyl ligands, L (RC(0))M T7 T7--CH=CHR PdL . Products isolated from the acylation of rr-allyl ligands were a,/3- and a,y-unsaturated ketones. 

Czisch, P., Erker, G., Korth, H. G., Sustmann, R. Transfer and coupling of zirconocene-bound alkenyl ligands. An alternative route to (s-trans-T -conjugated diene)ZrCp2 complexes. Organometallics 1984, 3, 945-947. 

A major drawback in the use of homocuprates, RaCuLi, is that one of the alkenyl or aryl groups can often be wasted clearly, however, this is not a serious problem if the cuprate is derived from a simple or-ganolithium. With reactive electrophiles such as allylic halides or a-halo ethers, both organic ligands are alkylated in the presence of 1 equiv. of HMPA, but with less reactive alkyl halides, at least 3 equiv. of (EtOlaP are required to stabilize the intermediate cuprate species during the much slower transfer of the second alkenyl ligand. ... 

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