Exchange reactions, boronic acid

This carboxyborane can undergo an amine exchange reaction with Hquid ammonia (eq. 7) to yield the boron analogue of glycine, the simplest alpha-amino acid (13). There has been a great deal of work on the pharmacological activity of these amino acid analogues (14). 

Selectivity in formation of protective groups may also be achieved by a proper choice of reaction conditions and catalyst. Thus formation of the 3-monothioketal from 3,6-diketones is achieved by dilution of the ethane-dithiol-boron trifluoride reaction mixture with acetic acid. 3-Monocyanohydrins are obtained in good yield from 3,20-diketo-(5a)-pregnanes by diluting the exchange reaction with ethanol. Similarly, dilution of the... 

A highly stereoselective synthesis of retinol via a Cm + C6 route was depicted by De Lera et al. [52]. A Suzuki reaction of a C14 alkenyliodide with a Cg alkenylboronic acid afforded retinol in 83% yield, with retention of the geometries of the coupling partners. The alkenyliodide was obtained by a zirconium-mediated methylalumination and a subsequent Al/I exchange by slow addition of ICN. Coupling with the C6 boronic acid (12 hrs to reach completion), afforded retinol in 83% yield [53], Fig. (21). 

A catalytic enantioselective arylation of arylaldehydes employs a chiral /3-amino alcohol and a boronic acid-diethylzinc exchange reaction to generate the reactive arylzinc species.247... 

An interesting polymer-assisted variant of the Suzuki-reaction was recently disclosed by Vaultier and coworkers (Scheme 17) [43]. Aryl boronic acids can be immobilized on an ion exchange resin. Under Suzuki-Miyaura coupling conditions bisaryl species are released into solution and isolated with minimum purification. The authors also demonstrated that this strategy can be employed for the synthesis of macroheterocycles. 

An example for the use of the boron-zinc exchange reaction for copper-mediated SN2 -substitutions of allylic electrophiles is the hydroboration of nitroolefin 130 with diethylborane, followed by successive transmetallation of the borane 131 with diethylzinc and CuCN-2LiCl, and final trapping with allyl bromide to give the product 133 with 83% yield over four steps (Scheme 34).34,34a This transformation again demonstrates the tolerance of the method towards functional groups and acidic hydrogen atoms. 

Boron Trichloride. Boron trichloride is prepared commercially by the chlorination of boron carbide (equation 15). Direct chlorination of boric acid or a sodium borate in the presence of carbon is an alternative method. Most of the boron trichloride produced is converted to filaments of elemental boron by chemical vapor deposition (CVD) on tungsten wire in a hydrogen atmosphere. Numerous laboratory preparations of boron trichloride have been reported. One of the most convenient is the halogen exchange reaction of aluminum chloride with boron trifluoride or a metal fluoroborate. 

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