Tert-Butyl bromide intermediate

The [4+1] annulation of 1-azadienes to pyrroles can also be achieved through their carbonyl iron complexes (Scheme 6). Novel complex (1,4-diphenyl-2-methyl-l-azabutadiene)tricarbonyliron (0) 24 was obtained in 40% yield from the corresponding azadiene 23 and Fe2(CO)9 then nucleophilic attack by methyl lithium and quenching with tert-butyl bromide, as the proton source, gave 2,5-dimethyl-l,3-diphenylpyrrole 26 in 70% yield, probably through the anionic intermediate complex 25 [88TL1425 90JCS(P1)761]. 

Addition of HBr to 2-methylpropene gives mainly tert-butyl bromide, because the product with the more stable carbocation intermediate always predominates in this type of reaction. 

But the addition product is 99+% tert-butyl bromide so the reaction clearly is kinetically controlled, tert-butyl bromide being formed considerably faster than isobutyl bromide. The slow, or rate-determining, step in this reaction is the formation of the intermediate cation rather than the reaction of the cation with bromide ion. So to account for the formation of tert-butyl bromide we have to consider why the tert-butyl cation is formed more rapidly than the isobutyl cation ... 

This is manifest in the reactivity of 180/180-Z1 which was generated from 3-bromo-41-f-pyran (283) by /3-elimination of hydrogen bromide with KOtBu (Scheme 6.61). Whether or not this reaction was conducted in the presence of styrene or furan, the only product identified was tert-butyl 4H-pyran-4-yl ether (284). This is in line with the relationship of the intermediate to a pyrylium ion. Thus, the addition of the tert-butoxide ion to 180/180-Zj has to be expected at the 4-position with formation of the vinyl anion 285, which is then protonated to give 284. Likewise, the attack of the nucleophile is predicted at C2and C6 leading to the vinyl anions 286, which... 

The El reaction involves the formation of a planar carbocation intermediate. Therefore, both syn and anti elimination can occur. If an elimination reaction removes two substituents from the same side of the C—C bond, the reaction is called a syn elimination. When the substituents are removed from opposite sides of the C—C bond, the reaction is called an anti elimination. Thus, depending on the substrates El reaction forms a mixture of cis (Z) and trans (E) products. For example, tert-hutyl bromide (3° alkyl halide) reacts with water to form 2-methylpropene, following an El mechanism. The reaction requires a good ionizing solvent and a weak base. When the carbocation is formed, SnI and El processes compete with each other, and often mixtures of elimination and substitution products occur. The reaction of t-butyl bromide and ethanol gives major product via El and minor product via SnI-... 

Imidazole or pyridine mediated silylation of l,2-0-[l-exo-ethoxy )ethylidene]-oc-D-glucopyranose failed to give a high yield of the 6-silyl ether due to some polymerization and side reactions. Activation of hydroxyl groups via a tributylstannyl intermediate followed by the tetrabutylammonium bromide catalyzed reaction with tert-butyl-chlorodiphenylsilane was more successful [231], the 6-0-silyl derivative being isolated in 87 % yield.. The lability of this protecting group under benzylation with benzyl bromide and sodium hydride at 0 °C has been observed [449]. 

On the basis of kinetic stndies the presence of a peroxo-intermediate was postdated and spectroscopic evidence for such an intermediate has been obtained. Brs", Br2, or HOBr appear to be the primary reaction prodncts of the enzyme-mediated peroxidation of bromide. The vanadium enzyme also nses phenylperacetic acid, m-chloroperoxybenzoic acid, and jo-nitroperoxybenzoic acid as oxidants, but alkyl peroxides such as ethylhydroperoxide, tert-butyl hydroperoxide, and cuminyl hydroperoxide are not substrates for the enzyme in the oxidation of bromide. The enzyme from the brown seaweed Ascophyllum nodosum does not display any specificity with regard to bromination of various organic nucleophilic acceptors which suggests a mechanism in which the oxidized bromine species are released into solution by the enzyme. Figure 1 gives a simple model for the reaction mechanism of the enzyme. 

In contrast, the hydrolysis of tm-butyl bromide (2-bromo-2-methylpropane) occurs in a stepwise manner (reaction 1.1b). In the first slow step, the C-Br bond breaks, with the bromine atom taking both electrons from the bond and leaving as a negatively charged bromide ion. The remainder of the molecule is the positively charged tert-butyl cation (2-methylprop-2-yl cation). This is a highly reactive intermediate, which reacts rapidly with the hydroxide ion to form the corresponding alcohol. 

The starting material, methoxytetralin (293), was converted into the a,(3-unsaturated ketone (294) by Birch reduction. The allene adduct (295) was prepared from ketone 294 and was then rearranged to the hydroxyl ketone (298) via intermediates 296 and 297. Compound 298 was methylated with sodium hydride and methyliodide to give the methoxyketone (299) in quanitative yield. Compound 299 was rearranged to 300 by refluxing with tert-butyl perbenzoate and cuprous bromide in absolute... 

Allyl bromide added drop wise with stirring to a soln. of tetramethylammonium di-tert-butyl phosphate in boiling dimethoxyethane, refluxed 3 hrs., the crude intermediate triester dissolved in benzene and trifluoroacetic acid, allowed to stand 18 hrs. at room temp. allyl phosphate. Y 83% as the anilinium salt. -This method appears to be particularly suited to the phosphorylation of polyfunctional halides containing no acid-labile groups. F. e. s. A. Zwierzak and M. Kluba, Tetrahedron 27, 3163 (1971). 

The conversion of aldehydes into vinyl iodides through the formation and subsequent iodination of hydrazones is known as the Barton vinyl iodide synthesis [202]. This synthesis can be quite challenging, and a practical approach to this chemistry has been developed using Af-tert-butyldimethylsilylhydrazones as intermediates (Scheme 7.139) [203], The initial formation of the A(-silylhydrazones was catalyzed by scandium(lll) inflate, and the iodination step was aided by the addition of 2-tert-butyl-l,l,3,3-tetramethylguanidine (BTMG). In addition to the generation of vinyl iodides, the chemistry was also extended to the synthesis of vinyl bromides. 

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