Effectiveness of the Oxygenation over Classical Protonolysis Reactions: A Case of Alkylzinc Complexes Incorporating an Aminoalcoholate Ligand.

Alkylzinc aminoalcoholates have emerged as powerful catalysts in organic synthesis and polymerization processes. Despite extensive research, difficulties in the rational design of these catalytic systems and in-depth understanding of their modes of action have hitherto been encountered. Most of the major obstacles stem largely from the relatively limited knowledge of the structure-activity relationship of zinc catalysts. In fact, the key active species are often generated in situ via the protonolysis of the alkylzinc precursors, which precludes their isolation and detailed characterization. Herein, the effectiveness of the oxygenation over the classical protonolysis in the synthesis of zinc alkylperoxides stabilized by an aminoalcoholate ligand is demonstrated. The controlled oxygenation of a tert-butylzinc complex incorporating a pridinolum (prinol) ligand leads to well-defined a dinuclear adduct of a (prinol)ZnOOtBu moiety with the parent tBuZn(prinol) complex and a novel dimer [tBuOOZn(prinol)]2 with terminal alkylperoxide groups. The observed reaction outcomes strongly depend on the reaction conditions. Although sparse examples of heteroleptic adducts of the [RZn(L)]x [ROOZn(L)]y -type are known, the herein reported homoleptic [ROOZn(L)]x aggregate is unprecedented. Strikingly, comparative studies involving reactions between tBuZn(prinol) and tert-butylhydroperoxide or ethanol revealed that the respective seemingly simple zinc alkylperoxides, or zinc alkoxides, respectively, are not accessible via the classical alcoholysis. We believe that these game-changing results concerning multifaceted chemistry of organozinc aminoalcoholates should pave the way for more rational development of various Zn-based catalytic systems.

Unravelling Structural Mysteries of Simple Organozinc Alkoxides.

Simple RZnOR' alkoxides are among the first known organozinc compounds, and widespread interest in their multifaced chemistry has been driven by their fundamental significance and potential applications including various catalytic reactions. Nevertheless, their chemistry in solution and in the solid state remains both relatively poorly understood and a subject of constant debate. Herein, the synthesis and structural characterization of long-sought structural forms, a roof-like trimer [(tBuZn)3 (µ-OC(H)Ph2 )2 (µ3 -OC(H)Ph2 )] and a ladder-type tetramer [(PhZn)4 (µ-OC(H)Ph2 )2 (µ3 -OC(H)Ph2 )2 ], incorporating diphenylmethanolate as a model alkoxide ligand, are reported. Both novel aggregates are robust in the solid state and resistant towards mechanical force. By using 1 H NMR and diffusion-order spectroscopy, it is demonstrated that new RZnOR' alkoxides are kinetically labile in solution and readily undergo ligand scrambling, such as in the case of Schlenk equilibrium. The elucidated key structural issues, which have remained undiscovered for decades, significantly advance the chemistry of RZnOR' alkoxides and should support the rational design of zinc alkoxide-based applications.

Towards deeper understanding of multifaceted chemistry of magnesium alkylperoxides.

Despite considerable progress in the multifaceted chemistry of non-redox-metal alkylperoxides, the knowledge about magnesium alkylperoxides is in its infancy and only started to gain momentum. Harnessing the well-defined dimeric magnesium tert-butylperoxide [(f5BDI)Mg(µ-η2:η1-OOtBu)]2 incorporating a fluorinated ß-diketiminate ligand, herein, we demonstrate its transformation at ambient temperature to a spiro-type, tetranuclear magnesium alkylperoxide [(f5BDI)2Mg4(µ-OOtBu)6]. The latter compound was characterized by single-crystal X-ray diffraction and its molecular structure can formally be considered as a homoleptic magnesium tert-butylperoxide [Mg(µ-OOtBu)2]2 terminated by two monomeric magnesium tert-butylperoxides. The formation of the tetranuclear magnesium alkylperoxide not only contradicts the notion of the high instability of magnesium alkylperoxides, but also highlights that there is much to be clarified with respect to the solution behaviour of these species. Finally, we probed the reactivity of the dimeric alkylperoxide in model oxygen transfer reactions like the commonly invoked metathesis reaction with the parent alkylmagnesium and the catalytic epoxidation of trans-chalcone with tert-butylhydroperoxide as an oxidant. The results showed that the investigated system is among the most active known catalysts for the epoxidation of enones.

From ethylzinc guanidinate to [Zn10O4]-supertetrahedron.

The controlled hydrolysis of an ethylzinc guanidinate complex affording an alkylzinc cluster containing a [Zn10O4]12+ supertetrahedron core stabilized by the guanidinate ligands is described. Accompanying investigations on the reactivity of this unprecedented cluster toward alcohols resulted in the formation of a mononuclear zinc alkoxide supported by the guanidinate ligands.

Unprecedented Variety of Outcomes in the Oxygenation of Dinuclear Alkylzinc Derivatives of an N,N-Coupled Bis(ß-diketimine).

Reactions between O2 and organometallics with non-redox-active metal centers have received continuous interest for over 150 years, although significant uncertainties concerning the character and details of the actual mechanism of these reactions persist. Harnessing dinuclear three-coordinate alkylzinc derivatives of an N,N-coupled bis(ß-diketimine) proligand (LH2 ) as a model system, we demonstrate for the first time that a slight modification of the reaction conditions might have a dramatic influence on the oxygenation reaction outcomes, leading to an unprecedented variety of products originating from a single reaction system, that is, partially and fully oxygenated zinc alkoxides, zinc alkylperoxides, and zinc hydroxide compounds. Our studies indicate that accessibility of the three-coordinate zinc center by the O2 molecule, coupled with the lower reactivity of Zn-Me vs. Zn-Et units towards dioxygen, are key factors in the oxygenation process, providing a novel tetranuclear methyl(methoxy)zinc {[L][ZnMe][Zn(µ-OMe]}2 and zinc ethoxide {[L][Zn(µ-OEt)]2 }2 . Remarkably, oxygenation of three-coordinate alkylzinc [L][ZnR]2 complexes at ambient temperature afforded a unique hydroxide {[L][Zn(µ-OH)]2 }2 . Oxygenation of the [L][ZnEt]2 complex in the presence of 4-methylpyridine (py-Me) at low temperature led to the isolation of a dinuclear zinc ethylperoxide [L][Zn(OOEt)(py-Me)]2 , which nicely substantiates the intermediacy of an unstable zinc alkylperoxide in the formation of the subsequent zinc alkoxide and hydroxide compounds. Finally, our investigations provide compelling evidence that a non-redox-active metal center plays a crucial role in the oxygenation process through assisting in single-electron transfer from an M-C bond to an O2 molecule. Although the oxygenation of zinc alkyls occurs by radical pathways, the reported results stand in clear contradiction to the widely accepted free-radical chain mechanism.

Oxygenation Chemistry of Magnesium Alkyls Incorporating ß-Diketiminate Ligands Revisited.

Despite the fact that extensive research has been carried out, the oxygenation of alkyl magnesium species still remains a highly unexplored research area and significant uncertainties concerning the mechanism of these reactions and the composition of the resulting products persist. This case study compares the viability of the controlled oxygenation of alkylmagnesium complexes supported by ß-diketiminates. The structural tracking of the reactivity of (N,N)MgR-type complexes towards O2 at low temperature showed that their oxygenation led exclusively to the formation of magnesium alkylperoxides (N,N)MgOOR. The results also highlight significant differences in the stability of the resulting alkylperoxides in solution and demonstrate that [(BDI)Mg(µ-η2 :η1 -OOBn)]2 (in which BDI=[(ArNCMe)2 CH]- and Ar=C6 H3 iPr2 -2,6) can be easily transformed to the corresponding magnesium alkoxide [(BDI)MgOBn]2 at ambient temperature, whilst [(F3 BDI)Mg(µ-OOtBu)]2 (in which F3 BDI=[(ArNCMe)2 CH]- and Ar=C6 H2 F3 -2,4,6) is stable under similar conditions. The observed selective oxygenation of (N,N)MgR-type complexes to the corresponding (N,N)MgOOR alkylperoxides strongly contradicts the widely accepted radical-chain mechanism for the oxygenation of the main-group-metal alkyls. Furthermore, either the observed transformation of the alkylperoxide [(BDI)MgOOBn]2 to the alkoxide [(BDI)MgOBn]2 as well as the formation of an intractable mixture of products in the control reaction between the alkylperoxide [(F3 BDI)MgOOtBu]2 and the parent alkylmagnesium [(F3 BDI)MgtBu] complex are not in line with the common wisdom that magnesium alkoxide complexes' formation results from the metathesis reaction between MgOOR and Mg-R species. In addition, a high catalytic activity of well-defined magnesium alkylperoxides, in combination with tert-butyl hydroperoxide (TBHP) as an oxygen source, in the epoxidation of trans-chalcone is presented.

Development of zinc alkyl/air systems as radical initiators for organic reactions.

This paper reports a series of comparative experiments on the activity of carbon- and oxygen-centred radical species in a model reaction of the radical addition of THF to imines mediated by a series of zinc alkyl/air reaction systems. The study strongly contradicts the notion that generally R˙ radicals are the initiating species in organic reactions mediated by R n M/air systems, and simultaneously demonstrates that oxygen-centred radical species are the key intermediates responsible for the initiation process. In addition, a new efficient RZn(L)/air initiating system for radical organic reactions exampled by a model reaction of radical addition of THF to imines is developed. Moreover, the isolation and structural characterization of the first zinc alkylperoxide supported by a carboxylate ligand, [Zn4(µ3-OOtBu)3(µ4-O)(O2CEt)3]2, as well as the novel octanuclear zinc oxo(alkoxide) aggregate with entrapped O-THF species, [Zn4(µ4-O)(µ3-2-O-THF)(O2CEt)5]2, provide clear mechanistic signatures for the mode of function of the RZn(O2CR')/air system.

Probing the role of π interactions in the reactivity of oxygen species: a case of ethylzinc aryloxides with different dispositions of aromatic rings toward the metal center.

Ethylzinc derivatives of ortho-hydroxybiphenyl and 2,6-diphenylphenol that bear different nuclearity and dispositions of aromatic rings toward the metal center were synthesized and structurally characterized in the solid state and solution. This family of well-defined compounds was examined as a model system for the activation of dioxygen mediated by using complexes that feature lack of a redox-active metal center. Experimental and theoretical studies indicate an essential role in the oxygenation process of intramolecular interactions that involve aromatic subunits. Additionally, novel results for the oxygenation chemistry of alkylzinc compounds, including the isolation and structural characterization of the unique octanuclear aryloxide (hydroxide) compound Zn8 (OAr)8 (OH)6 (O2 ) with an encapsulated peroxide species, are presented.

Probing secondary coordination sphere influence on the oxygenation of zinc alkyls: formation of a unique zinc peroxide species.

Reactions of ethylzinc derivatives of o-hydroxybiphenyl with O2 were investigated. The study revealed an essential role in the oxygenation process of intra-molecular interactions involving aromatic rings and provided a unique aryloxide (hydroxide) Zn8(OAr)8(OH)6(O2) cluster with an encapsulated peroxide species.

New insights into cinchonine-aluminium complexes and their application as chiral building blocks: unprecedented ligand-exchange processes in the presence of ZnR2 compounds.

Previous studies have demonstrated that [(CN)(2)AlCl] and [R(2)Al(µ-CN)](2) (CN=deprotonated cinchonine) complexes can effectively act as chiral, semirigid, N,N-ditopic metalloligands for Zn-containing nodes, and provide viable means for constructing new, homochiral, heterometallic, coordination polymers of zigzag and helical topologies. These findings have prompted further investigations on the organometallic analogues of the formula [(CN)(2)AlR], anticipating their utility as N,N-metalloligands for ZnR(2) units. Surprisingly, reactions of [(CN)(2)AlMe]-type metalloligands with ZnR(2) compounds (R=Me or Et) revealed unprecedented ligand-exchange processes, including zinc-to-aluminium and aluminium-to-zinc transmetalations of alkyl groups. The molecular and crystal structure of the resulting compounds was determined by X-ray diffraction analysis. From the reaction of [(CN)(2)AlMe] with ZnMe(2) a new pseudopolymorphic form of a noncovalent porous material based on [Me(2)Al(µ-CN)](2) molecules was isolated. Strikingly, the analogous reaction involving ZnEt(2) led to the generation of a new chiral 4N-tetratopic heterometalloligand [(CN)EtAl(µ-CN)(2)ZnEt]. The latter unit was successfully connected by alkyl-exchanged ZnMe(2) nodes to give an original homochiral heterometallic {[(CN)EtAl(µ-CN)(2)ZnEt]ZnMe(2)}(n) coordination polymer adopting a snake 1D motif. The outcome of the revealed reactions indicates the complicated multistep reaction route that involves redistribution of cinchonine and alkyl ligands among the Al and Zn centers, and a general reaction scheme is proposed. The results are in strong contrast with the previously studied inorganic-organic [(CN)(2)AlCl/ZnCl(2)] system, which exclusively affords a helical coordination polymer based on ZnCl(2) nodes and (CN)(2)AlCl metalloligands and lacks the exchange of CN ligands.

Synthesis, structure and unique reactivity of the ethylzinc derivative of a bicyclic guanidine.

An equimolar reaction between ZnEt(2) and 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) results in the formation of EtZn(hpp) (1) which crystallizes as a trinuclear agglomerate with the guanidinate ligands spanning 4-coordinate Zn centers. Exposure of a pre-formed THF solution of 1 to undried air leads to a ZnO-incorporating derivative 1(4)·ZnO, while an analogous experiment with CH(2)Cl(2) as solvent leads to a novel tetranuclear mixed aggregate formulated as [EtOZn(hpp)](2)[ClZn(hpp)](2) (2). The composition of 2 indicates that its formation proceeds via a complex multi-step reaction route that involves not only the oxygenation of ZnEt moieties, but also the activation of CH(2)Cl(2), causing the transfer of a chloride anion to the Zn center. Compounds were characterized by (1)H NMR spectroscopy and single-crystal X-ray diffraction analysis.