Structure and reaction pathway of TMP-Zincate: amido base or alkyl base?

Unlocking the Metalation Applications of TMP-powered Fe and Co(II) bis(amides): Synthesis, Structure and Mechanistic Insights

Typified by LiTMP and TMPMgCl.LiCl, (TMP=2,2,6,6-tetramethylpiperidide), s-block metal amides have found widespread applications in arene deprotonative metalation. On the contrary, transition metal amides lack sufficient basicity to activate these substrates. Breaking new ground in this field, here we present the synthesis and full characterisation of earth-abundant transition metals M(TMP)2 (M=Fe, Co). Uncovering a new reactivity profile towards fluoroarenes, these amide complexes can promote direct M-H exchange processes regioselectively using one or two of their basic amide arms. Remarkably, even when using a perfluorinated substrate, selective C-H metalation occurs leaving C-F bonds intact. Their kinetic basicity can be boosted by LiCl or NBu4Cl additives which enables formation of kinetically activated ate species. Combining spectroscopic and structural studies with DFT calculations, mechanistic insights have been gained on how these low polarity metalation processes take place. M(TMP)2 can also be used to access ferrocene and cobaltocene by direct deprotonation of cyclopentadiene and undergo efficient CO2 insertion of both amide groups under mild reaction conditions.

Enantioselective deprotometalation of alkyl ferrocenecarboxylates using bimetallic bases

Our attempts to deprotometalate alkyl ferrocenecarboxylates enantioselectively by using chiral lithium–zinc or lithium–cadmium bases are reported.

Programmable synthesis of multiply arylated cubanes through C-H metalation and arylation

Cubane (C₈H₈), a cubic alkane, has long attracted attention owing to its unique 3D structure. In order to utilize the cubane scaffold widely in science and technology, a powerful method for synthesizing diverse cubane derivatives is required. Herein, we report the synthesis of mono-, di-, tri-, and tetra-arylated cubanes. Directed ortho-metalation with lithium base/alkyl zinc and subsequent palladium-catalyzed arylation enabled C–H metalation and arylation of cubane. This reaction allows the late-stage and regioselective installation of a wide range of aryl groups, realizing the first programmable synthesis of diverse multiply arylated cubanes.

Cubane has attracted attention due to its unique 3D structure. Herein, we report the programmable synthesis of multiply arylated cubanes. The developed reaction allows the late-stage and regioselective installation of aryl groups.

A new route for the efficient metalation of unfunctionalized aromatics

The synthesis and isolation of a novel bimetallic species formed by reacting two equivalents of TMPLi with CuCl in the presence of Et₂O are reported. X-ray crystallography reveals the Et₂O-free tetranuclear aggregate (TMPCu)₂(TMPLi)₂ 1, which formally results from the catenation of dimers of TMPLi and TMPCu. NMR spectroscopy confirms that, upon dissolution in hydrocarbon media, the crystals fail to form a conventional Gilman cuprate dimer. Instead they exhibit a spectrum which is consistent with that recently proposed for an isomer of dimeric Gilman cuprate. Moreover, while pre-isolated Gilman cuprate is inert to benzene solvent, this new isomer smoothly affects aromatic deprotonation to give mainly Ph(TMP)₃Cu₂Li₂ 3, which is formally a heterodimer of Gilman cuprate TMPCu(μ-TMP)Li 2 and PhCu(μ-TMP)Li 4. Attempts to synthesise 3 through explicit combination of pre-isolated 2 and 4 were successful; additionally, this permitted the preparation of Ph(TMP)₃Cu₃Li 5 and Ph(TMP)₃CuLi₃ 7 when 4 was combined in 1 : 2 ratios with TMPCu or TMPLi, respectively. 5 was characterised as metallacyclic in the solid-state, its structural features resembling those in 3 but with reduced Li-π interactions. It also proved possible to perform Cu/Li exchange on 5 (using ^t BuOCu) to give a novel mixed organo(amido)copper species Ph(TMP)₃Cu₄ 6. Remarkably, the unprecedented reactivity of 1 towards benzene is reproduced by heating a 1 : 1 mixture of TMPLi and TMPCu in the same solvent; this gives predominantly 3. On the other hand, mixtures which are rich in either Cu or Li can lead to the selective in situ formation of 5 or 7. Though crystallographic data on 7 could not be obtained, DFT calculations accurately corroborated the observed structures of 3 and 5 and could be used to support 7 having the same structure type, albeit with enhanced Li-π interactions. This was consistent with NMR spectroscopic data. However, in contrast to 3 and 5, for which 2D NMR spectroscopy indicated only conformational changes, 7 was additionally found to exhibit fluxionality in a manner consistent with a dissociative process.

Enantioselective deprotometalation of N,N-dialkyl ferrocenecarboxamides using metal amides

Our attempts to deprotometalate N,N-dialkyl ferrocenecarboxamides enantioselectively by using chiral lithium or lithium–zinc dialkylamides are reported.

Trans-Metal-Trapping: Concealed Crossover Complexes En Route to Transmetallation?

Defined as the transfer of ligands from one metal to another, transmetallation is a common reaction in organometallic chemistry. Its chemical celebrity stems from its role in important catalytic cycles of cross-coupling reactions such as those of Negishi, Sonogashira, Stille, or Suzuki. This article focuses on trans-metal-trapping (TMT), which could be construed as partially complete transmetallations. On mixing two distinct organometallic compounds, of for example lithium with aluminium or gallium, the two metals meet in a crossover co-complex, but the reaction ceases at that point and full transmetallation is not reached. Though in its infancy, trans-metal-trapping shows promise in transforming failed lithiations into successful lithiations and in stabilising sensitive carbanions through cooperative bimetallic effects making them more amenable to onward reactivity.

Alkenylation of C(sp3 )-H Bonds by Zincation/Copper-Catalyzed Cross-Coupling with Iodonium Salts

α-Vinylation of phosphonates, phosphine oxides, sulfones, sulfonamides, and sulfoxides has been achieved by selective C-H zincation and copper-catalyzed C(sp³ )-C(sp² ) cross-coupling reaction using vinylphenyliodonium salts. The vinylation transformation proceeds in high efficiency and stereospecificity under mild reaction conditions. This zincative cross-coupling reaction represents a general alkenylation strategy, which is also applicable for α-alkenylation of esters, amides, and nitriles in the synthesis of β,γ-unsaturated carbonyl compounds.

Base-Controlled Regioselective Functionalization of Chloro-Substituted Quinolines

We prepared a number of di- and trifunctionalized quinolines by selective metalation of chloro-substituted quinolines with metal amides followed by reaction with different electrophiles. Metalation of the C-3 position of the quinolinic ring with lithium diisopropylamide at -70 °C is easy to achieve, whereas reaction with lithium-magnesium and lithium-zinc amides affords C-2 or C-8 functionalized derivatives in a regioselective fashion. These complementary methods could be rationalized by DFT calculations and are convenient strategies toward the synthesis of bioactive quinoline derivatives such as chloroquine analogues.

Metal exchange in lithiocuprates: implications for our understanding of structure and reactivity

New reagents have been sought for directed ortho cupration in which the use of cyanide reagents is eliminated. CuOCN reacts with excess TMPLi (TMP = 2,2,6,6-tetramethylpiperidide) in the presence of limited donor solvent to give crystals that are best represented as (TMP)2Cu0.1Li0.9(OCN)Li2(THF) 8, whereby both Lipshutz-type lithiocuprate (TMP)2Cu(OCN)Li2(THF) 8a and trinuclear (TMP)2(OCN)Li3(THF) 8b are expressed. Treatment of a hydrocarbon solution of TMP2CuLi 9a with LiOCN and THF gives pure 8a. Meanwhile, formation of 8b is systematized by reacting (TMPH2)OCN 10 with TMPH and nBuLi to give (TMP)2(OCN)Li3(THF)211. Important to the attribution of lower/higher order bonding in lithiocuprate chemistry is the observation that in crystalline 8, amide-bridging Cu and Li demonstrate clear preferences for di- and tricoordination, respectively. A large excess of Lewis base gives an 8-membered metallacycle that retains metal disorder and analyses as (TMP)2Cu1.35Li0.659 in the solid state. NMR spectroscopy identifies 9 as a mixture of (TMP)2CuLi 9a and other copper-rich species. Crystals from which the structure of 8 was obtained dissolve to yield evidence for 8b coexisting in solution with in situ-generated 9a, 11 and a kinetic variant on 9a ( i-9a), that is best viewed as an agglomerate of TMPLi and TMPCu. Moving to the use of DALi (DA = diisopropylamide), (DA)2Cu0.09Li0.91(Br)Li2(TMEDA)212 (TMEDA = N,N,N',N'-tetremethylethylenediamine) is isolated, wherein (DA)2Cu(Br)Li2(TMEDA)212a exhibits lower-order Cu coordination. The preparation of (DA)2Li(Br)Li2(TMEDA)212b was systematized using (DAH2)Br, DAH and nBuLi. Lastly, metal disorder is avoided in the 2 : 1 lithium amide : Lipshutz-type monomer adduct (DA)4Cu(OCN)Li4(TMEDA)213.

Templated deprotonative metalation of polyaryl systems: Facile access to simple, previously inaccessible multi-iodoarenes

The development of new methodologies to affect non-ortho-functionalization of arenes has emerged as a globally important arena for research, which is key to both fundamental studies and applied technologies. A range of simple arene feedstocks (namely, biphenyl, meta-terphenyl, para-terphenyl, 1,3,5-triphenylbenzene, and biphenylene) is transformed to hitherto unobtainable multi-iodoarenes via an s-block metal sodium magnesiate templated deprotonative approach. These iodoarenes have the potential to be used in a whole host of high-impact transformations, as precursors to key materials in the pharmaceutical, molecular electronic, and nanomaterials industries. To prove the concept, we transformed biphenyl to 3,5-bis(N-carbazolyl)-1,1'-biphenyl, a novel isomer of 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CPB), a compound which is currently widely used as a host material for organic light-emitting diodes.

Synthesis of a Sterically Demanding Dispiropiperidine and Its Application in Monoamidodialkyl Zincate Complexes

The new sterically hindered piperidine analog, dispiro[cyclohexane-2,2'-piperidine-6',2″-cyclohexane] (CPC(H), 2), and its N-methylated derivative CPC(Me) (3) were synthesized from commercially available starting materials in short steps. The N-lithiated amide LiCPC (4) was also isolated from 2 as a cyclictrimer in single crystals and showed slightly larger steric hindrance than that of lithium 2,2,6,6-tetramethylpiperidide (LiTMP) in the competitive methylation reaction with methyl trifluoromethanesulfonate. In addition, the heterobimetallic heteroleptic zincate complexes [Li(μ-NR₂)(μ-Et)Zn(Et)] (NR₂ = CPC, 5, and NR₂ = TMP, 6) were obtained as THF- and TMEDA-coordinated monomer 5·(THF)₂, 6·(THF)₂, 5·TMEDA, and 6·TMEDA (THF = tetrahydrofuran, TMEDA = N,N,N',N'-tetramethylethylenediamine). These molecular structures bearing different amido ligands in single crystals showed little structural differences from crystallographic studies. Diffusion-ordered spectroscopy (DOSY) revealed that the solution structures of the zincate complexes 5·(THF)₂ and 6·(THF)₂ only differ in the number of coordination THF molecules. In the deprotonation reactions with tert-butyl 3-bromobenzoate, the zincate complexes containing the CPC ligand [Li(μ-CPC)(μ-R)Zn(R)] (R = Et (5), tBu) showed moderately improved regioselectivity for the 6 position in comparison to those containing the TMP ligand [Li(μ-TMP)(μ-R)Zn(R)] (R = Et (6), tBu).

Synthetic and reactivity studies of hetero-tri-anionic sodium zincates

The blue, red and green spheres represent three different anions present within a series of novel hetero-trianionic sodium zincates. The syntheses and structures of the complexes are reported as well as their reactivities with important organic molecules.

Extending motifs in lithiocuprate chemistry: unexpected structural diversity in thiocyanate complexes

Lithio(thiocyanato)cuprates have been developed. These reveal planar, boat-shaped and chair-shaped Lipshutz-type dimers in the solid state, while Lipshutz-type and Gilman structures are seen in solution.

Heterobimetallic metallation studies of N,N-dimethylphenylethylamine (DMPEA): benzylic C–H bond cleavage/dimethylamino capture or intact DMPEA complex

The cleave and capture capacity of various bimetallic amido–alkyl mixtures is illustrated through reaction with the deprotonation sensitiveN,N-dimethylphenylethylamine.

TMP (2,2,6,6-tetramethylpiperidide)-aluminate bases: lithium-mediated alumination or lithiation–alkylaluminium-trapping reagents?

Surprisingly lithium TMP-aluminate reagents are not capable of directly aluminating anisole as previously thought but operateviasequential lithiation–alkylaluminium trapping.

New avenues in the directed deprotometallation of aromatics: recent advances in directed cupration

Advances in directed aromatic deprotometallation are reported in the context of recent developments in our understanding of lithium cuprates.

Novel surfactant-free route to delaminated all-silica and titanosilicate zeolites derived from a layered borosilicate MWW precursor

Layered borosilicate zeolite precursor ERB-1P (Si/B = 11) is delaminated via simultaneous deboronation and SDA removal, to yield material DZ-1 consisting of silanol nests, using a simple aqueous Zn(NO₃)₂ treatment.

Pre-inverse-crowns: synthetic, structural and reactivity studies of alkali metal magnesiates primed for inverse crown formation

The synthesis, structure and reactivity of donor-free sodium and potassium magnesiates are assessed culminating in the unique 1,4-dimetallation of naphthalene.

Combined directed ortho-zincation and palladium-catalyzed strategies: Synthesis of 4,n-dimethoxy-substituted benzo[b]furans

A new route to regioselectively dialkoxy-functionalized benzo[b]furan derivatives has been developed from 3-halo-2-iodoanisoles bearing an additional methoxy group, which have been accessed through an ortho-zincation/iodination reaction. Two palladium-catalyzed processes, namely a Sonogashira coupling followed by a tandem hydroxylation/cyclization sequence, give rise to new and interesting dimethoxy-substituted benzo[b]furans.