Hydroboration concerted mechanism

As can be seen in Figure 4, the hydroboration-oxidation of 1-methylcyclopentene produces only the trans-2-methyl-cyclopentanol with a yield of 86%.45 This result implies that the boron atom and the hydrogen atom are added to the double bond simultaneously on a syn mechanism.46 A concerted mechanism is invoked for these additions - it is shown in Figure 5.1... 

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. 

There is another reason we know that the concerted mechanism for hydroboration is correct, and it involves the stereochemistry of the addition reaction. 

B-alkyl-9-borabicyclo[3.3.1]nonanes undergo olefin-alkyl group exchange when refluxed with an olefin in THF. Kinetic and competition studies support a dehydroboration-hydroboration process rather than a concerted mechanism. Ab initio M.O. calculations show that the reaction between C2H4 and BH3 proceeds through a two-step process. A loose three-center (C—B—C) tt complex is formed which is then transformed into the product via a four-center transition state in a rate-determining step. 

Although it is less obvious than in preceding systems, hydroboration can be considered as a concerted mechanism hydrogen atoms in boranes have hydride ion character, and accordingly the stereo cis addition of boranes to unsymmetric double bonds proceeds regioselectively and in the anti-Markovnikoff sense. Several examples are reported in Scheme 10 (MVK is methyl vinyl ketone). 

Unexpected results can lead to new opportunities, but the provenance of the discovery is not always clear from research publications. Bunnett [21 ] admitted that the discovery of the S RN1 mechanism (Scheme 2.9) was serendipitous, and the discoveries of hydroboration [4] and of the conservation of orbital symmetry in concerted reactions (Woodward was working on the synthesis of vitamin B12) [29] originated in unexpected results. In another context, monitoring by NMR of reaction products as they were formed led to the chance observation of negative signals (emission of radiation instead of the usual absorption), explained by CIDNP in radical pairs (Scheme 2.6 earlier see also Chapter 10) [30]. However, as Pasteur remarked in the nineteenth century [31], chance favours only the prepared mind . 

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. 

Because syn addition to the double bond occurs and no carbocation rearrangements are observed, carbocations are not formed during hydroboration, as shown in Mechanism 10.5. The proposed mechanism involves a concerted addition of H and BH2 from the same side of the planar double bond the it bond and H-BH2 bond are broken as two new o bonds are formed. Because four atoms are involved, the transition state is said to be four-centered. 

Given the considerations mentioned above, the mechanism of Scheme 2 is proposed. This mechanism features a concerted addition of the Si-H bond to the C=C bond of the substrate, in a manner analogous to the B-C bond-forming step in the hydroboration of alkenes. Indeed,... 

They react with terminal alkynes by electrophilic addition of the empty p-orbital to the unsubstituted end of the triple bond 83. The intermediate would then be the more substituted vinyl cation 84. It is easier to draw this mechanism with R2BH than with the full structure for 9-BBN. The intermediate 84 is not fully formed before hydride transfer begins so that the reaction is semi-concerted and the transition state is something like 86. The result is a regioselective and stereospecific cis hydroboration of the triple bond to give the A-vinyl borane 85. The intermediate 84 is quite like the radical intermediate in hydrostannylation but the difference is that hydrogen transfer is intramolecular and stereospecific in hydroboration. 

More often in organometallic chemistry, the catalytic reaction occurs by a mechanism that is completely different from the mechanism of the uncatalyzed process. In this case, the reaction typically occurs by more steps, but the activation energy of each of the individual steps is lower than the activation energy of the imcatalyzed process. The overall barrier is then lower than that of the uncatalyzed reaction. A comparison of the uncatalyzed and catalyzed hydroboration of alkenes with a dialkoxyborane (ROl BH, such as cat-echolborane (see Chapter 16), illustrates this scenario. Qualitative reaction coordinates for tihe uncatalyzed and rhodium-catalyzed process are shown in Figure 14.4. In the absence of a catalyst, the B-H bond adds across the alkene through a concerted four-center transition state, albeit at elevated temperatures in neat alkene. hi contrast, late transition metal-catalyzed hydroborations first cleave the B-H bond by oxidative addition. Coordination... 

Hydroboration, as we have seen, can be classified as a concerted addition reaction in which no intermediate is formed. The mechanism is characteristic of a group of reactions called pericyclic (from the Greek, meaning around the circle) reactions, which involve a cyclic shift of electrons in and around the transition state.The mechanism proposed is further supported by the fact that rearrangements are not normally observed in hydroboration reactions, which implies that there are no carbocationic intermediates. 

We will begin by focusing on the first step of hydroboration, in which borane is attacked by a Tt bond, triggering a simultaneous hydride shift. In other words, formation of the C—BH2 bond and formation of the C—H bond occur together in a concerted process. This step of the proposed mechanism explains both the regioselectivity (Markovnikov addition) as well as the stereospecificity (syn addition) for this process. Each of these features will now be discussed in more detail. 

The observed stereospecificity for hydroboration-oxidation is consistent with the first step of the proposed mechanism, in which H and BH2 are simultaneously added across the it bond of the alkene. The concerted nature of this step requires that both groups add across the same face of the alkene, giving a syn addition. In this way, the proposed mechanism explains not only the regiochemistry but also the stereochemistry. 

VWiat is the expected stereospecificity (syn or anti) We have seen that hydroboration-oxidation produces a syn addition. Think about the mechanism of this process, and recall the reason for syn addition. In the first step, BH2 and H are added to the same face of the alkene in a concerted process. To determine if this syn requirement is even relevant in this case, we must analyze how many chirality centers are being formed in the process ... 

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 ... 

FIGURE 9.56 An intermediate carbocation of path b should lead to rearrangements in hydroboration, but no evidence of such can be found, which suggests that the concerted path a is the correct mechanism. 

The current mechanistic hypothesis for hydroboration, a concerted, one-step syn addition to alkenes in which the two new bonds are partially formed to different extents in the transition state, nicely rationalizes all the experimental data. A mechanism involving formation of an open cation followed by hydride transfer fails to account for the observed syn stereochemistry of addition, which is demanded by the one-step mechanism. 

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. 

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