1.3- dienes hydrovinylations

The rate of reaction of the other three dienes studied decreases with increasing ring size [27] in the case of cycloocta-1,3-diene, hydrovinylation is accompanied by isomerization or reaction with a second ethylene molecule, and the yield of 3-vinyl-cyclooct-l-ene never exceeds 50%. 

In addition to the extensive studies of hydrovinylations using styrene substrates, 1,3-dienes are also an especially useful substrate class. With phosphoramidite ligands, enantioselective diene hydrovinylation with ethylene provides a practical solution to the difficulties associated with the installation of exocyclic side chain stereochemistry, as the representative procedure below illustrates. ... 

At present, phosphoramidites represent the most promising ligand class for the hydrovinylation reaction, both for the excellent activities and enanioselectivities. The synthetic potential embraces not only styrene and styrene derivatives but also norbornene [54] and 1,3-dienes [55]. 

Conjugated dienes were thus selectively obtained by hydrovinylation of alkynes catalyzed by a cationic ruthenium alkylidene complex [43] (Eq. 31). This reaction is thought to be promoted by the ruthenium hydride species resulting from the deprotonation of the <5-methyl group of the metallic precursor, followed by the sequential insertion of alkyne and ethylene into the metal-hydride and metal-vinyl bonds. 

The optimal Nirligand ratio appears to be ligand- and substrate-dependent. In the case of the hydrovinylation of cyclohexa-1,3-diene in the presence of 11, changing the ratio from 1 1 to 1 10 has little effect upon the activity [34], while for PBU3 it is claimed that 1 2 is optimal [25], whereas with PPh3 the best results are obtained with a 1 1 ratio. In this last example, a 1 5 ratio leads to catalyst deactivation [27]. For many of the reactions involving chiral ligands a ratio of 1 1.2 has been chosen, but in the case of the hydrovinylation of cy-cloocta-1,3-diene in the presence of P(menthyl)2Pr the catalyst is still quite active at a 1 3.8 ratio [3, 39]. 

Table 1. The hydrovinylation of activated alkenes and cyclic 1,3-dienes. 

It is generally accepted that the nickel-catalyzed hydrovinylation of cyclic 1,3-dienes proceeds in an analogous manner to that discussed for styrene with an initial 1,2-addition of the Ni-H species. However, it should be stressed that an initial 1,4-addition has not been excluded. The observation of two isomeric products from the reaction involving hexadeuterocyclopentadiene suggests that the... 

Keywords Asymmetric hydrovinylation. Ethylene, Vinylarene, Diene, Norbornene, 2-Arylpro-panoic acid. Nickel, Palladium, tt-Allylnickel bromide, Phospholane, Ligand timing, Hemila-bile hgand. Catalyzed cychzation, 1,6-Diene... 

The RajanBabu group also applied the same catalytic system in the asymmetric hydrovinylation of 1,3-dienes and strained alkenes (Scheme 9.7). Satisfactory results could be obtained typically. 

Scheme 9.7 Asymmetric hydrovinylation of 1,3-dienes reported by RajanBabu.

In 2010, RajanBabu and co-workers developed a Co-catalyzed asymmetric hydrovinylation between 1,3-diene and ethylene. Intriguingly, the commercially available bisphosphine ligands DIOP and BDPP displayed superior activity in promoting the desired reactions. The reactions appeared to be quite general (Table 9.8). For a large array of substrates tested, nearly quantitative yields and up to 99% ee were obtained for the hydrovinylation products. 

Table 9.8 Co-catalyzed asymmetric hydrovinylation of 1,3-diene reported by RajanBabu.

Scheme 9.14 A possible mechanism of Co-catalyzed hydrovinylation of 1,4 diene.

Since the first enantioselective nickel-catalysed hydrovinylation of eyelo-hexa-l,3-diene reported by Buono et ah in 1985, which was based on the use... 

Scheme 5.11 Hydrovinylation of 1,3-dienes with an in situ generated nickel catalyst derived from Feringa s phosphoramidite ligand.

Scheme 5.12 Hydrovinylations of steroid 1,3-dienes with in situ generated nickel catalysts from phosphoramidite ligands.

The synthesis of 1,3-dienes via a ruthenacyclopentene intermediate is also a possible pathway. It results from intracyclic p-hydride ehmination, and this process takes place only when an exocychc p-elimination is not possible or not favored. Thus, the Cp RuCl(cod)-catalyzed hydrovinylation of ynamides proceeded in the presence of ethylene to selectively afford 2-amino-l,3-dienes [66] [Eq. (28)]. 

Allylic ethers may react with 2,3-disubstituted 1,3-butadienes in an iron-catalyzed process in two different modes depending on the ligand (Scheme 4-312). A [4+4] ene reaction is favored when a bipyridine(diene)iron complex is used as catalyst to form the corresponding vinyl ethers. If l,2-bis(diphenylphosphano)ethane is used as ligand, a 1,4-hydrovinylation of the diene is observed. The active Fe(0) catalysts are synthesized from Fe(ll) or Fe(III) salts by reduction with Grignard or trialkylaluminum reagents. ... 

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