α‐ and β‐Eliminations in Transition Metal Complexes: Strategies to Cleave Unstrained C−C and C−F Bonds

Abstract

Oxidative addition is the standard process for single‐bond activation in transition metal catalysis and it is known to operate for many types of bonds, but challenging σ‐bonds e. g. C(sp3)−F and C(sp3)−C(sp3) bonds are the exceptions in this respect. This short review aims at demonstrating how both α‐ and β‐eliminations may be better options for activation of unstrained C−F and C−C single bonds. Selected examples of such eliminations are presented with a mechanistic focus indicating how unstrained and unactivated C−C and C−F bonds can be broken by employing α‐ and β‐eliminations in transition metal hydrocarbyl ligands. Our examples show that the reaction barrier in β‐eliminations is controlled by the s‐character of the participating bonds where a higher s‐character gives a better overlap in the multi‐center transition state thereby increasing the reactivity; still β‐aryl eliminations can compete with the classical β‐hydrogen eliminations in certain cases.

Recent Advances in Alkenyl sp2 C–H and C–F Bond Functionalizations: Scope, Mechanism, and Applications

“…In contrast to the above-mentioned β-F elimination process, Chatani and colleagues in 2010 reported the elegant work of nickel-catalyzed cyclization of 1,1-difluoro-1,6-enynes with organozinc reagents involving α-F elimination. , Mechanistically, the process is initiated by an oxidative cyclization of both alkene and alkyne moieties with the Ni 0 complex, resulting in the formation of intermediary α,α-difluoronickelacyclopentenes. The authors claimed that the organozinc reagent serves as a bifunctional activator, a Lewis base with respect to Ni II and a Lewis acid with respect to the C–F bond, which greatly facilitated subsequent α-F elimination to generate the bicyclo[3.2.0]­heptene derivatives (Scheme ).…”

Catalytic Activation of Unstrained C(Aryl)–C(Alkyl) Bonds in 2,2′-Methylenediphenols

“…Alternatively, the catalytic cycle may involve rhodium carbene formation (path B). Directed activation of the benzylic methylene linker would afford the fused 6,6-rhodacyle 7 , which could hypothetically undergo α-aryl elimination to deliver the 5,6-spiro rhodacycle 12 . The subsequent C–H reductive elimination would generate one monomer, and the carbene species 13 could further react with H 2 to form the other monomer and close the catalytic cycle.…”

Synthesis and Characterization of Post-β-Carbon-Elimination Organopalladium Complexes

“…bond decomposes into a πbond-containing neutral ligand and a new anionic M−C′ complex with concomitant X−C′ bond cleavage (shown for the X = alkyl case in Scheme 1A). 2 β-Carbon elimination brings about a unique skeletal reorganization and as such plays an important role in organometallic polymerization, 1 smallmolecule catalysis, 3−6 and natural products synthesis, 7,8 with a growing number of recently published creative applications. 9 In particular, ring-opening β-carbon elimination of highly strained molecules, such as cyclopropane/butane or norbornene, is a prevalent strategy to obtain linear compounds by employing strain-release as a thermodynamic driving force to overcome the high barrier of the β-carbon elimination step.…”

“…During β-carbon elimination, an anionic M–X (X = C, N, O, etc.) bond decomposes into a π-bond-containing neutral ligand and a new anionic M–C′ complex with concomitant X–C′ bond cleavage (shown for the X = alkyl case in Scheme A) . β-Carbon elimination brings about a unique skeletal reorganization and as such plays an important role in organometallic polymerization, small-molecule catalysis, − and natural products synthesis, , with a growing number of recently published creative applications .…”