Evidence for a kinetic FLP reaction pathway in the activation of benzyl chlorides by alkali metal-phosphine pairs.

Kinetic frustrated Lewis pairs (FLP) allow facile cleavage of a number of E-H bonds (E = H, Si, C, B) where both the Lewis base and Lewis acid are involved in the bond activation transition state. More recently, kinetic FLP systems have been extended to the cleavage of C-X (X = F, Cl, Br) bonds. We report on the role of sodium tetrakis(pentafluorophenyl)borate in the benzylation of triarylphosphines, where the sodium cation and phosphine support a kinetic FLP type transition state.

A review of frustrated Lewis pair enabled monoselective C-F bond activation.

Frustrated Lewis pair (FLP) bond activation chemistry has greatly developed over the last two decades since the seminal report of metal-free reversible hydrogen activation. Recently, FLP systems have been utilized to allow monoselective C-F bond activation (at equivalent sites) in polyfluoroalkanes. The problem of 'over-defluorination' in the functionalization of polyfluoroalkanes (where multiple fluoro-positions are uncontrollably functionalized) has been a long-standing chemical problem in fluorocarbon chemistry for over 80 years.

Stereoselective Synthesis of Fluoroalkanes via FLP Mediated Monoselective C─F Activation of Geminal Difluoroalkanes.

A method of desymmetrization of geminal difluoroalkanes using frustrated Lewis pair (FLP) mediated monoselective C-F activation where a chiral sulfide is the Lewis base component is reported. The stereoselective reaction provides generally high yields of diastereomeric sulfonium salts with dr of up to 95:5. The distribution of diastereomers is found to be thermodynamically controlled via facile sulfide exchange.

Insights into the Mechanism of Aluminum-Catalyzed Halodefluorination.

Aluminum has been reported to catalyze halodefluorination reactions, where aliphatic fluorine is substituted with a heavier halogen. Although it is known that stoichiometric aluminum halide can perform this reaction, the role of catalytic aluminum halide and organyl alane reagents is not well understood. We investigate the mechanism of the halodefluorination reaction using catalytic aluminum halide and stoichiometric trimethylsilyl halide.

Experimental and computational insights into the mechanism of FLP mediated selective C-F bond activation.

Frustrated Lewis pairs (FLP) comprising of B(CF) (BCF) and 2,4,6-triphenylpyridine (TPPy), P(-Tol) or tetrahydrothiophene (THT) have been shown to mediate selective C-F activation in both geminal and chemically equivalent distal C-F sites. In comparison to other reported attempts of C-F activation using BCF, these reactions appear surprisingly facile. We investigate this reaction through a combination of experimental and computational chemistry to understand the mechanism of the initial C-F activation event and the origin of the selectivity that prevents subsequent C-F activation in the monoactivated salts.

Gauging the donor strength of iron(0) complexes their N-heterocyclic carbene gold(I) adducts.

We isolate and characterize the gold(I)-iron(0) adducts [(Pr-bimy)Au-Fe(CO)(PMe)][BAr] and [Au-{Fe(CO)(PMe)}][BAr] (Pr-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene, BAr = tetrakis(pentafluorophenyl)borate). DFT analysis reveals that the gold-iron interaction in [(Pr-bimy)Au-Fe(CO)(PMe)][BAr] is predominantly a σ-donation from iron to gold. We further extend this class of compounds to include [(Pr-bimy)Au-Fe(CO)(PR)][BAr] (PR = PPh, PCy, PCyPh, PMePh, PMePh, P(4-CHF)) and [(Pr-bimy)Au-Fe(CO)(PPh)][BAr] and correlate the Pr-bimy carbenic C NMR signal with the relative donor strength of the iron(0) ligand.

Fluorine-18 Labeling of Difluoromethyl and Trifluoromethyl Groups via Monoselective C-F Bond Activation.

We report a general method for the labeling of both CF and CF H groups in a broad range of chemical settings (aryl, oxide, sulfide). The method utilizes frustrated Lewis pair mediated selective C-F activation to formally substitute fluorine-19 with fluorine-18 in a two-step defluorination/radiofluorination process, and as such can utilize the target compounds as starting materials. The radiotracer precursors can be isolated as stable salts prior to radiofluorination.

Alkali Metal Adducts of an Iron(0) Complex and Their Synergistic FLP-Type Activation of Aliphatic C-X Bonds.

We report the formation and full characterization of weak adducts between Li and Na cations and a neutral iron(0) complex, [Fe(CO)(PMe)] (), supported by weakly coordinating [BAr] anions, [·M][BAr] (M = Li, Na). The adducts are found to synergistically activate aliphatic C-X bonds (X = F, Cl, Br, I, OMs, OTf), leading to the formation of iron(II) organyl compounds of the type [FeR(CO)(PMe)][BAr], of which several were isolated and fully characterized. Stoichiometric reactions with the resulting iron(II) organyl compounds show that this system can be utilized for homocoupling and cross-coupling reactions and the formation of new C-E bonds (E = C, H, O, N, S).

Access to and Reactivity of Fe , Fe , Fe , and Fe PC P Pincer Complexes.

Despite their promising metal-ligand cooperative reactivity, PC P pincer ligands are rarely reported for first-row transition-metal centres. Using a dehydration methodology, we report access to an Fe PC P pincer complex (1) that proceeds via an isolated α-hydroxylalkyl hydrido complex (3). Reversible carbonyl migration to the carbene position in 1 is found to allow coordination chemistry and E-H bond addition (E=H, B, Cl) across the iron-carbene linkage, representing a unique mechanism for metal-ligand cooperativity.

FLP-Catalyzed Monoselective C-F Functionalization in Polyfluorocarbons at Geminal or Distal Sites.

We report frustrated Lewis pair (FLP)-catalyzed monoselective C-F activation in a range of aliphatic polyfluorocarbons with equivalent geminal and distal C-F positions. This methodology can be applied to aromatic-, ether-, thioether-, and alkyl-supported fluoromethyl groups. We expand the range of FLP base partners that work with monoselective C-F activation to include sulfide.

Zero valent iron complexes as base partners in frustrated Lewis pair chemistry.

The prototypical iron(0) complex [Fe(CO)3(PMe3)2] (1) forms a frustrated Lewis pair (FLP) with B(C6F5)3 (BCF). In this FLP, the iron complex acts as the Lewis base partner, and the borane as the Lewis acid partner. This FLP is able to cleave H-H, H-Cl, H-O and H-S bonds in H2, HCl, H2O and HSPh.

C-N, C-S and S-S Bond Cleavage by Rhodium PC P Pincer Complexes.

Rhodium PC P complexes 1-L {L=PPh , PPh (C F )} react with isothiocyanate, carbodiimide and disulphide to enable C-S, C-N and S-S bond cleavage. The cleaved molecules are sequestered by the metal center and the pincer alkylidene linkage, forming η -coordinated sulfide or imide centered pincer complexes. When a C-S or S-S bond is cleaved, the resulting complexes can bridge two rhodium centers through sulphur forming dimeric complexes and eliminating a monodentate phosphine ligand.

Frustrated Lewis-Pair-Meditated Selective Single Fluoride Substitution in Trifluoromethyl Groups.

Single fluoride substitution in trifluoromethylarenes is an ongoing synthetic challenge that often leads to "over-reaction", where multiple fluorides are replaced. Development of this reaction would allow simple access to a vast range of difluoromethyl derivatives of current interest to pharmaceutical, agrochemistry, and materials sciences. Using a catalytic frustrated Lewis pair approach, we have developed a generic protocol that allows a single substitution of one fluoride in trifluoromethyl groups with neutral phosphine and pyridine bases.

Direct oxide transfer from an η-keto ligand to generate a cobalt PCP(O) pincer complex.

We report the direct carbonyl cleavage in a κ-P',(η-C,O),P'' ligand by a monomeric cobalt centre through metal-ligand cooperativity. C-O cleavage leads to the formation of a PCP(O) pincer ligand with a central alkylidene anchor. A DFT analysis, supported by kinetic studies, suggests that decoordination of a pincer phosphino arm to generate a kinetically accessible free phosphine may be critical in transfer of the oxide to a phosphorus position.

Applications of boroxide ligands in supporting small molecule activation by U(iii) and U(iv) complexes.

The boroxide ligand [OBAr2]- (Ar = Mes, Trip) is shown to be able to support both UIII and UIV centres for the first time. The synthesis and structures of homoleptic and heteroleptic UIII and UIV complexes are reported. The UX3 complex with larger substituents, [U(OBTrip2)3]2, exhibits greater thermal stability compared to less encumbered [U(OBMes2)3]2 but reacts with a smaller range of the small molecules tested to date.

Nucleophilic Substitution of Aliphatic Fluorides via Pseudohalide Intermediates.

A method for aliphatic fluoride functionalization with a variety of nucleophiles has been reported. Carbon-fluoride bond cleavage is thermodynamically driven by the use of silylated pseudohalides TMS-OMs or TMS-NTf , resulting in the formation of TMS-F and a trapped aliphatic pseudohalide intermediate. The rate of fluoride/pseudohalide exchange and the stability of this intermediate are such that little rearrangement is observed for terminal fluoride positions in linear aliphatic fluorides.

Synthesis and reactivity of a PCP cobalt(i) complex: the missing link in the cobalt PXP pincer series (X = B, C, N).

We report the first example of a cobalt PCP pincer complex () featuring a central alkylidene carbon donor accessed through the dehydration of an alcoholic POP proligand. Complex shares bonding similarities with cobalt PBP and PNP pincer complexes where the donor atom engages in π-bonding with the cobalt centre, and thus completes the PXP (X = B, C, N) pincer ligand series for cobalt (for X donors that partake in M-L π-bonding). As compared to PBP and PNP pincer complexes, which are known to be good hydride and proton acceptors (respectively), complex is found to be an effective hydrogen atom acceptor.

Selective Monodefluorination and Wittig Functionalization of gem-Difluoromethyl Groups to Generate Monofluoroalkenes.

Monodefluorination of gem-difluoromethyl groups is achieved using a frustrated Lewis pair (FLP) approach. Triarylphosphines and group 13 Lewis acids were surveyed as FLP components, with the combination of P( o-Tol) and B(CF) found to provide the best results, although the reaction is feasible with more economical components (PPh and BF·OEt). The α-fluoroalkylphosphonium products arising from the reaction were of lower activity, in regard to further fluoride abstraction, as compared to difluoride starting materials, leading to highly selective monodefluorination.

Stable Carbocation Generated via 2,5-Cyclohexadien-1-one Protonation.

Protonation of a substituted cyclohexadien-1-one (1) leads to the generation of carbocation [3], capable of effecting hydride abstraction and oxidation reactions. The molecular structure of [3] shows it to be structurally similar to [(p-MeO-CH)PhC]. The ability to easily access [3] from stable and available precursors, such as 1 and commercially available acids, may allow a wider application of the growing number of trityl-based reactions in organic syntheses.

Facile generation of iridium PCP pincer complexes via water elimination from an alcohol proligand.

We report the facile generation of Ir PCP pincer systems. These systems are accessed from the reaction between [IrCl(COD)] and a bis(diphenyl)phenylene P(OH)P proligand (1) with concomitant dehydration, followed by salt metathesis/ligand exchange in the case of cationic examples. In contrast to previously reported double C-H activation synthetic strategies to access similar complexes, accessing Ir PCP complexes through dehydration proceeds rapidly at room temperature and provides the first example of the incorporation of phosphino aryl substituents.

Aluminum-Catalyzed Cross-Coupling of Silylalkynes with Aliphatic C-F Bonds.

We report the generation of aliphatic and benzylic acetylenes via reaction of primary, secondary, and tertiary aliphatic fluorides with various trimethylsilyl acetylides. These reactions are catalyzed by Al and B Lewis acids, most effectively by the extremely fluorophilic tris(pentafluorophenyl)alane, representing the first example of catalytic incorporation of alkynes into aliphatic C-F positions. The fluorophilicity of the catalysts gives rise to fluorine selectivity over other halogens, allowing orthogonal reactivity pathways.

Characterisation of alkane σ-complexes.

Alkane σ-complexes have evolved from a curious phenomenon to an intermediate of intense interest, fuelling research into the area. Over the last fifteen years, metal alkane complex characterisation has evolved to incorporate reports employing UV/Vis, IR and NMR spectroscopy, and X-ray and neutron diffractometry. Previously, due to the sparse geometric characterisation of alkane σ-complexes, assumptions regarding bonding geometries and selectivities were made by comparison to related σ-complexes, or by analysis of C-H activation products.

Bis(phosphine)boronium salts. Synthesis, structures and coordination chemistry.

The synthesis of a range of bis(phosphine)boronium salts is reported [(R2HP)2BH2][X] (R = Ph, (t)Bu, Cy) in which the counter anion is also varied (X(-) = Br(-), [OTf](-), [BAr(F)4](-), Ar(F) = 3,5-(CF3)2C6H3). Characterization in the solid-state by X-ray diffraction suggests there are weak hydrogen bonds between the PH units of the boronium cation and the anion (X(-) = Br(-), [OTf](-)), while solution NMR spectroscopy also reveals hydrogen bonding occurs in the order [BAr(F)4](-) < [OTf](-) < Br(-). [(Ph2HP)2BH2][BAr(F)4] reacts with RhH(PPh3)3, by elimination of H2, forming [Rh(κ(1),η-PPh2BH2·PPh2H)(PPh3)2][BAr(F)4] which shows a β-B-agostic interaction from the resulting base stabilised phosphino-borane ligand.

Traceless chelation-controlled rhodium-catalyzed intermolecular alkene and alkyne hydroacylation.

A new functional-group tolerant, rhodium-catalyzed, sulfide-reduction process is combined with rhodium-catalyzed chelation-controlled hydroacylation reactions to give a traceless hydroacylation protocol. Aryl- and alkenyl aldehydes can be combined with both alkenes, alkynes and allenes to give traceless products in high yields. A preliminary mechanistic proposal is also presented.