The direct anti-Markovnikov addition of strong Brønsted acids to alkenes remains an unsolved problem in synthetic chemistry. Here, we report an efficient organic photoredox catalyst system for the addition of HCl, HF and also phosphoric and sulfonic acids to alkenes, with complete regioselectivity. These transformations were developed using a photoredox catalyst in conjunction with a redox-active hydrogen atom donor. The nucleophile counterion plays a critical role by ensuring high reactivity, with 2,6-lutidinium salts typically furnishing the best results. The nature of the redox-active hydrogen atom donor is also consequential, with 4-methoxythiophenol providing the best reactivity when 2,6-lutidinium salts are used. A novel acridinium sensitizer provides enhanced reactivity within several of the more challenging reaction manifolds. This Article demonstrates how nucleophilic addition reactions mediated by photoredox catalysis can change the way electrophilic and homofugal precursors are constructed.

Herein is presented a direct method for the metal-free hydrotrifluoromethylation of alkenes. The method relies on the single electron oxidation of a commercially available sodium trifluoromethanesulfinate salt (CF 3 SO 2 Na, Langlois reagent) by N-Me-9-mesityl acridinium as a photoredox catalyst. Methyl thiosalicylate is used as a substoichiometric H-atom donor for aliphatic alkenes, and thiophenol is used as a stoichiometric H-atom donor for styrenyl substrates. The substrate scope for the transformation is broad, including mono-, di-and trisubstituted aliphatic and styrenyl alkenes, with high regioselectivity in nearly all cases examined.Scheme 1 Proposed hydrotrifluoromethylation method using photoredox catalysis.

An operationally simple nickel-catalyzed hydroarylation reaction for alkynes is described. This threecomponent coupling reaction utilizes commercially available alkynes and aryl bromides, along with water and Zn. An airstable and easily synthesized Ni(II) precatalyst is the only entity used in the reaction that is not commercially available. This reductive cross-coupling reaction displays a fairly unusual anti selectivity when aryl bromides with ortho substituents are used. In addition to optimization data and a preliminary substrate scope, complementary experiments including deuterium labeling studies are used to provide a tentative catalytic mechanism. We believe this report should inspire and inform other Ni-catalyzed carbofunctionalization reactions.

Herein we report energy transfer studies in a series of Ru(II) and Os(II) linked coiled-coil peptides in which the supramolecular scaffold controls the functional properties of the assembly. A general and convergent method for the site-specific incorporation of bipyridyl Ru(II) and Os(II) complexes using solid-phase peptide synthesis and the copper-catalyzed azide-alkyne cycloaddition is reported. Supramolecular assembly positions the chromophores for energy transfer. Using time-resolved emission spectroscopy we measured position-dependent energy transfer that can be varied through changes in the sequence of the peptide scaffold. High level molecular dynamics simulations were used in conjunction with the spectroscopic techniques to gain molecular-level insight into the observed trends in energy transfer. The most efficient pair of Ru(II) and Os(II) linked peptides as predicted by molecular modeling also exhibited the fastest rate of energy transfer (with k(EnT) = 2.3 × 10(7) s(-1) (42 ns)). Additionally, the emission quenching for the Ru(II) and Os(II) peptides can be fit to binding models that agree with the dissociation constants determined for the peptides via chemical denaturation.

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