Luca Russo

Cleave and capture chemistry illustrated through bimetallic-induced fragmentation of tetrahydrofuran

The cleavage of ethers is commonly encountered in organometallic chemistry, although rarely studied in the context of new, emerging bimetallic reagents. Recently, it was reported that a bimetallic sodium-zinc base can deprotonate cyclic tetrahydrofuran under mild conditions without opening its heterocyclic (OC(4)) ring. In marked contrast to this synergic sedation, herein we show that switching to the more reactive sodium-magnesium or sodium-manganese bases promotes cleavage of at least six bonds in tetrahydrofuran, but uniquely the ring fragments are captured in separate crystalline complexes. Oxide fragments occupy guest positions in bimetallic, inverse crown ethers and C(4) fragments ultimately appear in bimetallated butadiene molecules. These results demonstrate the special synergic reactivity that can be executed by bimetallic reagents, which include the ability to capture and control, and thereby study, reactive fragments from sensitive substrates.

Closer Insight into the Reactivity of TMP−Dialkyl Zincates in Directed ortho-Zincation of Anisole: Experimental Evidence of Amido Basicity and Structural Elucidation of Key Reaction Intermediates

The new dialkyl(aryl) lithium zincates [(THF)2Li(C6H4−OMe)MeZnMe] (4), [(TMEDA)Li(C6H4−OMe)MeZnMe] (6), [(THF)3Li(C6H4−OMe)tBuZntBu] (7), and [(PMDETA)Li(C6H4−OMe)tBuZntBu] (8) have been prepared by co-complexation reactions of lithiated anisole with the relevant dialkylzinc compound and the relevant Lewis base. These new heterobimetallic compounds have been characterized in solution using 1H, 13C{H}, and 7Li NMR spectroscopy, and the molecular structures of 6 and 8 have been elucidated by X-ray crystallography. In 6 the distinct metals are connected through the anisole ligand which binds in an ambidentate fashion (through carbon−zinc and oxygen−lithium contacts) and also through one of the methyl groups, to close a [LiOCCZnC] six-membered ring; whereas 8 displays an open structure where anisole connects the two metals (in the same mode as in 6) but with the tert-butyl groups exclusively bonded terminally to zinc. Reactivity studies of zincates 4 and 7 with the amine TMP(H) supply experimental evidence that these heterobimetallic compounds are intermediates in the two-step deprotonation reaction of anisole by TMP−dialkyl zincates and show the relevance of the alkyl groups in the efficiency of TMP−dialkyl zincate bases. In addition, important solvent effects have also been evaluated. When hexane is added to THF solutions of compounds 4 or 7, the homoleptic tetraorganozincate [(THF)2Li2Zn(C6H4−OMe)4] (5) is obtained as the result of a disproportionation process. This lithium-rich zincate has also been spectroscopically and crystallographically characterized.

Zinc–salophen complexes as selective receptors for tertiary amines

Zinc–salophen compounds 1–3 incorporating in the given order 1,2-diaminobenzene, 2,3-diaminonaphthalene, and 9,10-diaminophenantrene moieties were synthesised. Their binding properties toward a series of tertiary amines were assessed by UV-Vis and fluorescence spectroscopy in chloroform solution. Unprecedented selectivities of quinuclidine vs. triethylamine higher than 105 were recorded, thereby revealing the dramatic influence of steric effects on axial coordination of tertiary amines. X-Ray diffraction analyses showed that in the solid state compound 2 is dimeric, but its 1 : 1 quinuclidine complex is monomeric. Strong indications were obtained that both free receptors and their amine adducts are monomeric in dilute chloroform solution.

Further investigations into tetrahedral M4L6 cage complexes containing guest anions: new structures and NMR spectroscopic studies

A series of ligands LPh, Lnaph and Lanth, which contain two bidentate pyrazolyl–pyridine termini separated by an aromatic (1,2-phenyl, 2,3-naphthyl or 2,3-anthracenyl, respectively) spacer have been used to prepare tetrahedral cage complexes of the form [M4L6]Xn, in which a bis-bidentate bridging ligand spans each of the six edges of the M4 tetrahedron and one anion is bound in the central cavity. Several new examples have been structurally characterised, including an example with a new ligand (Lanth), the first example with a second-row transition metal ion [Cd(II)], and the first example of a cage containing a dianionic guest (hexafluorosilicate). The series of structurally similar Co(II) complexes [Co4L6(BF4)](BF4)7 (L = LPh, Lnaph and Lanth) have been examined in detail by NMR spectroscopy. The 1H NMR spectra are highly shifted between −110 and +90 ppm, but the spectra can be completely assigned by correlation of measured T1 relaxation times with distances of the protons in the complexes from the paramagnetic Co(II) centres. 1H DOSY measurements have been used to estimate diffusion constants which confirm the structural integrity of the cages in solution, and 19F DOSY measurements on the anions show that (i) the trapped [BF4]− anion diffuses at the same rate as the cage superstructure surrounding it, indicating that it is trapped inside the cage cavity; and (ii) the ‘free’ [BF4]− anions have diffusion rates consistent with substantial retardation due to ion-pairing with the 7+ complex cation.

Structural Basis for Regioisomerization in the Alkali-Metal-Mediated Zincation (AMMZn) of Trifluoromethyl Benzene by Isolation of Kinetic and Thermodynamic Intermediates

Performed with a desire to advance knowledge of the structures and mechanisms governing alkali-metal-mediated zincation, this study monitors the reaction between the TMP-dialkylzincate reagent [(TMEDA)Na(TMP)(tBu)Zn(tBu)] 1 and trifluoromethyl benzene C6H5CF32. A complicated mixture of products is observed at room temperature. X-ray crystallography has identified two of these products as ortho- and meta-regioisomers of heterotrianionic [(TMEDA)Na(TMP)(C6H4-CF3)Zn(tBu)], 3-ortho and 3-meta, respectively. Multinuclear NMR data of the bulk crystalline product confirm the presence of these two regioisomers as well as a third isomer, 3-para, in a respective ratio of 20:11:1, and an additional product 4, which also exhibits ortho-zincation of the aryl substrate. Repeating the reaction at 0 °C gave exclusively 4, which was crystallographically characterized as [{(TMEDA)2Na}+{Zn(C6H4-CF3)(tBu)2}−]. Mimicking the original room-temperature reaction, this kinetic product was subsequently reacted with TMP(H) to afford a complicated mixture of products, including significantly the three regioisomers of 3. Surprisingly, 4 adopts a solvent-separated ion pair arrangement in contrast to the contacted ion variants of 3-ortho and 3-meta. Aided by DFT calculations on model systems, discussion focuses on the different basicities, amido or alkyl, and steps, exhibited in these reactions, and how the structures and bonding within these isolated key metallic intermediates (prior to any electrophilic interception step), specifically the interactions involving the alkali metal, influence the regioselectivity of the Zn−H exchange process.

Alkali‐Metal‐Mediated Manganation(II) of Functionalized Arenes and Applications of ortho‐Manganated Products in Pd‐Catalyzed Cross‐Coupling Reactions with Iodobenzene

Extending the recently introduced concept of “alkali-metal-mediated manganation” to functionalised arenes, the heteroleptic sodium manganate reagent [(tmeda)Na(tmp)(R)Mn(tmp)] (1; TMEDA = N,N,N′,N′-tetra-methylethylenediamine, TMP = 2,2,6,6-tetramethylpiperidide, R = CH2 SiMe3) has been treated with anisole or N,N-diisopropylbenzamide in a 1:1 stoichiometry in hexane. These reactions afforded the crystalline products [(tmeda)Na(tmp)(o-C6H4OMe)Mn(tmp)] (2) and [(tmeda)Na(tmp){o-{C(O)N(iPr)2C6H4}Mn(CH2SiMe3)] (3), respectively, as determined from X-ray crystallographic studies. On the basis of these products, it can be surmised that reagent 1 has acted, at least partially and ultimately, as an alkyl base in the first reaction liberating the silane Me4Si, but as an amido base in the second reaction liberating the amine TMPH. Both of these paramagnetic products 2 and 3 have contacted ion-pair structures, the key features of which are six-atom, five-element (NaNMnCCO) and seven-atom, five-element (NaNMnCCCO) rings, respectively. Manganates 2 and 3 were successfully cross-coupled with iodobenzene under [PdCl2(dppf)] (dppf=1,1′-bis(diphenylphosphino)ferrocene) catalysis to generate unsymmetrical biaryl compounds in yields of 98.0 and 66.2%, respectively. Emphasizing the importance of alkali-metal mediation in these manganation reactions, the bisalkyl Mn reagent on its own fails to metalate the said benzamide, but instead produces the monomeric, donor–acceptor complex [Mn(R)2{(iPr)2NC(Ph)(=O)}2] (5), which has also been crystallographically characterised. During one attempt to repeat the synthesis of 2, the butoxide-contaminated complex [{(tmeda)Na(R)(OBu)(o-C6H4OMe)Mn}2] (6) was obtained. In contrast to 2 and 3, due to reduced steric constraints, this complex adopts a dimeric arrangement in the crystal, the centrepiece of which is a twelve atom (NaOCCMnC)2 ring.

Tuning the Basicity of Synergic Bimetallic Reagents: Switching the Regioselectivity of the Direct Dimetalation of Toluene from 2,5‐ to 3,5‐Positions

Meta-meta metalation: Remarkably, toluene can be directly dimanganated or dimagnesiated at the 3,5-positions using bimetallic bases with active Me3SiCH2 ligands (see scheme, blue). In contrast, n-butyl ligands lead to 2,5-metalation (red). tmp=2,2,6,6-tetramethylpiperidide.

Structural and magnetic insights into the trinuclear ferrocenophane and unexpected hydrido inverse crown products of alkali-metal-mediated manganation(II) of ferrocene.

With the aim of introducing the diisopropylamide [NiPr(2)](-) ligand to alkali-metal-mediated manganation (AMMMn) chemistry, the temperature-dependent reactions of a 1:1:3 mixture of butylsodium, bis(trimethylsilylmethyl)manganese(II), and diisopropylamine with ferrocene in hexane/toluene have been investigated. Performed at reflux temperature, the reaction affords the surprising, ferrocene-free, hydrido product [Na(2)Mn(2) (mu-H)(2){N(iPr)(2)}(4)]2 toluene (1), the first Mn hydrido inverse crown complex. Repeating the reaction rationally, excluding ferrocene, produces 1 in an isolated crystalline yield of 62 %. At lower temperatures, the same bimetallic amide mixture leads to the manganation of ferrocene to generate the first trimanganese, trinuclear ferrocenophane, [{Fe(C(5)H(4))(2)}(3){Mn(3)Na(2)(NiPr(2))(2) (HNiPr(2))(2)}] (2) in an isolated crystalline yield of 81 %. Both 1 and 2 have been characterised by X-ray crystallographic studies. The magnetic properties of paramagnetic 1 and 2 have also been examined by variable-temperature magnetisation measurements on powdered samples. For 1, the room-temperature value for chiT is 3.45 cm(3) K mol(-1), and on lowering the temperature a strong antiferromagnetic coupling between the two Mn ions is observed. For 2, the room-temperature value for chiT is 4.06 cm(3) K mol(-1), which is significantly lower than the expected value for three isolated paramagnetic Mn(II) ions.

Direct C-H metalation with chromium(ii) and iron(ii) : transition-metal host/benzenediide guest magnetic inverse-crown complexes

Abstract Check M(etal)ate: The chessboard and the figures represent a special reaction in which different low-polarity metals can metalate arenes directly when they are brought into the right position. In a combination of queen (sodium) and knight (chromium or iron), it is possible for the knight (usually the weaker piece) to make a direct deadly hit on the king (benzene) in this game of elemental chess.

Expanding Mg–Zn Hybrid Chemistry: Inorganic Salt Effects in Addition Reactions of Organozinc Reagents to Trifluoroacetophenone and the Implications for a Synergistic Lithium–Magnesium–Zinc Activation

Numerous organic transformations rely on organozinc compounds made through salt-metathesis (exchange) reactions from organolithium or Grignard reagents with a suitable zinc precursor. By combining X-ray crystallography, NMR spectroscopy and DFT calculations, this study sheds new light on the constitution of the organometallic species involved in this important synthetic tool. Investigations into the metathesis reactions of equimolar amounts of Grignard reagents (RMgX) and ZnCl(2) in THF led to the isolation of novel magnesium-zinc hybrids, [{(thf)(2)Mg(μ-Cl)(3)ZnR}(2)] (R=Et, tBu, nBu or o-OMe-C(6)H(4)), which exhibit an unprecedented structural motif in mixed magnesium-zinc chemistry. Furthermore, theoretical modelling of the reaction of EtMgCl with ZnCl(2) reveals that formation of the mixed-metal compound is thermodynamically preferred to that of the expected homometallic products, RZnCl and MgCl(2). This study also assesses the alkylating ability of hybrid 3 towards the sensitive ketone trifluoroacetophenone, revealing a dramatic increase in the chemoselectivity of the reaction when LiCl is introduced as an additive. This observation, combined with recent related breakthroughs in synthesis, points towards the existence of a trilateral Li/Mg/Zn synergistic effect.