Catalytic activation of unstrained C(aryl)–C(aryl) bonds in 2,2′-biphenols

Abstract

Transition-metal catalysis has emerged as an important means for C-C activation allowing mild and selective transformations. However, the current scope of C-C bonds that can be activated is primarily restricted to either highly strained systems or more polarized C-C bonds. In contrast, catalytic activation of nonpolar and unstrained C-C moieties remains an unmet challenge. Here we report a general approach for catalytic activation of the unstrained C(aryl)-C(aryl) bonds in 2,2’-biphenols. The key is utilizing the phenol moiety as a handle to install phosphinites as a recyclable directing group. Using hydrogen gas as the reductant, mono-phenols are obtained with a low catalyst loading and high functional group tolerance. This approach has also been applied to the synthesis of 2,3,4-trisubstituted phenols. Further mechanistic study suggests that the C-C activation step is mediated by a rhodium(I) mono-hydride species. Finally, a preliminary study on breaking the inert biphenolic moieties in lignin models is illustrated.

An Introduction to Model Compounds of Lignin Linking Motifs; Synthesis and Selection Considerations for Reactivity Studies

“…[213,214] In such cases CÀC-linkeda romatic dimers with alkyl or 5-5 linking motifs might be more suitable. [215] Such models are not the focus of this Review as the actual identityo ft he linking motifs in such technical lignins is highly diverse and for al arge part unknown. [52] More analysiso ft echnical lignins is required to guide the synthesis of an ew generation of model compounds for use in the valorization of these recalcitrant lignins.…”

Catalytic Scissoring of Lignin into Aryl Monomers

“…Currently, only one successful example for the selective cleavage of C ary C ary bond in 5‐5′ linkage has been documented. Dong and co‐workers recently reported a general approach for the catalytic activation of the unstrained C aryl C aryl bonds in 2,2′‐biphenols over a Rh catalyst (Figure ). The phenol moiety served as a handle to install a phosphonite‐directing group that guided the Rh metal insertion into C aryl C aryl .…”

Redox-neutral photocatalytic strategy for selective C–C bond cleavage of lignin and lignin models via PCET process

“…Various methods have been developed for effective cleavage of C-O bonds in both lignin and lignin model compounds [6][7][8][9][10][11][12][13][14]. In comparison with the comprehensive studies of C-O bond cleavage, only a handful of examples were reported for selective C-C bond cleavage in lignin or lignin model compounds due to its higher dissociation energies [15][16][17] than that of C-O bond. Such as in 2015, Klankermayer group [18] reported a selective redox-neutral cleavage of C a -C b in b-O-4 lignin models catalyzed by rutheniumtriphos complexes at 160°C (Scheme 1a).…”