Organoboranes boron halides

The main chemical products produced from these minerals are (a) boron oxides, boric acid and borates, (b) esters of boric acid, (c) refractory boron compounds (borides, eu .), (d) boron halides, (e) boranes and carbaboranes and (f) organoboranes. The main industrial and domestic uses of boron compounds in Europe (USA in parentheses) are ... 

A number of types of organoboranes, including aryl, benzyl, /-butyl and methyl derivatives, cannot be obtained via hydroboration. Their preparation can be achieved by a number of approaches, the most general of which involves the reaction of a reactive organometallic reagent with a boron halide or alkox-ide 4 the organometallic reagent may sometimes be prepared in situ. 

Organoborane intermediates can also be used to synthesize alkyl halides. Replacement of boron by iodine is rapid in the presence of base.150 The best yields are obtained with sodium methoxide in methanol.151 If less basic conditions are desirable, the use of iodine monochloride and sodium acetate gives good yields.152 As is the case in hydroboration-oxidation, the regioselectivity of hydroboration-halogenation is opposite to that observed for direct ionic addition of hydrogen halides to alkenes. Terminal alkenes give primary halides. 

Cross-coupling reactions between organoboranes and heteroaryl halides are effectively catalyzed by Pd(0) in the presence of a base. Couplings in simple pyrimidines are illustrated by the reaction between 2-chloropyrimidine and 2- or 3-thiophene- and selenophene-boronic acids which give the corresponding 2-substituted pyrimidines (921) (Scheme 72). In 2,4-dichloro- or 2,4-dibromo-pyrimidine it is the 4-halo substituent which is the more reactive. 

The Suzuki Coupling, which is the palladium-catalysed cross coupling between organoboronic acid and halides. Recent catalyst and methods developments have broadened the possible applications enormously, so that the scope of the reaction partners is not restricted to aryls, but includes alkyls, alkenyls and alkynyls. Potassium trifluoroborates and organoboranes or boronate esters may be used in place of boronic acids. Some pseudohalides (for example triflates) may also be used as coupling partners. 

Aluminum alkyls, aUcylaluminum halides, and alkylalu-minum hydrides strongly associate in solution and in the solid state." The association of trialkylaluminum compounds is in stark contrast to the monomeric nature of trialkylboron see Boron Organoboranes) and trialkylgaUium (see GalUum Organometallic Chemistry) compounds. 

The reaction of alkenes with borane, monoalkyl and dialkylboranes leads to a new organoborane (see 15-16). Treatment of organoboranes with alkaline H2O2 oxidizes trialkylboranes to esters of boric acid." This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles that may be present elsewhere in the molecule. There is no rearrangement of the R group itself, and this reaction is a step in the hydroboration method of converting alkenes to alcohols (15-16). The mechanism has been formulated as involving initial formation of an ate complex when the hydroperoxide anion attacks the electrophilic boron... 

In general, all types of boron-stabilized carbanions are readily alkylated by primary alkyl halides. The situation for the i ylation of dimesitylboryl compounds is sununarized in Scheme 2. These reactions represent highly efficient homologation processes. Oxidation of the tertiary all l organoboranes is slow, but use of 4-methoxy-2,6-dimethylphenyl groups instead of mesityl groups renders the products sensitive to solvolysis. ... 

The mechanism by which these alkylations occur is fundamentally similar to the oxidation of organoboranes to alcohols. It is believed that the enolate of the haloester or haloketone reacts with the borane. Subsequently, elimination of halide, followed by migration of one of the boron substituents, occurs. In agreement with this mechanism, retention of configuration of the migrating group is observed. ... 

Boron.- The chemistry of organoboranes has seen significant developments across a range of different fronts. As with zinc, ultrasound has had an impact in the boron area. Alkyl and aryl halides react with Bp2.Et20 and Mg under ultrasonic conditions to... 

Other approaches to trisubstituted aikenes via organoboranes involve alkynyltrialkyl borates, alkenyltrialkyl borates or the cross-coupling reaction of alkenylboronic acids with alkyl halides. Both ( )- and (2)-l,2-disubstituted alkenyl bromides can also be prepared by the methodology shown in eq 7. The boron trifiuoride etherate-mediated 1,4-addition of 1,2-disubstituted alkenylboronates affords y,5-unsaturated ketones (eq 9). The boronates can also be converted into chiral enoiborates for the enantioselective addition to aldehydes. ... 

Organoboranes have many advantages as the organometallic component (R M) for the Pd-catalyzed carbonylative cross-coupling reaction with organic halides (RX). They can be prepared in many ways and are compatible with a variety of functional groups of synthetic importance. Most boronic acids and esters are stable to air and moisture. 

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