Conjugated diene complexes rearrangement

The preparation of conjugated diene complexes will be presented by groups. In addition, isomerization reactions, or degenerate rearrangements with activation energies >25 kcalmol-1, will be considered in this section. 

Conjugated dienes sucb as butadiene and its open-chain analogues can act as 17 ligands the complexes are u.sunlly prepared from melal carbonyl complexes by direct replacement of 2CO by the diene. Isomerization or rearrangement of the diene may occur a.s indicated schematically below ... 

Hydrocyanation of aliphatic conjugated dienes in the presence of Ni(0) complexes gives diene rearrangement products and /i.y-unsaUiratcd nitriles in 10-90% yields10. Dienes other than 1,3-butadiene do not produce terminal nitriles, implying that the more highly substituted jr-allyl nickel complex is favored. Thus, reaction of 1-phenylbuta-l,3-diene (1) affords ( )-2-methyl-4-phenylbut-3-enenitrile (2) as the sole product (equation 5). The... 

The complete shift of a conjugated diene to a new conjugated diene system has also been observed in the -ionone system (33). Such diene rearrangements require a stoichiometric quantity of Fe(C0)5. The rearranged diene ligand is conveniently liberated from the Fe by oxidation of the complex with FeCls-... 

Nonconjugated dienes are rearranged to iron carbonyl complexes of conjugated dienes when treated with FefCOjs or Fe3(CO)i2. Thus 1,4-pentadiene gives tmw -l,3-pentadieneiron tricarbonyl (34) possibly by the route ... 

The alkoxymercuration of conjugated dienes is more complex since these compounds are less reactive than simple alkenes and the products often react further to afford dimercurated products whose regio- or stereo-chemistry is strongly affected by the initial product. For example, 1,3-butadiene, 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene and 1,3-cyclohexadiene react with meicuiy(II) acetate in methanol to produce 1,2-adducts (equation 265).443-446 These adducts sometimes rearrange with time.446 1,3-Butadiene reacts further to afford primarily the meso adduct of double 1,2-additions.447 Surprisingly, little additional work has been reported on reactions of conjugated dienes. 

The first isolated example of a bis(olefin)zirconocene has been prepared by this method. Photolysis of 10 in the presence of 2,3,5,6-tetrakis(methylene)bicyclo[2.2.2]octane yields 11, which at elevated temperature rearranges to the (conjugated-diene)zirconocene complex 12... 

Insertions of vinylic palladium halides with olefins not conjugated with carbonyl groups is more complex. Rearrangement of the initial insertion product to a i-allyl-palladium derivative usually occurs as in Eq. (p). Since the a -allylic complexes are relatively stable, catalytic reactions to form dienes with tertiary amines as bases are slow and inefficient . A useful catalytic reaction occurs, however, if a nucleophilic secondary amine is used as base instead of the tertiary amine. The a-allylic palladium intermediates are attacked by the amine to form tertiary allylic amines and regenerate the catalyst ... 

Not less useful is the photochemistry of non-conjugated dienes. Thus, 1,4-dienes undergo a typical photo-rearrangement to vinyl cyclopropanes, which may be a valid way for building a three-membered ring, particularly in a complex structure (this is known as the di-jt-methane rearrangement, see Scheme 2.2). p,y-Unsaturated ketones and imines rearrange in the same way to cyclopropylketones and imines respectively [9]. 

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