Halogenation rearrangements

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

Preparation from Nitrene Intermediates. A convenient, small-scale method for the conversion of carboxyhc acid derivatives into isocyanates involves electron sextet rearrangements, such as the ones described by Hofmann and Curtius (12). For example, treatment of ben2amide [55-21-0] with halogens leads to an A/-haloamide (2) which, in the presence of base, forms a nitrene intermediate (3). The nitrene intermediate undergoes rapid rearrangement to yield an isocyanate. Ureas can also be formed in the process if water is present (18,19). 

Other Reactions. Primary amyl alcohols can be halogenated to the corresponding chlorides by reaction with hydrogen chloride in hexamethylphosphoramide (87). Neopentyl chloride [753-89-9] is formed without contamination by rearrangement products. A convenient method for preparing / f/-amyl bromide and iodide involves reaction of / f/-amyl alcohol with hydrobromic or hydroiodic acid in the presence of Li or Ca haUde (88). The metal haUdes increase the yields (85 —95%) and product purity. 

Introduction of a 3-bromosubstituent onto thiophene is accompHshed by initial tribromination, followed by reduction of the a-bromines by treatment with zinc/acetic acid, thereby utilizing only one of three bromines introduced. The so-called halogen dance sequence of reactions, whereby bromothiophenes are treated with base, causing proton abstraction and rearrangement of bromine to the produce the most-stable anion, has also been used to introduce a bromine atom at position 3. The formation of 3-bromotbiopbene [872-31-1] from this sequence of reactions (17) is an efficient use of bromine. Vapor-phase techniques have also been proposed to achieve this halogen migration (18), but with less specificity. Table 3 summarizes properties of some brominated thiophenes. 

A-Halogenated pyrazoles are unstable compounds (Cl>Br>I) that are seldom isolated. 1-Bromopyrazoles resemble NBS and may be important in the process of C-bromination, not because of an A to C rearrangement but by acting as a source of the powerfully electrophilic brominium ion (Section 4.04.2.1.4(v)). 4-Substituted pyrazoles can form... 

Imtdazo[4,5-c]pyridtne, 4,5,6,7-tetrahydro-synthesis, 5, 623, 640, 641 Imidazo[4,5-c]pyridine-6-carboxylic acid, 4,5,6,7-tetrahydro-synthesis, 5, 623, 641 Imidazopyridines as anthelmintic, 1, 202 synthesis, 5, 462 Imidazo[l,2-n]pyridines deuterium exchange, 5, 611 diazo coupling, 5, 614 Dimroth rearrangement, 5, 613 halogenation, 5, 611 hydrogenation, 5, 614 Mannich reaction, 5, 612 nitration, 5, 612 1-oxides... 

Purine, 1,6-dihydro-8,9-dimethyl-6-thioxo-synthesis, 5, 583 Purine, 2,6-dimethoxy-synthesis, 5, 596 Purine, 2,6-dimethoxy-7-methyl-rearrangement, 5, 558 Purine, 2,7-dimethyl-halogenation, 5, 547 Purine, 7,9-dimethyl-UV spectra, 5, 517 Purine, 8,8-dimethyl-synthesis, 5, 580 Purine, 6-dimethylamino-mass spectra, 5, 519 occurrence, 5, 600 Purine, 6-dimethylamino-9-benzyl-alkylation, 5, 531 Purine, 3,7-dimethyl-6,8-dioxo-methylation, 5, 534 Purine, 6,8-dioxo-alkylation, 5, 534... 

Pyrimidine, 5-acetyI-2,4-dimethyI-synthesis, 3, 125 Pyrimidine, acylamino-deaeylation, 3, 85 Pyrimidine, alkoxy-hydrolysis, 3, 91 Primary Synthesis, 3, 134 synthesis, 3, 132, 134 Pyrimidine, 2-alkoxy-aminolysis, 3, 92 rearrangement, 3, 92, 135 synthesis, 3, 134 transalkoxylation, 3, 92 Pyrimidine, 4-alkoxy-aminolysis, 3, 92 rearrangement, 3, 92, 135 synthesis, 3, 134 transalkoxylation, 3, 92 Pyrimidine, 6-alkoxy-aminolysis, 3, 92 rearrangement, 3, 92, 135 synthesis, 3, 134 transalkoxylation, 3, 92 Pyrimidine, alkyl-halogenation, 3, 76 nitration, 3, 77 oxidation, 3, 76 synthesis, 3, 124... 

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