A-Halo boronic acids

Cyclic esters of a-halo boronic acids in asymmetric synthesis 98T10555. 

Vinylboranes react with acidic hydrogen peroxide to give aldehydes, whilst the oxidation of a-halo-boronic acids in the presence of DNP/H2SO4 to give aldehyde-2,4-DI s in good yields was used as a proof of structure (equation 21). ... 

Catalyzed hydroborationt. S prepared in the presence of 10 nnol Many catalysts have been discc Sonication with activated nickel ether) significantly increases the 1 fioselectivity for the hydroboraiioi reactions using bis(mesityl)niobiui cause the major hydroborating ages a-Halo boronic acids. The catecholborane without solvent. 

Organoboranes can also be used to construct carbon-carbon bonds by several other types of reactions that involve migration of a boron substituent to carbon. One such reaction involves a-halo carbonyl compounds.20 For example, ethyl bromoac-etate reacts with trialkylboranes in the presence of base to give alkylated acetic acid derivatives in excellent yield. The reaction is most efficiently carried out with a 9-BBN derivative. These reactions can also be effected with (3-alkenyl derivatives of 9-BBN to give (3,y-unsaturated esters.21... 

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. 

Grigg et al. [61] have recently reported a one-pot reaction involving the initial three-component condensation with a 2-halo-benzylamine 112, ethyl glyoxylate 113 and an aryl boronic acid 114, followed by Pd cyclization in the presence of carbon monoxide to give 116, or in the presence of allene to form dihydroisoquinoline amino acid derivatives 117 (Scheme 7.15). 

As a consequence of the mild reaction conditions in the sequence to 3-halo furans 55 the palladium catalyst should be still intact to trigger another Pd-catalyzed coupling in the sense of a sequentially Pd-catalyzed process [31]. As a consequence, a sequential Sonogashira-deprotection-addition-cyclocondensation-Suzuki reaction, where the same catalyst system is applied in two consecutive significantly different cross-coupling reactions in the same reaction vessel should be feasible. Therefore, upon consecutive reactions of (hetero)aroyl chlorides 7 and THP-protected propargyl alcohols 54, Nal and PTSA, and finally, addition of 1.05 equiv of boronic acids 60 and sodium carbonate, the substituted 3-aryl furans 61 can be obtained in decent yields (Scheme 35). 

For syntheses requiring the syn adducts, it is more practical to use the boron enolate without additional Lewis acids [75], since the auxiliary is available as either enantiomer and is recoverable. On the other hand, the anti adducts are (so far) only available by the diethylaluminim chloride/boron enolate protocol [66]. Similar principles may be used to prepare syn and anti halohydrins by aldol addition of a-halo acetate enolates of Evans imides [90,91]. 

Monoprotected P-keto-aldehydes may be prepared by the a-dialkoxymethyl-ation of pre-formed enolates or enamines with trimethyl orthoformate and boron trifluoride diethyl etherate. Regiospecificity is maintained when the enolate is released from the silyl enol ether with methyl-lithium. P-Keto-acetals or P-diketones may also be formed by acylation of enol ethers with acid chlorides. High yields depended on the use of activated acid chlorides, such as a-halo- or a-cyano-acetyl chlorides. Enaminosilanes are acylated by a wide range of acid chlorides in the presence of potassium fluoride and a crown ether, giving very high yields of the enaminone [equation (46)]. Under the reaction conditions,... 

Metalated ferrocenes have served as valuable intermediates for the synthesis of a number of other derivatives. Treatment of lithiated ferrocenes with tributyl borate followed by hydrolysis leads to ferroceneboronic acid (XXXIII) as well as the diboronic acid (73). Ferroceneboronic acid, like benzeneboronic acid, is readily cleaved by cupric bromide or cupric chloride to form the corresponding halo derivatives (XXXIV). Ferrocene-l,l -diboronic acid reacts in the same manner, and either one or two carbon-boron bonds can be cleaved. Further reactions of this type have led to a variety of mixed dihaloferrocenes (73, 75). 

---