Boronic acids, metal catalyzed formation

The Suzuki-Miyaura cross-coupling reaction is a standard method for carbon-carbon bond formation between an aryl halide or triflate and a boronic acid derivative, catalyzed by a palladium-metal complex. As with the Mizoroki-Heck reaction, this cross-coupling reaction has been developed in ionic liquids in order to recycle and reuse the catalyst. In 2000, the first cross-coupling of a halide derivative with phenylboronic acid in [bmim] [BF4] was described. As expected, the reaction proceeded much faster with bromobenzene and iodobenzene, whereas almost no biphenyl 91 was obtained using the chloride derivative (Scheme 36). The ionic liquid allowed the reactivity to be increased, with a turnover number between 72 and 78. Furthermore, the catalyst could be reused repeatedly without loss of activity, even when the reaction was performed under air. Cross-coupling with chlorobenzene was later achieved - although with only a moderate yield (42%) - using ultrasound activation. 

In consideration of conceivable strategies for the more direct construction of these derivatives, nitriles can be regarded as simple starting materials with which the 3+2 cycloaddition of acylcarbenes would, in a formal sense, provide the desired oxazoles. Oxazoles, in fact, have previously been obtained by the reaction of diazocarbonyl compounds with nitriles through the use of boron trifluoride etherate as a Lewis acid promoter. Other methods for attaining oxazoles involve thermal, photochemical, or metal-catalyzed conditions.12 Several recent studies have indicated that many types of rhodium-catalyzed reactions of diazocarbonyl compounds proceed via formation of electrophilic rhodium carbene complexes as key intermediates rather than free carbenes or other types of reactive intermediates.13 If this postulate holds for the reactions described here, then the mechanism outlined in Scheme 2 may be proposed, in which the carbene complex 3 and the adduct 4 are formed as intermediates.14... 

In addition, metal-catalyzed carbon-carbon bond formation and subsequent enzymatic transformations turned out to be compatible. This has been demonstrated by the Groger and Hummel groups [59], combining the Suzuki reaction as an example for a paUadium-catalyzed cross<oupling reaction with an asymmetric ADH-catalyzed ketone reduction in an aqueous reaction medium (Scheme 19.24). In such a one-pot process, the amount of boronic acid turned out to be critical because of strong inhibition of the enzyme. Thus, a two-step one-pot strategy was developed which was based on the use of 1 equiv of boronic acid in the Suzuki reaction, and addition of the enzyme directly to the reaction mixture after consumption... 

The application of the ideas of Lewis on acids, which correlate a wide range of phenomena in qualitative fashion, has as yet led to very few quantitative studies of reaction velocity but has led to detailed speculations as to mechanisms (Luder and Zuffanti, 118). Friedel-Crafts reactions are considered to be acid-catalyzed, the formation of a car-bonium ion being the first step. The carbonium ion then acts as an acid relative to the base benzene which, upon loss of a proton, yields the alkylated product. Isomerizations of isoparaffins can be explained in similar fashion (Schneider and Kennedy, 119). An alkyl halide yields a carbonium ion on reaction with acids such as boron trifluoride, aluminum chloride, and other metal halides. 

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