Selective, radical-free activation of benzylic C–H bonds in methylarenes

Abstract

We report rare examples of exclusive benzylic C-H oxidative addition in industrially important methylarenes using simple η4-arene iridium complexes. Mechanistic studies showed that coordinatively unsaturated η2-arene intermediates are responsible for...

Selective ortho-C–H Activation in Arenes without Functional Groups

“…The arene with the smallest alkyl substituent, methylbenzene, gave no ortho -C–H activation product but instead gave benzyl hydride complex 2k . 9 The observed order of ortho regioselectivity, sec -alkyl > n -alkyl ≫ methyl ( Figure 2 D), is opposite to that of classical electrophilic substitution 7a , 7b and contrasts with that of known oxidative additions of C–H bonds in alkylarenes, which favor meta and para but not ortho products. 10 The same counterintuitive trend holds for the C–H activation of para -substituted dialkylarene ligands ( Figure 2 C): for example, in p -isopropylmethylbenzene, aromatic C–H activation occurs exclusively next to the isopropyl substituent and not next to the methyl substitutent ( 2m ).…”

“…An exception was the bulkiest primary alkylarene in the test, neopentylbenzene, which yielded C–H activation product 2j with 91% ortho selectivity. The arene with the smallest alkyl substituent, methylbenzene, gave no ortho -C–H activation product but instead gave benzyl hydride complex 2k . The observed order of ortho regioselectivity, sec -alkyl > n -alkyl ≫ methyl (Figure D), is opposite to that of classical electrophilic substitution , and contrasts with that of known oxidative additions of C–H bonds in alkylarenes, which favor meta and para but not ortho products .…”

“…We recently demonstrated that Cp*Ir(η 4 -methylarene) complexes promote selective benzylic C–H activation of the methylarene ligand in the presence of a phosphine ligand. 9 Our attempt to extend this reactivity to primary and secondary alkylarenes led to an unexpected switch in selectivity and formation of ortho -C–H activation products ( Figure 2 ). The reaction of isopropylbenzene complex 1a with PMe 3 in n -hexane at 100 °C yielded the product of oxidative addition of an ortho -C–H bond, Ir aryl hydride complex 2a , in 99% yield ( Figure 2 A).…”

“…Bulkier p -diisopropylbenzene gave ortho metalation product 2l exclusively, while smaller p -dimethylbenzene yielded a 32:68 mixture of ortho and benzyl C–H metalation products 2n . 9 …”

“…We recently demonstrated that Cp*Ir­(η 4 -methylarene) complexes promote selective benzylic C–H activation of the methylarene ligand in the presence of a phosphine ligand Our attempt to extend this reactivity to primary and secondary alkylarenes led to an unexpected switch in selectivity and formation of ortho -C–H activation products (Figure ).…”