Hydroboration of 1-Methylcyclopentene

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

Step 2 The tt complex rearranges to an organoborane. Hydrogen migrates from boron to carbon, carrying with it the two electrons in its bond to boron. 

Transition state for hydride migration Product of addition of borane (BH3) 

The mechanistic complexity of hydroboration-oxidation stands in contrast to the simplicity with which these reactions are carried out experimentally. Both the hydroboration and oxidation steps are extremely rapid reactions and are performed at room temperature with conventional laboratory equipment. Ease of operation, along with the fact that hydroboration-oxidation leads to syn hydration of alkenes with a regioselec-tivity opposite to Markovnikov s rule, makes this procedure one of great value to the synthetic chemist. 

Step 1 is consistent with the regioselectivity of hydroboration. Boron, with its attached substituents is more sterically demanding than hydrogen, and bonds to the less crowded carbon of the double bond hydrogen bonds to the more crowded one. Electronic effects are believed to be less important than steric ones, but point in the same direction. Hydrogen is transferred with a pair of electrons to the carbon atom that bears more of the positive charge in the tt complex, namely, the one that bears the methyl group. 

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We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

FIGURE 6 10 Orbital inter actions and electron flow in the hydroboration of 1 methylcyclopentene... 

Hvdroboration always gives sy.n addition of the clcmcnts.-of -boron—and hydrogen to a double bond. For example, hydroboration of 1-methylcyclopentene gives only the product shown in Equation 7.19.61... 

The asymmetric synthesis of enantiomerically pure primary amines has received considerable attention in recent years due to applications of the chiral amines, either as chiral auxiliaries for the synthesis of optically active molecules [33] or as a deri-vatizing agent for the resolution of racemic carboxylic acids [34], Hydroboration -amination is also a convenient synthetic route to epimerically clean amine derivatives in a simple one-stage reaction. Interestingly, rrans-2-phenylcyclopentylamine (cypenamine), which is an antidepressant [35], can be obtained as a pure isomer in good yields by the hydroboration of 1-methylcyclopentene [7,10,36] (Scheme 13). 

In the hydroboration of 1-methylcyclopentene shown in Solved Problem 8-3, the reagents are achiral, and the products are chiral. The product is a racemic mixture of frans-2-methylcy-clopentanol, but only one enantiomer is shown. Show how the other enantiomer is formed. 

Either material when used for hydroboration of 1-methylcyclopentene gives, after oxidation, frans-2-methylcyclopentanol in 70-75% optical purify. 

Hydroboration of 1-methylcyclopentene (1) with diborane gives tris-(frans-2-methyl-cyclopentyl)borane (2), which on oxidation gives tra/is-2-methylcyclopentanol (3).1 Addition of sodium cyanide to a solution of (2), followed by treatment with trifluoro-acetic anhydride (TFAA) and then by oxidation with alkaline hydrogen peroxide, gives... 

Mechanism of the hydroboration of 1-methylcyclopentene. The favored transition state is the one that places the partial positive charge on the more highly... 

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... 

Hydroboration of 1-Methylcyclopentene 248 6.10 Acid-Catalyzed Dimerization of 2-Methylpropene 262... 

We can see the results of a syn addition in our example involving the hydroboration of 1-methylcyclopentene. Formation of the enantiomer, tvhich is equally likely, results when the boron hydride adds to the top face of the 1-methylcyclopentene ring ... 

Both the regioselectivity and syn stereoselectivity are illustrated by hydroboration of 1-methylcyclopentene. 

Scheme 6.25. A representation of the oxidative hydroboration of 1-methylcyclopentene (as a typical alkene). The addition of boron and hydride (H ) is suprafacial with the double bond attacking the boron so as to build up charge on that carbon of the double bond that would be the most stable carbocation it is to this carbon of the double bond that the hydride (H") adds. In the oxidation step, the anion of hydrogen peroxide (H02 ) attacks the boron. Subsequent rearrangement of carbon to oxygen with oxygen-oxygen bond cleavage produces the alcohol. Only one ligand to boron is shown to emphasize the oxidation process.

Acid-Catalyzed Hydration of 2-Methylpropene 227 Hydroboration of 1-Methylcyclopentene 233 Oxidation of an Organoborane 235 Bromine Addition to Cyclopentene 237 Epoxidation of Bicyclo[2.2.1]-2-heptene 240 Free-Radical Addition of Hydrogen Bromide to 1-Butene 243... 

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