Boranes with halogens

Reactions of tris(organothio)boranes with a-halogenated nitriles are very slow (c.f. (II)). Analogous reactions of tris(organothio)boranes with acetonitrile could not be accomplished. 

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). 

Many boron hydrides, especially the higher boranes, undergo halogenation, alkylation and other substitution reactions when treated with electrophiles. Such reactions are catalyzed by acids, yielding a variety of stable products. 

The reaction can be extended to ot,ot-dihalo esters1557 and a,a-dihalo nitriles.1558 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 applied to dihalo nitriles, the two alkyl groups can be primary or secondary, but with dihalo esters, dialkylation is limited to primary R. Another extension is the reaction of boranes with -y-halo-a.p-unsaturated esters.1559 Alkylation takes place in the -y position, but the double bond migrates, e.g.,... 

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. ... 

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). 

Enantiomerically pure phosphine borane 26 was deprotonated at low temperature and halogenated with 1,2-dibromo- or 1,2-diiodoethane to furnish compounds 27, which could be isolated as crystalline solids in good yields. In analogy to chlorophosphine boranes (see Section 4.3.2.2), they are not configurationally stable and racemise slowly at room temperature. Therefore, compounds 27 were not isolated but used in situ with several carbanionic reagents to afford the corresponding tertiary phosphine borane with inversion (28) or retention of configuration (28 ) (Table 5.5). 

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... 

Dialkylquinolinyl boranes 83 and 86 were prepared from halogen/metal exchange of 3-bromoquinoline (70) with n-BuLi followed by quenching with Et2BOMe and Br-9-BBN, respectively. They are then coupled with bromides 84 and 87 to give 3-substituted quinoline derivatives 85 and 88, respectively (85H2375). 

Such functional groups as OR, OH, NH2, SMe, halogen, and COOR may be present in the molecule," but not groups that are reducible by borane. Hydroboration of enamines with 9-BBN provides an indirect method for reducing an aldehyde or ketone to an alkene, for example. 

Infrared spectroscopy is an excellent tool in iminoborane chemistry, which readily permits, to distinguish between iminoboranes and nitrile-borane adducts and to identify monomeric and dimeric forms of iminoboranes. This event is due to the fact that the i>CN of CN multiple bonds absorbs outside the fingerprint region and can be considered to be a valuable group frequency even when mixed with other vibrational modes. In some cases other vibrations like NH, BH, B-halogen or B-S stretching modes are helpful for determining the structure of iminoboranes. 

Heteroboranes are those in which one or more non-boron atoms replace a BH vertex, together with groups that may be attached to these heteroatoms. Boranes that contain CH vertices constitute the vast family of carbaboranes. The possibility for carbon to participate in electron-deficient frameworks contradicted the former prejudice of the always electron-precise carbon as the well-behaved brother of naughty boron. So far, most elements have been introduced as heteroatoms into borane frameworks, with the exception of the halogens and the noble gases. 

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