Stabilization of a Two-Coordinate [GeCl]⁺ Cation by Simultaneous σ and π Donation from a Monodentate Carbodiphosphorane

Give me four: The synthesis, structure, and reactivity of [GeCl]+ and [SnCl]+ cations bearing a carbodiphosphorane as ancillary ligand are presented. In the Ge compound, simultaneous σ and π donation from the carbodiphosphorane to Ge is observed (see HOMO (left) and HOMO−1 (right); C black, H white, P orange, Cl green, Ge purple (not visible)), whereas only the σ dative component is present in the Sn analogue

Carbon-Based Frustrated Lewis Pairs

The use of FLPs offers a new and versatile strategy to activate small molecules such as H2, CO2, acetylenes, disulfides, olefins, and nitrogen oxides. This chapter describes the recent advances reported on the design and reactivity of new frustrated systems constituted by at least one carbon-based partner

Synthesis, Structure, and Applications of Pyridiniophosphines

A new family of cationic ligands, N-alkyl/aryl pyridiniophosphines, has been synthesized through a short, scalable, and highly modular route. Evaluation of their electronic properties evidenced weak σ-donor and quite strong π-acceptor character when used as ancillary ligands. These attributes confer a substantially enhanced π-acidity to the PtII and AuI complexes thereof derived and, as result, they depict an improved ability to activate alkynes towards nucleophilic attack. This superior performance has been demonstrated along several mechanistically diverse PtII- and AuI-catalyzed transformations

Polyfluorinated Cyclopentadienones as Lewis Acids

The ability of 2,3,4,5-tetrakis(trifluoromethyl)cyclopenta-2,4-dien-1-one and 2,3,4,5-tetrakis(pentafluorophenyl)cyclopenta-2,4-dien-1-one to act as organic Lewis acids in the field of frustrated Lewis pair (FLP) chemistry was evaluated. Whereas the former ketone formed zwitterionic adducts with all phosphines studied, the latter did not react with bulky phosphines and, instead, gave completely organic FLPs. Unfortunately, these did not activate dihydrogen, even under high pressures

Coordination Chemistry of Ene-1,1-diamines and a Prototype "Carbodicarbene"

Carbophilic Lewis acids can polarize a coordinated π-bond by a slippage mechanism. A series of stable ylid- or enolate gold complexes of ene-1,1-diamines not only emulate this property, but also reveal the exceptional donor capacity of such electron-rich olefin ligands. Moreover, the first metal complex of a tetraaminoallene is reported, which features a prototype “carbodicarbene” ligand bound to a transition-metal template

Tuning the Lewis Acidity of Boranes in Frustrated Lewis Pair Chemistry: Implications for the Hydrogenation of Electron-Poor Alkenes

An analysis of the metal-free reduction of electron deficient olefins by frustrated Lewis pairs indicates that the rate-determining step might be either the heterolytic cleavage of H2 to form an -onium borohydride salt, or the subsequent transfer of the hydride moiety to the substrate following a Michael-type addition reaction. While the use of strong Lewis acids such as B(C6F5)3 facilitates the first of these processes, hydride transfer to the olefin should be contrarily favoured by the use of weak Lewis acids which, for this very same reason, might be unable to promote the prior H2 split. After systematic testing of several boranes of different Lewis acidity (assessed by using the Childs’ method) and steric demand, an optimal situation that employs tris(2,4,6-trifluorophenyl)borane was reached. Mixtures of this borane with 1,4-diazabicyclo[2.2.2]octane (DABCO) exhibited excellent catalytic activity for the hydrogenation of alkylidene malonates. In fact, this transformation could be achieved under milder conditions than those we reported previously. Moreover, the reaction scope could be expanded to other electron deficient olefins containing esters, sulfones or nitro functionalities as electron-withdrawing substituents

Steering the Surprisingly Modular π-Acceptor Properties of N-Heterocyclic Carbenes: Implications for Gold Catalysis

Thumbnail image of graphical abstract Accept it! Although the π-acceptor properties of N-heterocyclic carbenes (NHCs) are often considered negligible, they can be enhanced to the extent that they start to dominate the catalytic behavior of gold–NHC complexes (see scheme). As it seems to be easier to tune the π acidity of NHCs than to alter their σ-donor properties, the general perception of this important class of ancillary ligands needs to be revised

On the Reactivity of Tetrakis(trifluoromethyl)cyclopentadienone towards Carbon-Based Lewis Bases

The reactivitiy of tetrakis(trifluoromethyl)cyclopentadienone towards different C-based Lewis bases, such as N-heterocyclic carbenes (NHC), ylides and isonitriles, are reported. While sterically not hindered carbenes were found to yield kinetic adducts by regiospecific nucleophilic attack at the position adjacent to the carbonyl group of the ketone, bulkier nucleophiles afforded the thermodynamically more stable O-bridged zwitterions. Interestingly, isonitriles were found to dimerize and trimerize under the same reaction conditions, forming bicyclic products that evolve differently depending on the nature of the substituents

Protecting-Group-Free and Catalysis-Based Total Synthesis of the Ecklonialactones

A concise and protecting-group-free total synthesis of optically pure ecklonialactones A (1) and B (2) is described. The successful route to these oxylipins isolated from various brown algae involves five transition-metal-catalyzed transformations in the longest linear sequence of 13 steps. The first chiral center was set by a rhodium-catalyzed 1,4-addition of an alkenyl boronate to the commercial butenolide 11, which was controlled by Carreira’s carvone-derived diene ligand 21. Other key steps involve a ring-closing olefin metathesis effected by the ruthenium indenylidene complex 22 for the formation of the five-membered carbocycle, a vanadium-catalyzed, hydroxy-directed epoxidation, and a ring-closing alkyne metathesis (RCAM) to forge the macrocyclic ring. Because of the unusually high propensity of the oxirane of the ecklonialactones for ring-opening, this transformation was best achieved with [(Ph3SiO)3Mo≡CPh]·OEt2 (34) as the catalyst, which is a representative of a new generation of highly tolerant yet remarkably efficient molybdenum alkylidyne complexes. The ancillary triphenylsilanolate ligands in 34 temper the Lewis acidity of the molybdenum center and are not able to nucleophilically open the fragile epoxide ring. The final reduction of the cycloalkyne formed in the RCAM step to the required (Z)-alkene was accomplished either by Lindlar reduction or with the aid of nickel boride

Synthesis and Reactivity of Metal Complexes with Acyclic (Amino)(Ylide)Carbene Ligands

No cycle required: The straightforward synthesis of acyclic (amino)(ylide)carbene gold complexes was achieved by reaction of isocyanide gold complexes with phosphorus and arsenic ylides as well as electron-rich olefins. Their ability to form bimetallic species and to act as ligand-transfer reagents has also been established