Four-electron reduction of benzene by a samarium(II)-alkyl without the addition of external reducing agents.

Benzene reduction by molecular complexes remains an important synthetic challenge, requiring harsh reaction conditions involving group I metals. Reductions of benzene, to date, typically result in a loss of aromaticity, although the benzene tetra-anion, a 10π-electron system, has been calculated to be stable and aromatic. Due to the lack of sufficiently potent reductants, four-electron reduction of benzene usually requires the use of group I metals.

Natural and Semisynthetic Immunomodulatory Luakuliide Labdane Diterpenoids.

Spectroscopy-guided isolation of extracts of the Tongan marine sponge cf. (Lamarck, 1814) has resulted in the reisolation of the labdane diterpenoid luakuliide A () and one new congener, luakulialactam A (). In addition to establishing the absolute configuration of , synthetic modifications to the luakuliide framework at key positions has created a set of six derivatives (-) which were used to interrogate a structure-activity relationship relating to the immunomodulatory effects of luakuliide A.

Synthesis and Reactivity of Discrete Europium(II) Hydride Complexes.

The bulky β-diketiminate ligand frameworks [BDI] and [BDI] (BDI=[HC{C(Me)N-Dipp/Ar}] (Dipp=2,6-diisopropylphenyl (Dipp); Ar=2,6-dicyclohexylphyenyl (DCHP) or 2,4,6-tricyclohexylphyenyl (TCHP)) have been developed for the kinetic stabilisation of the first europium (II) hydride complexes, [(BDI)Eu(μ-H)], [(BDI)Eu(μ-H)] and [(BDI)Eu(μ-H)], respectively. These complexes represent the first step beyond the current lanthanide(II) hydrides that are all based on ytterbium. Tuning the steric profile of β-diketiminate ligands from a symmetrical to unsymmetrical disposition, enhanced solubility and stability in the solution-state.

Reductive Coupling of a Diazoalkane Derivative Promoted by a Potassium Aluminyl and Elimination of Dinitrogen to Generate a Reactive Aluminium Ketimide.

The reaction of 9-diazo-9H-fluorene (fluN ) with the potassium aluminyl K[Al(NON)] ([NON] =[O(SiMe NDipp) ] , Dipp=2,6-iPr C H ) affords K[Al(NON)(κN ,N -{(fluN ) })] (1). Structural analysis shows a near planar 1,4-di(9H-fluoren-9-ylidene)tetraazadiide ligand that chelates to the aluminium. The thermally induced elimination of dinitrogen from 1 affords the neutral aluminium ketimide complex, Al(NON)(N=flu)(THF) (2) and the 1,2-di(9H-fluoren-9-yl)diazene dianion as the potassium salt, [K (THF) ][fluN=Nflu] (3).

Controlled reductive C-C coupling of isocyanides promoted by an aluminyl anion.

We report the reaction of the potassium aluminyl, K[Al(NON)] ([NON] = [O(SiMeNDipp)], Dipp = 2,6-iPrCH) with a series of isocyanide substrates (R-NC). In the case of Bu-NC, degradation of the isocyanide was observed generating an isomeric mixture of the corresponding aluminium cyanido-κ and -κ compounds, K[Al(NON)(H)(CN)]/K[Al(NON)(H)(NC)]. The reaction with 2,6-dimethylphenyl isocyanide (Dmp-NC), gave a C-homologation product, which in addition to C-C bond formation showed dearomatisation of one of the aromatic substituents.

Isolating elusive 'Al(μ-O)M' intermediates in CO reduction by bimetallic Al-M complexes (M = Zn, Mg).

The reaction of compounds containing Al-Mg and Al-Zn bonds with NO enabled isolation of the corresponding Al(μ-O)M complexes. Electronic structure analysis identified largely ionic Al-O and O-M bonds, featuring an anionic μ-oxo centre. Reaction with CO confirmed that these species correspond to the proposed intermediates in the formation of μ-carbonate compounds.

Carbon-chalcogen bond formation initiated by [Al(NON)(E)] anions containing Al-E{16} (E{16} = S, Se) multiple bonds.

Multiply-bonded main group metal compounds are of interest as a new class of reactive species able to activate and functionalize a wide range of substrates. The aluminium sulfido compound K[Al(NON)(S)] (NON = [O(SiMeNDipp)], Dipp = 2,6-PrCH), completing the series of [Al(NON)(E)] anions containing Al-E{16} multiple bonds (E{16} = O, S, Se, Te), was accessed desulfurisation of K[Al(NON)(S)] using triphenylphosphane. The crystal structure showed a tetrameric aggregate joined by multiple K⋯S and K⋯π(arene) interactions that were disrupted by the addition of 2.

Extending chain growth beyond C → C in CO homologation: aluminyl promoted formation of the [CO] ligand.

(NON)Al-K(TMEDA) (NON = [O(SiMeNDipp)], Dipp = 2,6-PrCH), containing an Al-K bond, activates and reductively couples cabon monoxide gas to form the [CO] ligand. This oxocarbon anion is thermally isomerised in the presence of CO and TMEDA. In contrast, the dimeric potassium aluminyl [K{Al(NON)}] yields an aluminium complex containing the hitherto unknown [CO] ligand.

Potassium Aluminyl Promoted Carbonylation of Ethene.

The potassium aluminyl [K{Al(NON )}] ([NON ] =[O{SiMe NDipp} ] , Dipp=2,6-iPr C H ) activates ethene towards carbonylation with CO under mild conditions. An isolated bis-aluminacyclopropane compound reacted with CO via carbonylation of an Al-C bond, followed by an intramolecular hydrogen shift to form K [Al(NON )(μ-CH CH=CO-1κ C -2κO)Al(NON )Et]. Restricting the chemistry to a mono-aluminium system allowed isolation of [Al(NON )(CH CH CO-κ C )] , which undergoes thermal isomerisation to form the [Al(NON )(CH CH=CHO-κ C,O)] anion.

Controlling Al- Interactions in Group 1 Metal Aluminyls ( = Li, Na, and K). Facile Conversion of Dimers to Monomeric and Separated Ion Pairs.

The aluminyl compounds [{Al(NON)}] (NON = [O(SiMeNDipp)], Dipp = 2,6-PrCH), which exist as contacted dimeric pairs in both the solution and solid states, have been converted to monomeric ion pairs and separated ion pairs for each of the group 1 metals, = Li, Na, and K. The monomeric ion pairs contain discrete, highly polarized Al- bonds between the aluminum and the group 1 metal and have been isolated with monodentate (THF, = Li and Na) or bidentate (TMEDA, = Li, Na, and K) ligands at . The separated ion pairs comprise group 1 cations that are encapsulated by polydentate ligands, rendering the aluminyl anion, [Al(NON)] "naked".

Dihydrogen Activation by Lithium- and Sodium-Aluminyls.

To date, aluminyl anions have been exclusively isolated as their potassium salts. We report herein the synthesis of the lithium and sodium aluminyls, M [Al(NON )] (M=Li, Na. NON =[O(SiMe NDipp) ] ; Dipp=2,6-iPr C H ).

Ytterbium (II) Hydride as a Powerful Multielectron Reductant.

A dimeric β-diketiminato ytterbium(II) hydride affects both the two-electron aromatization of 1,3,5,7-cyclooctatetraene (COT) and the more challenging two-electron reduction of polyaromatic hydrocarbons, including naphthalene (E =-2.60 V). Confirmed by Density Functional Theory calculations, these reactions proceed via consecutive polarized Yb-H/C=C insertion and deprotonation steps to provide the respective ytterbium (II) inverse sandwich complexes and hydrogen gas.

Hydroarylation of olefins catalysed by a dimeric ytterbium(II) alkyl.

Although the nucleophilic alkylation of aromatics has recently been achieved with a variety of potent main group reagents, all of this reactivity is limited to a stoichiometric regime. We now report that the ytterbium(II) hydride, [BDIYbH] (BDI = CH[C(CH)NDipp], Dipp = 2,6-diisopropylphenyl), reacts with ethene and propene to provide the ytterbium(II) n-alkyls, [BDIYbR] (R = Et or Pr), both of which alkylate benzene at room temperature. Density functional theory (DFT) calculations indicate that this latter process operates through the nucleophilic (S2) displacement of hydride, while the resultant regeneration of [BDIYbH] facilitates further reaction with ethene or propene and enables the direct catalytic (anti-Markovnikov) hydroarylation of both alkenes with a benzene C-H bond.

Oxidative Addition of Hydridic, Protic, and Nonpolar E-H Bonds (E = Si, P, N, or O) to an Aluminyl Anion.

The aluminyl anion K[Al(NON)] {NON = [O(SiMeNDipp)]; Dipp = 2,6-PrCH} engages in oxidative additions with the E-H (E = Si, P, N, or O) bonds of phenylsilane (PhSiH), mesityl phosphane (MesPH; Mes = 2,4,6-MeCH), 2,6-di--propylaniline (DippNH), and 2,6-di--butyl-4-methylphenol (ArOH). The resulting (hydrido)aluminate salts are formed regardless of the E-H bond polarity. All of the products were characterized by nuclear magnetic resonance and infrared spectroscopic techniques and single-crystal X-ray diffraction.

Double insertion of CO into an Al-Te multiple bond.

We report the [Al(NON)(Te)(THF)] anion containing a terminal aluminium telluride bond. DFT calculations confirm appreciable Al-Te multiple bond character and reaction with CO proceeds via a double insertion to afford the previously unknown tellurodicarbonate ligand.

Carbon-Carbon Bond Forming Reactions Promoted by Aluminyl and Alumoxane Anions: Introducing the Ethenetetraolate Ligand.

[K{Al(NON )}] (NON =[O(SiMe NDipp) ] , Dipp=2,6-iPr C H ) reacts with CS to afford the trithiocarbonate species [K(OEt )][Al(NON )(CS )] 1 or the ethenetetrathiolate complex, [K{Al(NON )(S C)}] [3] . The dimeric alumoxane [K{Al(NON )(O)}] reacts with carbon monoxide to afford the oxygen analogue of 3, [K{Al(NON )(O C)}] [4] containing the hitherto unknown ethenetetraolate ligand, [C O ] .

Synthesis and reactivity of a terminal aluminium-imide bond.

The reaction of the potassium aluminyl K[Al(NONDipp)] (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3) with an organic azide generates the aluminium imide complex, K[Al(NONDipp)(NMes)] (Mes = mesityl = 2,4,6-Me3C6H2). DFT calculations indicate the Al-Nimide interaction is polarized but has appreciable multiple-bond character. This is demonstrated experimentally by the reaction with carbon dioxide, giving a rare example of a main group carbamate dianion via a [2+2] cycloaddition reaction.

The "Metallo"-Diels-Alder Reactions: Examining the Metalloid Behavior of Germanimines.

Addition of MesN (Mes=2,4,6-Me C H ) to germylene [(NON )Ge] (NON =O(SiMe NtBu) ) (1) gives germanimine, [(NON )Ge=NMes] (2). Compound 2 behaves as a metalloid, showing reactivity reminiscent of both transition metal-imido complexes, undergoing [2+2] addition with heterocumulenes and protic sources, as well as an activated diene, undergoing a [4+2] cycloaddition, or "metallo"-Diels-Alder, reaction. In the latter case, the diene includes the Ge=N bond and π-system of the Mes substituent, which is reactive towards dienophiles including benzaldehyde, benzophenone, styrene, and phenylacetylene.

Reactions of In-Zn bonds with organic azides: products that result from hetero- and homo-bimetallic behaviour.

The indyl anion, K[In(NON)] (NON = [O(SiMeNDipp)], Dipp = 2,6-iPrCH) reacts with group 12 compounds M(BDI)Cl (M = Zn, Cd; BDI = [HC{C(Me)NR}], R = 2,4,6-MeCH (Mes), Dipp) to afford the heterobimetallic compounds (NON)In-M(BDI) that contain the first In-Zn and In-Cd bonds. The reactivity of the In-Zn bonds towards organic azides, R'N (R' = Mes, Dipp, Ph) was investigated. (NON)In-Zn(BDI) reduces MesNvia an isolable triazenide intermediate to generate the bridging imido compound, (NON)In-(μ-NMes)-Zn(BDI).

Aluminium-Mediated Carbon Dioxide Reduction by an Isolated Monoalumoxane Anion.

The deoxygenative conversion of carbon dioxide to carbon monoxide is promoted by the aluminyl anion [Al(NON )] (NON =[O(SiMe NAr) ] , Ar=2,6-iPr C H ). The reaction proceeds via the isolable monoalumoxane anion [Al(NON )(O)] , containing a terminal aluminum-oxygen bond. This species reacts with a second equivalent of carbon dioxide to afford the carbonate [Al(NON )(CO )] , and with nitrous oxide to generate the hyponitrite anion, [Al(NON )(κ O,O'-N O )] .

Isoelectronic Aluminium Analogues of Carbonyl and Dioxirane Moieties.

We report the anion [Al(NON )(Se)] (NON =[O(SiMe NAr) ] , Ar=2,6-iPr C H ), which is an isoelectronic Group 13 metal analogue of the carbonyl group containing an aluminium-selenium multiple bond. It was synthesized in a single step from the reaction of the aluminyl anion [Al(NON )] with elemental selenium. Spectroscopic, crystallographic, and computational analysis confirmed multiple bonding between aluminium and selenium.

Reduction of organic azides by indyl-anions. Isolation and reactivity studies of indium-nitrogen multiple bonds.

The synthesis of a new potassium-indyl complex, K[In(NON)] (NON = [O(SiMeNAr)], Ar = 2,6-PrCH) and its reactivity with organic azides RN is reported. When R = 2,6-bis(diphenylmethyl)-4- Bu-phenyl, a dianionic alkyl-amide ligand is formed C-H activation across a transient In-N bond. Reducing the size of the R-group to 2,4,6-trimethylphenyl (mesityl, Mes) enables oxidation of the indium and elimination of dinitrogen to afford the imide species, K[In(NON)(NMes)].

Reduction vs. Addition: The Reaction of an Aluminyl Anion with 1,3,5,7-Cyclooctatetraene.

The potassium aluminyl complex K[Al(NON )] (NON=NON =[O(SiMe NAr) ] , Ar=2,6-iPr C H ) reacts with 1,3,5,7-cyclooctatetraene (COT) to give K[Al(NON )(COT)]. The COT-ligand is present in the asymmetric unit as a planar μ -η :η -bridge between Al and K, with additional K⋅⋅⋅π-aryl interactions to neighboring molecules that generate a helical chain. DFT calculations indicate significant aromatic character, consistent with reduction to [COT] .

Coordination of arenes and phosphines by charge separated alkaline earth cations.

Generation of β-diketiminato group 2 cations, [(MeBDI)Ae]+ and [(t-BuBDI)Ae]+ (MeBDI = HC{(Me)CN-2,6-i-Pr2C6H3}2; t-BuBDI = HC{(t-Bu)CN-2,6-i-Pr2C6H3}2; Ae = Mg or Ca), in conjunction with the weakly coordinating anion, [Al{OC(CF3)3}4]-, allows the characterisation of charge separated alkaline earth η6-π adducts to toluene or benzene when crystallised from the arene solvents. Addition of 1,4-difluorobenzene to [(MeBDI)Mg]+ results in the isolation of [(MeBDI)Mg(1,4-F2C6H4)3]+ in which the fluorobenzene molecules coordinate via κ1-F-M interactions. Although DFT analysis indicates that the polyhapto arene binding to Mg is effectively electrostatic in origin, the interactions with Ca (Sr and Ba) are observed to invoke small but significant π overlap of the arene HOMOs with the alkaline earth valence nd orbitals.

Indyllithium and the Indyl Anion [InL] : Heavy Analogues of N-Heterocyclic Carbenes.

Reduction of the indate complex In(NON )(μ-Cl) Li(OEt ) (NON =[O(SiMe NAr) ] ; Ar=2,6-iPr C H ) with sodium generates the In diindane species [In(NON )] . Further reduction with a mixture of potassium and [2.2.