Mechanism of Hydroboration-Oxidation

A second aspect of hydroboration-oxidation concerns its stereochemistry. As illustrated for the case of 1-methylcyclopentene, H and OH add to the same face of the double bond. 

Overall, the reaction leads to syn addition of H and OH to the double bond. This fact has an important bearing on the mechanism of the process. 

Hydroboration-oxidation of a-pinene (page 231), like catalytic hydrogenation, is stereoselective. Addition takes place at the less hindered face of the double bond, and a single alcohol is produced in high yield (89%). Suggest a reasonable structure for this alcohol. 

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

The regioselectivity of addition is consistent with the electron distribution in the complex. Hydrogen is transferred with a pair of electrons to the carbon atom that can best support a positive charge, namely, the one that bears the methyl group. 

Steric effects may be an even more important factor in controlling the regioselectivity of addition. Boron, with its attached substituents, is much larger than a hydrogen atom and becomes bonded to the less crowded carbon of the double bond, whereas hydrogen becomes bonded to the more crowded carbon. 

Step 1 A molecule of borane (BH3) attacks the alkene. Electrons flow from the Jl orbital of the alkene to the 2p orbital of boron. A 71 complex is formed. 

FIGURE 6.10 Orbital interactions and electron redistribution in the hydroboration of 1-methylcyclopentene. 

In order to simplify our presentation, we U regard the hydroborating agent as if it were borane (BHj) itself rather than B2H6 or the borane-tetrahydrofuran complex. BH3 is electrophilic it has a vacant 2p orbital that interacts with the -it -electron pair of the alkene as shown in step 1 of Mechanism 6.4. The product of this step is an unstable intermediate 

Borane (BH3) does not exist as such at room temperature and atmospheric pressure. Two moiecuies of BH3 combine to give diborane (B2H6), which is the more stable form. 

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This mechanism should remind you of the mechanism of the oxidation of boranes, which often follows on from hydroboration— see Chapter 19, p. 446. 

In the following sections we shall consider details of the mechanism that lead to the anti-Markovnikov regiochemistry and syn stereochemistry of hydroboration-oxidation. 

Hydroboration/oxidation is a process that yields an alcohol that is the product of overall anti-Markovnikov addition. The mechanism of hydroboration is complex, but several lines of evidence have led to the picture we have of a concerted reaction with an unsymmetrical transition state in which one of the alkene s carbon atoms becomes partially positively charged (Figs. 9.55-9.60). The synthetic utility of this reaction is not complex at all. For unsymmetrical aikenes, hydroboration/oxidation leads to the less substituted alcohol. 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

Although the hydroboration-oxidation reaction gives a product with a regiochemistry opposite to that predicted by Markovnikov s rule, the regiochemistry is in accord with the mechanistic version of this rule—that is, the electrophile adds to the less substituted carbon. Let s look at the mechanism of this reaction. 

This process is sometimes abbreviated to S f2 at silicon to save space. The intermediate is a trigonal bipyramid with negatively charged pentacovalent silicon. It is often omitted in drawings because it is formed slowly and decomposes quickly. This mechanism is similar to nucleophilic substitution at boron except that the intermediate is pentacovalent (Si) rather than tetrahedral (B). The hydrolysis of a boron ester at the end of a hydroboration-oxidation sequence would be an example of an analogous boron reaction. 

This chapter focuses on the chemistry ofbiomimetic copper nitrosyl complexes relevant to the NO-copper interactions in proteins that are central players in dissimilatory nitrogen oxide reduction (denitrification). The current state of knowledge of NO-copper interactions in nitrite reductase, a key denitrifying enzyme, is briefly surveyed the syntheses, structures, and reactivity of copper nitrosyl model complexes prepared to date are presented and the insight these model studies provide into the mechanisms of denitrification and the structures of other copper protein nitrosyl intermediates are discussed. Emphasis is placed on analysis of the geometric features, electronic structures, and biomimetic reactivity with NO or NOf of the only structurally characterized copper nitrosyls, a dicopper(II) complex bridged by NO and a mononuclear tris(pyrazolyl)hydroborate complex having a Cu(I)-NO formulation. 

As was already mentioned, the standard procedure for acid catalyzed alkene hydration exhibits a rather low selectivity. On the other hand, the use of a hydroxymercuration-reduction sequence leads to the exclusive formation of Markovnikov s alcohols. A nearly exclusive anti-Markovnikov s hydration is achieved via a hydroboration-oxidation reaction (see Section 2.4). The result in both these cases is the net addition of H2O, but the basic differences in the reaction mechanisms unambiguously determine a reversed regioselectivity pattern. 

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