1 -methylcyclopentene hydroboration

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... 

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... 

The hydroboration is a y -stereospecific reaction. For example reaction with 1-methylcyclopentene 11 yields the l,2-tra 5-disubstituted product 12 only ... 

One of the features that makes the hydrobora ( ion reaction so useful is the regiochemistry that results when an unsymmetrical alkene is hydroborated. For example, hydroboration/oxidation of 1-methylcyclopentene yields trans-2-methylcydopentanol. Boron and hydrogen both add to the alkene from the same face of the double bond—that is, with syn stereochemistry, the opposite of anti—with boron attaching to the less highly substituted carbon. During the oxidation step, the boron is replaced by an -OH with the same stereochemistry, resulting in an overall syn non-Markovnikov addition of water. This stereochemical result is particularly useful because it is complementary to the Markovnikov regiochemistry observed for oxymercuration. 

Figure 11.1 The hydroboration-oxidation of 1-methylcyclopentene. The first reaction is a syn addition of borane. (In this illustration we have shown the boron and hydrogen both entering from the bottom side of 1-methylcyclopentene. The reaction also takes place from the top side at an equal rate to produce the enantiomer.) In the second reaction the boron atom is replaced by a hydroxyl group with retention of configuration. The product is a trans compound (trans-2-methyl-cyclopentanol), and the overall result is the syn addition of -H and -OH.

Hydroborating reagent 3-Methylcyclopentene 3 -Methy lcyclohexene 7,7-Dimethylnorbomene ... 

Hydroboration of an alkene and subsequent reactions of the product trialkylborane, either with hydrogen peroxide or with acid, appear to be highly stereospecific. For example, 1-methylcyclopentene gives exclusively trans-2-methylcyclopentanol on hydroboration followed by reaction with alkaline... 

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... 

Hydroboration-oxidation converts 1-methylcyclopentene to the desired alcohol by anti-Markovnikov syn hydration of the double bond. The resulting alcohol is then converted to its p-toluenesulfonate ester and treated with acetate ion as in part (a) to give m-2-methyl-cyclopentyl acetate. 

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). 

Working backward, use hydroboration-oxidation to form 2-methyl-cyclopentanol from 1-methylcyclopentene. The use of (1) and (2) above and below the reaction arrow indicates individual steps in a two-step sequence. 

The stereochemistry of the hydroboration-oxidation of 1-methylcyclopentene is shown next. Boron and hydrogen add to the same face of the double bond (syn) to form a trialkylborane. Oxidation of the trialkylborane replaces boron with a hydroxyl group in the same stereochemical position. The product is frracemic mixture is expected because a chiral product is formed from achiral reagents. 

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. 

Problem 7.9 What product will result from hydroboration/oxidationof l-methylcyclopentene with... 

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... 

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... 

An alternative approach to monoalkylboranes via hydroboration is based on thexylborane. The 1 1 reaction of di- and more reactive trisubstituted olefins, e.g., 1-methylcyclopentene at —20° to — 25°C stops at the thexylmonoalkylborane stage . Less reactive trisubstituted olefins, e.g., 1-methylcyclohexene and a-pinene, react slowly, and the corresponding thexylmonoalkylboranes are formed only in 80% and 75 % yields, respectively. At 0°C, dehydroboration of 2,3-dimethyl-2-butene occurs owing to the equilibrium ... 

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

Problem 7.9 What product will result from hydroboration/oxidation of l-methylcyclopentene with deuterated borane, BD3 Show both the stereochemistry /spatial arrangement) and the regiochemistry (orientation of the product. 

Methylcyclohexanol, dehydration of, 183 1-Methylcyclopentene addition of hydrogen chloride, 215 hydroboration-oxidation, 230-233 Methylenecyclohexane, 677 Methylene group, 57 prefix, 170... 

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