Catechol boronic esters

Some effected the coupling of phenyl-, 2-fuiyl-. and 1-hexenylboronic acids with 4-thallated indole-3-carboxaldehyde (Pd(OAc),/DMF) to give 4-substituted 3-formylin-doles [148]. Regioselective thallation of indole-3-carboxaldehyde is achieved using thallium tris-trifluoroacetate in 77% yield. Indole 129, which is available by the Buchwald zirconium indoline synthesis, was used by Buchwald to synthesize 130 via a Suzuki protocol [149]. Boronate ester 130 is prepared by the hydroboration of3-methyl-1-butyne with catechol borane. Indole 131 had been used in earlier studies to synthesize the clavicipitic acids. 

Evans and co-workers developed rhodium catalyzed hydroboration reactions, which enable the use of catechol borane and other boronate esters as hydroborating agents to afford organoboronic esters as products.9 These products have increased stability and can be used directly in palladium cross-coupling applications. However, the rhodium-catalyzed method is most effective for the hydroboration of monosubstitued olefins (i.e., 16), as lower reactivity is observed with more substituted alkenes. 

Boronic ester formation is an alternative process suitable for dynamic covalent chemistry. Nishimura and Kobayashi made use of the reversible condensation between an aryl-boronic acid and catechol in the development of dynamic hemicarcerand 42. Heating a 2 4 mixture of a tetra-boronic acid cavitand and biscatechol 43 resulted in the quantitative formation of 42. Again, the high efficiency is a result of the proper choice of the building blocks. Biscatechol 43, which in its lowest energy conformation is a perfect 120° ditopic bis-l,2-diol unit, is complementary to the orientation of the boronic acids in the cavitand building block to yield 42 with little or no strain. 

Severin s group also prepared the first examples of boron-based rotaxanes utilizing boronic esters as stoppers. The MCR of l,2-di(4-pyridyl) ethylene 16, catechol 17, 3,5-bis(trifluoromethyl)phenylboronic acid 18, and 1,5-dinaph-tho-38-crown-lO 19 resulted in the formation of rotaxane 20 (Scheme 4.12) [14]. 

Chan and co-workers have demonstrated tiiat boronic esters can be used in place of boronic acids in both O- and N-arylations in the parent phenyl case (Scheme 5.26) [10]. In botii arylations, boronic esters 24-27 are even more efficient than the parent acid. However, catechol ester 28 and pinacolate 29 were less efficient, perhaps due to instability (possibility of catechol O-arylation) and steric hindrance respectively. The overall superior performance of the triphenylboroxine (30) is noteworthy since it is... 

Different approaches were investigated to overcome this problem. The first strategy reported in 1991 was based on the use of a chiral aminodiol. Donation of electrons from nitrogen to boron activates the diene and accelerates the addition to activated olefins compared to its boronic ester analogue [70]. The endo cycloadducts were exclusively produced with N-phenyl maleimide at room temperature. A low asymmetric induction was observed with dienes derived from N-substituted amino acid (12% ee) [71]. Ate complexes prepared by quatemarization of a catechol derivative with a stoichiometric amount of CsF are highly reactive in these cycloaddition reactions (Scheme 9.32) [72]. 

X 10 S cm". This approach is seemingly espedally useful for battery electrolytes, because the transference number of the lithium ion is increased recently, Weng et al. [519] reported an improved synthesis of tetrafluoro-catechol [105] that is a starting material for the fluorinated boronate ester, 2-(pentafluorophenyl)-tetrafluoro-l,3,2-benzodioxaborole (PFPTFBB). PEPTEBB acts as an anion receptor. In addition, it is an effective redox shuttle for overcharge protection of lithium-ion batteries. Conceptually, this approach is similar to the use of lithium salts with large anions or the immobilization of anions at polymer backbones. 

Witulski has reported the first hydroboration of 1-alkynylamides. Thus, the hydroboration of ynamide 112 with catechol borane in THF proceeded chemo- and regioselectively yielding only the monohydroboration product, alkenyl boronic ester 113 [90]. However, the isolation of the boronic ester 113 was complicated due to its instability and difficulties of storage and purification. Therefore, it was directly subjected to Suzuki-Miyaura cross-coupling yielding (E)-y3-arylenamide and 3-(2 -amidovinyl)indoles such as 114 (Scheme 3.58). 

Catalyst, alumina, 34, 79, 35, 73 ammonium acetate, 31, 25, 27 boron tnfluonde etherate, 38, 26 copper chromite, 31, 32, 36, 12 cupric acetate monohydrate, 38, 14 cuprous oxide silver oxide, 36, 36, 37 ferric nitrate, hydrated, 31, 53 phosphoric acid, 38, 25 piperidine, 31, 35 piperidine acetate, 31, 57 Raney nickel, 36, 21, 38, 22 sulfuric acid, 34, 26 Catechol, 33, 74 Cetylmalonic acid, 34, 16 Cetylmalonic ester, 34,13 Chlorination, by sulfuryl chloride, 33, 45, 37, 8... 

Yang et al. used catechol to immobihze borane and boronic acids [58], since support-bound boronic catachol ester hnkage is stable enough to perform amidation reactions. Custom-derivatized boronic acids could be obtained, which could be hberated from the support with THF/H2O/ACOH 90 5 5 (v/v/v). Hebei et al. reported Suzuki-mediated release of biaryls from polyglycerol esters of various boronic acids and aryl bromides [59]. 

These two milestone syntheses were soon followed by others, and activity in this field continued to be driven by interest in the biologically active esters of cephalotaxine. In 1986, Hanaoka et al. (27) reported the stereoselective synthesis of ( )-cephalotaxine and its analog, as shown in Scheme 4. The amide acid 52, prepared by condensation of ethyl prolinate with 3,4-dimethoxyphenylacetyl chloride, followed by hydrolysis of the ethyl ester, was cyclized to the pyrrolobenzazepine 53 by treatment with polyphos-phoric acid, followed by selective O-alkylation with 2,3-dichloropropene (54) in the presence of sodium hydride. The resulting enol ether 55 underwent Claisen rearrangement on heating to provide C-allylated compound 56, whose reduction with sodium borohydride yielded the alcohol, which on treatment with 90% sulfuric acid underwent cationic cyclization to give the tetracyclic ketone 57. Presumably, this sequence represents the intramolecular version of the Wichterle reaction. On treatment with boron tribromide, ketone 57 afforded the free catechol, which was reacted with dibromometh-ane and potassium fluoride to give methylenedioxy derivative 58, suited for the final transformations to cephalotaxine. Oxidation of ketone 58... 

A standard way to make vinyl boronic acids 237 is to hydroborate an alkyne with catechol borane38 235 and again hydrolyse the ester products 236. The empty orbital of the boron atom attacks the electron-rich terminus of the alkyne where is the larger HOMO of the Jt-bond. One detailed example39 includes Suzuki coupling to Z-styryl bromide. Another diester group commonly used in the Suzuki coupling is derived from pinacol - examples appear later. 

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