Pinacol Cross-Coupling Promoted by an Aluminyl Anion.

A simple sequential addition protocol for the reductive coupling of ketones and aldehydes by a potassium aluminyl grants access to unsymmetrical pinacolate derivatives. Isolation of an aluminium ketyl complex presents evidence for the accessibility of radical species. Product release from the aluminium centre was achieved using an iodosilane, forming the disilylated 1,2-diol and a neutral aluminium iodide, thereby demonstrating the steps required to generate a closed synthetic cycle for pinacol (cross) coupling at an aluminyl anion.

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 (fluN2 ) with the potassium aluminyl K[Al(NON)] ([NON]2- =[O(SiMe2 NDipp)2 ]2- , Dipp=2,6-iPr2 C6 H3 ) affords K[Al(NON)(κN1 ,N3 -{(fluN2 )2 })] (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, [K2 (THF)3 ][fluN=Nflu] (3). The reaction of 2 with N,N'-diisopropylcarbodiimide (iPrN=C=NiPr) affords the aluminium guanidinate complex, Al(NON){N(iPr)C(N=CMe2 )N(CHflu)} (4), showing a rare example of reactivity at a metal ketimide ligand. Density functional theory (DFT) calculations have been used to examine the bonding in the newly formed [(fluN2 )2 ]2- ligand in 1 and the ketimide bonding in 2. The mechanism leading to the formation of 4 has also been studied using this technique.

Three Oxidative Addition Routes of Alkali Metal Aluminyls to Dihydridoaluminates and Reactivity with CO2.

Three distinct routes are reported to the soluble, dihydridoaluminate compounds, AM[Al(NONDipp )(H)2 ] (AM=Li, Na, K, Rb, Cs; [NONDipp ]2- =[O(SiMe2 NDipp)2 ]2- ; Dipp=2,6-iPr2 C6 H3 ) starting from the alkali metal aluminyls, AM[Al(NONDipp )]. Direct H2 hydrogenation of the heavier analogues (AM=Rb, Cs) produced the first examples of structurally characterized rubidium and caesium dihydridoaluminates, although harsh conditions were required for complete conversion. Using 1,4-cyclohexadiene (1,4-CHD) as an alternative hydrogen source in transfer hydrogenation reactions provided a lower energy pathway to the full series of products for AM=Li-Cs. A further moderation in conditions was noted for the thermal decomposition of the (silyl)(hydrido)aluminates, AM[Al(NONDipp )(H)(SiH2 Ph)]. Probing the reaction of Cs[Al(NONDipp )] with 1,4-CHD provided access to a novel inverse sandwich complex, [{Cs(Et2 O)}2 {Al(NONDipp )(H)}2 (C6 H6 )], containing the 1,4-dialuminated [C6 H6 ]2- dianion and representing the first time that an intermediate in the commonly utilized oxidation process of 1,4-CHD to benzene has been trapped. The synthetic utility of the newly installed Al-H bonds has been demonstrated by their ability to reduce CO2 under mild conditions to form the bis-formate AM[Al(NONDipp )(O2 CH)2 ] compounds, which exhibit a diverse series of eyecatching bimetallacyclic structures.

Synthesis, Characterization, and Structural Analysis of AM[Al(NONDipp)(H)(SiH2Ph)] (AM = Li, Na, K, Rb, Cs) Compounds, Made Via Oxidative Addition of Phenylsilane to Alkali Metal Aluminyls.

We report the oxidative addition of phenylsilane to the complete series of alkali metal (AM) aluminyls [AM{Al(NONDipp)}]2 (AM = Li, Na, K, Rb, and Cs). Crystalline products (1-AM) have been isolated as ether or THF adducts, [AM(L)n][Al(NONDipp)(H)(SiH2Ph)] (AM = Li, Na, K, Rb, L = Et2O, n = 1; AM = Cs, L = THF, n = 2). Further to this series, the novel rubidium rubidiate, [{Rb(THF)4}2(Rb{Al(NONDipp)(H)(SiH2Ph)}2)]+ [Rb{Al(NONDipp)(H)(SiH2Ph)}2]-, was isolated during an attempted recrystallization of Rb[Al(NONDipp)(H)(SiH2Ph)] from a hexane/THF mixture. Structural and spectroscopic characterizations of the series 1-AM confirm the presence of µ-hydrides that bridge the aluminum and alkali metals (AM), with multiple stabilizing AM···π(arene) interactions to either the Dipp- or Ph-substituents. These products form a complete series of soluble, alkali metal (hydrido) aluminates that present a platform for further reactivity studies.

Potassium Aluminyl Promoted Carbonylation of Ethene.

The potassium aluminyl [K{Al(NONDipp )}]2 ([NONDipp ]2- =[O{SiMe2 NDipp}2 ]2- , Dipp=2,6-iPr2 C6 H3 ) 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 K2 [Al(NONDipp )(µ-CH2 CH=CO-1κ2 C1,3 -2κO)Al(NONDipp )Et]. Restricting the chemistry to a mono-aluminium system allowed isolation of [Al(NONDipp )(CH2 CH2 CO-κ2 C1,3 )]- , which undergoes thermal isomerisation to form the [Al(NONDipp )(CH2 CH=CHO-κ2 C,O)]- anion. DFT calculations highlight the stabilising influence of incorporated benzene at multiple steps in the reaction pathways.

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(NONDipp)] {NONDipp = [O(SiMe2NDipp)2]2-; Dipp = 2,6-iPr2C6H3} engages in oxidative additions with the E-H (E = Si, P, N, or O) bonds of phenylsilane (PhSiH3), mesityl phosphane (MesPH2; Mes = 2,4,6-Me3C6H2), 2,6-di-iso-propylaniline (DippNH2), and 2,6-di-tert-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. This study highlights the versatility of aluminyl anions to activate hydridic, acidic, and (essentially) nonpolar E-H bonds.

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

The aluminyl compounds [M{Al(NONDipp)}]2 (NONDipp = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2C6H3), 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, M = Li, Na, and K. The monomeric ion pairs contain discrete, highly polarized Al-M bonds between the aluminum and the group 1 metal and have been isolated with monodentate (THF, M = Li and Na) or bidentate (TMEDA, M = Li, Na, and K) ligands at M. The separated ion pairs comprise group 1 cations that are encapsulated by polydentate ligands, rendering the aluminyl anion, [Al(NONDipp)]- "naked". For M = Li, this structure type was isolated as the [Li(TMEDA)2]+ salt directly from a solution of the corresponding contacted dimeric pair in neat TMEDA, while the polydentate [2.2.2]cryptand ligand was used to generate the separated ion pairs for the heavier group 1 metals M = Na and K. This work shows that starting from the corresponding contacted dimeric pairs, the extent of the Al-M interaction in these aluminyl systems can be readily controlled with appropriate chelating reagents.

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, M2 [Al(NONDipp )]2 (M=Li, Na. NONDipp =[O(SiMe2 NDipp)2 ]2- ; Dipp=2,6-iPr2 C6 H3 ). Both compounds crystallize from non-coordinating solvent as "slipped" contacted dimeric pairs with strong M⋅⋅⋅π(aryl) interactions. Isolation from Et2 O solution affords the monomeric ion pairs (NONDipp )Al-M(Et2 O)2 , which contain discrete Al-Li and Al-Na bonds. The ability of the full series of Li, Na and K aluminyls to activate dihydrogen is reported.

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

[K{Al(NONDipp )}]2 (NONDipp =[O(SiMe2 NDipp)2 ]2- , Dipp=2,6-iPr2 C6 H3 ) reacts with CS2 to afford the trithiocarbonate species [K(OEt2 )][Al(NONDipp )(CS3 )] 1 or the ethenetetrathiolate complex, [K{Al(NONDipp )(S2 C)}]2 [3]2 . The dimeric alumoxane [K{Al(NONDipp )(O)}]2 reacts with carbon monoxide to afford the oxygen analogue of 3, [K{Al(NONDipp )(O2 C)}]2 [4]2 containing the hitherto unknown ethenetetraolate ligand, [C2 O4 ]4- .

A Stable Calcium Alumanyl.

A seven-membered N,N'-heterocyclic potassium alumanyl nucleophile is introduced and utilised in the metathetical synthesis of Mg-Al and Ca-Al bonded derivatives. Both species have been characterised by experimental and theoretical means, allowing a rationalisation of the greater reactivity of the heavier group 2 species implied by an initial assay of their reactivity.

Distibanes and Distibenes from Reduction of Sb(NONR )Cl by using MgI Reagents.

The bis(amidodimethyl)disiloxane antimony chlorides Sb(NONR )Cl (NONR =[O(SiMe2 NR)2 ]2- ; R=tBu, Ph, 2,6-Me2 C6 H3 =Dmp, 2,6-iPr2 C6 H3 =Dipp, 2,6-(CHPh2 )2 -4-tBuC6 H2 =tBu-Bhp) are reduced to SbII and SbI species by using MgI reagents, [Mg(BDIR' )]2 (BDI=[HC{C(Me)NR'}2 ]- ; R'=2,4,6-Me3 C6 H2 =Mes, Dipp). Stoichiometric reactions with Sb(NONR )Cl (R=tBu, Ph) form dimeric SbII stibanes [Sb(NONR )]2 , shown crystallographically to contain Sb-Sb single bonds. The analogous distibane with R=Dmp substituents has an exceptionally long Sb-Sb interaction and exhibits spectroscopic and reactivity properties consistent with radical character in solution. When R=Dipp, reductions with MgI reagents directly give distibenes [Sb(µ-NONDipp )Mg(BDIR' )(THF)n ]2 (R'=Mes, n=1; R'=Dipp, n=0). Crystallographic analysis shows a trans-substitution of the Sb=Sb double bond, with bridging NONDipp -ligands between the SbI and MgII centres. An attempt to access the NONPh -analogue using the same protocol afforded the polystibide cluster Sb8 [µ4 ,η2:2:2:2 -Mg(BDIMes )]4 , which co-crystallized with the ligand transfer product, [Mg(BDIMes )]2 (µ-NONPh ).

Isoelectronic Aluminium Analogues of Carbonyl and Dioxirane Moieties.

We report the anion [Al(NONAr )(Se)]- (NONAr =[O(SiMe2 NAr)2 ]2- , Ar=2,6-iPr2 C6 H3 ), 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(NONAr )]- with elemental selenium. Spectroscopic, crystallographic, and computational analysis confirmed multiple bonding between aluminium and selenium. Addition of a second equivalent of selenium afforded the diselenirane, [Al(NONAr )(κ2 -Se2 )]- , which is an isoelectronic analogue of the dioxirane group.

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(NONAr )]- (NONAr =[O(SiMe2 NAr)2 ]2- , Ar=2,6-iPr2 C6 H3 ). The reaction proceeds via the isolable monoalumoxane anion [Al(NONAr )(O)]- , containing a terminal aluminum-oxygen bond. This species reacts with a second equivalent of carbon dioxide to afford the carbonate [Al(NONAr )(CO3 )]- , and with nitrous oxide to generate the hyponitrite anion, [Al(NONAr )(κ2 O,O'-N2 O2 )]- .

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

The potassium aluminyl complex K[Al(NONAr )] (NON=NONAr =[O(SiMe2 NAr)2 ]2- , Ar=2,6-iPr2 C6 H3 ) reacts with 1,3,5,7-cyclooctatetraene (COT) to give K[Al(NONAr )(COT)]. The COT-ligand is present in the asymmetric unit as a planar µ2 -η2 :η8 -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]2- . Addition of 18-crown-6 causes a rearrangement of the C8 -carbocycle to form the isomeric 9-aluminabicyclo[4.2.1]nona-2,4,7-triene anion.

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

Reduction of the indate complex In(NONAr )(µ-Cl)2 Li(OEt2 )2 (NONAr =[O(SiMe2 NAr)2 ]2- ; Ar=2,6-iPr2 C6 H3 ) with sodium generates the InII diindane species [In(NONAr )]2 . Further reduction with a mixture of potassium and [2.2.2]crypt affords the InI N-heterocyclic indyl anion [In(NONAr )]- , which crystallizes with a non-contacted [K([2.2.2]crypt)]+ cation. The indyl anion can also be isolated as the indyllithium compound In(NONAr )(Li{THF}3 ), which contains an In-Li bond. Density functional theory calculations show that the HOMO of the indyl anion is a metal-centred lone pair, and preliminary reactivity studies confirm its nucleophilic behaviour.

Bismuth(III) Complex of the [S4]•- Radical Anion: Dimer Formation via Pancake Bonds.

Reaction of bismuth(II) compounds with sulfur gives mixtures of [Bi(NONR)]2(µ2-Sn) (NONR = [O(SiMe2NR)2]2-). Examples for n = 1 and 3 have been crystallographically verified for R = 2,6-iPr2C6H3 (Dipp) and R = tBu, and the pentasulfide (n = 5) for R = Dipp. The corresponding product from reaction with the new Bi(II) radical Bi(NONAr‡)• (Ar‡ = C6H2(CHPh2)2-tBu-2,6,4) exists as the dimer [Bi(NONAr‡)(S4)]2, with π*(SOMO)-π*(SOMO) interactions linking the sulfur chains through trans-antarafacial pancake bonds.

The Reactivity of Germanium Phosphanides with Chalcogens.

The reactivity of germanium phosphanido complexes with elemental chalcogens is reported. Addition of sulfur to [(BDI)GePCy2] (BDI = CH{(CH3)CN-2,6-iPr2C6H3}2) results in oxidation at germanium to form germanium(IV) sulfide [(BDI)Ge(S)PCy2] and oxidation at both germanium and phosphorus to form germanium(IV) sulfide dicylohexylphosphinodithioate complex [(BDI)Ge(S)SP(S)Cy2], whereas addition of tellurium to [(BDI)GePCy2] only gives the chalcogen inserted product, [(BDI)GeTePCy2]. This reactivity is different from that observed between [(BDI)GePCy2] and selenium. Addition of selenium to the diphenylphosphanido germanium complex, [(BDI)GePPh2], results in insertion of selenium into the Ge-P bond to form [(BDI)GeSePCy2] as well as the oxidation at phosphorus to give [(BDI)GeSeP(Se)Ph2]. In contrast, addition of selenium to the bis(trimethylsilyl)phosphanido germanium complex, [(BDI)GeP(SiMe3)2], yields the germanium(IV) selenide [(BDI)Ge(Se)P(SiMe3)2].

Tin and Lead Phosphanido Complexes: Reactivity with Chalcogens.

The reactivity of tin and lead phosphanido complexes with chalogens is reported. The addition of sulfur to [(BDI)MPCy2] (M = Sn, Pb; BDI = CH{(CH3)CN-2,6-iPr2C6H3}2) results in the formation of phosphinodithioates [(BDI)MSP(S)Cy2] regardless of the conditions; however, when selenium is added to [(BDI)MPCy2], a selenium insertion product, phosphinoselenoite [(BDI)MSePCy2], can be isolated. This compound readily reacts with additional selenium to form the phosphinodiselenoate complex [(BDI)MSeP(Se)Cy2]. In contrast, the addition of selenium to [(BDI)SnP(SiMe3)2] results in the formation of the heavy ether [(BDI)SnSeSiMe3]. Differences in the solution and solid-state molecular species of tin phosphinoselenoite and phosphinodiselenoate complexes were probed using multinuclear solution and solid-state NMR spectroscopy.

Isolation and characterization of roridin E.

The commonly occurring, high-cytotoxicity macrolide roridin E has been re-isolated from Stachybotrys chartarum and characterized by 1-D and 2-D NMR spectroscopy. Assignment of the spectral data for roridin E revealed differences from the accepted literature, and spectra are reported herein to aid in future identification. For the first time confirmation of structure was provided by a crystallographic solution for roridin E. Copyright © 2016 John Wiley & Sons, Ltd.

(15)N NMR Spectroscopy, X-ray and Neutron Diffraction, Quantum-Chemical Calculations, and UV/vis-Spectrophotometric Titrations as Complementary Techniques for the Analysis of Pyridine-Supported Bicyclic Guanidine Superbases.

Pyridine substituted with one and two bicyclic guanidine groups has been studied as a potential source of superbases. 2-{hpp}C5H4N (I) and 2,6-{hpp}2C5H3N (II) (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) were protonated using [HNEt3][BPh4] to afford [I-H][BPh4] (1a), [II-H][BPh4] (2), and [II-H2][BPh4]2 (3). Solution-state (1)H and (15)N NMR spectroscopy shows a symmetrical cation in 2, indicating a facile proton-exchange process in solution. Solid-state (15)N NMR data differentiates between the two groups, indicating a mixed guanidine/guanidinium. X-ray diffraction data are consistent with protonation at the imine nitrogen, confirmed for 1a by single-crystal neutron diffraction. The crystal structure of 1a shows association of two [I-H](+) cations within a cage of [BPh4](-) anions. Computational analysis performed in the gas phase and in MeCN solution shows that the free energy barrier to transfer a proton between imino centers in [II-H](+) is 1 order of magnitude lower in MeCN than in the gas phase. The results provide evidence that linking hpp groups with the pyridyl group stabilizes the protonation center, thereby increasing the intrinsic basicity in the gas phase, while the bulk prevents efficient cation solvation, resulting in diminished pKa(MeCN) values. Spectrophotometrically measured pKa values are in excellent agreement with calculated values and confirm that I and II are superbases in solution.