Functionalisation of Carbon-Fluorine Bonds with Main Group Reagents

Synthetic approaches to produce reactive chemical building blocks from fluorinated molecules by the functionalization of carbon–fluorine bonds with main group reagents are reviewed. The reaction types can be categorized as: (i) the formal 1,2-addition of C–F bonds across Si–Si, B–B, or Mg–Mg bonds; (ii) the oxidative addition of C–F bonds to Si(II), Ge(II), and Al(I) centres; and (iii) the dehydrogenative coupling of C–F bonds with Al–H or B–H bonds. Many of the advances have emerged between 2015–2016 and are largely focused upon aromatic substrates that contain sp2 C–F bonds

Functionalization and Hydrogenation of Carbon Chains Derived from CO

Selective reactions that combine H 2 , CO and organic electrophiles (aldehyde, ketones, isocyanide) to form hydrogenated C 3 and C 4 carbon chains are reported. These reactions proceed by CO homologation mediated by [W(CO) 6 ] and an aluminum(I) reductant, followed by functionalization and hydrogenation of the chain ends. A combination of kinetics (rates, KIEs) and DFT calculations has been used to gain insight into a key step which involves hydrogenation of a metallocarbene intermediate. These findings expand the extremely small scope of systems that combine H 2 and CO to make well-defined products with complete control over chain length and functionality

Organometallic chemistry using partially fluorinated benzenes

Fluorobenzenes, in particular fluorobenzene (FB) and 1,2-difluorobenzene (1,2-DiFB), are increasingly becoming recognised as versatile solvents for conducting organometallic chemistry and transition-metal-based catalysis. The presence of fluorine substituents reduces the ability to donate π-electron density from the arene and consequently fluorobenzenes generally bind weakly to metal centres, allowing them to be used as essentially non-coordinating solvents or as readily displaced ligands. In this context, examples of well-defined complexes of fluorobenzenes are discussed, including trends in binding strength with increasing fluorination and different substitution patterns. Compared to more highly fluorinated benzenes, FB and 1,2-DiFB typically demonstrate greater chemical inertness, however, C–H and C–F bond activation reactions can be induced using appropriately reactive transition metal complexes. Such reactions are surveyed, including catalytic examples, not only to provide perspective for the use of FB and 1,2-DiFB as innocent solvent media, but also to highlight opportunities for their exploitation in contemporary organic synthesis.We thank the Herchel Smith Fund (S. D. P.) and Royal Society (M. R. C., A. B. C.) for research fellowships. M. R. C. and A. B. C. also gratefully acknowledge the European Research Council for provision of Starting Grants (677367 and 637313) to support their current work on C–F and C–H bond activation, respectively

A combined experimental and computational study on the reaction of fluoroarenes with Mg–Mg, Mg–Zn, Mg–Al and Al–Zn bonds

Through a combined experimental and computational (DFT) approach, the reaction mechanism of the addition of fluoroarenes to Mg–Mg bonds has been determined as a concerted S_{N}Ar-like pathway in which one Mg centre acts as a nucleophile and the other an electrophile. The experimentally determined Gibbs activation energy for the addition of C₆F₆ to a Mg–Mg bond of a molecular complex, ΔG‡_{₂₉₈K}(experiment) = 21.3 kcal mol⁻¹ is modelled by DFT with the ωB97X functional, ΔG‡₂₉₈ {K}_(DFT) = 25.7 kcal mol⁻¹. The transition state for C–F activation involves a polarisation of the Mg–Mg bond and significant negative charge localisation on the fluoroarene moiety. This transition state is augmented by stabilising closed-shell Mg⋯F_{ortho} interactions that, in combination with the known trends in C–F and C–M bond strengths in fluoroarenes, provide an explanation for the experimentally determined preference for C–F bond activation to occur at sites flanked by ortho-fluorine atoms. The effect of modification of both the ligand coordination sphere and the nature and polarity of the M–M bond (M = Mg, Zn, Al) on C–F activation has been investigated. A series of highly novel β-diketiminate stabilised complexes containing Zn–Mg, Zn–Zn–Zn, Zn–Al and Mg–Al bonds has been prepared, including the first crystallographic characterisation of a Mg–Al bond. Reactions of these new M–M containing complexes with perfluoroarenes were conducted and modelled by DFT. C–F bond activation is dictated by the steric accessibility, and not the polarity, of the M–M bond. The more open coordination complexes lead to enhanced Mg⋯F_{ortho} interactions which in turn lower the energy of the transition states for C–F bond activation

Reversible dihydrogen activation and catalytic H/D exchange with group 10 heterometallic complexes

Reaction of a hexagonal planar palladium complex featuring a [PdMg3H3] core with H2 is reversible and leads to the formation of a new [PdMg2H4] tetrahydride species alongside an equivalent of a magnesium hydride co-product [MgH]. While the reversibility of this process prevented isolation of [PdMg2H4], analogous [PtMg2H4] and [PtZn2H4] complexes could be isolated and characterised through independent syntheses. Computational analysis (DFT, AIM, NCIPlot) of the bonding in a series of heterometallic tetrahydride compounds (Ni–Pt; Mg and Zn) suggests that these complexes are best described as square planar with marginal metal-metal interactions; the strength of which increases slightly as group 10 is descended and increases from Mg to Zn. DFT calculations support a mechanism for H2 activation involving a ligand-assisted oxidative addition to Pd. These findings were exploited to develop a catalytic protocol for H/D exchange into magnesium hydride and zinc hydride bonds

Reactions of an aluminum(I) reagent with 1,2-, 1,3-, and 1,5-dienes: dearomatization, reversibility, and a pericyclic mechanism

Addition of the aluminum(I) reagent [{(ArNCMe)2CH}Al] (Ar = 2,6-di-iso-propylphenyl) to a series of cyclic and acyclic 1,2-, 1,3-, and 1,5-dienes is reported. In the case of 1,3-dienes, the reaction occurs by a pericyclic reaction mechanism, specifically a cheletropic cycloaddition, to form aluminocyclopentene-containing products. This mechanism has been examined by stereochemical experiments and DFT calculations. The stereochemical experiments show that the (4 + 1) cycloaddition follows a suprafacial topology, while calculations support a concerted albeit asynchronous pathway in which the transition state demonstrates aromatic character. Remarkably, the substrate scope of the (4 + 1) cycloaddition includes styene, 1,1-diphenylethylene, and anthracene. In these cases, the diene motif is either in part, or entirely, contained within an aromatic ring and reactions occur with dearomatisation of the substrate and can be reversible. In the case of 1,2-cyclononadiene or 1,5-cyclooctadiene, complementary reactivity is observed; the orthogonal nature of the C═C π-bonds (1,2-diene) and the homoconjugated system (1,5-diene) both disfavor a (4 + 1) cycloaddition. Rather, reaction pathways are determined by an initial (2 + 1) cycloaddition to form an aluminocyclopropane intermediate which can in turn undergo insertion of a further C═C π-bond, leading to complex organometallic products that incorporate fused hydrocarbon rings

Oxidative addition of carbon-fluorine and carbon-oxygen bonds to Al(I)

Addition of fluoroarenes, fluoroalkanes or benzofuran to [{(2,6-iPr2C6H3NCMe)2CH}Al] results in facile oxidative addition of either a C–F or C–O bond to the Al(I) centre