Remarkable γ-Effect of Tin: Acid-Promoted Cyclopropanation Reactions of α-((Alkoxycarbonyl)oxy)stannanes with Alkenes

Modular Enantioselective Synthesis of cis-Cyclopropanes through Self-Sensitized Stereoselective Photodecarboxylation with Benzothiazolines

Chiral cis-cyclopropanes are strained rigid analogues of alkyl chains, whose study and application are limited by their difficult synthesis. A modular approach from olefin materials is enabled by the discovery of the electron donor-acceptor (EDA) interaction between 2-substituted benzothiazolines and N-hydroxyphthalimide esters. These complexes are activated by visible light without photocatalysts, and the benzothiazoline reagent plays a triple role as a photoreductant, a stereoselective hydrogen-atom donor, and a Brønsted acid. Beyond the enantioselective synthesis of cis-cyclopropanes, these results introduce benzothiazolines as accessible and easily tunable self-sensitized photoreductants.

Photoredox-catalyzed oxo-amination of aryl cyclopropanes

Cyclopropanes represent a class of versatile building blocks in modern organic synthesis. While the release of ring strain offers a thermodynamic driving force, the control of selectivity for C–C bond cleavage and the subsequent regiochemistry of the functionalization remains difficult, especially for unactivated cyclopropanes. Here we report a photoredox-coupled ring-opening oxo-amination of electronically unbiased cyclopropanes, which enables the expedient construction of a host of structurally diverse β-amino ketone derivatives. Through one electron oxidation, the relatively inert aryl cyclopropanes are readily converted into reactive radical cation intermediates, which in turn participate in the ensuing ring-opening functionalizations. Based on mechanistic studies, the present oxo-amination is proposed to proceed through an S_N2-like nucleophilic attack/ring-opening manifold. This protocol features wide substrate scope, mild reaction conditions, and use of dioxygen as an oxidant both for catalyst regeneration and oxygen-incorporation. Moreover, a one-pot formal aminoacylation of olefins is described through a sequential cyclopropanation/oxo-amination.

The ring-opening and functionalization of electronically unbiased cyclopropanes is highly challenging to achieve in a regioselective fashion. Here, the authors report a mild photoredox-coupled oxoamination of electronically unactivated aryl cyclopropanes with simple azaarenes and molecular oxygen.

Generality and Strength of Transition Metal β-Effects

Using computation, we examine the generality and strength of β-effects from transition metal centers on β-elimination. In particular, we find that a β-Pd(II) substituent imparts over twice the stabilization to a carbocation as a Si substituent, representative of the well-known β-silicon effect. We established efficient and practical computational parameters to investigate the σσ conjugation in an experimentally relevant system: N, N-picolinamide vinyl metalacycles with β-substituents that can undergo elimination. We have found that the β-Pd effect depends on the nature of the C_β substituent (X): This effect is negligible for X = H, Me, OH, and F, but is significant for X = Cl, Br, and I. We have also extended these studies to the β-effect in N, N-picolinamide vinyl metalacycles with β-substituents of other transition metals-Fe(II), Ru(II), Os(II), Co(III), Rh(III), Ir(III), Ni(II), Pd(II), Pt(II), Cu(III), Ag(III), and Au(III). We found that the electronegativity of the metals correlates reasonably well with the relative β-effects, with first-row transition metals exerting the strongest influence. Overall, it is our anticipation that a more profound appreciation of transition metal β-effects will facilitate the design of novel reactions, including new variants of transition metal catalyzed C-H functionalization.

Silver-Catalyzed Cyclopropanation of Alkenes Using N-Nosylhydrazones as Diazo Surrogates

An efficient silver-catalyzed [2 + 1] cyclopropanation of sterically hindered internal alkenes with diazo compounds in which room-temperature-decomposable N-nosylhydrazones are used as diazo surrogates is reported. The unexpected unique catalytic activity of silver was ascribed to its dual role as a Lewis acid activating alkene substrates and as a transition metal forming silver carbenoids. A wide range of internal alkenes, including challenging diarylethenes, were suitable for this protocol, thereby affording a variety of cyclopropanes with high efficiency in a stereoselective manner under mild conditions.

Three-Dimensional Six-Connecting Organic Building Blocks Based on Polychlorotriphenylmethyl Units—Synthesis, Self-Assembly, and Magnetic Properties

The synthesis of a three-dimensional, six-connecting, organic building block based on a robust, rigid, and open-shell polychlorotriphenylmethyl (PTM) unit (radical 1) is reported, and its self-assembly properties are described in detail. The tendencies of this highly polar molecule and its hydrogenated precursor, compound 4, to form hydrogen bonds with oxygenated solvents ([1THF(6)] and [4THF(6)]) were reduced by replacing THF with diethyl ether in the crystallization process to yield two-dimensional (2D) hydrogen-bonded structures ([1(Et(2)O)(3)] and [4(Et(2)O)(3)]). The presence of direct hydrogen bonds between the radicals in the latter phase of 1 gives rise to very weak ferromagnetic intermolecular interactions at low temperatures, whereas when the radicals are isolated by THF molecules these interactions are antiferromagnetic and very weak. The role played by the carboxylic groups not only in the self-assembly properties but also in the transmission of the magnetic interactions has been illustrated by determination of the crystal structure and measurement of the magnetic properties of the corresponding hexaester radical 6, in which the close packing of molecular units gives rise to weak antiferromagnetic intermolecular interactions. Attempts to avoid solvation of the molecules in the solid state and to increase the structural and magnetic dimensionality were pursued by recrystallization of both compounds 1 and 4 from concentrated nitric acid, affording two three-dimensional (3D) robust hydrogen-bonded structures. While the structure obtained with compound 4 is characterized by the presence of polar channels and boxes containing water guest molecules along the c axis, radical 1 was oxidized to the corresponding fuchsone 10, which presented a completely different close-packed, guest-free structure.

The Si−Si Effect on Ionization of β-Disilanyl Sulfides and Selenides

The ionization energies of conformationally constrained, newly synthesized beta-disilanyl sulfides and selenides were determined by photoelectron spectroscopy. These ionization energies reflect substantial (0.53-0.75 eV) orbital destabilizations. The basis for these destabilizations was investigated by theoretical calculations, which reveal geometry-dependent interaction between sulfur or selenium lone pair orbitals and sigma-orbitals, especially Si-Si sigma-orbitals. These results presage facile redox chemistry for these compounds and significantly extend the concept of sigma-stabilization of electron-deficient centers.

Evaluation of beta- and gamma-effects of group 14 elements using intramolecular competition

To evaluate beta-effects and gamma-effects of group 14 elements, we have devised a system in which the intramolecular competition between gamma-elimination of tin and beta-elimination of silicon, germanium, and tin can be examined. Thus, the reactions of alpha-acetoxy(arylmethyl)stannanes with allylmetals (metal = Si, Ge, Sn) in the presence of BF(3).OEt(2) were carried out. The reactions seem to proceed by the initial formation of an alpha-stannyl-substituted carbocation, which adds to an allylmetal to give the carbocation that is beta to the metal and gamma to tin. The beta-elimination of the metal gives the corresponding allylated product, and the gamma-elimination of tin gives the cyclopropane derivative. In the case of allylsilane, the cyclopropane derivative was formed as a major product, whereas in the case of allylgermane the allylated product was formed predominantly. In the case of the allystannane the allylated product was formed exclusively. These results indicate that the gamma-elimination of tin is faster than the beta-elimination of silicon, but slower than the beta-elimination of germanium and tin. The theoretical studies using ab initio molecular orbital calculations of the carbocation intermediates are consistent with the experimental results. The effect of substituents on silicon was also studied. The introduction of sterically demanding substituents on silicon disfavored the beta-elimination of silicon probably because of the retardation of nucleophilic attack on silicon to cleave the carbon-silicon bond.