Regioselective Aromatic Borylation in an Inert Solvent

Abstract

[reaction: see text]. A protocol for performing Rh catalyzed aromatic borylations in cyclohexane has been devised. Borylation at the 5-position of several 1,3-substituted aromatic species ranging from electron-rich (1,3-(NMe(2))(2)C(6)H(4)) to electron-deficient (1,3-(CF(3))(2)C(6)H(4)) yields the corresponding aryl boronate esters. Veratrole was selectively borylated at the 4-position, thus extending regioselectivity to 1,2-substituted benzenes. Selective borylation at the 3-position of an N-protected pyrrole has also been demonstrated, providing a valuable reagent for cross-coupling reactions in a single step.

Iridium-catalyzed borylation of thiophenes: versatile, synthetic elaboration founded on selective C–H functionalization

“…26 Recognizing that this transformation's simplicity could offer advantages over traditional routes to arylboron compounds, we explored the generality of this reaction with arenes, including the first extensions to heterocyclic substrates. [27][28][29] Despite improvements in catalyst generation, 28,30 application of this methodology to substituted thiophenes is limited to five substrates: 2-methylthiophene, [31][32][33] 2-cyanothiophene, 32 2-bromothiophene, 32 2-methoxythiophene, 33 and 2-trifluoromethylthiophene. 33 These reactions yield 5-borylated products exclusively in accord with the preference for borylation of C-H bonds adjacent to formally sp 3 -hybridized heteroatoms in 5-membered heterocycles.…”

Iridium-catalyzed C–H coupling reaction of heteroaromatic compounds with bis(pinacolato)diboron: regioselective synthesis of heteroarylboronates

“…(2) and (3)). 12 The triisopropylsilyl group can be deprotected by treatment with TBAF or CF 3 CO 2 H to provide isomerically pure 3-borylpyrrole or -indole. 13 <<Eqs.…”

Transition Metal‐Catalyzed Borylation of Alkanes and Arenes via C—H Activation

“…and Rh(III) complexes. Oxidative addition of benzylic C-H bonds giving a η 3 -benzyl complex predominates for a mono(boryl)Rh(I) complex, whereas a tris(boryl)Rh(III) species activates the aromatic C-H bond rather than the weaker benzylic C-H bonds [38,42]. It is interesting to note that catalytic dehydrogenative coupling of pinBH to pin 2 B 2 was observed with the present catalyst system.…”

“…The reactions of unsubstituted pyrrole, furan and thiophene resulted in slightly lower yields than those of benzo-fused derivatives because they were accompanied by 2,5-bis(boryl)heteroarenes in a range of 5 to 10%. The regiochemistry can be made to vary by steric hindrance of substituents to selectively synthesize 3-borylated products for 1-triisopropylsilylpyrrole and -indole [38]. [28].…”

“…Aromatic borylation by pinBH with Hartwig's catalyst Cp*Rh(η 4 -C 6 Me 6 ) was amply demonstrated by Smith (Scheme 25) [38,42]. In a cyclohexane solvent at 150 °C in a sealed ampule, 1,3-disubstituted arenes were selectively borylated at the 5-position and 1,2-disubstituted arenes were selectively borylated at the 4-position.…”