Stable Bis(diisopropylamino)cyclopropenylidene (BAC) as Ligand for Transition Metal Complexes

Dimerization and ring-opening in bis(diisopropylamino)cyclopropenylidene (BAC) mediated by [U(NR2)3(CCPh)] (R = SiMe3)

Addition of 2 equiv. of bis(diisopropylamino)cyclopropenylidene (BAC) to [U(NR2)3(CCPh)] (1, R = SiMe3), in Et2O, results in formation of [cyclo-N(iPr)C(Me)2CH(NiPr2)C{CHC3(NiPr2)2}][U(NR2)2(N(SiMe3)SiMe2CH2)(CCPh)] (2) in moderate isolated yield. Complex 2 is the result of coupling and protonation of two BAC molecules, where complex 1 contributes the required proton. It was characterized by NMR spectroscopy and X-ray crystallography and represents a new mode of reactivity of the cyclopropenylidene fragment.

A Density Functional Theory Study on Et-BAC-Catalyzed 1,6-Conjugate Addition of p-Chlorobenzaldehyde to p-Quinone Methide for the Synthesis of α,α'-Diarylated Ketones

Umpolung-based organocatalysis has made a remarkable breakthrough in the field of synthetic organic chemistry. Among a plethora of umpolung catalysts, bis(amino)cyclopropenylidenes (BACs) have emerged as efficient organocatalysts with potential applications in synthesizing numerous essential organic moieties. In this study, a plausible mechanism for bis(diethylamino)cyclopropenylidene (Et-BAC)-catalyzed synthesis of α,α'-diarylated ketones has been established using the density functional theory (DFT) method. The proposed catalytic cycle of the studied reaction initiates with the nucleophilic interaction of Et-BAC with p-chlorobenzaldehyde to form a zwitterionic intermediate, which is then transformed to a reactive Breslow intermediate. The Breslow intermediate further undergoes a chemoselective and stereoselective 1,6-conjugate addition reaction with p-quinone methide to form a new C-C bond connection. Finally, the generated adduct undergoes a proton shift reaction with the assistance of both 8-diazabicyclo(5.4.0)undec-7-ene (DBU) and protonated DBU to yield the desired product. Conceptual DFT-derived reactivity indices and frontier molecular orbital theory analysis have been successfully utilized to unravel the role of Et-BAC in this studied reaction. In addition to Et-BAC, DBU and protonated DBU also play a very important role in lowering the activation energy barrier of proton transfer steps. This investigation will help in the rational designing of simple nonheterocyclic carbene-mediated novel organic transformations.

Bis(amino)cyclopropenium Ion as a Hydrogen-Bond Donor Catalyst for 1,6-Conjugate Addition Reactions

The catalytic application of the bis(amino)cyclopropenium ion has been investigated in conjugate addition reactions. The hydrogen atom, which is attached to the cyclopropene ring of bis(amino)cyclopropenium salts, is moderately acidic and can potentially serve as a hydrogen-bond donor catalyst in some organic transformations. This hypothesis has been successfully realized in the 1,6-conjugate addition reactions of p-quinone methides with various nucleophiles such as indole, 2-naphthol, thiols, phenols, and so forth. The spectroscopic studies (NMR and UV-vis) as well as the deuterium isotope labeling studies clearly revealed that the hydrogen atom (C-H) that is present in the cyclopropene ring of the catalyst is indeed solely responsible for catalyzing these transformations. In addition, these studies also strongly indicate that the C-H hydrogen of the cyclopropene ring activates the carbonyl group of the p-quinone methide through hydrogen bonding.

Molecular Orbital Insights of Transition Metal-Stabilized Carbocations

Transition metal-stabilized carbocations are characterized by synthetically valuable interactions, yet, to date there are no comprehensive reports of the many bonding modes that can exist between a metal and carbocation. This review summarizes developments in these complexes to provide a clear picture of their properties and reactivities. In order to strategically exploit them, we propose this summary of the different bonding modes for transition metal-carbocation complexes. These models will help chemists understand the orbital interactions involved in these compounds so that they can approach their synthetic goals most effectively. Multiple transition metals and carbocations will be discussed.

Electronic and ligating properties of carbocyclic carbenes: A theoretical investigation

Carbocyclic carbenes (CCCs) are a class of nucleophilic carbenes which are very similar to N-heterocyclic carbenes (NHCs) in terms of their reactivity, but they do not contain a stabilizing heteroatom in their cyclic ring system. In this study, 17 representative known CCCs and 34 newly designed CCCs are evaluated using quantum chemical methods, and the results are compared in terms of their stability, nucleophilicity, and proton affinity (PA) parameters. The results are divided on the basis of ring size of the known and reported CCCs. The stability, nucleophilicity, PA, complexation energy, and bond strength-related parameters were estimated using M06/6-311++G(d,p) method. The results indicated that the CCCs known in the literature are strong σ-electron donating species and have considerable π-accepting properties. This study led to the design and identification of a few new CCCs with dimethylamine and diaminomethynyl substituents which can be singlet stable and are substantially nucleophilic. © 2018 Wiley Periodicals, Inc.

Carbeniophosphines versus Phosphoniocarbenes: The Role of the Positive Charge

The chemistry of carbeniophosphines and phosphoniocarbenes, which have general structures derived formally from the three-component "carbene/phosphine/positive charge" association, is presented. These two complementary classes of carbon-phosphorus-based ligands, defined by the presence of an inverted cationic coordinating structure (C+ ∼P: vs. P+ ∼C:) have the common purpose of positioning a positive charge in the vicinity of the metal center. Through selected examples, the synthetic methods, coordination properties, and general reactivity of both cationic species is described. Particular emphasis is placed on the influence of the positive charge on the respective chemical behavior of the two classes of compound.

Exploring bis-(amino)cyclopropenylidene as a non-covalent Brønsted base catalyst in conjugate addition reactions

The catalytic application of bis-(amino)cyclopropenylidene, as a non-covalent Brønsted base, in the 1,4- and 1,6-conjugate addition of carbon nucleophiles to enones andp-quinone methides, respectively, is described.

Gas-phase reactions of cyclopropenylidene with protonated alkyl amines

Reactions of c-C₃H₂ with protonated amines are driven by its high gas-phase basicity, forming proton-bound dimer as the first step.

Aza-Morita–Baylis–Hillman reactions catalyzed by a cyclopropenylidene

Catalysis using a bis(dialkylamino)cyclopropenylidene (BAC) has been developed, which relies on a formal umpolung activation of Michael acceptor pro-nucleophiles.

Phase-transfer and other types of catalysis with cyclopropenium ions

This work establishes the cyclopropenium ion as a viable platform for efficient phase-transfer catalysis of a diverse range of organic transformations. The amenability of these catalysts to large-scale synthesis and structural modification is demonstrated. Evaluation of the molecular structure of an optimal catalyst reveals some unique structural features of these systems. Finally, a discussion of electronic charge distribution underscores an important consideration for catalyst design.

A simple access to transition metal cyclopropenylidene complexes

The first example of a BAC–Cu complex, its outstanding catalytic activity in Click chemistry and its use as a carbene-transfer reagent to easily access Au-, Pd–, Ir– and Rh–BAC compounds are reported.

Theoretical study of the ring expansion reaction mechanism of cyclopropenylidene with azetidine

The mechanism for the ring expansion reaction between cyclopropenylidene and azetidine was systematically investigated employing second-order Møller-Plesset perturbation theory (MP2) in order to better understand the reactivity of cyclopropenylidene with the four-membered ring compound azetidine. Geometry optimizations and vibrational analyses were performed for the stationary points on the potential energy surfaces of the system. The results of our calculations show that cyclopropenylidene can insert into azetidine at its C-N or C-C bond. From a kinetic viewpoint, it is easier for cyclopropenylidene to insert into the C-N bond of azetidine than into the C-C bond. During the first insertion step and the second ring-opening step, it forms spiro and carbene intermediates, respectively. In the following two H-transfer steps, the carbene intermediate forms allenes and alkynes, respectively, as products. From a thermodynamic perspective, allenes are the dominant product because the reaction is exothermic by 373.4 kJ/mol⁻¹.

Synthesis, structure, and reactivity of carbene-stabilized phosphorus(III)-centered trications [L3P]3+

Carbene-stabilized [L(3)P](+3) cations have been synthesized for the first time by a reaction between 1-chloro-2,3-bis(dialkylamino)cyclopropenium salts and P(SiMe(3))(3). In addition, the first structural characterization of such an entity is reported. Consistent with the X-ray data, density functional calculations indicate that these P-centered cations, despite their high positive charge, still feature a nonbonding electron pair on the P-atom (HOMO) and a very low-lying LUMO depicting them as poor σ-donors and excellent π-acceptors.

Recent developments of cyclopropene chemistry

This critical review discusses recent developments in the field of cyclopropene chemistry. Although several excellent reviews that mainly focused on the thermolysis and pyrolysis as well as metal-mediated reactions of cyclopropenes have been published, significant new developments have also been achieved in recent years. This brand new review provides an overview of the progress from 2007 to 2011 on the syntheses and transformations of cyclopropenes as well as their related mechanistic studies (238 references).

Introduction to N-Heterocyclic Carbenes: Synthesis and Stereoelectronic Parameters

N-Heterocyclic carbenes (NHCs) are cyclic compounds containing a divalent carbon atom bound to at least one nitrogen atom within the heterocycle. Variation of the size of the carbene ring, the substituents on the nitrogen atoms or the additional atoms within the heterocycle lead to an array of different NHCs exhibiting a broad range of electronic properties. Their ability to act as donors and the resulting stable bounds to most transition metals make them excellent ligands in coordination chemistry. In addition, free NHCs have found applications as organocatalysts in metal free chemical transformations. In this Chapter synthetic procedures leading to different NHCs and important structural and electronic features of this class of compounds are discussed.

Stable cyclic carbenes and related species beyond diaminocarbenes

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

A general strategy for regiocontrol in nickel-catalyzed reductive couplings of aldehydes and alkynes

A strategy for catalyst-controlled regioselectivity in aldehyde-alkyne reductive couplings has been developed. This strategy is the first where either regiochemical outcome may be selected for a broad range of couplings, without relying on substrate biases or directing effects. The complementary use of small cyclopropenylidene carbene ligands or highly hindered N-heterocyclic carbene ligands allows the regiochemical reversal with unbiased internal alkynes, aromatic internal alkynes, conjugated enynes, or terminal alkynes.

Reactivity of cyclic (alkyl)(amino)carbenes (CAACs) and bis(amino)cyclopropenylidenes (BACs) with heteroallenes: comparisons with their N-heterocyclic carbene (NHCs) counterparts

Similarly to NHCs, CAAC(a) and BAC(a) react with CO2 to give the corresponding betaines. Based on the carbonyl stretching frequencies of cis-[RhCl(CO)2(L)] complexes, the order of electron donor ability was predicted to be CAAC(a) approximately BAC(a)>NHCs. When the betaines nu(asym)(CO2) values are used, the apparent ordering is BAC(a)>NHCs approximately CAAC(a) that indicates a limitation for the use of IR spectroscopy in the ranking of ligand sigma-donating ability. Although all carbenes react with carbon disulfide to give the corresponding betaines, a second equivalent of CS2 reacts with the BAC-CS2 leading to a bicyclic thieno[2,3-diamino]-1,3-dithiole-2-thione, which results from a novel ring expansion process. Surprisingly, in contrast to NHCs, CAAC(a) does not react with carbodiimide, whereas BAC(a) exclusively gives a ring expanded product, analogous to that obtained with CS2. The intermediate amidinate can be trapped, using the lithium tetrafluoroborate adduct of BAC(b) as a carbene surrogate.