Catalytic Asymmetric Diamination of Styrenes.

An enantioselective catalytic vicinal diamination of styrenes is reported, which proceeds under entirely intermolecular reaction control. It relies on a chirally modified aryliodine(I) catalyst and proceeds within an iodine(I/III) manifold with conventional 3-chloroperbenzoic acid as a terminal oxidant. An environmentally benign solvent combination not only adds to the attractiveness of the process but also slows down the rate of the undesired background reaction. A total of 30 examples are presented, which consistently provide high enantiomeric excesses in the range 91-98%.

DFT Rationalization of the Diverse Outcomes of the Iodine(III)-Mediated Oxidative Amination of Alkenes.

A computational study of the mechanism for the iodine(III)-mediated oxidative amination of alkenes explains the experimentally observed substrate dependence on product distribution. Calculations with the M06 functional have been carried out on the reaction between PhI(N(SO2 Me)2 )2 and three different representative substrates: styrene, α-methylstyrene, and (E)-methylstilbene. All reactions start with electrophilic attack by a cationic PhI(N(SO2 Me)2 )(+) unit on the double bond, and formation of an intermediate with a single C-I bond and a planar sp(2) carbocationic center. The major path, leading to 1,2-diamination, proceeds through a mechanism in which the bissulfonimide initially adds to the alkene through an oxygen atom of one sulfonyl group. This behavior is now corroborated by experimental evidence. An alternative path, leading to an allylic amination product, takes place through deprotonation at an allylic C-H position in the common intermediate. The regioselectivity of this amination depends on the availability of the resonant structures of an alternate carbocationic intermediate. Only in cases where a high electronic delocalization is possible, as in (E)-methylstilbene, does the allylic amination occur without migration of the double bond.

Substitution Effects of Hypervalent Iodine(III) Reagents in the Diamination of Styrene.

The influence of electronic parameters on the reaction performance of hypervalent iodine(III) reagents in the vicinal diamination of styrene has been investigated. It demonstrates the influence of the relative electron density on the aryl substituent of the hypervalent iodine reagent. In these cases, compounds with donor substituents outperform the corresponding acceptor-substituted systems. In line with this observation, a rapid enantioselective diamination was observed for a preformed chiral bisimidoiodine(III) reagent. For the first time, X-ray structural data could be obtained for an isolated chiral bisimidoiodine(III).

Formoxyboranes as hydroborane surrogates for the catalytic reduction of carbonyls through transfer hydroboration.

A new class of Lewis base stabilized formoxyboranes demonstrates the feasibility of catalytic transfer hydroboration. In the presence of a ruthenium catalyst, they have shown broad applicability for reducing carbonyl compounds. Various borylated alcohols are obtained in high selectivity and yields up to 99%, tolerating several functional groups. Computational studies enabled to propose a mechanism for this transformation, revealing the role of the ruthenium catalyst and the absence of hydroborane intermediates.

Silyl formates as hydrosilane surrogates for the transfer hydrosilylation of ketones.

A transfer hydrosilylation of ketones employing silyl formates as hydrosilane surrogates under mild conditions is presented. A total of 24 examples of ketones have been successfully converted to their corresponding silyl ethers with 61-99% yields in the presence of a PNHP-based ruthenium catalyst and silyl formate reagent. The crucial role of the ligand for the transformation is demonstrated.

A Practical Aryliodine(I/III) Catalysis for the Vicinal Diamination of Styrenes.

2,6-Disubstituted iodoarenes bearing amide-functionalized side arms are reported as new structures in redox-active iodine(I/III) catalysis. In combination with bis-sulfonimides as nitrogen sources and 3-chloroperbenzoic acid (mCPBA) as benign terminal oxidant they catalyze the vicinal diamination of styrenes. The obtained reactivity and selectivity outperform other iodoarene catalyst candidates. This protocol provides a sustainable alternative to previous related protocols for diamination that are based on stoichiometric iodine(III) reagents.

Vicinal difunctionalization of alkenes with iodine(III) reagents and catalysts.

Hypervalent iodine(III) reagents have been known for over a century, and their reaction profile is still actively investigated. Recent years have seen impressive improvements in the area of alkene difunctionalization reactions, where new methodologies have become available. Especially chiral non-racemic hypervalent iodine(III) reagents and catalysts have emerged as versatile tools for the realization of important enantioselective transformations.