Metalated epoxides as carbenoids. Competing C-H and CC insertion in α-alkoxy epoxide systems

In a reinvestigation of the reactivity of carbenoids derived from epoxides, we studied the factors that could influence the chemoselectivity of the carbenoid insertion into vicinal C-H or CC bond in cyclic α-alkoxy epoxides bearing an alkenyl side chain. This reaction gives access to bi- or tricyclic systems, respectively.     

Synthesis and Reactivity of Six‐Membered Oxa‐Nickelacycles: A Ring‐Opening Reaction of Cyclopropyl Ketones

Cyclopropanecarboxaldehyde ( 1 a ), cyclopropyl methyl ketone ( 1 b ), and cyclopropyl phenyl ketone ( 1 c ) were reacted with [Ni(cod)₂] (cod=1,5‐cyclooctadiene) and PBu₃ at 100 °C to give η²‐enonenickel complexes ( 2 a – c ). In the presence of PCy₃ (Cy=cyclohexyl), 1 a and 1 b reacted with [Ni(cod)₂] to give the corresponding μ‐η²:η¹‐enonenickel complexes ( 3 a , 3 b ). However, the reaction of 1 c under the same reaction conditions gave a mixture of 3 c and cyclopentane derivatives ( 4 c , 4 c′ ), that is, a [3+2] cycloaddition product of 1 c with (E)‐1‐phenylbut‐2‐en‐1‐one, an isomer of 1 c . In the presence of a catalytic amount of [Ni(cod)₂] and PCy₃, [3+2] homo‐cycloaddition proceeded to give a mixture of 4 c (76 %) and 4 c′ (17 %). At room temperature, a possible intermediate, 6 c , was observed and isolated by reprecipitation at ?20 °C. In the presence of 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr), both 1 a and 1 c rapidly underwent oxidative addition to nickel(0) to give the corresponding six‐membered oxa‐nickelacycles ( 6 ai , 6 ci ). On the other hand, 1 b reacted with nickel(0) to give the corresponding μ‐η²:η¹‐enonenickel complex ( 3 bi ). The molecular structures of 6 ai and 6 ci were confirmed by X‐ray crystallography. The molecular structure of 6 ai shows a dimeric η¹‐nickelenolate structure. However, the molecular structure of 6 ci shows a monomeric η¹‐nickelenolate structure, and the nickel(II) 14‐electron center is regarded as having “an unusual T‐shaped planar” coordination geometry. The insertion of enones into monomeric η¹‐nickelenolate complexes 6 c and 6 ci occurred at room temperature to generate η³‐oxa‐allylnickel complexes ( 8 , 9 ), whereas insertion into dimeric η¹‐nickelenolate complex 6 ai did not take place. The diastereoselectivity of the insertion of an enone into 6 c having PCy₃ as a ligand differs from that into 6 ci having IPr as a ligand. In addition, the stereochemistry of η³‐oxa‐allylnickel complexes having IPr as a ligand is retained during reductive elimination to yield the corresponding [3+2] cycloaddition product, which is consistent with the diastereoselectivity observed in Ni⁰/IPr‐catalyzed [3+2] cycloaddition reactions of cyclopropyl ketones with enones. In contrast, reductive elimination from the η³‐oxa‐allylnickel having PCy₃ as a ligand proceeds with inversion of stereochemistry. This is probably due to rapid isomerization between syn and anti isomers prior to reductive elimination.     

[4+3] Cycloaddition of Donor–Acceptor Cyclopropanes with Amphiphilic Benzodithioloimine as Surrogate for ortho‐Bisthioquinone

Donor–acceptor cyclopropanes were reacted with amphiphilic benzodithioloimine to give seven‐membered heterocycles with two sulfur atoms. Formally, this transformation can be regarded as a [4+3] cycloaddition reaction of the three‐membered ring and ortho‐bisthioquinone. The benzodithioloimine serves as a surrogate for this highly reactive diene. The structure of the products was confirmed by X‐ray crystallography. Broad signals in ¹³C NMR studies suggest that several conformers, slowly interconverting on the NMR timescale, are present at room temperature.     

Alkyne Insertion into the M?P and M?H Bonds (M=Pd,Ni, Pt,and Rh): A Theoretical Mechanistic Study of the C?P and C?H Bond‐Formation Steps

In hydrogen‐metal‐phosphorus (H M P) transition metal complexes (proposed as intermediates of H P bond addition to alkynes in the catalytic hydrophosphorylation, hydrophosphinylation, and hydrophospination reactions), alkyne insertion into the metal‐hydrogen bond was found much more facile compared to alkyne insertion into the metal‐phosphorus bond. The conclusion was verified for different metals (Pd, Ni, Pt, and Rh), ligands, and phosphorus groups at various theory levels (B3LYP, B3PW91, BLYP, MP2, and ONIOM). The relative reactivity of the metal complexes in the reaction with alkynes was estimated and decreased in the order of Ni>Pd>Rh>Pt. A trend in relative reactivity was established for various types of phosphorus groups: PR₂>P(O)R₂>P(O)(OR)₂, which showed a decrease in rate upon increasing the number of the oxygen atoms attached to the phosphorus center.     

Oxidative Addition at a Carbene Center: Synthesis of an Iminoboryl–CAAC Adduct

The reaction of a cyclic (alkyl)(amino)carbene (CAAC) with dichloro‐ and dibromobis(trimethylsilyl)aminoborane results in the formation of haloiminoborane–CAAC adducts. When the iodo analogue is used, an oxidative addition at the carbene center affords a cationic iminoboryl–CAAC adduct, featuring a boron–nitrogen triple bond. Similar salts are also obtained by halide abstraction from the chloro‐ and bromoiminoborane–CAAC adducts. The reactivity of all of these compounds towards CO₂ is discussed.     

Regioselectivity in iron-catalyzed [2+2+1] cycloadditions: a DFT investigation of substituent effects in 1,4-diazabutadienes

The transition-metal-catalyzed [2+2+1] cycloaddition reaction of 1,4-diazabutadienes, in which the imine-carbon atoms are part of an oxazine ring system, with ethylene and carbon monoxide leads to the regioselective formation of pyrrolidinone derivatives. To explain this regioselectivity, the transition states and intermediates of the rate-determining step of the catalysis are determined by high-level DFT calculations. The experimentally observed regioselectivity is consistent with the lower activation energy of the addition of ethylene towards the carbon atom next to the oxazine oxygen atom. Furthermore, the activation barrier of a conceivable back reaction is higher for the intermediates with the experimentally observed regioselectivity. These thermodynamic and kinetic arguments at first sight appear to be confirmed by the calculated NPA charges in the transition states, which reveal that the differences in these charges are greatest for those transition states that lead to the formation of the energetically favored transition structures. Nevertheless, calculations of analogous transition structures and reaction products starting from 1,4-diazabutadienes with a 2-fluoro, 2-hydroxo or 2-amino substituent revealed that the regioselectivity is not determined by the electronegativity of the heteroatom and thus by the differences in the NPA charges or the resulting Coulombic interactions in the transition structures. The main reason for the observed regioselectivities is the pi-donor ability of the substituent to contribute to a delocalized pi system incorporating the adjacent imine moiety. The increasing pi-donor capability results in decreased reactivity of this moiety and increases the (relative) reactivity of the second imine group. This effect can even overcompensate for strong intramolecular Coulombic attractions in the transition structures.     

Facile Access to Dative,Single, and Double Silicon−Metal Bonds Through M−Cl Insertion Reactions of Base-Stabilized SiII Cations

Silicon(II) cations can offer fascinating reactivity patterns due to their unique electronic structure: a lone pair of electrons, two empty p orbitals and a positive charge combined on a single silicon center. We now report the facile insertion of N-heterocyclic carbene (NHC)-stabilized silyliumylidene ions into M−Cl bonds (M=Ru, Rh), forming a series of novel chlorosilylene transition-metal complexes. Theoretical investigations revealed a reaction mechanism in which the insertion into the M−Cl bond with concomitant 1,2-migration of a silicon-bound NHC to the transition metal takes place after formation of an initial silyliumylidene transition-metal complex. The mechanism could be verified experimentally through characterization of the intermediate complexes. Furthermore, the obtained chlorosilylene complexes can be conveniently utilized as synthons to access Si−M and Si=M bonding motifs bonds through reductive dehalogenation.     

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

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.