Boronate ester functionalization

Preparation of a similar copolymer 2.28 containing sexithiophene units which are alternately linked by a hydrophobic alkyl chain and a hydrophilic PEG chain was achieved by Suzuki-type cross-coupling of dibromo derivative 2.23 with bis-boronic ester 2.27 (Scheme 1.6) [94]. The latter compound was synthesized by Stille-type cross-coupling of bis-stannylated derivative 2.26 and a bromobithiophene endowed with boronic ester functionality at other terminal a-position. However, the synthesized copolymer 2.28 was found to be insoluble in common organic solvents. 

Weakly basic atoms, especially halides, cannot be present in the p-position of 1 or 4, because elimination of such species together with the boronic ester function is rapid in the presence of any reagent as basic as water [15]. 

Further transformations of cyclopropylboronates by creation of a new carbon-carbon or carbon-oxygen bond have been largely developed, as well as the modification of the side-chain in the presence of the boronic ester functionality [46]. 

Gobalt-catalyzed Diels-Alder reactions offer an interesting alternative since the cycloadditions are carried out under mild conditions. With isoprene, alkynylboronic esters predominantly gave the regioisomer in which the methyl group of the diene and the boronic ester functionality are meta related [64], Subsequent Suzuki coupling reactions can be performed without isolation of the intermediates (Scheme 9.28). 

In 2011, Hartwig and coworkers reported the total synthesis of taiwaniaquinol B (55, Scheme 11.9), a member of a family of diterpenoids that are derived from the abietane skeleton [36]. A key aspect of the Hartwig synthesis of taiwaniaquinol B was the use of the iridium-catalyzed borylation reaction to accomplish the C(5) functionalization of resorcinol derivative 53. This regioselectivity for the overall bromination is complementary to that which would be obtained using a standard electrophilic aromatic substitution (EAS) reaction. In the transformation of 53 to 54, a sterically controlled borylation was first accomplished, which was then followed by treatment of the boronic ester intermediate with cupric bromide to... 

The rhodium-catalyzed borylation of alkanes is applied to regiospecific functionalization of polyolefines.165,165a The reaction of polypropylenes (atactic, isotactic, and syndiotactic) with B2pin2 in the presence of Cp Rh( 74-C6Me6) catalyst at 200 °G affords the borylated polymers, which are treated with basic hydrogen peroxide in a mixture of THF and H20 to oxidize the boronate esters to the corresponding alcohols (Scheme 20). The hydroxylated polymers contain 0.2-1.5% hydroxymethyl side-chains. 

Boronic esters have been used in a wide range of transformations. These useful reagents have been transformed into numerous functional groups and are essential reagents for several C-C bond-forming reactions. Transition metal-catalyzed hydroboration of olefins often leads to mixtures of branched and linear products. Several groups have reported asymmetric reductions of vinyl boronic esters [50-52] with chiral rhodium P,P complexes however, the first iridium-catalyzed reduction was reported by Paptchikhine et al (Scheme 10) [53]. 

These syntheses provide further examples of the efficient nucleophilic displacement of halide from ot-halo boronic esters, even by very hindered strong bases, as well as the functional group compatibility of the synthesis. 

Preparation of some azulenylmagnesium species was achieved by the halogen-magnesium exchange reactions of iodoazulenes with lithium tributylmagnesate at low temperatures (equations 29-33) . The reactions offer access to a variety of functionalized azulenes including azulenylphosphine, -stannane and -boronic ester. 

In summary, the direct insertion of zinc dust to organic halides is an excellent method for preparing a broad range of polyfunctional organozinc halides bearing various functional groups like an ester" , an ether, an acetate" , a ketone, cyano", halide" , N,N-bis(trimethylsilyl)amino °, primary and secondary amino, amide, phthalimide , sulfide, sulfoxide and sulfone , boronic ester , enone " or a phosphonate . An alternative method is based on transmetalation reactions. 

The cross-coupling of alkynylzinc halides or fluorinated alkenylzinc halides with fluori-nated alkenyl iodides allows the preparation of a range of fluorinated dienes or enynes - Functionalized allylic boronic esters can be prepared by the cross-coupling of (dialkylbo-ryl)methylzinc iodide 428 with functionalized alkenyl iodides. The intramolecular reaction provides cyclized products, such as 429 (Scheme 109) ° °. In some cases, reduction reactions or halogen-zinc exchange reactions are observed. 

An asymmetric aza-Diels-Alder reaction between functionalized 1,3-dienes and imines is mediated by a binaphthol-modified boronic ester (Scheme 108) (267). Unfortunately, at the present time the reaction requires a stoichiometric amount of the chiral promoter. 

EPR experiments on carbon-centred radicals with either a- or /J-boronic ester substituents have been reported.168 While the a-substituted radicals were modestly thermodynamically stable, the /J-substituted radicals underwent easy /J-climination. An EPR experiment on the photo-oxidation of phenolic compounds containing at least one free ortho position has indicated the formation of persistent secondary radicals derived from dimerization or polymerization from C-0 coupling.169 The structure of the succinimidyl radical has been re-examined using density functional theory with a variety of basis sets. The electronic ground state was found to be of cr-symmetry allowing for facile -scission. These conclusions were also predicted using MP2 but... 

The major improvement compared to the Rh(I) catalysts was the ability of the Cu(I)/RB (OR )2 system to also carboxylate alkenyl-boronic esters in very good yield, under similar conditions. Interestingly, the system functioned better under ligandless conditions, producing the corresponding unsaturated carboxylic acids in good yields. 

We also found that ester functionality was compatible with this reduction sequence. Thus, treatment of the ester-lactone 81 with sodium borohydride and boron trifluoride etherate provided the tetrahydropyran 123 in 55% purified yield. Upon reaction with sodium hydroxide, 81 underwent simple ester hydrolysis to furnish the carboxylic acid 127 (see Table 3). 

High activity of / -PrMgCI-LiCI in the exchange reaction allows the preparation of highly functionalized aryl- and hetaryl pinacolborates by the exchange reaction in the presence of boronic ester (Scheme 31).85... 

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