Irradiation of Cp2TiCl2 with green light leads to electronically excited [Cp2TiCl2]*. This complex constitutes an efficient photoredox catalyst for the reduction of epoxides and for 5‐exo cyclizations of suitably unsaturated epoxides. To the best of our knowledge, our system is the first example of a molecular titanium photoredox catalyst.
We
describe a combination of catalysts that allows the coupling
of titanocene(III) catalysis with photoredox catalysis. Oxidation
of radical intermediates by a photoredox catalyst opens novel catalytic
mechanisms for reductive epoxide ring opening and redox-neutral epoxide
radical arylation. In the former case, the requirement of metallic
reductants and stoichiometric acidic additives is bypassed.
We report here a mechanistically
distinct approach to achieve Suzuki–Miyaura-type
cross-couplings between alkyl iodides and aryl organoborons. This
process requires a copper catalyst but, in contrast with previous
approaches based on palladium and nickel systems, does not utilizes
the metal for the activation of the alkyl electrophile. Instead, this
strategy exploits the halogen-atom-transfer ability of α-aminoalkyl
radicals to convert secondary alkyl iodides into the corresponding
alkyl radicals that then are coupled with aryl, vinyl, alkynyl, benzyl,
and allyl boronate species. These novel coupling reactions feature
a simple setup and conditions (1 h at room temperature) and facilitate
access to privileged motifs targeted by the pharmaceutical sector.
Cp2Ti(TFA) is a broadly applicable catalyst for the [2+2] cycloaddition of bisenones by inner‐sphere electron transfer. The attractiveness of this mechanism is shown by comparison with outer‐sphere ET methods. DFT calculations show that the reaction proceeds through a unique unfavorable 5‐exo (the rate‐determining step) and a favorable 4‐exo cyclization.
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