2-methyl-2-butene. reaction with borane

When the alkene precursor is hindered (highly substituted), it may be possible to isolate monoalkylboranes or dialkylboranes. Three alkylboranes should be discussed because they are also used in reactions with alkenes. When 2-methyl-2-butene reacts with borane, the product is the dialkylborane 60, and its common name is disiamylborane (from the common name di-secoradoo -isoamylborane). Similar reaction of borane with 2,3-dimethyl-2-butene leads to a monoalkylborane, 61. The common name of this product is thexylborane (from tertiary-he lborane). 

If the hydroboration reaction is to be used to convert 1-alkynes into aldehydes, some way to stop the addition at the vinylborane stage is needed. The problem is that there is not enough steric hindrance at the end carbon of the vinylborane. The solution is to build extra steric hindrance into the other alkyl groups attached to the boron of the vinylborane. A borane, R2BH, with two bulky R groups already attached to the boron is used as the hydroboration reagent. One such reagent is prepared by the reaction of two equivalents of 2-methyl-2-butene (also known by the common name of isoamylene) with borane to produce a dialkylborane called di si amyl borane (a shortened version of diisoamylborane) ... 

BUTEN-3-OL, 3-METHYL (115-18-4) Forms explosive mixture with air (flash point 56°F/13°C). Violent reaction with strong oxidizers. Reacts violently with aliphatic amines, alkalis, ammonium persulfate, boranes, bromine dioxide, isocyanates, nitric acid, perchlorates, permanganates, peroxides, sodium peroxide, sulfuric acid, uranium fluoride. 

When the reaction is done with borane it is usually carried to the trialkylborane stage, but by controlling the stoichiometry the dialkyl or monoalkylborane can be obtained, especially from highly substituted alkenes such as 2-methyl-2-butene or 2,3-dimethylbutene. The resulting dialkyl and monoalkyl boranes are known as disiamylb-orane and thexylborane. Another useful dialkyl borane is 9-borabicyclo[3.3.1]nonane, known as 9-BBN. 

D. Bis-(3-methyl-2-butyl)borane ( Disiamylhorane ).5 A dry 500-ml. three-necked flask is equipped as described in Section A. The apparatus is flushed with nitrogen, and the flask is charged with 92 ml. (0.11 mole) of a 1.23f solution of borane in tetrahydrofuran. The flask is cooled with an ice bath, and to the stirred solution is added a solution of 15.4 g. (0.22 mole) of 2-methyl-2-butene (Note 12) in 40 ml. of anhydrous tetrahydrofuran over a 30-minute period. After the addition is complete, the reaction mixture is kept below approximately 10° for 2 hours. The resulting solution is used directly in the next step. 

The hydroboration of a trisubstituted olefin, exemplified by the reaction of 2-methyl-2-butene with diborane, is conveniently stopped at the dialkylborane stage to produce disiamyl-borane. As a result of its rather large steric requirements this... 

A typical reaction that illustrates Markovnikov addition is the reaction of HBr with 2-methyl-2-butene to give 2-bromo-2-methylbutane (1, sec. 2.10.A). This reaction proceeds by formation of the more stable carbo-cation, which reacts with the nucleophilic bromide ion. If the anti-Markovnikov bromide (the bromine resides on the less substituted carbon) is desired, a different mechanistic pathway must be followed. A typical anti-Markovnikov addition reaction is addition of borane to the alkene, giving primary alcohol (2) after oxidation of the intermediate alkylborane (sec. 5.4.A). This alcohol can be converted to the anti-Markovnikov bromide, 3, by treatment with PBr3. The key to controlling such reactions is a fundamental... 

It is possible to prevent the second hydroboration step and, in effect, stop the reaction at the alkenylborane stage by using a sterically hindered disubstituted borane. One of the most widely used of these is di-sec-isoamyl borane, (sia)2BH, prepared by treating borane with two equivalents of 2-methyl-2-butene (amyl is an older common name... 

The initial products of hydroboration are prone to further hydroboration reactions. This complication can be avoided by using a sterically hindered borane so that further reaction is slowed. A favorite suitably hindered reagent is the borane formed by reaction of two equivalents of 2-methyl-2-butene with BH3 (Fig. 10.74). 

In contrast, a terminal alkyne reacts with mercury(ll) acetate to give a methyl ketone. The hydroboration reaction actually requires a substituted, hindered borane rather than diborane itself. With diborane, the alkenylborane can react with a second equivalent of diborane. Di( 1,2-dimethylpropyl)borane—also called di(r< c-isoamyl)borane and abbreviated disiamylborane—is prepared by adding borane to 2-methyl-2-butene. 

For many purposes, borane does a great job in terms of both reactivity (high) and selectivity (not always so high). However, there are processes where a less reactive and more selective borane will be preferred, and numerous compounds are available. If borane is reacted with 2,3-dimethyl-2-butene, only a single addition takes place, because of the steric hindrance that would result from a further addition. The product is termed thexylborane (11.27). Similarly, disiamylborane (SiajBH, 11.28) is prepared from 2-methyl-2-butene. The number of times the borane reacts is clearly a function of steric hindrance. 9-Borabicyclo[3.3.1]nonane (9-BBNH, 11.29) is prepared from 1,5-cyclooctadiene, and catecholborane (HBcat, 11.30) from catechol (1,2-benzenediol) and borane. All of these have been used to improve selectivity for specific reactions—in general, the more hindered the borane, the more selective the reaction. 

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