Arylboron reagents

Acetylenic esters react with arylboron reagents in the presence of rhodium diphosphine catalyst to give cyclic ketones.409 Equation (61) shows an example which may involve ortfe-metallation and ketone formation. A catalytic, enantioselective reaction was also achieved (Equation (62)). These processes presumably involve unprecedented addition of organorhodium species to the ester carbonyl group. 

Complementary to the use of zinc reagents for the introduction of (functionalized) alkyl groups is the rhodium-catalyzed conjugate addition of aryl- and alkenylboron reagents. This method rapidly became popular, also because arylboron reagents are air and moisture stable and a large variety of them is commercially available . 

For a Rh-catalyzed asymmetric conjugate addition of arylboronic reagents to a,/)-unsaturated esters, see S. Sakuma, M. Sakai, R. Itooka and N. Miyaura, J. Org. Chem., 65, 5951 (2000) and references cited therein. 

Alkynes in combination with a catalytic amount of a nickel complex have been found to catalyse the conjugate addition of arylboron reagents to ,/3-unsaturated carbonyl compounds.238... 

Scheme 33 Rh-catalyzed directed C-H arylation of arenes with arylboron reagents by Miyamura et al. [124]...

Miyamura S, Tsurugi H, Satoh T, Miura M (2008) Rhodium-catalyzed regioselective aryla-tion of phenylazoles and related compounds with arylboron reagents via C-H bond cleavage. J Organomet Chem 693 2438-2442... 

Carbon-carbon bond formation using arylboron reagents with rhodium catalysts in aqueous media... 

CARBON-CARBON BOND FORMATION USING ARYLBORON REAGENTS WITH RHODIUM(I)... 

Rhodium catalysts solubilised in an aqueous phase through the use of water-soluble ligands (1) and (2) (Figure 7.3) allow for the addition of arylboron reagents to... 

Figure 7.3 Addition of arylboron reagents to alkenes and alkynes.

The use of arylboron reagents for direct C-H arylation involves, conceptually, more s)mthetic steps for their preparation, usually from the correspond haloarene. Furthermore, in the absence of haloarene an additional stoichiometric oxidant is necessary. However, these methods tend to involve milder and lower temperature conditions. 

Palladium-catalyzed cross-coupling reactions of alkyl bromides with arylboron reagents represent another breakthrough for the development of a general method for arylation of alkyl halides. Fu found that the palladiumprimary alkyl bromides with various arylboronic acids (Equation 5.6) [10]. An air-stable commercially available preligand, [HP(t-Bu)2Me]BF4 5, also worked well with a variety of arylboronic acids and gave the same products in comparable yields, as shown in the parentheses in Equation 5.6. 

The direct borylation of arenes is an attractive strategy for accessing synthetically useM arylboron reagents. Iridium complexes have emerged as the catalyst of choice for the selective borylation of arenes using HB(pin) or Bj(pin)j [67], Extensive studies by the Hartwig, Ishiyama, and Miyanra groups have led to the identification of the Ir OMe(cod)j/dtbpy as the optimal catalyst system for these transformations [68-70], As illustrated in Scheme 24.59, electronically diverse arenes are borylated at room temperature to afford the products in excellent yields. The site-selectivity of these... 

Uozumi, Y., Danjo, H. and Hayashi, T., Cross-coupling of aryl halides and allyl acetates with arylboron reagents in water using an amphiphilic resin-supported palladium-catalyst, /. Org. Chem., 1999,64, 3384-... 

Arylboronate reagents have also been used for the cross coupling at the sp C-H bond of benzylic amines in the presence of Ru3(CO)i2 and pinacolone as solvent. The reaction was not only directed by a pyridine substituent but also required substitution of this pyridyl group at the 3-position [17, 18]. 

Lately, Kuwano and coworkers reported on the 1,2-migration in the rhodium catalyzed reaction of alkenyl acetates and arylboron reagents, producing the isomerized frani-alkenes effectively. The catalytic system proved less sensitive to the steric interactions of the Cl substituent resulting in a broad scope and good regioselectivities (Scheme 2.5) [16]. 

SCHEME 10.15 Ligand-enabled cross coupling of Y-C(sp )—H bonds in triflyl-protected amino acid esters with arylboron reagents. 

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