Hydroboration, alkene mechanism

When the formation of two new bonds occurs in a concerted fashion (i.e., in a single step with a cyclic transition state), then those two bonds will be added to the same face of the substrate, which is called syn addition. Examples of this include dihydroxylation with OSO4, epoxidation, cyclopropanation, and hydroboration. The mechanism of catalytic hydrogenation of an alkene (or of an alkyne with Lindlar s catalyst) is another example of a syn addition, since both hydrogens will be sequentially added to the same face of the pi bond. 

Hydroboration-oxidation of alkynes is believed to proceed via a mechanism that is similar to the mechanism invoked for hydroboration-oxidation of alkenes (Mechanism 9.3). Specifically, borane adds to the alkyne in a concerted process that gives an is ft-Markovnikov addition. There is, however, one critical difference. Unlike an alkene, which only possesses one it bond, an alkyne possesses two it bonds. As a result, two molecules of BH3 can add across the alkyne. To prevent the second addition, a dialkyl borane (R2BH) is employed instead of BH3. The two alkyl groups provide steric hindrance that prevents the second addition. Two commonly used dialkyl boranes are disiamylborane and 9-BBN ... 

The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

Both ( )- and (Z)-l-halo-l-alkenes can be prepared by hydroboration of 1-alkynes or 1-halo-l-alkynes followed by halogenation of the intermediate boronic esters (244,245). Differences in the addition—elimination mechanisms operating in these reactions lead to the opposite configurations of iodides as compared to bromides and chlorides. 

Alkylboranes can be coupled by treatment with silver nitrate and base." Since alkylboranes are easily prepared from alkenes (15-16), this is essentially a way of coupling and reducing alkenes in fact, alkenes can be hydroborated and coupled in the same flask. For symmetrical coupling (R = R ) yields range from 60 to 80% for terminal alkenes and from 35 to 50% for internal ones. Unsymmetrical coupling has also been carried out, but with lower yields. Arylboranes react similarly, yielding biaryls. The mechanism is probably of the free-radical type. 

Although alkyl groups in general increase the rates of electrophilic addition, we have already mentioned (p. 974) that there is a different pattern depending on whether the intermediate is a bridged ion or an open carbocation. For brominations and other electrophilic additions in which the first step of the mechanism is rate determining, the rates for substituted alkenes correlate well with the ionization potentials of the alkenes, which means that steric effects are not important. Where the second step is rate determining [e.g., oxymercuration (15-3), hydroboration (15-17)], steric effects are important. ... 

Hartwig65 reported that dimethyltitanocene is an efficient catalyst for the hydroboration of alkenes and examined the mechanism of titanocene dicarbonyl-catalyzed the hydroboration of alkynes (Scheme 5).66... 

In comparison with the hydroboration and diborafion reactions, thioboration reactions are relatively limited. In 1993, Suzuki and co-workers reported the Pd(0)-catalyzed addition of 9-(alkylthio)-9-BBN (BBN = borabicyclo [3.3.1] nonane) derivatives to terminal alkynes to produce (alkylthio)boranes, which are known as versatile reagents to introduce alkylthio groups into organic molecules [21], Experimental results indicate that the thioboration reactions, specific to terminal alkynes, are preferentially catalyzed by Pd(0) complexes, e.g. Pd(PPh3)4, producing (thioboryl)alkene products, in which the Z-isomers are dominant. A mechanism proposed by Suzuki and co-workers for the reactions involves an oxidative addition of the B-S bond to the Pd(0) complex, the insertion of an alkyne into the Pd-B or Pd-S bond, and the reductive elimination of the (thioboryl)alkene product. 

A multistep pathway analogous to the mechanism of alkene hydrogenation has been shown to be operative in the rhodium-catalyzed hydroboration of alkenes.363 Deuterium labeling studies furnished evidence that the reversibility of the elementary steps is strongly substrate-dependent. The key step is hydride rather than boron migration to the rhodium-bound alkene. 

Hydroboration provides yet another mechanism to produce an alcohol from an alkene. This is an anti-Markovnikov and a syn addition. 

Mechanism 8-4 Acid-Catalyzed Hydration of an Alkene 338 8-5 Hydration by Oxymercuration-Demercuration 340 Mechanism 8-5 Oxymercuration of an Alkene 340 8-6 Alkoxymercuration-Demercuration 342 8-7 Hydroboration of Alkenes 343... 

Mechanism 8-6 Hydroboration of an Alkene 345 8-8 Addition of Halogens to Alkenes 349... 

The same alcohol could be made by the Baeyer-Villiger rearrangement but the stereochemistry would have to be set up before the Baeyer-Villiger step. Hydroboration has the advantage that stereochemistry is created in the hydroboration step. We have discussed the details of this step. In drawing the mechanism it is usually best to draw it as a simple concerted four-centre mechanism providing you remember that the regioselectivity is controlled by the initial interaction between the nucleophilic end of the alkene and the empty p orbital on boron. 

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