Hydrogenation II Alkenes and Dienes

The extent of double bond isomerization also varies with the nature of the catalyst. The degree of isomerization over metal catalysts usually decreases in the order Pd Ni Rh Ru Os =Ir =Pt.5.6 The extensive double bond isomerization observed with palladium and, to some extent, with nickel catalysts can be attributed to the formation of the adsorbed 7t-allyl species with these catalysts. While double bond isomerization may not be important in a routine alkene hydrogenation, it may influence a selective hydrogenation because the isomerized olefin can have different adsorption characteristics from those of the 

In many cases double bond isomerization is only observed when the reaction is interrupted before complete saturation takes place, but sometimes the isomerized product is sufficiently difficult to saturate so it simply accumulates in 

Electronic effects can also influence the ease of double bond hydrogenation. Compounds such as A cyclohexenecarboxaldehyde (4) and other 3-carbonyl substituted cyclohexenes, such as 5 and 6, are more difficult to hydrogenate than the non-carbonyl containing materials. This decrease in activity has been attributed to an interaction between the carbonyl carbon and the 7t cloud of the double bond, shown by 7, which has been termed a supra-annular effect.7-  

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The pentacyanocobaltate(II) ion has long been known to catalyze alkene hydrogenation, mainly of conjugated dienes. A review of the early work is available.45 The catalyst system shows negligible activity for the hydrogenation of non-activated monoenes. A major disadvantage is that the system is inhibited by excess substrate, and the turnover numbers obtained are generally less than 2. 

The ease with which nucleophiles add to alkenes and alkynes coordinated to Pd(II) combined with the tendency of Pd to form rr-allyl complexes accounts for the limited number of Pd(II)-alkene, Pd(II)-diene, or Pd(II)-alkyne complexes. Indeed, as detailed in the next section, depending on the conditions or their structural features, alkenes, dienes, and alkynes often lead to the formation of 7r-allylpalladium complexes instead of rf or rf rr-complexes. This is particularly true in the case of alkenes bearing hydrogens a to the double bond, 1,3-dienes, such as 1,3-butadiene, and alkynes of low steric hindrance, hi marked contrast, Pd(l,5-hexadiene)Cl2 is obtained from the reaction of Cl2Pd(PhCN)2 with allyl chloride. ... 

Unfortunately, ( PDI)Mn(THF)2 exhibited little activity for the hydrogenation of alkenes or [2 + 2] cyclization of dienes. In an attempt to synthesize an active bis(imino)pyridine manganese precatalyst, alternative reduction conditions were explored. Sodium amalgam reduction of ( PDI)MnCl2 in pentane yielded the red bis(chelate) complex ( PDI)2Mn. X-ray diffraction established a cis-divacant octahedral compound where one imine arm on each of the chelates is dissociated from the metal center. The metrical parameters from X-ray diffraction in conjunction with SQUID magnetic and EPR spectroscopic data established that the overall S = 3/2 compound is best described as a high-spin Mn(II) species (Sy = 5/2) with two bis(imino)pyridine radical anions. 

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