4-methylcyclopentene

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... 

Methylenecyclopentane has the less substituted double bond and is the major product The reported isomer distribution is 91% methylenecyclopentane and 9% 1 methylcyclopentene ... 

The reaction course taken by photoexcited cycloalkenes in hydroxylic solvents depends on ring size. 1-Methylcyclohexene, 1-methylcycloheptene, and 1-methylcyclooc-tene all add methanol, but neither 1-methylcyclopentene nor norbomene does so. The key intermediate in the addition reactions is believed to be the highly reactive -isomer of the cycloalkene. 

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. 

The stereochemistry of addition of hydrogen halides to alkenes depends on the structure of the alkene and also on the reaction conditions. Addition of hydrogen bromide to cyclohexene and to E- and Z-2-butene is anti.6 The addition of hydrogen chloride to 1 -methylcyclopentene is entirely anti when carried out at 25° C in nitromethane.7... 

Attempts to extend this reaction to the five-membered ring olefins 1-methylcyclopentene and norbomene resulted in 1-methylcyclopentane and methylenecyclopentane for the former and products (43)-(48) for the latter(80) ... 

A summary of addition reactions of alkenes with 1-methylcyclopentene as the organic substrate. A bond designated means that the stereochemistry of the group is unspecified. For brevity the structure of only one enantiomer of the product is shown, even though racemic mixtures would be produced in all instances in which the product is chiral. 

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.

---