Hydrogenation more substituted double

The cobalt(I) complex CoBr(PPh3)3 as a boron trifluoride etherate selectively hydrogenates conjugated dienes to monoenes via an unusual 1,2-hydrogen addition at the more-substituted double bond (186). 

The hydrogenation of dienes to monoenes introduces several problems of selectivity. Regioselective saturation of one of the double bonds is governed basically by the same effects that determine the relative reactivities of monoalkenes in a binary mixture that is, a terminal double bond is reduced preferentially to other, more substituted double bonds. During the reduction of a diene, a new competition also emerges since the newly formed monoene and the unreacted diene compete for... 

The difference in reactivity of different double bonds allows selective hydrogenation of dienes. Conjugated dienes with at least one trisubstituted double bond may be completely reduced by using excess reagents222 [Eq. (11.71)]. Regioselective hydrogenation of the more substituted double bond occurs in 1,5-dienes. The hydrogenation of 1,4-dienes, however, is not selective [Eq. (11.72)] ... 

For hydrogenation of more substituted double bonds over Raney Ni, application of elevated temperature and pressure may be advantageous to complete the reaction within a reasonable time, as seen in the hydrogenation of ethylidenecyclobutane in eq. 3.4, although the reaction time was not described.51 Hydrogenation of iso-propylidenecyclobutane at 80-100°C and 0.34-0.41 MPa H2 proceeded less readily than in the case of ethylidenecyclobutane. 

Path [1] occurs faster than Path [2] because it results in removal of the less hindered 2° hydrogen, forming an enolate on the less substituted a carbon. Path [2] results in removal of a 3° hydrogen, forming the more stable enolate with the more substituted double bond. This enolate predominates at equilibrium. 

While most catalysts reduce the least-substituted double bond preferentially, the novel catalyst system CoBr(PPh3)3 BF30Et2 can selectively hydrogenate conjugated dienes to monoenes via 1,2-hydrogen addition to the more substituted double bond (Scheme 92). Unfortunately, hydrogenation of functionalized alkenes, such as methyl vinyl ketone, methyl acrylate and -butyl vinyl ether, does not occur even under forcing conditions. 

Isomerization of allylic aicohois. This cationic complex (1) is a very active catalyst for hydrogenation of alkenes, even of tri- and tetrasubstituted alkenes. Since it can sometimes effect isomerization of the substrate as well, it was examined as a catalyst for isomerization of allylic alcohols to carbonyl compounds, a reaction that ordinarily requires fairly drastic conditions. After activation with hydrogen, 1 can effect this isomerization in THF at 20°. Primary and secondary allylic alcohols with monosubstituted double bonds are isomcrized in quantitative yield at 20°, Alcohols with more substituted double bonds require more catalyst and/or higher temperatures (60°). Dismutation is not observed in this system. Examples ... 

When we studied E2 reactions of alkyl halides in Section 9.7, we saw that a jS-hydrogen must be anti to the leaving group. If only one /8-hydrogen meets this requirement, then the double bond is formed in that direction. If, however, two /8-hydrogens meet this requirement, then elimination follows Zaitsev s rule ehmination occurs preferentially to form the more substituted double bond. 

FIGURE 10.5 It is possible to hydrogenate less-substituted double bonds in the presence of more-hindered, more-substituted double bonds.The catalyst in this example is a ruthenium-carbon complex. 

Deprotonation of an unsytnmetrical ketone such as 2-methylcyclopentanone may lead to two isomeric species, a more substituted and a less substituted enolate. The former, featuring the more substituted double bond, is more stable than the latter (Section 11-5). As in the elimination reaction of a haloalkane via the E2 mechanism (Section 7-7), the choice of base and reaction conditions determines which one is formed. For example, addition of 2-methylcyclopentanone to a cold solution of LDA in THF gives predominantly the less substituted, less stable enolate. The reason is that LDA is a bulky base and prefers to remove a hydrogen from the less-hindered a-carbon, generating the less stable anion, termed the kinetic enolate. Under these conditions, namely, the absence of a proton source and at low tanperatures, equilibration with the more stable enolate does not occur, and the kinetic enolate can be used as such in practical further transformations. 

Uses ndReactions. Dihydromyrcene is used primarily for manufacture of dihydromyrcenol (25), but there are no known uses for the pseudocitroneUene. Dihydromyrcene can be catalyticaUy hydrated to dihydromyrcenol by a variety of methods (103). Reaction takes place at the more reactive tri-substituted double bond. Reaction of dihydromyrcene with formic acid gives a mixture of the alcohol and the formate ester and hydrolysis of the mixture with base yields dihydromyrcenol (104). The mixture of the alcohol and its formate ester is also a commercially avaUable product known as Dimyrcetol. Sulfuric acid is reported to have advantages over formic acid and hydrogen chloride in that it is less compUcated and gives a higher yield of dihydromyrcenol (105). 

More recent examples of experiments with sterically congested molecules are Mylroie and Stenberg s hydrogenation of the sterically hindered substituted tryptycenes,60 and hydrogenation of the double bond in tetraethyl bicy-clo[2.2.2]oct-7-ene-2-syn,3-syn,5-syn,6-syn-tetracarboxylate (4) which occurs over 5% Pd/C in ethyl acetate at room temperature and under 1 atm of hydrogen in 48 hours.61... 

A quite consistent relationship is found in these and related data. Conditions of kinetic control usually favor the less substituted enolate. The principal reason for this result is that removal of the less hindered hydrogen is faster, for steric reasons, than removal of more hindered protons. Removal of the less hindered proton leads to the less substituted enolate. Steric factors in ketone deprotonation can be accentuated by using more highly hindered bases. The most widely used base is the hexamethyldisilylamide ion, as a lithium or sodium salt. Even more hindered disilylamides such as hexaethyldisilylamide7 and bis(dimethylphenylsilyl)amide8 may be useful for specific cases. On the other hand, at equilibrium the more substituted enolate is usually the dominant species. The stability of carbon-carbon double bonds increases with increasing substitution, and this effect leads to the greater stability of the more substituted enolate. 

In conclusion, the anti selectivity for hydrogen abstraction of the ene reaction of trisubstituted olefins is related (a) to the degree of crowdedness of the more substituted side of the olefin (b) to the non-bonded interactions during the new double bond formation and (c) to the lack of interaction of oxygen with two allylic hydrogens. 

Hydroboration-oxidation of alkenes preparation of alcohols Addition of water to alkenes by hydroboration-oxidation gives alcohols via anti-Markovnikov addition. This addition is opposite to the acid-catalysed addition of water. Hydrohoration is regioselective and syn stereospecific. In the addition reaction, borane bonds to the less substituted carbon, and hydrogen to the more substituted carbon of the double bond. For example, propene reacts with borane and THF complex, followed by oxidation with basic hydrogen peroxide (H2O2), to yield propanol. 

Metal-assisted reductions with NaBFLt can be used to hydrogenate various functional groups41,42. The Co2+-NaBH4 system selectively reduces limonene at the less substituted double bond300 though W-4 Raney Ni proved to be more effective301 (equation 21). 

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