Four-center reactions, hydroboration

Why does alkene hydroboration take place with non-Markovnikov rc o-chemiatry, yielding the less highly substituted alcohol Hydroboration differs from many other alkene oddition reactions in that it occurs in a single step without a carlK>cation intemnediate. We can view the reaction as taking place through a four-center, cyclic transition state, as shown in Figure 7.6 Cp. 244 j. Since both 0-H and C-B bonds form at the same time and from the same face of the alkene, syn stereochemistry is observed. 

Diborane as well as mono- and dialkylboranes give cis addition in reactions with alkenes, as noted above, which is a result of the four-center transition state in which the boron and hydrogen are delivered from the same face, illustrated by hydroboration of 1-methylcyclopentene with R2BH to give 19. The methyl-bearing... 

The hydroboration reaction with alkenes to produce alkylboranes (sec. 5.2) proceeds by a four-center transition state rather than a cationic intermediate. The regiochemistry of the final alkylborane product is controlled by the nonbonded steric interactions of the groups attached to boron (in this case sec-isoamyl from the disiamylborane) and the groups on the alkene. Oxidation with basic hydrogen peroxide converts the borane to the anti-Markovnikov alcohol (2). The difference in regiochemistry between 1 and 2 arose because the mechanism for generating each relied on difference factors. 

In Chapter 5, the cis addition inherent to hydroboration is the result of a four-center transition state (sec. 5.2.A.ii), which delivered boron and hydrogen from the same face. Subsequent transformations that involve boron lead to a new group on the same face as boron. Reaction of methylcyclopentene with 9-BBN, for example, gave exclusively the trans borane (96), and that was converted to aldehyde 97 by carbonylation. The addition of the boron dictated the stereochemical relationship of the methyl group and the 9-BBN moiety. The carbonylation and oxidation reactions proceeded with net retention of configuration, maintaining the trans geometry in the isolated product. 

The hydroboration reaction is also very predictable with regard to the stereochemistry of addition. The addition occurs stereospecifically syn through a four-center transition state with essentially simultaneous bonding to boron and hydrogen. Both the new C-B and C-H bonds are therefore formed from the same side of the multiple bond. In molecular orbital terms, the addition reaction is viewed as taking place by interaction of the olefin rr-orbital with the empty p-orbital on trivalent boron. Formation of the carbon-boron bond is accompanied by concerted rupture of a B-H bond ... 

While the four-center transition structure for BH3 addition is a widely used model, other reaction pathways have also been considered. In a synthesis of optically active (-)-l-butanol-l-d, Streitwieser and co-workers used the optically active borane formed from diborane and (-l- )-a-pinene (R2BH) to carry out the hydroboration-oxidation of (Z)-l-butene-l-d. To explain the observed stereochemistry of the reduction, they proposed that hydroboration involves a n complex between R2BH and the alkene (Figure 9.41). Note the close resemblance of such a complex to the cyclic structures... 

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... 

STEP 1. Reaction of a nucleophile and an electrophile to form a new covalent bond. The addition of borane to an alkene is initiated by coordination of the vacant 2p orbital of boron (an electrophile) with the electron pair of the pi bond (a nucleophile). Chemists account for the stereoselectivity of hydroboration by proposing the formation of a cyclic, four-center transition state. Boron and hydro-... 

Note that the B and the H of the borane are on the same side of the ring in 65, which is a consequence of the four-center transition state and concerted asynchronous delivery of B and H to the C=C unit. This reaction constitutes a cis addition of borane to the alkene, where the B and the H add cis (on the same side). In the case of methylcyclopentene, the cis addition of B and H leads to a trans relationship between the BH2 unit and the methyl group in 65, but the hydroboration reaction is a cis addition. 

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. 

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