Bromides s. Halides

Bridges s. Methylene bridg..., Steroids, bridged Bromides s. Halides Bromination s. Halogenation, Replacement of hydrogen by halogen... 

Si-Bridges, elimination 21, 970 Bromide ion 21, 879 Bromides s. Halides, Replacement... 

Bromate s.a. Dichlorobromate Bromides s. Halides Bromination (s.a. Halogenation)... 

Bridges, formation, stereospecific 23, 276 S-Bridges 22, 607, 622 23, 601 Si-Bridges, elimination 21, 970 Bromide ion 21, 879 Bromides s. Halides, Replacement... 

The following qualitative observations on the action of liquid ammonia on organic compounds are mainly by E. C. Franklin and C. A. Kraus, those in brackets are by G. Gore. Aliphatic compounds.—Halides methyl iodide, m. chloroform, reacts, and m. bromoform, m. iodoform, v.s., ethyl bromide and iodide, s. ethylene bromide, s. ethylidene chloride, m. isobutyl bromide, s. amyl bromide, s.s. tribromomethane, v.s. nitrotriohloromethane, m. perehloroethane (n.s.) perchloroethylene (m.) dichloroacetylene (s.). Alcohols methyl, m. ethyl, m. propyl, m. normal butyl,... 

Chiral (E)-enolethers. A degassed soln. of (5R)-5-cyclohexyl-2-ethenyl-l,3-dioxolan-4-one (2 1 cisjtrans) in THF added to ca. 1 eq. of a suspension of bis(l,5-cycloocta-diene)nickel(0) in the same solvent under N2, stirred until the complex dissolved (10 min), after 3h the resulting rust-coloured precipitate collected, suspended in methylene chloride, treated with MejSiCl, and stirred for 30 min intermediate 7c-allylnickel complex (Y 78%), in benzene treated with DMF and 5 eqs. 1-iodo-propane, irradiated with a sunlamp (GE 275 W Model RSW) for 2,5 h at 10°, stirred for a further 3 h, diluted with pentane to precipitate nickel halide, and stirred for a further 4 h product (Y 82% E/Z 9 1). Subsequent treatment with acetals afforded 2-p-alkoxy-l,3-dioxolan-4-ones with asym. induction, thereby providing an alternative to asym. aldol condensation. F.e. inch reaction with ar. and a,P-ethylene-bromides s. D.J. Krysan, P.B. Mackenzie, J. Am. Chem. Soc. 110, 6273 (1988). 

The oxidative addition of methyl iodide to Vaska s complex, shown in Equation 7.2, is a classic example of the oxidative addition of alkyl halides by an mechanism. Strong electrophiles that are sterically accessible, such as methyl iodide, benzyl bromide, allyl halides, and chloromethyl ethers, react with Lj(CO)IrX species by this pathway. A series of data supports addition of these electrophiles by an mechanism. For example, the trans stereochemistry of the kinetic product from addition of methyl iodide is inconsistent with a concerted three-centered mechanism radical traps do not affect the rate or products of the reaction the reaction rates are faster in more polar solvents - and the reactions are first order in both metal and electrophile. Higher aUcyl halides add by more complex mechanisms presented below. 

Reactions.—Kinetic resolution of secondary alkyl bromides and iodides can be achieved by reaction of two molar equivalents of the halide with the chiral lithio-oxazoline (69) and isolation of excess halide (Scheme 33). The lithio derivative (69) reacts preferentially with (S )-halide [as in (70) with smaller than R J leaving enriched (R)-halide of 30-50% enantiomeric purity. 

Jew, Park, and coworkers extended the substrate scope of phase-transfer catalytic alkylation to the oxazoline system in order to synthesize chiral a-alkyl serine derivatives. The authors chose phenyl oxazoline derivative 74 [106] as the substrate (Scheme 12.6). In the presence of Cj-symmetric chiral quaternary ammonium bromide (S,S)-31c, the asymmetric phase-transfer alkylation of 74 with alkyl halides, followed by acidic hydrolysis, provided chiral a-alkyl serines in very high yields and enantioselecti vities. 

MarkownikofT s rule The rule states that in the addition of hydrogen halides to an ethyl-enic double bond, the halogen attaches itself to the carbon atom united to the smaller number of hydrogen atoms. The rule may generally be relied on to predict the major product of such an addition and may be easily understood by considering the relative stabilities of the alternative carbenium ions produced by protonation of the alkene in some cases some of the alternative compound is formed. The rule usually breaks down for hydrogen bromide addition reactions if traces of peroxides are present (anti-MarkownikofT addition). 

Place a mixture of 0-5 g. of finely powdered thiourea, 0-5 g. of the alkyl halide and 5 ml. of alcohol in a test-tube or small flask equipped with a reflux condenser. Reflux the mixture for a j)eriod depending upon the nature of the halide primary alkyl bromides and iodides, 10-20 minutes (according to the molecular weight) secondary alkyl bromides or iodides, 2-3 hours alkyl chlorides, 3-5 hours polymethy lene dibromides or di-iodides, 20-50 minutes. Then add 0 5 g. of picric acid, boil until a clear solution is obtained, and cool. If no precipitate is obtained, add a few drops of water. RecrystaUise the resulting S-alkyl-iso-thiuronium picrate from alcohol. 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Among the hydrogen halides only hydrogen bromide reacts with alkenes by both electrophilic and free radical addition mechanisms Hydrogen iodide and hydrogen chlo ride always add to alkenes by electrophilic addition and follow Markovmkov s rule Hydrogen bromide normally reacts by electrophilic addition but if peroxides are pres ent or if the reaction is initiated photochemically the free radical mechanism is followed... 

Hydrogen bromide is unique among the hydrogen halides m that it can add to alkenes either by electrophilic or free radical addition Under photochemical conditions or m the presence of peroxides free radical addition is observed and HBr adds to the double bond with a regio selectivity opposite to that of Markovmkov s rule... 

Substitution can take place by the S l or the 8 2 mechanism elimination by El or E2 How can we predict whether substitution or elimination will be the principal reac tion observed with a particular combination of reactants The two most important fac tors are the structure of the alkyl halide and the basicity of the anion It is useful to approach the question from the premise that the characteristic reaction of alkyl halides with Lewis bases is elimination and that substitution predominates only under certain special circumstances In a typical reaction a typical secondary alkyl halide such as iso propyl bromide reacts with a typical Lewis base such as sodium ethoxide mainly by elimination... 

Organosulfur Halides. When sulfur is directly linked only to an organic radical and to a halogen atom, the radical name is attached to the word sulfur and the name(s) and number of the halide(s) are stated as a separate word. Alternatively, the name can be formed from R—SOH, a sulfenic acid whose radical prefix is sulfenyl-. For example, CH3CH2—S — Br would be named either ethylsulfur monobromide or ethanesulfenyl bromide. When another principal group is present, a composite prefix is formed from the number and substitutive name(s) of the halogen atoms in front of the syllable thio. For example, BrS—COOH is (bromothio)formic acid. 

Electrophilic attack on the sulfur atom of thiiranes by alkyl halides does not give thiiranium salts but rather products derived from attack of the halide ion on the intermediate cyclic salt (B-81MI50602). Treatment of a s-2,3-dimethylthiirane with methyl iodide yields cis-2-butene by two possible mechanisms (Scheme 31). A stereoselective isomerization of alkenes is accomplished by conversion to a thiirane of opposite stereochemistry followed by desulfurization by methyl iodide (75TL2709). Treatment of thiiranes with alkyl chlorides and bromides gives 2-chloro- or 2-bromo-ethyl sulfides (Scheme 32). Intramolecular alkylation of the sulfur atom of a thiirane may occur if the geometry is favorable the intermediate sulfonium ions are unstable to nucleophilic attack and rearrangement may occur (Scheme 33). 

Halide ions may attack 5-substituted thiiranium ions at three sites the sulfur atom (Section 5.06.3.4.5), a ring carbon atom or an 5-alkyl carbon atom. In the highly sterically hindered salt (46) attack occurs only on sulfur (Scheme 62) or the S-methyl group (Scheme 89). The demethylation of (46) by bromide and chloride ion is the only example of attack on the carbon atom of the sulfur substituent in any thiiranium salt (78CC630). Iodide and fluoride ion (the latter in the presence of a crown ether) prefer to attack the sulfur atom of (46). cis-l-Methyl-2,3-di-t-butylthiiranium fluorosulfonate, despite being somewhat hindered, nevertheless is attacked at a ring carbon atom by chloride and bromide ions. The trans isomer could not be prepared its behavior to nucleophiles is therefore unknown (74JA3146). 

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