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)((t)Bu)Zn((t)Bu)] 1 and trifluoromethyl benzene C(6)H(5)CF(3) 2. 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)(C(6)H(4)-CF(3))Zn((t)Bu)], 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 degrees C gave exclusively 4, which was crystallographically characterized as [{(TMEDA)(2)Na}(+){Zn(C(6)H(4)-CF(3))((t)Bu)(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.

A regioselectively 1,1',3,3'-tetrazincated ferrocene complex displaying core and peripheral reactivity.

Regioselective 1,1',3,3'-tetrazincation [C-H to C-Zn(tBu)] of ferrocene has been achieved by reaction of a fourfold excess of di-t-butylzinc (tBu2Zn) with sodium 2,2,6,6-tetramethylpiperidide (NaTMP) in hexane solution manifested in the trimetallic iron-sodium-zinc complex [Na4(TMP)4Zn4(tBu)4{(C5H3)2Fe}], 1. X-ray crystallographic studies supported by DFT modelling reveal the structure to be an open inverse crown in which two [Na(TMP)Zn(tBu)Na(TMP)Zn(tBu)]2+ cationic units surround a {(C5H3)2Fe}4- tetraanion. Detailed C6D6 NMR studies have assigned the plethora of 1H and 13C chemical shifts of this complex. It exists in a major form in which capping and bridging TMP groups interchange, as well as a minor form that appears to be an intermediate in this complicated exchange phenomenon. Investigation of 1 has uncovered two distinct reactivities. Two of its peripheral t-butyl carbanions formally deprotonate toluene at the lateral methyl group to generate benzyl ligands that replace these carbanions in [Na4(TMP)4Zn4(tBu)2(CH2Ph)2{(C5H3)2Fe}], 2, which retains its tetrazincated ferrocenyl core. Benzyl-Na π-arene interactions are a notable feature of 2. In contrast, reaction with pyridine affords the crystalline product {[Na·4py][Zn(py*)2(tBu)·py]}∞, 3, where py is neutral pyridine (C5H5N) and py* is the anion (4-C5H4N), a rare example of pyridine deprotonated/metallated at the 4-position. This ferrocene-free complex appears to be a product of core reactivity in that the core-positioned ferrocenyl anions of 1, in company with TMP anions, have formally deprotonated the heterocycle.

Synthetic and structural insights into the zincation of toluene: direct synergic ring metallation versus indirect nonsynergic lateral metallation.

Toluene has been directly zincated by reaction with the heteroleptic sodium zincate [(tmeda)Na(tBu)(tmp)Zn(tBu)] (1) to afford a statistical mixture of the meta and para regioisomers of [(tmeda)Na(C(6)H(4)Me)(tmp)Zn(tBu)] (2) (TMEDA = N,N,N',N'-tetramethylethylenediamine, TMP = 2,2,6,6-tetramethylpiperidide). Interestingly, the methyl group of toluene has been left untouched by the mixed-metal base. The crystal structures of both regioisomers have been elucidated by using X-ray crystallography, which revealed a common motif, in which zinc lies in the same plane as the tolyl ring and binds strongly to the carbon that has experienced the deprotonation, leading to a covalent sigma bond; whereas sodium adopts a perpendicular disposition, pi-engaging with the aromatic ring. DFT studies of the reaction of 1 with toluene showed that of the four possible regioisomers, the experimentally observed meta and para ones are the most thermodynamically preferred products due to a large extent to the combined (synergic) effect of the zinc and sodium and the different way in which each metal binds to the tolyl anion. In contrast, the more straightforward benzyl-based zincate [(tmeda)(2)Na(CH(2)Ph)Zn(tBu)(2)] (3) is obtained through an indirect route, in which toluene is first laterally metallated by monometallic butylsodium to afford benzylsodium, which then forms a mixed-metal compound by co-complexation with tBu(2)Zn. Zincate 3 has been characterised by X-ray crystallography in the solid state and by NMR spectroscopy in deuterated THF solution.

Transamination chemistry of sodium TMP-zincate: synthesis and crystal structure of a chiral amidozincate.

In a new type of reactivity for sodium TMP-zincate [(TMEDA)NaZn((t)Bu)(2)(TMP)] (1), transamination reactions with the amines diisopropylamine, DA(H), hexamethyldisilazane, HMDS(H) and chiral (R)-N-benzyl-alpha-methylbenzylamine have produced new sodium amido-di-tert-butyl zincates (all structurally characterised) with concomitant loss of TMP(H).

Dizincation and dimagnesiation of benzene using alkali-metal-mediated metallation.

Benzene can be easily 1,4-dideprotonated stoichiometrically on reaction with two equivalents of a synergic mixture of tBu2Zn, NaTMP and TMEDA to give a unique 1,4-dizincated benzene product which has been characterised by X-ray crystallography and NMR spectroscopy as well as modelled theoretically by DFT computational studies; a related synergic dimagnesiation of benzene is also reported.

Organometallic polymers assembled from cation-pi interactions: use of ferrocene as a ditopic linker within the homologous series [{(Me3Si)2NM}2.(Cp2Fe)]infinity (M=Na, K, Rb, Cs; Cp=cyclopentadienyl).

Addition of ferrocene to solutions of alkali metal hexamethyldisilazides M(HMDS) in arenes (in which M=Na, K, Rb, Cs) allows the subsequent crystallization of the homologous series of compounds [{(Me(3)Si)(2)NM}(2) (Cp(2)Fe)](infinity) (1-4). Similar reactions using LiHMDS led to the recrystallization of the starting materials. The crystal structures of 1-4 reveal the formation of one-dimensional chains composed of dimeric [{M(HMDS)}(2)] aggregates, which are bridged through neutral ferrocene molecules by eta(5)-cation-pi interactions. In addition, compounds 3 and 4 also contain interchain agostic M--C interactions, producing two-dimensional 4(4)-nets. Whereas 1 and 2 were prepared from toluene, the syntheses of 3 and 4 required the use of tert-butylbenzene as the reaction media. The attempted crystallization of 3 and 4 from toluene resulted in formation of the mixed toluene/ferrocene solvated complexes [{(Me(3)Si)(2)NM)(2)}(2) (Cp(2)Fe)(x)(Tol)(y)](infinity) (in which M=Rb, x=0.6, y=0.8, 5; M=Cs, x=0.5, y=1, 6). The extended solid-state structures of 5 and 6 are closely related to the 4(4)-sheets 3 and 4, but are now assembled from a combination of cation-pi, agostic, and pi-pi interactions. The charge-separated complex [K{(C(6)H(6))(2)Cr}(1.5)(Mes)][Mg(HMDS)(3)] (15) was also structurally characterized and found to adopt an anionic two-dimensional 6(3)-network through doubly eta(3)-coordinated bis(benzene)chromium molecules. DFT calculations at the B3 LYP/6-31G* level of theory indicate that the binding energies of both ferrocene and toluene to the M(HMDS) dimers increases in the sequence Li

Alkali-metal-mediated zincation of anisole: synthesis and structures of three instructive ortho-zincated complexes.

The new concept of alkali-metal-mediated zincation (AMMZ), formally a zinc-hydrogen exchange reaction but one that requires the participation of an alkali metal, is applied here to the alkyl aryl ether anisole, an important molecule for studying directed ortho-metalation (DoM) chemistry. Treating one molar equivalent of anisole with the lithium dialkyl-TMP zincate reagent [THF.Li(mu-TMP)(mu-tBu)Zn(tBu)] (1) in hexane solution affords the mono-ortho-zincated complex [THF.Li(mu-TMP)(mu-o-C6H4OMe)Zn(tBu)] (2), which establishes that 1 functions as an alkyl base although previously it was regarded as an amido (TMP) base in other DoM applications. Treating two molar equivalents of anisole with 1, and increasing the reaction time, affords the bis-ortho-zincated complex [THF.Li(mu-TMP)(mu-o-C6H4OMe)Zn(o-C6H4OMe)] (3), which establishes that 1 can also function as a dual alkyl base. Omitting THF and rerunning the reaction with one or two molar equivalents of anisole affords [Ph(Me)O.Li(mu-TMP)(mu-o-C6H4OMe)Zn(tBu)] (4), which remarkably contains a combination of neutral and ortho-deprotonated anisole ligands. On isolating crystalline 4 from solution and adding THF, it converts to 2 and then to 3 on further stirring of the solution, as determined by NMR studies. This fact, along with other observations, would suggest that a complex-induced proximity effect does not need to be invoked to explain the observed zincation of anisole. The crystal structures of 2-4 are presented, as are their 1H, 13C, and 7Li NMR spectra recorded in C6D6 solution.

Trapping, stabilization, and characterization of an enolate anion of a 1,6-adduct of benzophenone chelated by a sodium alkylamidozincate cation.

There has been a recent upsurge of activity in the study of alkali metal zincate reagents due to their often special reactivity/selectivity in, for example, deprotonative metalation and nucleophilic addition reactions. Heteroleptic dialkylamidozincates, [M+Zn(R)2(NR'2)-], usually transfer selectivity of the amide ligand to electrophiles. Here, in contrast, it is reported that the sodium zincate [TMEDA.Na(mu-tBu)(mu-TMP)Zn(tBu)] reacts as an alkylating agent toward the diaryl ketone benzophenone (Ph2C=O), selectively adding one of its tBu ligands to the para-C atom of one of the Ph rings. The reaction can be carried out at room temperature, which is a decided advantage over lithium reagents as these are generally utilized at subambient temperatures. The stabilizing effect of the bimetallic (Na, Zn) cationic residue of the starting zincate reagent in coordinating to the dearomatized enolate anion of the 1,6-addition adduct allows the adduct to be isolated in a pure crystalline form. An X-ray crystallographic study of the adduct reveals a molecular structure based on a near-planar, four-element (NaOZnN) ring with a TMP-N and an enolato-O bridge. The Na and Zn atoms also carry terminal TMEDA (N,N'-attached) and tBu (C-attached) ligands, respectively. Also included are 1H/13C NMR spectroscopic data for the adduct when dissolved in cyclohexane-d12 solution.

Sodium dialkyl-amidozincates: alkyl or amido bases? An experimental and theoretical case study.

Alkali metal zincate reagents are attracting considerable attention at present in respect to their often special reactivity/selectivity in hydrogen-metal and halogen-metal interconversion reactions. Heteroleptic diorgano-amidozincates, typified by lithium di-tert-butyltetramethylpiperidinozincate, have proved to be especially useful reagents in such applications. In this paper the related sodium TMP-zincate, prepared as its TMEDA (N,N,N',N'-tetramethylethylenediamine) adduct, [TMEDA.Na(mu-tBu)(mu-TMP)Zn(tBu)], 1, is introduced. This new zincate was synthesized from a 1:1:1 mixture of tBu2Zn, NaTMP, and TMEDA in hexane solution, as a colorless crystalline solid in an isolated yield of 58%. It has been characterized in solution by 1H and 13C NMR spectroscopic studies. An X-ray crystallographic study reveals that 1 adopts a five-membered (NaNZnCC) ring system featuring a TMP bridge and an unusual, asymmetrical tBu bridge involving a Na...Me agostic contact. Probing the basicity of 1, reaction with benzene affords the new hetero(tri)leptic zincate [TMEDA.Na(mu-Ph)(mu-TMP)Zn(tBu)], 2, which has also been crystallographically characterized. Thus, in this hydrogen-metal exchange reaction 1 functions as an alkyl base, with the elimination of butane, as opposed to an amido base. Also reported are DFT calculations using B3LYP functionals and the 6-311G** basis set on model zincate systems, which intimate that the preference of 1 for tBu ligand transfer over TMP ligand transfer in the reaction toward benzene is due to favorable thermodynamic factors.