Methylbutane, 2-, bromination

A base-induced bromination has been reported. 2-Methyl butane reacts with 50% aqueous NaOH and CBr4, in a phase-transfer catalyst, to give a modest yields of 2-bromo-2-methylbutane. ... 

Bromination of an optically active form of the corresponding chloro compound (l-chloro-2-methylbutane) also results in an optically active product, and retention of configuration. It may be that an actual bridged radical is formed, but a somewhat less concrete interaction seems more likely, as halogenation with the more reactive chlorine is found to lead wholly to racemisation. 

Pentafluorocthyl iodide is of practical interest, particularly as a precursor of higher perfluoroal-kyl iodides. There are several patents for the preparation of the key compound from tetra-fluoroethene, iodine pentafluoride and iodine at 75-80 C in the presence of catalysts anti-mony(III) fluoride, titanium(lV) chloride, boron trifluoride, vanadium(V) fluoride, niobium(V) fluoride, and molybdenum(Vl) fluoride.11-13 The agents iodine monofluoride" and bromine monofluoride" can add to branched pcrfluoroalkcnes, e.g. perfluoro-2-methylbut-2-ene gives perfluoro-2-iodo-2-methylbutane.1415... 

This example demonstrates that the rule of thumb is an estimate at best. Most importantly, it is a reminder that the tertiary product will not necessarily be the primary product. Bromine is more selective than chlorine and substituting bromine for chlorine in the same reaction will result in predominately 2-bromo-2-methylbutane. Fluorine, on the other hand, is so reactive that the primary7 product would pre-dominate. 

Reactions with 3-methylbutanal (82) (for ipsenol, 83) or senecio aldehyde (dimethylacrolein, 84, for ipsdienol, 85) were discussed previously (Vol. 4, p. 467). Many further publications on the subject have s peared. Isoprene can be tribrominated, first with bromine in carbon tetrachloride, then with N-bromosuccinimide. This is one route to 2-bromomethyl-l,3-butadiene (9, R = Br), obtained from the tribromide with zinc amalgam reduction, but in fact the tribromide reacted directly with the aldehydes 82 or 84 to give ipsenol (83) or ipsdienol (85). A reagent used for the reaction of 9 (R = Br) with the aldehydes... 

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... 

Methyl-2-pentene added with stirring under Ng to slightly more than the equivalent amount of 9-borabicyclo[3.3.1]nonane in tetrahydrofuran, refluxed 1 hr., tetrahydrofuran replaced by methylene chloride, cooled to 0, bromine added during 1 min. in the dark by syringe through a septum inlet, stirred 0.5 hr. at 0° and 1 hr. at 25°, all in the dark 2-bromo-4-methylpentane. Y 74%. -Similarly 2-Methylbut-2-ene 2-bromo-3-methylbutane. Y 88% by GLC. F. e. s. C. F. Lane and H. C. Brown, J. Organometal. Chem. 2(5, C 51 (1971). 

One way to test for the intermediacy of carbocations in reaction mechanisms is to look for rearrangements, for example, from a 2° carbocation to a 3° carbocation. In the addition of bromine to 3,3-dimethyl-l-butene (7) in methanol, however, the only products observed were l,2-dibromo-3,3-di-methylbutane (8), 45%, and 2-bromo-l-methoxy-3,3-dimethylbutane (9), 44%. There was no evidence for products such as 10, which might have been expected if a free 2° carbocation were formed and then rmderwent a methyl shift to yield a 3° carbocation. Therefore, the intermediate in the addition of bromine to alkyl-substituted alkenes appears not to behave like a carbocation. 

Because homolysis of the bromine-bromine bond in Br2 is more facile (DH° = 193kJmoT [46.1 kcal moT ]) than the bond between chlorine atoms in CI2 (DH° = 242.4 kJmol [58.9kcalmoT ]), it should come as no surprise that when bromine is allowed to react with 2-methylbutane, only 2-bromo-2-methylbutane... 

A substitution reaction at a stereogenic center can lead to a racemic mixture of products. For example, in the free radical reaction of bromine with (5) l-chloro-2-methylbutane, a bromine atom replaces a hydrogen atom at the tertiary stereogenic center to give a racemic mixture of R) and (5)-2-bromo-l-chloro-2-methylbutane. 

In 2-bromo-2-methylbutane the carbon atoms adjacent to the carbon atom bearing the bromine atom are two methyl groups and a methylene group. The products are 2-methyl-1-butene and 2-methyl-... 

Now let s draw the forward scheme. The 3° alcohol is converted to 2-methylpropene using strong acid. Anti-Markovnikov addition of HBr (with peroxides) produces l-bromo-2-methylpropane. Subsequent reaction with sodium acetylide (produced from the 1° alcohol by dehydration, bromination and double elimation/deprotonation as shown) produces 4-methyl-1-pentyne. Deprotonation with sodium amide followed by reaction with 1-bromopentane (made from the 2° alcohol by tosylation, elimination and anfi -Markovnikov addition) yields 2-methyl-4-decyne. Reduction using sodium in liquid ammonia produces the E alkene. Ozonolysis followed by treatment with dimethylsulfide produces an equimolar ratio of the two products, 3-methylbutanal and hexanal. 

Now, let s draw the forward scheme. Radical bromination of 2-methylbutane produces the tertiary alkyl hahde, selectively. Then, elimination with NaOEt, followed by awti-Markovnikov addition (HBr / peroxides), and then elimination with iert-butoxide, followed by another awri-Markovnikov addition (HBr / peroxides) produces l-bromo-3-methylbutane. This alkyl hahde will then undergo an Sn2 reaction when treated with an acetylide ion to give 5-methyl-1-hexyne. Ozonolysis of this terminal alkyne cleaves the CC triple bond, producing the carboxylic acid. Deprotonation (with NaOH) produces a carboxylate nucleophile that subsequently reacts with bromomethane in an Sn2 reaction to give the desired ester. 

The major product is produced following the rearrangement of the 2° carbocation that is formed when the carbon-bromine bond in 2-bromo-3-methylbutane breaks to release a Br ion. As suggested below, two possible rearrangements, labeled (a) and (b), can occur. Both rearrangements involve the movement of a hydrogen atom. These rearrangements are examples of a hydride shift. 

The condensed structural formulas for 2-bromo-3-methylbutane and ethanol are CH3CHBrCH(CH3)2 and CH3CH2OH. The carbon bonded to bromine in the haloalkane is a 2° carbon atom and Br is a very weak base and a good leaving group. CH3CH2OH is a weak base and also a weak nucleophile. The solvent is polar protic. On the basis of this information, we conclude that S l and El reactions will occur (see Figure 27-13). The first step in both the SnI and El mechanisms is the formation of the carbocation ... 

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