Boranes reaction with halogen

NaCl-soln. 2-phenyl-l-propylmercuric chloride. Y 97%. F. e., also with dialkyl-boranes, s. R. C. Larock and H. C. Brown, Am. Soc. 92, 2467 (1970) anti-Markownikoff hydrohalogenation (cf. Synth. Meth. 24, 560) by reaction with halogen, also reduction to hydrocarbons (cf. Synth. Meth. 15,69) cf. J. J. Tufariello and M. M. Hovey, Am. Soc. 92, 3221 (1970). 

Solovyev et al. [60] have shown that the IPr-borane complex can be substituted in a number of different feshions. For instance, reduction of alkyl halides or alkyl sulfonates yields the corresponding NHC-stabilized boryl-halide or sulfonate. Reaction with halogen-based electrophiles also yields boryl-halides. Reactions with Lewis or Bronsted acids (e.g., triflic acid) have also proven successful. These various reactions are especially useful since they result in the addition of a good leaving group, which paves the way for subsequent nucleophilic... 

In the late 1960s, methods were developed for the synthesis of alkylated ketones, esters, and amides via the reaction of trialkyl-boranes with a-diazocarbonyl compounds (50,51), halogen-substituted enolates (52), and sulfur ylids (53) (eqs. [33]-[35]). Only one study has addressed the stereochemical aspects of these reactions in detail. Masamune (54) reported that diazoketones 56 (Ri = CH3, CH2Ph, Ph), upon reaction with tributylborane, afford almost exclusively the ( )-enolate, in qualitative agreement with an earlier report by Pasto (55). It was also found that E) - (Z)-enolate isomerization could be accomplished with a catalytic amount of lithium phenoxide (CgHg, 16 hr, 22°C) (54). 

Both diethyl (3-pyridyl)borane and diethyl (4-pyridyl)borane are readily accessible from 3-bromopyridine and 4-bromopyridine, respectively, via halogen-metal exchange and reaction with triethylborane [29-32], The two pyridylboranes have been coupled with a variety of aryl and heteroaryl halides, as exemplified by the coupling of diethyl (3-pyridyl)borane and 2-nitrophenylbromide to form phenylpyridine 39 [30, 31]. 

Bayer and Siebert reported the reaction of 3,3-dimethyl-l-butyne with -BuLi in pentane, followed by the addition of boron trichloride to afford the intermediate dichloro(3,3-dimethylbut-l-ynyl)borane that was hydroborated with dichloroborane formed in situ to give l,l-bis(dichloroboryl)-3,3-dimethylbutene 51 (Scheme 6) <2002ZN1125>. When the latter was subjected to halogen exchange with boron triiodide, it was converted into the corresponding tetraiodide 52, which underwent a redox reaction with hex-3-yne to give the 2-(2,2-dimethylpropylidene)-l,3-diborole 22 in 73% yield. 

It is possible to replace just one halogen or both. In the latter case the two alkyl groups can be the same or different. When dialkylation is apphed to dihalo nitriles, the two alkyl groups can be primary or secondary, but with dUialo esters, dialkylation is limited to primary R. Another extension is the reaction of boranes (BR3) with y-halo-a,p-unsaturated esters.Alkylation takes place in the y position, but the double bond migrates out of conjugation with the COOEt unit [BrCH2 CH=CHCOOEt RCH=CHCH2COOEt]. In this case, however, double-bond... 

Iridium-catalysed C-H-boronation can be carried out using either pinacol borane or pinacol diborane, both methods giving comparable results. Reaction occurs at a-positions of five-membered rings and is compatible with halogen substituents, as exemplified below. ... 

There is a lot of information on the mercuration of heterocycles in the old literature, but it is seldom used nowadays due to the major disadvantages of toxicity and associated waste management it can be, however, a very useful reaction. The advantages of mercuration are that it can be carried out in hydroxylic and acidic solvents and in the presence of air, and that mercury in the product is easily replaced by ipso-substitution with other electrophiles, such as halogens, and gives boronic acids by reaction with borane. 

If boron of an alkylborane could be replaced with a halogen, the product would be an alkyl halide. However, reaction of alkylboranes (neat) with chlorine, bromine, or iodine is very difficult. a when halogenation is done with bromine or iodine dissolved in dichloromethane, however, the reaction is fast and is synthetically useful.A simple example is the reaction of alkenes with boranes followed by addition of bromine, which leads to the alkyl bromide. An example is taken from the synthesis of 2-bromobutane (70) from 2-butene in 88% yield. 0 jhe bromination occurs by a free radical mechanism. Initial reaction with bromine generates a... 

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