Cyclic Alkyl Amino Carbene Research Articles

Bis(Cyclic Alkyl Amino Carbene) Ruthenium Complexes: A Versatile, Highly Efficient Tool for Olefin Metathesis.

The state‐of‐the‐art in olefin metathesis is application of N‐heterocyclic carbene (NHC)‐containing ruthenium alkylidenes for the formation of internal C=C bonds and of cyclic alkyl amino carbene (CAAC)‐containing ruthenium benzylidenes in the production of terminal olefins. A straightforward synthesis of bis(CAAC)Ru indenylidene complexes, which are highly effective in the formation of both terminal and internal C=C bonds at loadings as low as 1 ppm, is now reported.

Bonding analysis of ylidone complexes EL2 (E = C–Pb) with phosphine and carbene ligands L

Quantum chemical calculations using DFT and ab initio methods have been carried out of the compounds EL2 with atoms E = C–Pb and the ligands L = PPh3, N-heterocyclic carbene (NHC), bicyclic NHC, and cyclic alkyl-amino carbene (cAAC). The equilibrium structures are reported and the bonding situation was analyzed with a variety of charge- and energy decomposition methods. Some of the molecules are experimentally known, but most molecules have not yet been prepared. The bonding analysis suggests that all but one molecule should be considered as ylidones that possess dative bonds L→E←L. The sole exception is C(cAAC)2, which has a linear structure and classical double bonds (cAAC)=C=(cAAC). The ylidones EL2 have two lone-pair orbitals at atom E with σ and π symmetry. The π lone pair is somewhat delocalized due to L←E→L π-backdonation. The contribution of L←E→L π-backdonation relative to L→E←L σ-donation increases for the heavier elements. The compounds have rather large first and second proton affinities, which is a characteristic feature of ylidones. They are thus double Lewis bases that could be utilized in chemical reactions.

Cp*-Substituted Boron Cations: The Effect of NHC, NHO, and CAAC Ligands.

The effect of a ligand on the electron deficiency and Lewis acidity of the Cp*-substituted boron dication has been investigated experimentally and theoretically. In addition to the reported IMes- and N-heterocyclic olefin (NHO)-stabilized boron dications, the related cyclic alkylamino carbene (CAAC)-coordinated boron mono- and dications have also been synthesized and structurally characterized. An electrochemical study of dications [3a-3c]2+ confirms the higher electron deficiency of the dicationic system than the related boron monocations. Moreover, the presence of a π-acidic CAAC ligand is critical for realizing stable radical species generated from the chemical reduction of boron cations. The nature of the axial ligand also significantly affects the selectivity of the hydride addition reaction of boron dications. While bulky superhydride reacts with [3a-3c]2+ in the same manner to give the cyclic boreniums, [BH4]- attacks three different electrophilic sites of boron dications: the sp2 carbon of Cp* of the IMes-coordinated system ([3a]2+), the central boron atom of the NHO-stabilized analogue ([3b]2+), and the ylidene carbon of the CAAC-containing boron dication ([3c]2+).

Stable phosphorus carbene analog debuts

Carbenes were once thought to be transient species featuring a neutral carbon with a pair of unshared electrons that formed during chemical reactions. Given the instability, the best chemists could do was invoke them by proposed reaction mechanisms. With time, researchers designed molecular frameworks to house carbenes so that they were persistent enough to be isolated, namely N-heterocyclic carbenes and cyclic alkyl-amino carbenes. These molecules now serve as ligands to bind and stabilize metals and as organocatalysts. Phosphinidenes, the phosphorus analogs of carbenes, are now following a similar course of development. After years of working with transient phosphinidenes, a team led by Guy Bertrand of the UCSD-CNRS Joint Research Chemistry Laboratory at the University of California, San Diego, has prepared the first phosphinidene that’s stable for weeks at room temperature (Chem 2016, DOI: 10.1016/j.chempr.2016.04.001). As with carbenes, the secret to making a stable phosphinidene has been finding just...

Why do N-heterocyclic carbenes and silylenes activate white phosphorus differently?

We explored the possible reaction of three model carbenes and two model silylenes with white phosphorus leading to insertion, bis-adduct, and tris-adduct products. The energetics of the calculated mechanisms are in accordance with the experimental observations explaining key features like carbene–P4 ratio-dependent reactivity of carbonyl decorated bisamidocarbene and cyclic alkyl-amino carbene. The analysis of the bonding motif of C–P and Si–P bonds in the key intermediate revealed the importance of double-bond and donor–acceptor resonance structures. We found correlation between the σ-donor and π-acceptor strength of investigated carbenes and silylenes and their reactivity with white phosphorus. The combination of low σ-donor and high π-acceptor strength in N-heterocyclic silylene (NHSi) indicated the instability of the key intermediate, bis-adduct, and tris-adduct products and the stability of the insertion product, while high σ-donor and low π-acceptor strength of N-heterocyclic carbene (NHC) resulted in reversed stability order of the previous compounds explaining the different reaction of NHC and NHSi with white phosphorus. Donor-stabilized silylene (DSSi) has high σ-donor and low π-acceptor strength similar to NHC supporting their similar reactivity and the difference between NHSi and DSSi.

Studies on Three-Coordinate [Co{N(SiMe3)2}2L] Complexes, L = N-Heterocyclic Carbene

A series of three-coordinate N-heterocyclic carbene bis(trimethylsilyl)amide complexes of cobalt, [Co{N(SiMe3)2}2(NHC)] (NHC = N,N′-diarylimidazolin-2-ylidene, aryl = 2,6-diisopropylphenyl, SIPr (1), aryl = mesityl, SIMes (2); κ1-N,N′-diphosphanylimidazol-2-ylidene, PCP (5); κ1-N-phosphanyl-N′-2,6-diisopropylphenylimidazol-2-ylidene, PC (6); cyclic alkyl amino carbene :C(Cy)CH2CMe2N-2,6-Pri2C6H3, cAACCy (7)) were prepared by the reaction of [Co{N(SiMe3)2}2] with the corresponding NHCs. The complexes exhibited interesting transamination reactivity in reactions with bulky 2,6-diisopropylaniline (NH2(DiPP)), where adjustment of the stoichiometry and reaction conditions resulted in the substitution of one or two N(SiMe3)2 ligands by the anilido ligand NH(DiPP), giving [Co{N(SiMe3)2}{NH(DiPP)}(NHC)] (NHC = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, IPr (8)) and [Co{NH(DiPP)}2(NHC)] (NHC = SIPr (9); IPr (10); PCP (11)). X-ray crystallography revealed trigonal-planar coordination geometry at Co for all o...

Olefins from seed oils at industrial scales

Organic Chemistry![Figure][1] PHOTO: © MARTIN SHIELDS/ALAMY Linear-chain olefins with terminal double bonds (linear α-olefins, or LAOs) are essential building blocks for both commodity products, such as lubricants and thermoplastics, and fine chemicals and drugs. Many fatty acids derived from seed oils have a linear tail containing a central double bond, and exchanging this tail with ethylene (ethenolysis) would provide a renewable source of LAOs. However, most catalysts for this reaction deactivate after thousands of reaction cycles (turnovers) and require unacceptable amounts of catalyst. Marx et al. now report a ruthenium ethenolysis catalyst bearing a cyclic alkyl amino carbene ligand that achieves more than 100,000 turnovers at a catalyst loading of only three parts per million. Angew. Chem. Int. Ed. 10.1002/anie.201410797 (2014). [1]: pending:yes

The fate of NHC-stabilized dicarbon.

The attempted synthesis of NHC-stabilized dicarbon (NHC=C=C=NHC) through deprotonation of a doubly protonated precursor ([NHC-CH=CH-NHC](2+) ) is reported. Rather than deprotonation, a clean reduction to NHC=CH-CH=NHC is observed with a variety of bases. The apparent resistance towards deprotonation to the target compound led to a reinvestigation of the electronic structure of NHC→CC←NHC, which showed that the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) gap is likely too small to allow for isolation of this species. This is in contrast to the recent isolation of the cyclic alkylaminocarbene analogue (cAAC=C=C=cAAC), which has a large HOMO-LUMO gap. A detailed theoretical study illuminates the differences in electronic structures between these molecules, highlighting another case of the potential advantages of using cAAC rather than NHC as a ligand. The bonding analysis suggests that the dicarbon compounds are well represented in terms of donor-acceptor interactions L→C2 ←L (L=NHC, cAAC).

Cr(i)Cl as well as Cr+ are stabilised between two cyclic alkyl amino carbenes.

Cr(i)Cl is a very unstable species. The present work describes the stabilisation of Cr(i)Cl in the low coordinate environment of cyclic alkyl(amino) carbene ligands and its synthetic application to yield an unprecedented cationic complex with a two coordinate Cr(i). One electron reduction of (cAAC)2CrCl2 (1) with equivalent amount of KC8 results in the formation of (cAAC)2CrCl (2), with a distorted trigonal planar configuration at the metal centre. SQUID, EPR and theoretical studies reveal a Cr(i) centre with S = 5/2 spin ground state for 2. It represents the first example of a mononuclear Cr complex showing slow relaxation of magnetisation under an applied magnetic field. The chlorine atom in 2 is expected to be prone to further reactions with appropriate reagents. This qualifies 2 as a promising precursor for the preparation of various interesting complexes with Cr(i) in a low coordinate environment. The first example of this metathesis reaction is observed when 2 is treated with Na[B(C6H3(CF3)2)4] resulting in [(cAAC)2Cr]+[B(C6H3(CF3)2)4]-, a linear cationic complex with two coordinate Cr(i) and an S = 5/2 spin ground state.

Cyclic alkyl amino carbene (CAAC) ruthenium complexes as remarkably active catalysts for ethenolysis.

An expanded family of ruthenium-based metathesis catalysts bearing cyclic alkyl amino carbene (CAAC) ligands was prepared. These catalysts exhibited exceptional activity in the ethenolysis of the seed-oil derivative methyl oleate. In many cases, catalyst turnover numbers (TONs) of more than 100,000 were achieved, at a catalyst loading of only 3 ppm. Remarkably, the most active catalyst system was able to achieve a TON of 340,000, at a catalyst loading of only 1 ppm. This is the first time a series of metathesis catalysts has exhibited such high performance in cross-metathesis reactions employing ethylene gas, with activities sufficient to render ethenolysis applicable to the industrial-scale production of linear α-olefins (LAOs) and other terminal-olefin products.

Cyclic Alkyl Amino Carbene (CAAC) Ruthenium Complexes as Remarkably Active Catalysts for Ethenolysis

AbstractAn expanded family of ruthenium‐based metathesis catalysts bearing cyclic alkyl amino carbene (CAAC) ligands was prepared. These catalysts exhibited exceptional activity in the ethenolysis of the seed‐oil derivative methyl oleate. In many cases, catalyst turnover numbers (TONs) of more than 100 000 were achieved, at a catalyst loading of only 3 ppm. Remarkably, the most active catalyst system was able to achieve a TON of 340 000, at a catalyst loading of only 1 ppm. This is the first time a series of metathesis catalysts has exhibited such high performance in cross‐metathesis reactions employing ethylene gas, with activities sufficient to render ethenolysis applicable to the industrial‐scale production of linear α‐olefins (LAOs) and other terminal‐olefin products.

The reactivities of iminoboranes with carbenes: BN isosteres of carbene-alkyne adducts.

The first examples of adducts of cyclic alkyl(amino) carbenes (CAAC) and N-heterocyclic carbenes (NHCs) with iminoboranes have been synthesized and isolated at low temperature (-45 °C). The adducts show short BN bonds and planarity at boron, mimicking the structures of the isoelectronic imine functionality. When di-tert-butyliminoborane was reacted with 1,3-bis(isopropyl)imidazol-2-ylidene (IPr), the initially formed Lewis acid-base adduct quickly rearranged to form a new carbene substituted with an aminoborane at the 4-position. Warming the iminoborane-CAAC adduct to room temperature resulted in an intramolecular cyclization to give a bicyclic 1,2-azaborilidine compound.

Computational study of unsaturated and saturated cyclic (alkyl) (amino) carbene borane complexes

In this study, several density functional theory (DFT) and CBS-QB3 calculations were performed to explore the properties of different conformers of cyclic (alkyl) (amino) carbene (CAAC) borane complexes with the chemical formula BH(C4NH5)2. In 2011, Kinjo et al. successfully synthesized a Lewis-basic boron derivative using two saturated CAACs (Kinjo et al., 2009). In this work, the properties of the boron atom attached to two unsaturated CAACs and two saturated CAACs were compared. According to our results, the unsaturated CAAC shows reduced proton affinity of the boron atom as compared to the saturated CAAC; the insertion of two CC double bonds into CAAC makes the BCcarbenic bonds stronger. The results of natural bond orbital analysis indicated that the lone pair of the central boron atom can be viewed as a pure π-type p orbital.

Stabilization of a two-coordinate mononuclear cobalt(0) compound.

Compound (Me2 -cAAC:)2 Co(0) (2; Me2 -cAAC:=cyclic (alkyl) amino carbene; :C(CH2 )(CMe2 )2 N-2,6-iPr2 C6 H3 ) was synthesized by the reduction of the precursor (Me2 -cAAC:)2 Co(I) Cl (1) with KC8 in THF. The cyclic voltammogram of 1 exhibited one-electron reduction, which suggests that synthesis of a bent 2-metallaallene (2) from 1 should be possible. Compound 2 contains one cobalt atom in the formal oxidation state zero, which is stabilized by two Me2 -cAAC: ligands. Bond lengths from X-ray diffraction are 1.871(2) and 1.877(2) Å with a C-Co-C bond angle of 170.12(8)°. The EPR spectrum of 2 exhibited a broad resonance attributed to the unique quasi-linear structure, which favors near degeneracy and gives rise to very rapid relaxation conditions. The cAACCo bond in 2 can be considered as a typical Dewar-Chatt-Duncanson type of bonding, which in turn retains 2.5 electron pairs on the Co atom as nonbonding electrons.

A Cyclization–Rearrangement Cascade for the Synthesis of Structurally Complex Chiral Gold(I)–Aminocarbene Complexes

AbstractA facile synthesis of chiral cyclic alkyl aminocarbene–gold(I) complexes from gold‐free 1,7‐enyne substrates was developed. The novel cyclization–rearrangement reaction sequence is triggered by the addition of (Me2S)AuCl to different 1,7‐enynes and leads to structurally unique carbene–gold(I) complexes in high yields. These novel complexes are catalytically active and inhibit the proliferation of different human cancer cell lines.

Stabilization of a Cobalt–Cobalt Bond by Two Cyclic Alkyl Amino Carbenes

(Me2-cAAC:)2Co2 (2, where Me2-cAAC: = cyclic alkyl amino carbene, :C(CH2)(CMe2)2N-2,6-iPr2C6H3)) was synthesized via the reduction of precursor (Me2-cAAC:Co(II)(μ-Cl)Cl)2 (1) with KC8. 2 contains two cobalt atoms in the formal oxidation state zero. Magnetic measurement revealed that 2 has a singlet spin ground state S = 0. The cyclic voltammogram of 2 exhibits both one-electron oxidation and reduction, indicating the possible synthesis of stable species containing 2(•-) and 2(•+) ions. The latter was synthesized via reduction of 1 with required equivalents of KC8 and characterized as [(Me2-cAAC:)2Co2](•+)OTf(-) (2(•+)OTf(-)). Electron paramagnetic resonance spectroscopy of 2(•+) reveals the coupling of the electron spin with 2 equiv (59)Co isotopes, leading to a (Co(0.5))2 state. The experimental Co1-Co2 bond distances are 2.6550(6) and 2.4610(6) Å for 2 and 2(•+)OTf(-), respectively. Theoretical investigation revealed that both 2 and 2(•+)OTf(-) possess a Co-Co bond with an average value of 2.585 Å. A slight increase of the Co-Co bond length in 2 is more likely to be caused by the strong π-accepting property of cAAC. 2(•+) is only 0.8 kcal/mol higher in energy than the energy minimum. The shortening of the Co-Co bond of 2(•+) is caused by intermolecular interactions.

Carbonylation of Cyclotrisilenes

Carbon monoxide (CO) is a common ligand in transitionmetal chemistry. In organic chemistry, it is frequently used to introduce a C1 unit, although carbonylations, such as hydroformylation or the Pauson–Khand reaction, generally require a transition-metal catalyst, even for strained (and thus highly reactive) cyclic compounds. Only a few reports on the reactivity of CO towards compounds of main-group elements other than carbon are available. Although known since the pioneering work by Schlesinger and Burg in the 1930s, borane–CO adducts are usually unstable at room temperature. Only recently, Piers et al. reported a stable adduct between the very Lewis acidic perfluorinated pentaphenylborole and CO. The hydroborating Lewis acid (F5C6)2BH incorporates CO assisted by a alkenyl-functionalized phosphine in a frustrated Lewis pair (FLP) mechanism. A B(C6F5)3-based FLP is capable of stoichiometric CO reduction with dihydrogen. A few coordinatively unsaturated and thus Lewis acidic organic species, such as transient triplet carbenes, and Bertrand s cyclic alkyl amino carbenes react with CO in the absence of transition metals, forming ketenes. Despite growing appreciation of the capability of main group species to activate small molecules, such as H2 and NH3, [10] reactions of stable low-valent compounds of the heavier elements with CO have not, to our knowledge, been reported. Transient silylene–CO adducts, however, have been observed in the gas phase and in cold matrices. We have recently shown that reactions of cyclotrisilenes with N-heterocyclic carbenes (NHCs) reversibly afford cyclotrisilene–NHC adducts, which in the case of 1a can undergo reversible ring-opening to form a highly unsaturated NHCstabilized disilenylsilylene. Isonitriles also react with cyclotrisilenes, and, mindful of the isoelectronic relationship to CO, we became interested in the reactivity of the latter. Herein, we report the facile reaction of carbon monoxide with cyclotrisilenes in the absence of a catalyst to form highly functionalized cyclic silenes. Exposure of a benzene solution of cyclotrisilene 1a to 1 atm of CO at 25 8C results in precipitation of a yellow powder within a few hours, which owing to its insolubility cannot be characterized spectroscopically. A reaction for several days without stirring under otherwise identical conditions afforded bright yellow single crystals. Elemental analysis confirmed the incorporation of one equivalent of CO per molecule of 1a. X-ray crystallography revealed the product to be the tricyclic Brook-type bis(silene) 2a, formally arising from two molecules each of 1a and CO (Scheme 1, Figure 1). With a p-donating oxygen atom at carbon, 2a is reminiscent of donor-stabilized silenes with inverse polarization,

Relationship between Gallium Pyramidalization in L·GaCl3 Complexes and the Electronic Ligand Properties

Six new molecular GaCl3 adducts of electron rich compounds of the carbone (carbodiphosphorane, tetraaminoallene) and cyclic alkyl amino carbene (CAAC) families have been synthesized and characterized by X-ray crystallography. The sum of their Cl-Ga-Cl angles has been compared to those of 20 other complexes exhibiting various oxygen-, nitrogen-, phosphorus-, and carbon-donor ligands for which good quality X-ray analyses have been reported. The pyramidalization of the GaCl3 moiety in L·GaCl3 complexes has been checked against the computed antisymmetric stretching of the Ga-Cl bonds. It has also been compared to the symmetric stretching of the C-O bonds of the corresponding L·Ni(CO)3 complexes (Tolman Electronic Parameter). On this basis, a relationship between the pyramidalization observed in the gallium complexes and the electronic ligand properties has been established.

Deprotonation of a Borohydride: Synthesis of a Carbene‐Stabilized Boryl Anion

An acidic hydride! Thanks to the presence of a π-acceptor cyclic alkyl amino carbene and of two electron-withdrawing nitrile groups, a borohydride reacts with a base to give a carbene-stabilized boryl anion, which reacts with carbon and metal electrophiles at the boron center. Dipp = 2,6-diisopropylphenyl, KHMDS = potassium bis(trimethylsilyl)amide.

A Singlet Biradicaloid Zinc Compound and Its Nonradical Counterpart

Metal ions with radical centers in their coordination sphere are key participants in biological and catalytic processes. In the present study, we describe the synthesis of the cAAC:ZnCl2 adduct (1) using a cyclic alkylaminocarbene (cAAC) as donor ligand. Compound 1 was treated with 2 equiv of KC8 and LiB(sec-Bu)3H to yield a deep blue-colored dicarbene zinc compound (cAAC)2Zn (2) and the colorless hydrogenated zinc compound (cAACH)2Zn (3), respectively. Compounds 2 and 3 were well characterized by spectroscopic methods and single-crystal X-ray structural analysis. Density functional theory calculations were performed for 2 which indicate that this molecule possesses a singlet biradicaloid character. Moreover, we show the application of 2 in CO2 activation, which yields a zwitterionic cAAC·CO2 adduct.