Augmenting metallobasicity to modulate gold hydrogen bonding.

We report the synthesis and characterization of two phosphine gold carbinol species designed to support intramolecular Au⋯H-O hydrogen bonding. Increasing the metallobasicity of gold through chloride to phenyl ligand substitution produced an observable increase in this hydrogen bond's strength which was analyzed experimentally and computationally.

Ligand-enforced intimacy between a gold cation and a carbenium ion: impact on stability and reactivity.

Controlling the reactivity of transition metal complexes by positioning non-innocent functionalities around the catalytic pocket is a concept that has led to significant advances in catalysis. Here we describe our efforts toward the synthesis of dicationic phosphine gold complexes of general formula [(o-Ph2P(C6H4)Carb)Au(tht)]2+ decorated by a carbenium moiety (Carb) positioned in the immediate vicinity of the gold center. While the most acidic examples of such compounds have limited stability, the dicationic complexes with Carb+ = 9-N-methylacridinium and Carb+ = [C(Ar N )2]+ (Ar N = p-(C6H4)NMe2) are active as catalysts for the cycloisomerization of N-propargyl-4-fluorobenzamide, a substrate chosen to benchmark reactivity. The dicationic complex [(o-Ph2P(C6H4)C(Ar N )2)Au(tht)]2+, which also promotes hydroarylation and enyne cyclization reactions, displays a higher catalytic activity than its acridinium analog, indicating that the electrophilic reactivity of these complexes scales with the Lewis acidity of the carbenium moiety. These results support the role of the carbenium unit as a non-innocent functionality which can readily enhance the activity of the adjacent metal center. Finally, we also describe our efforts toward the generation and isolation of free γ-cationic phosphines of general formula [(o-Ph2P(C6H4)Carb)]+. While cyclization into phosphonium species is observed for Carb+ = [C(Ar N )2]+, [C(Ph)(Ar N )]+, and 9-xanthylium, [(o-Ph2P(C6H4)-9-N-methylacridinium)]+ can be isolated as an air stable, biphilic derivative with uncompromised Lewis acidic and basic properties.

Stabilized Carbenium Ions as Latent, Z-type Ligands.

Controlling the reactivity of transition metals using secondary, σ-accepting ligands is an active area of investigation that is impacting molecular catalysis. Herein we describe the phosphine gold complexes [(o-Ph2 P(C6 H4 )Acr)AuCl]+ ([3]+ ; Acr=9-N-methylacridinium) and [(o-Ph2 P(C6 H4 )Xan)AuCl]+ ([4]+ ; Xan=9-xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes, the more Lewis acidic complex [4]+ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative (7) in which the metal atom is covalently bound to the former carbocationic center. This anion-induced AuI /AuIII oxidation is accompanied by a conversion of the Lewis acidic carbocationic center in [4]+ into an X-type ligand in 7. We conclude that the carbenium moiety of this complex acts as a latent Z-type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.

Synthesis, structure and anion binding properties of 1,8-bis(dimesitylboryl)anthracene and its monoborylated analog.

Two boranes, 1-(dimesitylboryl)anthracene (1) and 1,8-bis(dimesitylboryl)anthracene (2), have been synthesized with the spectrophysical properties showing how the inclusion of one or two boron atoms progressively perturbs the π-system of the anthracene backbone. This perturbation is caused by conjugation of the anthracene-π* orbital with the vacant p-orbital on boron. Additionally, both 1 and 2 have a high affinity for fluoride and cyanide anions which are complexed in a 1 : 1 guest-host ratio. The mono-borane 1 is particularly well-suited for cyanide binding, displaying a binding constant of 3 × 107 in THF. Furthermore, as a result of their unique electronic structures, these boranes display a fluorescence response to fluoride anion characterized by a blue shift in the case of 1 and a red shift in the case of 2.

Synthesis and photophysical properties of imine borane adducts towards vapochromic materials.

A series of alkynyl aryl conjugated aldehydes and imines were prepared and their adducts with various Lewis acidic boranes have been studied via NMR, absorption, and luminescence spectroscopies in solution. The imine-B(C6F5)3 adduct showed remarkable solution stability, and was then trialled in vapochromism experiments using simple impregnated paper strips to examine the fluorescence responses.

Alternative Strategies with Iodine: Fast Access to Previously Inaccessible Iodine(III) Compounds.

Non-iodinated arenes can be easily and selectively converted into (diacetoxyiodo)arenes in a single step under mild conditions by using iodine triacetates as reagents. The oxidative step is decoupled from the synthesis of hypervalent iodine(III) reagents, which can now be prepared conveniently in a one-pot synthesis for subsequent reactions without prior purification. The chemistry of iodine triacetates was also expanded to heteroatom ligand exchanges to form novel inorganic hypervalent iodine compounds.

Divergent Elementoboration: 1,3-Haloboration versus 1,1-Carboboration of Propargyl Esters.

This work showcases the 1,3-haloboration reaction of alkynes in which boron and chlorine add to propargyl systems in a proposed sequential oxazoliumborate formation with subsequent ring-opening and chloride migration. In addition, the functionalization of these propargyl esters with dimethyl groups in the propargylic position leads to stark differences in reactivity whereby a formal 1,1-carboboration prevails to give the 2,2-dichloro-3,4-dihydrodioxaborinine products as an intramolecular chelate. Density functional theory calculations are used to rationalize the distinct carboboration and haloboration pathways. Significantly, this method represents a metal-free route to highly functionalized compounds in a single step to give structurally complex products.

Amidine functionalized phosphines: tuneable ligands for transition metals.

Attachment of racemic 1,3,5,7-tetramethyl-2,4,6-trioxa-8-phosphatricyclo[3.3.1.13,7]decane (α,ß-CgP) to (1R,5S)-1,8,8-trimethyl-2,4-diazabicyclo[3.2.1]oct-2-ene gave a diastereomeric mixture of a novel amidine-phosphine ligand, α,ß-CgPAm. The phosphination was completely selective for the 4-position of the bicyclic amidine and there was no subsequent 1,2-migration of the α,ß-CgP group. Methylation of the non-phosphinated nitrogen gave the amidinium salt [α,ß-CgPAmMe]BF4 as a diastereomeric mixture. The donating ability of α,ß-CgPAm and [α,ß-CgPAmMe]+ has been assessed through the synthesis and characterization of appropriate Rh(i), Au(i) and Pt(ii) complexes. As expected α,ß-CgPAm is a better net donor than the cationic derivative as shown by the magnitude of the νCO stretches in the IR spectra of the [Rh(L)(CO)(acac)]0/+ complexes and through determination of the relative energies of the HOMO and LUMO orbitals for both ligands by DFT. Attempts to resolve [Au(α,ß-CgPAmMe)Cl]BF4, [Pt(α,ß-CgPAmMe)Cl3], [Rh(α,ß-CgPAmMe)(acac)(CO)]BF4 and [Rh(α,ß-CgPAmH)(acac)(CO)]BF4 by fractional crystallization were unsuccessful as diastereomeric mixtures were obtained in every case; the structures of the last three complexes have been determined by single-crystal X-ray techniques and compared with related literature complexes.

Reactions of biologically inspired hydride sources with B(C6F5)3.

The combination of 1-benzyl-1,4-dihydropyridines with the strong Lewis acid, B(C6F5)3, generates a stable pyridinium borohydride species in high yields (94%) in as little as 10 min. This use of biologically inspired hydride sources further builds on the recent work of new hydride donors in the formation of borohydrides. When functionalizing the dihydropyridine with an amide or carboxylic acid moiety, a disproportionation reaction composed of a series of protonation/reduction steps is observed upon the addition of B(C6F5)3 As a result, one equivalent of dihydropyridine undergoes net hydrogenation, whereas the other is dehydrogenated yielding the pyridinium counterpart in a transfer hydrogenation-type mechanism.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

Tris(2,4,6-trifluorophenyl)borane: An Efficient Hydroboration Catalyst.

The metal-free catalyst tris(2,4,6-trifluorophenyl)borane has demonstrated its extensive applications in the 1,2-hydroboration of numerous unsaturated reagents, namely alkynes, aldehydes and imines, consisting of a wide array of electron-withdrawing and donating functionalities. A range of over 50 borylated products are reported, with many reactions proceeding with low catalyst loading under ambient conditions. These pinacol boronate esters, in the case of aldehydes and imines, can be readily hydrolyzed to leave the respective alcohol and amine, whereas alkynyl substrates result in vinyl boranes. This is of great synthetic use to the organic chemist.

Stoichiometric and Catalytic C-C and C-H Bond Formation with B(C6 F5 )3 via Cationic Intermediates.

This work showcases a new catalytic cyclization reaction using a highly Lewis acidic borane with concomitant C-H or C-C bond formation. The activation of alkyne-containing substrates with B(C6 F5 )3 enabled the first catalytic intramolecular cyclizations of carboxylic acid substrates using this Lewis acid. In addition, intramolecular cyclizations of esters enable C-C bond formation as catalytic B(C6 F5 )3 can be used to effect formal 1,5-alkyl migrations from the ester functional groups to unsaturated carbon-carbon frameworks. This metal-free method was used for the catalytic formation of complex dihydropyrones and isocoumarins in very good yields under relatively mild conditions with excellent atom efficiency.

Synthesis and reactivity of N,N'-1,4-diazabutadiene derived borocations.

A series of borocations have been synthesised from the addition of haloboranes to synthetically accessible N,N'-1,4-diazabutadiene precursors, which are derived from commercially available anilines. The synthesis and structural studies of the borocations are described.

The Propargyl Rearrangement to Functionalised Allyl-Boron and Borocation Compounds.

A diverse range of Lewis acidic alkyl, vinyl and aryl boranes and borenium compounds that are capable of new carbon-carbon bond formation through selective migratory group transfer have been synthesised. Utilising a series of heteroleptic boranes [PhB(C6 F5 )2 (1), PhCH2 CH2 B(C6 F5 )2 (2), and E-B(C6 F5 )2 (C6 F5 )C=C(I)R (R=Ph 3 a, nBu 3 b)] and borenium cations [phenylquinolatoborenium cation ([QOBPh][AlCl4 ], 4)], it has been shown that these boron-based compounds are capable of producing novel allyl- boron and boronium compounds through complex rearrangement reactions with various propargyl esters and carbamates. These reactions yield highly functionalised, synthetically useful boron substituted organic compounds with substantial molecular complexity in a one-pot reaction.

Contrasting Frustrated Lewis Pair Reactivity with Selenium- and Boron-Based Lewis Acids.

The activation of π-bonds in diynyl esters has been investigated by using soft and hard Lewis acids. In the case of the soft selenium Lewis acid PhSeCl, sequential activation of the alkyne bonds leads initially to an isocoumarin (1 equiv PhSeCl) and then to a tetracyclic conjugated structure with the isocoumarin subunit fused to a benzoselenopyran (3 equiv PhSeCl). Conversely, the reaction with the hard Lewis acidic borane B(C6 F5 )3 initiates a cascade reaction to yield a complex π-conjugated system containing phthalide and indene subunits.

Lewis acid-base 1,2-addition reactions: synthesis of pyrylium borates from en-ynoate precursors.

Treatment of methyl (Z)-2-alken-4-ynoates with the strong Lewis acid tris(pentafluorophenyl) borane, B(C6F5)3, yield substituted zwitterionic pyrylium borate species via an intramolecular 6-endo-dig cyclisation reaction.

Σ- versus π-activation of alkynyl benzoates using B(C6F5)3.

We have prepared a range of alkynyl benzoates in high yields and have investigated their reactivities with the strong Lewis acid B(C6F5)3. In such molecules both σ-activation of the carbonyl and π-activation of the alkyne are possible. In contrast to the reactivity of propargyl esters with B(C6F5)3 which proceed via 1,2-addition of the ester and B(C6F5)3 across the alkyne, the inclusion of an additional CH2 spacer switches off the intramolecular cyclization and selective σ-activation of the carbonyl group is observed through adduct formation. This change in reactivity appears due to the instability of the species which would be formed through B(C6F5)3 activation of the alkyne.