Treatment of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) with a group 1 metal (Li, Na, K, Rb, or Cs), resulted in the reduction of this important radical to the TEMPO(-) anion--the first examples of elemental-metal single electron reduction of the radical to its anionic form. The synthesis and characterization of seven alkali metal TEMPO(-) complexes are reported. A variety of structural motifs are encountered depending on the choice of metal and/or solvent. (THF)(2) x [Li(+)(TEMPO(-))](4) 1 crystallized from THF as a cyclic (Li(4)O(4)) molecule. Two Li centers are stabilized by coordination to a THF molecule; the others by intramolecular coordination to N(TEMPO) atoms. [(THF) x Na(+)(TEMPO(-))](4) 2 exists as a distorted cubane where each Na center is coordinated to a THF molecule. No appreciable Na-N(TEMPO) coordination is observed. [(THF)(2) x Na(+)(3)(TEMPO(-))(2)(OH)](2) 3 was serendipitously prepared and exists as a distorted bis(cubane). It is envisaged that 3 is formed from 2 by insertion of a (Na-OH)(2) double bridge into its framework. [Na(+)(4)(mu(3)-TEMPO(-))(2)(mu(2)-TEMPO(-))(2)(TMEDA)(2)] 4, adopts a four-runged ladder structure, whereby the two outer Na centers are coordinated to TMEDA, in addition to two mu(2)-O and a N atom. The inner metal atoms are bound to three mu(3)-O atoms and a N atom. [(THF) x K(+)(TEMPO(-))](4) 5 resembles the motif found for 2; however, presumably because of the larger size of the metal, K-N(TEMPO) interactions are present in 5. The asymmetric unit of [(TMEDA) x Rb(+)(2)(TEMPO(-))(2)](2) 6 comprises a Rb(4)O(4) cubane with half a molecule of TMEDA coordinated to each metal. From a supramolecular perspective, 6 exists as a polymeric array of cubane units connected by TMEDA bridges. Completing the series, [Cs(+)(TEMPO)](infinity) 7 crystallizes from hexane to form a donor-free polymeric complex. Complexes 1, 2, and 4-7 are soluble in D(8)-THF solution, and their NMR spectra are reported. The solution structures in donor solvent appear virtually identical.

The new mixed lithium-zinc enolate compounds [(TMEDA) 2 Li 2 Zn{OC(dCH 2 )Mes} 4 ] (2) and [{TMP(H)} 2 Li 2 Zn{OC(dCH 2 )Mes} 4 ] (3) were prepared by reaction of the sterically demanding ketone 2,4,6-trimethylacetophenone (1) with the all-amido homoleptic zincate [LiZn(TMP) 3 ] (TMP ) 2,2,6,6tetramethylpiperidide). X-ray crystallographic studies revealed that these compounds adopt a trinuclear Li • • • Zn • • • Li chain arrangement with enolate O bridges. In contrast, the metalation of 1 with heteroleptic [(TMEDA)LiZn(TMP)Me 2 ] afforded the dimeric lithium enolate [(TMEDA) 2 Li 2 {OC(dCH 2 )Mes} 2 ] (4) as a crystalline solid, which has been characterized in the solid state by X-ray crystallography, and Me 2 Zn • TMEDA and TMP(H) as coproducts, showing that the dimethylamido zincate behaves as an amide base. The homoleptic zinc enolate [(TMEDA)Zn{OC(dCH 2 )Mes} 2 ] (5) was obtained by reaction of 1 with the zinc amide Zn(TMP) 2 , and its structure was determined by X-ray crystallography. 5 adopts a rarely observed monomeric arrangement where the two enolate groups bind terminally to the zinc. New enolates 2-5 have also been characterized by 1 H, 13 C, and 7 Li NMR spectroscopy in C 6 D 6 solution. DFT studies of the metalation of 1 by Zn(TMP) 2 and Et 2 Zn revealed that the former amide has a much greater kinetic basicity than the latter alkyl reagent.

The zinc-hydrogen exchange reaction has undergone a remarkable transformation in recent times from obscurity to a novel alternative, and in many cases the preferred option, to long-established lithiation methods for generating aromatic organometallic intermediates suitable for subsequent functionalization. Simple zinc reagents (alkyls, amides), as kinetically sluggish bases, are generally useless for such applications. Installing high metalation power within a zinc reagent usually requires a more complex composition, in which reactivity is boosted through cooperative effects between its different components. Modified from their magnesiating "turbo-Grignard" reagents, Knochels three-component systems [(TMP) 2 Zn·2MgCl 2 ·2 LiCl] [1] and [{iPr(tBu)N} 2 Zn· 2MgCl 2 ·2 LiCl][2] are excellent complex zincators for both aromatic and heteroaromatic substrates (TMP is 2,2,6,6-tetramethylpiperidide). Formally a two-component lithium amide-zinc alkyl mixed complex, "[LiZn(TMP)(tBu) 2 ]" introduced by Kondo and Uchiyama, is also a potent chemo-and regioselective zincator for similar substrates.[3]Our own group has contributed the related sodium TMPzincate [(TMEDA)·Na(m-TMP)(m-tBu)Zn(tBu)] [4] (1), which depending on the organic substrate can execute regioselective ortho-, meta-, or dizincation in reactions that have been structurally defined.[5]Here we report the first investigation of the surprising reactivity of 1 towards an aryl halide, namely chlorobenzene. Fully characterized by X-ray crystallography and NMR spectroscopy, the unexpected final product isolated from this reaction is the remarkable zwitterionic benzannulated bicyclic zinc complex [{1-Zn(tBu)} À -{2-N(Me)(CH 2 )CH 2 -CH 2 NMe 2 } + -C 6 H 4 ] (2). Formation of 2 can be rationalized by a novel four-step ortho-zincation, zincate (or sodium chloride) elimination, azazincation-addition, amine a-zincation sequence. An intermediate along this path formed prior to the final step (amine a-zincation), [{1-Zn(tBu) 2 } À -{2-N(Me) 2 CH 2 CH 2 NMe 2 } + -C 6 H 4 ] (Int), has also been isolated from the reaction and crystallographically characterized.In the few previous studies of reactions of aryl halides and TMP-zincates, [6] no experimental evidence has been collected on the chemistry taking place between the limits of the starting materials and the zinc-free quenched products. Therefore, to shed light on the critical metal (and bimetal) activity stage of these reactions, our primary objective was to get inside these limits by isolating and characterizing representative zinc-containing intermediates. This was realized through the synthesis of 2, which were isolated as small colorless block crystals in 20.7 % yield from the equimolar reaction of 1 and chlorobenzene in hexane solution.Since it was reported earlier [6] that reaction of [LiZn-(TMP)(tBu) 2 ] with haloarenes followed by electrophilic trapping produced polyfunctional haloarenes (i.e., with retention of the original halogen substituent), we expected 2 to be an ortho-zincated chlorobenzene derivative. Sur...

SummaryPreviously we reported that direct zincation of N,N-dimethylaniline by the mixed-metal zincate reagent 1 ((TMEDA)Na(TMP)(t-Bu)Zn(t-Bu)) surprisingly led to meta-metallation (zincation) of the aniline, as manifested in the crystalline complex 2 ((TMEDA)Na(TMP)(m-C6H4-NMe2)Zn(t-Bu)), and that iodination of these isolated crystals produced the meta-isomer N,N-dimethyl-3-iodoaniline quantitatively. Completing the study here we find that treating the reaction solution with iodine produces a 72% conversion and results in a mixture of regioisomers of N,N-dimethyliodoaniline, with the meta-isomer still the major product (ortho:meta:para ratio, 6:73:21), as determined by NMR. In contrast to this bimetallic method, sodiation of N,N-dimethylaniline with n-BuNa produced the dimeric, ortho-sodiated complex 3 (((TMEDA)Na(o-C6H4-NMe2))2), as characterised by X-ray crystallography and NMR. No regioisomers were observed in the reaction solution. Introducing t-Bu2Zn to this reaction solution afforded a cocrystalline product in the solid-state, composed of the bis-anilide 4 ((TMEDA)Na(o-C6H4-NMe2)2Zn(t-Bu)) and the Me2N–C cleavage product 5 ({(TMEDA)2Na}+{(t-Bu2Zn)2(µ-NMe2)}−), which was characterised by X-ray crystallography. NMR studies of the reaction mixture that produces 4 and 5 revealed one additional species, but the mixture as a whole contained only ortho-species and a trace amount of para-species as established by iodine quenching. In an indirect variation of the bimetallic reaction, TMP(H) was added at room temperature to the reaction mixture that afforded 4 and 5. This gave the crystalline product 6 ((TMEDA)Na(TMP)(o-C6H4-NMe2)Zn(t-Bu)), the ortho-isomer of the meta-complex 2, as determined from X-ray crystallographic and NMR data. Monitoring the regioselectivity of the reaction by iodination revealed a 16.6:1.6:1.0 ortho:meta:para ratio. Interestingly, when the TMP(H) containing solution was heated under reflux for 18 hours more meta-isomer was produced (corresponding ratio 3.7:4.2:1.0). It is likely that this change has its origin in a retro reaction that produces the original base 1 as an intermediate. Theoretical calculations at the DFT level using the B3LYP method and the 6-311G** basis set were used to probe the energetics of both monometallic and bimetallic systems. In accord with the experimental results, it was found that ortho-metallation was favoured by sodiation; whereas meta- (closely followed by para-) metallation was favoured by direct sodium-mediated zincation.

More usually thought of as a base, the sodium zincate [(TMEDA)·Na(μ-TMP)(μ-(t)Bu)Zn((t)Bu)] 1 can undergo single electron transfer with TEMPO to give [(TMEDA)·Na(μ-TMP)(μ-TEMPO(-))Zn((t)Bu)] 2 and [(TMEDA)·Na(μ-TEMPO(-))(2)Zn((t)Bu)] 3; and with chalcone [PhCOCH=CHPh] gives [{(TMEDA)·Na(μ-TMP)Zn((t)Bu)}(2)(μ-OCPhCH=CHPhCHPhCH=CPh-μ-O)] which contains two chalcone units C-C coupled though their benzylic C atoms.

As a further contribution to alkali-metal-mediated metalation, a method for converting C−H bonds directly to C−Zn bonds without the need for an additional salt metathesis step, reactions of the sodium TMP-zincate [(TMEDA)Na(μ-TMP)(μ- t Bu)Zn( t Bu)] (1) with three different electron-rich aromatic substrates, have been investigated. Under ambient-temperature conditions, N,N-diethylbenzamide, N,N-diethyl-3-methoxybenzamide, and N,N-diethyl phenyl O-carbamate were zincated ortho to the substituent group (in between both substituents in the second case) in the crystalline products [(TMEDA)Na(μ-TMP){μ-2-[1-C(O)NEt2]C6H4}Zn( t Bu)] (3), [(TMEDA)Na(μ-TMP){μ-2-(1-C(O)NEt2)(3-OMe)C6H3}Zn( t Bu)] (4), and [(TMEDA)Na(μ-TMP){μ-2-(1-OC(O)NEt2)C6H4}Zn( t Bu)] (6). X-ray crystallography established that, in each case, the deprotonated aromatic fragment is captured by the residue of the bimetallic base, giving rise to seven-membered (NaNZnCCCO) ring structures for 3 and 4 and an eight-membered (NaNZnCCOCO) ring structure for 6. The new zincated aromatics were also characterized by solution-state 1H and 13C NMR spectroscopy. Reactivity studies of 3, 4, and 6 were also performed with iodine. In each case, three molar equivalents of iodine in THF solution gave the ortho-iodo products N,N-diethyl-2-iodobenzamide (7), N,N-diethyl-2-iodo-3-methoxybenzamide (8), and N,N-diethyl-2-iodophenyl-O-carbamate (9) in quantitative, 71% and 75% yields as determined by NMR analysis, respectively.

Alkali metal zincates are among an increasing number of mixedmetal organoreagents that are attracting widespread attention because of their ability to exhibit synergic reactivity. Such special behaviour can be defined as reactions arising from the cooperative effects of the two distinct metals, the hard alkali metal and soft zinc, within the multicomponent zincate that cannot be reproduced by either single alkali metal or zinc component on its own. This synergism has been particularly prominent in metalation (metal-hydrogen exchange) applications. [1] Alkylzinc (R 2 Zn) or amidozinc [RZn(NR' 2 )/Zn(NR' 2 ) 2 ] reagents are generally notoriously poor kinetic bases incapable of directly metalating (zincating) aromatic substrates to any synthetically useful extent, but combined with an alkali metal compound [2] or related component [3] they can transform into highly reactive "zincators". Kondo and Uchiyama's "LiZn(TMP)tBu 2 " (TMP = 2,2,6,6-tetramethylpiperidide), Mongin's "LiZn(TMP) 3 ", [2a-j] and our own [(TMEDA)Na(TMP)(tBu)Zn(tBu)] [2p-t] (TMEDA = N,N,N',N'-tetramethylethylenediamine) belong in this category and although fundamental differences exist between these powerful amide-based zincators, their Zn H exchange reactions can be grouped together as alkali-metal-mediated zincations "AMMZn's". [1] Exemplified by TMP, the amide components involved in AMMZn chemistry are invariably monoanionic ligands derived from secondary amines containing one acidic N H bond. Secondary diamines containing two acidic N H bonds and therefore having potentially at least two metalation sites would offer an interesting contrast but until this work they have not been investigated in this context. Reported herein, our first venture in introducing diamines to this chemistry by reacting a mixture of nBuLi and tBu 2 Zn (or Me 2 Zn) with N, N'-diisopropylethylenediamine iPrN(H)CH 2 CH 2 N(H)iPr has uncovered a surprising new synergic chemistry which leads ultimately to the transformation of the saturated ethylenediamine to a dianionic unsaturated diazaethene. To gauge whether a bimetallic mixture exhibits synergic activity, its separated monometallic components should be reacted with the substrate in control reactions. As expected, we found that tBu 2 Zn is too weak a base to deprotonate N, N'-diisopropylethylenediamine, but instead forms the simple Lewis acid-Lewis base adduct [tBu 2 Zn {iPrN(H)CH 2 CH 2 N(H)iPr}] 1 (Scheme 1 : see Supporting Information for full details). While we easily generated a crystalline product from a 1:1 reaction of the more powerful base nBuLi and the diamine, and characterised it spectroscopically as [{LiN(iPr)CH 2 CH 2 N(H)iPr}], we deemed it unnecessary to determine its crystal structure as Gardiner and Raston had previously reported [4] that the same reaction with the t-butyl homolog tBuN(H)CH 2 CH 2 N(H)tBu produced lithiation of one N H unit in [cis-{Li[μ-N(tBu)CH 2 CH 2 N(H)tBu]} 2 ] 2, which is dimeric with a 5,4,5-fused ring system having a (LiN) 2 core (Scheme 1). From the precedents of th...

Metal detector: a bowl-shaped nanomolecule (see picture; S yellow, C gray, Zn blue) containing an unprecedented 16-atom [ZnC(3)](4) "anti-crown" ring has been unearthed by isolating a dizincated 2-substituted thiophene intermediate that would normally be hidden in tandem functionalization methodology.