Hydrovinylation of 1,3-dienes

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.

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

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

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. 

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)]. 

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. 

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%. 

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... 

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. ... 

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

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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