Transition‐Metal‐Mediated Cleavage of C−C Bonds in Aromatic Rings

Abstract

Metal‐mediated cleavage of aromatic C−C bonds has a range of potential synthetic applications: from direct coal liquefaction to synthesis of natural products. However, in contrast to the activation of aromatic C−H bonds, which has already been widely studied and exploited in diverse set of functionalization reactions, cleavage of aromatic C−C bonds remains Terra incognita. This Minireview summarizes the recent progress in this field and outlines key challenges to be overcome to develop synthetic methods based on this fundamental organometallic transformation.

Boron-Mediated Carbon–Carbon Bond Cleavage and Rearrangement of Benzene Forming the Borepinyl Radical and Borole Derivatives

“…The results indicate that atomic boron is quite reactive toward the aromatic benzene molecule in forming the borepinyl radical and borole derivatives via carbon–carbon bond cleavage and rearrangement reactions, which are predicted to be highly exothermic due to the formation of strong B–C bonds. In contrast, most transition-metal atoms with partially filled valence d orbitals available prefer to form stable sandwich or half-sandwich benzene complexes via dative bonding. − Transition-metal insertion into the aromatic C–C bonds is thermodynamically not very favorable due to relatively weak metal–carbon bonding …”

“…9−15 Transition-metal insertion into the aromatic C− C bonds is thermodynamically not very favorable due to relatively weak metal−carbon bonding. 56…”

Selective cleavage of unactivated arene ring C–C bonds by iridium: key roles of benzylic C–H activation and metal–metal cooperativity

“…[1][2][3][4][5][6] In contrast, functionalisations that involve breaking the aromatic ring are rare despite their tremendous synthetic potential to provide convenient access to a range of ring opening, contraction and expansion products from cheap hydrocarbons. [7][8][9][10] The main challenge in developing such transformations is the slow and unselective oxidative addition of aromatic C-C bonds due to their higher kinetic and thermodynamic stability compared to that of C-H bonds. 9,10 As a result, insertion of metal complexes into aromatic C-H bonds is overwhelmingly more common that insertion into aromatic C-C bonds.…”

“…[7][8][9][10] The main challenge in developing such transformations is the slow and unselective oxidative addition of aromatic C-C bonds due to their higher kinetic and thermodynamic stability compared to that of C-H bonds. 9,10 As a result, insertion of metal complexes into aromatic C-H bonds is overwhelmingly more common that insertion into aromatic C-C bonds. Of the seven examples of arene ring scission by a well-defined metal complexes reported to date, 11,12 only cleavage of C 6 (CF 3 ) 6 by a Pt complex, 13 and benzene and biphenylene by Al complexes 14,15 proceed selectively.…”

“… 7–10 The main challenge in developing such transformations is the slow and unselective oxidative addition of aromatic C–C bonds due to their higher kinetic and thermodynamic stability compared to that of C–H bonds. 9,10 As a result, insertion of metal complexes into aromatic C–H bonds is overwhelmingly more common that insertion into aromatic C–C bonds.…”