Halogenating reagents

Entry Substituents Halogenation reagent Position Yield (%) Ref. 

Other Electrophilic Reactants. ReversibHity of the electrophilic reactions enables substituted dye derivatives to be obtained. Thus, the halogenation of cyanines, oxonoles, and merocyanines has been studied (3,65,66). Halogen atoms are mobHe in the polymethine chain, and the derivatives themselves can function as halogenation reagents. 

Af-Halogeno-, fV-nitroso- and hence fV-amino-azetidines have been prepared from azetidines by reaction with positive halogen reagents and nitrosating agents, respectively (Section 5.09.2.2.3). 

Other interactions of /3-lactams with electrophiles include the oxidative decarboxylation of the azetidin-2-one-4-carboxylic acid (85) on treatment with LTA and pyridine (81M867), and the reaction of the azetidin-2-one-4-sulfinic acid (86) with positive halogen reagents. This affords a mixture of cis- and trans-4-halogeno-/3-lactams (87), the latter undergoing cyclization to give the bicyclic /3-lactam (88) (8UOC3568). 

The last two of these reactions also feature a reduction in oxidation state. A closely related route is halogen exchange usually in the presence of an excess of the halogenating reagent , e.g. ... 

Positive halogen reagents can cyclize y- and 8-hydroxyalkenes to tetrahydro-furan and tetrahydropyran derivatives, respectively.85 Iodocyclization of homoal-lylic alcohols generates 3-iodotetrahydrofiirans when conducted in anhydrous acetonitrile.86 The reactions are stereospecific, with the /(-alcohols generating the irons and the Z-isomer the cis product. These are endo-5 cyclizations, which are preferred to exo-4 reactions. 

Trivalent phosphorus-halogen reagents have been noted to be of use in obtaining simple Abramov-type products with chloral199 200 and with aldimines.201 Similarly, phosphorus-carboxylate anhydrides have been useful in overcoming the stereochemical difficulties associated with alkyl transfer for obtaining Abramov-type products in a direct manner.202 205... 

In addition to carbonyl substrates, imines have been used extensively with phosphorus-halogen reagents for the preparation of a variety of phosphonates and phosphinates. Combined in a reaction medium, secondary amines react with formaldehyde and phosphorus trichloride134 135 or alkyldichlorophosphines136 to produce N,N-disubstituted aminomethylphosphonates or -phosphinates. These reactions occur under mild conditions with good yield. Similarly, aliphatic carboxylic amides react with aldehydes to generate imines, which can be used in situ with diphenylchlorophosphine to produce... 

In the case of the tantalum complexes 100, reversible hydrogen migration may occur at room temperature in the presence of carbon monoxide, or at 70°C with dihydrogen or dimethylphosphino ethane to afford complexes 106, 107, and 108, respectively.92,95 In contrast, the phosphametallacycle remains intact when 100 is treated with halogenated reagents such as CH3X (X = Cl, Br, I).92,95... 

The reactivity of the methoxyethy1idene derivatives with nucleophiles other than water has been examined. It has been already found (13) that they may be readily opened by halogenating reagents ( N -b romo succ i n i-mide for instance) to give a or 3-ha 1 oacetates (respectively from five- or six-membered-ring orthoesters), and this reaction may be compare with the familiar Hanessian procedure for conversion of henzylidene acetals iiito bro-mo-deoXybenzoates (68). ... 

Electrophilic iodine reagents have also been employed in iodocyclization. Several salts of pyridine complexes with I+ such as bis(pyiidinium)iodonium tetrafluoroborate and bis(collidine)iodonium hexafluorophosphate have proven especially effective.61 y-Hydroxy- and d-hydroxyalkcncs can be cyclized to tetrahydrofuran and tetrahydropyran derivatives, respectively, by positive halogen reagents.62 (see entries 6 and 8 in Scheme... 

Chloramine-T has also been employed, both as a halogenating reagent and base the reaction proceeds in good yield with aromatic as well as with aliphatic aldoximes (81). The role of chloramine-T probably involves an initial chlorination of the aldoxime to give the hydroximoyl chloride, followed by base-catalyzed hydrogen chloride elimination to afford the nitrile oxide (81). 

B. Classon, Z. Liu, and B. Samuelsson, New halogenation reagent systems useful for the mild one-step conversion of alcohols into iodides or bromides, J. Org. Chem. 53 6126 (1988). 

Pyrimidines undergo halogenation at position 5 (Eq. 5-17), while guanine reacts at position 8. Adenine is quite unreactive. Elemental halogens or a variety of other halogenating reagents may be used. Of special... 

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