Dihalo

The same disconnection is also effective for cyclopropanes but the reagent for the earbene synthon is a diazocompound RCHN2 or a dihalo compound treated with a metal e.g. 

Treatment of geminal dihalocyclopropyl compounds with a strong base such as butyl lithium has been for several years the most versatile method for cumulenes. The dihalo compounds are easily obtained by addition of dihalocarbenes to double--bond systems If the dihalocyclopropanes are reacted at low temperatures with alkyllithium, a cyclopropane carbenoid is formed, which in general decomposes above -40 to -50°C to afford the cumulene. Although at present a number of alternative methods are available , the above-mentioned synthesis is the only suitable one for cyclic cumulenes [e.g. 1,2-cyclononadiene and 1,2,3-cyclodecatriene] and substituted non-cyclic cumulenes [e.g. (CH3)2C=C=C=C(CH3)2]. 

The frequencies of suite III are usually lower for disubstituted (especially 2,4-dihalo and 2,5-disubstituted thiazoles) and those of the corresponding monosubstituted derivatives. 

Halogens add to butenediol, giving 2,3-dihalo-l,4-butanediol (90,91). In a reaction typical of aHyhc alcohols, hydrogen haUdes cause substitution of halogen for hydroxyl (103). 

Reactions of the Aromatic Ring. The aromatic ring of hydroxybenzaldehydes participates in several typical aromatic electrophilic reactions. Ha.logena.tlon, Chlorination and bromination yield mono- and dihalo derivatives, depending on reaction conditions. Bromination of / -hydroxy-benzaldehyde in chloroform yields 65—75% of the product shown (39). 

Hydroxybenzaldehyde has extensive use as an intermediate in the synthesis of a variety of agricultural chemicals. Halogenation of Nhydroxybenzaldehyde, followed by conversion to the oxime, and subsequent dehydration results in the formation of 3,5-dihalo-4-hydroxybenzonitrile (2). Both the dibromo- and dhodo-compounds are commercially important contact herbicides, hromoxynil [1689-84-5] (2) where X = Br, and ioxynil [1689-83-4]( where X = I respectively (74). Several hydrazone derivatives have also been shown to be active herbicides (70). 

Halogenation. Halogens add directly to the double bond of maleic anhydride to give dihalo succinic acids. However, different procedures are used for dihalomaleic anhydride derivatives. Fluorinated substrates offer access to difluoromaleic anhydride [669-78-3] (64). 

Direct halogenation of S-hydroxyquiaoline has been used as a route to 5,7-dihalo derivatives. Compounds of this type reported iaclude... 

A number of compounds of the types RSbY2 and R2SbY, where Y is an anionic group other than halogen, have been prepared by the reaction of dihalo- or halostibines with lithium, sodium, or ammonium alkoxides (118,119), amides (120), azides (121), carboxylates (122), dithiocarbamates (123), mercaptides (124,125), or phenoxides (118). Dihalo- and halostibines can also be converted to compounds in which an antimony is linked to a main group (126) or transition metal (127). 

In addition to the trialkyldihalo- and triaryldihalo antimony compounds, mixed dihalo compounds such as chi oroiodo triphenyl antimony [7289-82-9], (CgH3)2SbClI, have been reported (182). It has been shown, however, that such compounds disproportionate ia solutioa to give a mixture of startiag material plus products (183) ... 

Trialkyl- and triaryldihaloarsoranes have been studied to a much greater extent than the tri- and tetrahaloatsotanes. The dihalo compounds ate stable crystalline species, although they decompose on heating ... 

Addition to the Double Bond. Chlorine, bromine, and iodine react with aHyl chloride at temperatures below the inception of the substitution reaction to produce the 1,2,3-trihaLides. High temperature halogenation by a free-radical mechanism leads to unsaturated dihalides CH2=CHCHC1X. Hypochlorous and hypobromous acids add to form glycerol dihalohydrins, principally the 2,3-dihalo isomer. Dehydrohalogenation with alkah to epicbl orobydrin [106-89-8] is ofgreat industrial importance. 

Halogens add to the double bond of the alcohol to afford the corresponding dihalo derivatives, eg, CgH CHXCHXCH20H, where X = Cl or Br. The allyHc chloride C H Cl [2687-12-9] can be obtained by treatment of the alcohol with hydrochloric acid, thionyl chloride, or carbon tetrachloride—triphenylphosphine as the halogen donor. 

The reactions of haloquinoxalines in which the halogen atom is bonded to the benzenoid ring have not been well studied, but by analogy with examples in the phenazine series it would seem probable that they are unlikely to be displaced with the same ease as those bonded directly to the heterocyclic ring. It is evident from the foregoing discussion that A-oxidation has a pronounced effect on their reactivity, and, by this means, considerable latitude in the specific functionalization of dihalo or polyhalo derivatives may be exercised. 

When 6/3-aminopenicillanic acid (6-APA) is diazotized in the presence of chloride ion, the principal product obtained is 6a-chloropenicillanic acid (38) (62JOC2668), presumably by way of the diazo intermediate (39 Scheme 29) (72JCS(P1)895). If the diazotization is carried out in the presence of excess bromide instead of chloride, significant amounts of the 6,6-dibromo derivative are obtained, and in the case of excess iodide the 6,6-diiodopenicillanic acid becomes the predominant product (69JCS(C)2123). The 6,6-dihalo products presumably arise from nitrous acid oxidation of halide to halogen, which then reacts with (39). 

The 6a-halo- and 6,6-dihalo-penicillanates have been shown to undergo halogen-metal exchange to form enolates which can then react with acetaldehyde to form 6-(l-hydroxyethyOpenicillanates (Scheme 34) (77JOC2960, 79TL3805). From (41) the isomeric products were obtained in the ratio (48) (49) (50) = 24 49 27. From (42) the isomeric products were obtained in the ratio (51) (52) (53) = 39 1.5 60. Product ratios were very... 

Aziridine, cis-7V-t-butyl-3-ethynyl-2-vinyl-rearrangement, 7, 540 Aziridine, 2-chlorocarbonyl-ring expansion, 7, 42 Aziridine, 2-chloromethyl-synthesis, 7, 42 Aziridine, Mcyanodiphenyl-irradiation, 7, 61 Aziridine, dihalo-reduction, 7, 74 thermolysis, 7, 73... 

Furoxans, diethyl-synthesis, 6, 423 Furoxans, dihalo-synthesis, 6, 423 Furoxans, dimethyl-NMR, 6, 397 O NMR, 6, 398 Furoxans, diphenylring cleavage, 6, 404 synthesis, 6, 423 Furoxans, hydroxy-reactions, 6, 414 Furoxans, mercapto-reaotions, 6, 414 Furoxans, nitro-as explosives, 6, 426 reactions, 6, 413-414 reduction, 6, 423 Furoxans, phenyl-reactions... 

Pyrazolo[3,4-d][l,2]diazepines synthesis, 7, 597 Pyrazolop, 4- 6][ 1,4]diazepines synthesis, 5, 272 Pyrazolo[l, 4]diazepinones as anticonvulsant, 1, 170 Pyrazolo[2,3-e]diazepinones synthesis, 5, 272 1 H-Pyrazolo[l,5-6]imidazoles synthesis, 6, 992 Pyrazolo[2,3-a]imidazoles biological activity, 6, 1024 Pyrazolo[2,3-c]imidazoles reactions, 6, 1041 synthesis, 6, 1047 Pyrazolo[2,3-imidazoles synthesis, 6, 991 Pyrazolo[3,2- njisoquinolines synthesis, 5, 339 Pyrazolop, 4-c]isoquinolines synthesis, 5, 273 Pyrazolonaphthyri dines synthesis, 5, 339 Pyrazolone, diazophotolysis, 5, 252 Pyrazolone, 4,4-dihalo-rearrangements, 5, 250 Pyrazolone, ethoxy-hydrazinolysis, 5, 253 Pyrazolone, 4-halo-... 

Pyridazine, 4-amino-3,6-dihalo-ring contraction, 3, 29 Pyridazine, 4-amino-3,6-dimethoxy-nitration, 3, 20... 

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