2-Bromo-4-methyl-3-hexene

Substitution reactions with (E)- or (Z)-vinylic halides usually show predominant retention of structure in the olefin substitution as they do in the carboalkoxylation, but the specificity is quite dependent on reaction conditions. Low reaction temperature, excess organophosphine, and dilution with excess trialkylamine and/or olefin all appear to improve the specificity. Under favorable conditions, for example, (Z)-l-bromo-l-hexene and methyl acrylate give an 82% yield of the (E, Z) and only 10% of the (E, E) isomer of methyl 2,4-nonadienoate 29). 

There is also the possibility that one combination of reactants will give more isomeric products than the other. For example, this may occur when electron-donating substituents are present in the olefinic reactant. This effect is seen in the reciprocal reactions of methyl /3-bromomethacrylate with 1-hexene and of Z-l-bromo-l-hexene with... 

In a more recent study Co(dppe)I2 was used as a catalyst for reductive additions of primary, secondary, and tertiary alkyl bromides or iodides 249 to alkyl acrylates, acrylonitrile, methyl vinyl ketone, or vinylsulfone 248 in an acetonitrile/water mixture using zinc as a stoichiometric reducing agent [305]. The yields of the resulting esters 252 were mostly good. The authors tested radical probes, such as cyclopropylmethyl bromide or 6-bromo-1-hexene (cf. Part 1, Fig. 8). However, the latter did not cyclize, but isomerized during addition, while the former afforded complicated mixtures. On this basis the authors proposed a traditional two-electron mechanism to be operative the results do not, however, exclude a radical-based Co(I) catalytic cycle convincingly (Fig. 61). 

Bromo-l-hexene, B-60264 (Bromomethyl)cyclopentane, B-70241 5-Bromo-3-methyl-1 -pentene, B-60299... 

Methyl propargyl ketone, 193 - (48) 5-Chloro-3-hexen-l-yne and 5-bromo-3-hexen-l-yne, 193 - (49) 4,5-Hexadien-2-yn-l-ol, 194 -(so) l-Phenylthio-3-methyl-l,2-butadiene, 195... 

Preparation by reaction of 6-bromo-l-hexene with 5-allyl-2,4-dihydroxyace-tophenone in the presence of potassium carbonate and potassium iodide in refluxing methyl ethyl ketone (30%) [2671,2678,2679],... 

What dienes containing isolated double bonds are capable of being formed, but are not observed, in the two preceding equations describing elimination in 3-methyl-5-hexen-3-ol and 4-bromo-4-methyl-1-hexene ... 

One of the classical solutions to overcome the low reactivity is to render the carbomagnesiation intramolecular. For example, Utimoto and coworkers reported that the reaction of ( )-6-bromo-3-methyl-l-trimethylsilyl-l-hexene (85) with magnesium produces the corresponding Grignard reagent 86, which intramolecularly adds to the vinylsilane moiety from the less hindered side affording a single stereoisomer of cycUzed product 87 (Scheme 60) °°. 

Intermolecular carbometallations are typically accomplished with organolithiums however, dia-stereocontrolled intramolecular carbometallation can be attained with appropriate Grignard reagents, as exemplified in Scheme 88, the suprafacial and 5-exo-trig closure of ( )-6-bromo-3-methyl-1 -trimethylsi-lyl-1-hexene (267) to fraru-l-methyl-2-(trimethylsilyl)methylcyclopentane (268).203... 

It is interesting and of preparative value that although 1-bromo-2-methyl-l-propene and similar halides add to 1-hexene at both double-bond carbons, only one of the two 7r-allylic intermediates reacts with the amine. The result is that a mixture of six isomeric dienes is formed, but only one allylic amine is produced. Therefore the reaction is useful since the dienes and the amine are separated easily (2). 

The effect of chain length on the catalytic performance was investigated using a series of co-bromo-2-methylalkenes. In all cases the predominant enantiomer produced had the -configuration except for 3-bromo-2-methylpropene oxide, which was predominantly in the S-form due to the priority switch [274], The short propene and butene derivatives were converted quantitatively whereas the longer pentene, hexene and heptene substrates failed to convert completely. Many other functional groups such as carboxylic ester, methoxy, acetoxy and carbonic ester are accepted by the system. The epoxidation fails, however, for 4-hydroxy-2-methyl-l-butene as substrate [270]. 

As a result of an extensive study, it has been found that methylene groups are attacked much more readily than a methyl group. For example, 2-methyl-2-butene requires 16 hours for completion of the reaction, whereas, 2-methyl-2-hexene requites 10 minutes. The conversion of cyclohexene to 3-broraocyclohexene is accomplished in 20 minutes in 87% yield. It is noteworthy that the bromination of 1-octene with N-btomo-succinimide yields a mixture of l-bromo-2-octene and 3-bromo-1-octene and that the proportion of these isomers is in close agreement with the equilibrium mixture formed at 100° by analogous bromides. ... 

For example, addition of 1,3-dibromopropane to the THF solution of 8 at — 78"C formed the intermediate 9a, which cyclized, when warmed to room temperature, to yield 1-methyl-l-(l-methylethenyl)cyclopentane 13 [11]. On the other hand, an acidic workup of 9a at — 35 C gave a single monoalkylated product, 6-bromo-2,3,3-trimethyl-l-hexene 12, in 72% isolated yield. Similar chemistry was observed with 1,4-dibromobutane, except that no cyclization would occur without refluxing conditions. Interestingly, the cyclization of 9a to 13 represents a cross-coupling of an organomagnesium reagent with an alkyl bromide, which is normally observed only in the presence of certain transition metal salts or complexes [12]. 

Hydrogen halide is also removed when an unsaturated halide is distilled from its mixture with a solid alkali hydroxide 4-methyl-1,3-hexadiene, for example, was prepared in this way from 4-bromo-4-methyl-l-hexene.109... 

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