Cis hydrogenation

In general, hydroboration—protonolysis is a stereoselective noncatalytic method of cis-hydrogenation providing access to alkanes, alkenes, dienes, and enynes from olefinic and acetylenic precursors (108,212). Procedures for the protonolysis of alkenylboranes containing acid-sensitive functional groups under neutral or basic conditions have been developed (213,214). 

Heretofore, most homogeneous and all heterogeneous catalysts have given cis hydrogenation. 

A potentially useful route to substituted (Z)-allylboron derivatives involves the selective cis hydrogenation of propynylboron derivatives. One recent report applied this approach in the synthesis of a (Z)-3-trimethylsilyl-2-propenylboronate, which cannot be prepared by the allyl-organometallic route discussed in Section 1.3.3.3.3.1.1.1. The selectivity for the Z-isomer was only 9 1 21. The scope of this method remains to be fully documented35. 

In total, there are three barrierless entrance pathways frontside and backside addition of hydrogen atom to vinyl, and abstraction of the cis hydrogen. All the pathways discussed so far are on the singlet PES. There are also addition pathways on the triplet surface, but these are impeded by small barriers and are not expected to be important compared to the singlet surface. 

For (203), models indicated that the isomer containing cis-syn-cis hydrogen atoms on the cyclohexane ring should be able to form clam-type complexes, provided the cyclohexane ring is in the flexible or twist conformation. The models suggested that the cavity defined by the ten oxygen donors would be ideal for K+. However, for the potassium and barium thiocyanate complexes, configurations of type (204) do not occur in the solid state. Instead, two molecules of the bis-crown coordinate simultaneously to two alkali metal ions - both these 2 2 complexes have structures of type (205). 

Apart from Cr(0) carbonyl complexes, similar Mo, W and Co complexes also catalyze 1,4-cis-hydrogenation of dienes, though the selectivity of these catalysts is relatively low [63]. 

Conformation 8 agrees with the chemical transformations of sophoridine obtained earlier (203). Matrine isomers are known to react faster by dehydrogenation over palladized asbestos or with mercuric acetate if they contain cis hydrogens. Sophoridine has been shown to dehydrogenate faster than allomatrine (5), but slower than matrine (4) (196). Since there are three cis hydrogens (H-5, H-6, H-7) in matrine and none in allomatrine, the sophoridine molecule is supposed to contain two cis hydrogens a cis orientation of H-6 and H-7 in 8 meets this requirement. 

Cyclobutanes are less reactive compounds, undergoing ring opening according to the 1,2-dicarbene mechanism through a,a,(3, (3-tetraadsorbed intermediates (40) when CH2—CH2 bonds are present.297,298 One particular exception is cis-1,2-dimethylcyclobutane, which always yields 2,3-dimethylbutane as the main product. This was interpreted to occur via a jr-adsorbed alkene that is readily formed from the cis compound [Eq. (11.83)] due to the presence of vicinal cis hydrogens, but not from the trans isomer ... 

Very little systematic work has been conducted with Raney-nickel cathodes. Junghans reported on electrocatalytic hydrogenation of steroids, and he stresses that electrohydrogenation with Raney nickel yields higher selectivities (tram- vs cis-hydrogenation) than the usual catalytic hydrogenations (202, 203). 

Very selective cis-hydrogenations are also achieved by reduction with diimine (N2H2, S. Hunig, 1965 C.E. Miller, 1965 D.J. Pasto, 1991). The reagent can be used at low temperatures and has been employed in the selective reduction of C C double bonds, e.g. in the presence of a sensitive peroxidic function (W. Adam, 1978). 

Isocrotonic acid can be prepared by the stereospecific cis-hydrogenation of tetrolic acid3 or, mixed with the acid with sodium amalgam.4 The eis-acid can also be prepared in small amounts by isomerization of the frana-acid.6 The method herein described is much less laborious than the older procedures.2... 

Alkenes can also be produced effectively by die reduction of alkynes. The reduction can be carried out stereospecifically to give either cis or trails olefins as desired. This is a very useful method because of the stereocontrol. The P-2 nickel catalyst for the cis hydrogenation is produced in situ by the reduction... 

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