Isolation and structural elucidation of a key aluminoaromatic intermediate and evidence for dismutation phenomena in TMP-alumination chemistry

Lithium TMP-aluminate "(i)Bu(3)Al(TMP)Li" undergoes dismutation in THF solution to precipitate the tetraalkylaluminate [{Li.(THF)(4)}(+){Al((i)Bu)(4)}(-)], but reacts kinetically as a TMP base towards N,N-diisopropylbenzamide to afford the crystalline ortho-aluminated species [(THF)(3).Li{O([=C)N((i)Pr)(2)(C(6)H(4))}Al((i)Bu)(3)] and TMPH.     

Alkali‐Metal‐Mediated Magnesiations of an N‐Heterocyclic Carbene: Normal,Abnormal, and “Paranormal” Reactivity in a Single Tritopic Molecule

Herein the sodium alkylmagnesium amide [Na₄Mg₂(TMP)₆(nBu)₂] (TMP=2,2,6,6‐tetramethylpiperidide), a template base as its deprotonating action is dictated primarily by its 12 atom ring structure, is studied with the common N‐heterocyclic carbene (NHC) IPr [1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]. Remarkably, magnesiation of IPr occurs at the para‐position of an aryl substituent, sodiation occurs at the abnormal C4 position, and a dative bond occurs between normal C2 and sodium, all within a 20 atom ring structure accommodating two IPr²⁻. Studies with different K/Mg and Na/Mg bimetallic bases led to two other magnesiated NHC structures containing two or three IPr⁻ monoanions bound to Mg through abnormal C4 sites. Synergistic in that magnesiation can only work through alkali‐metal mediation, these reactions add magnesium to the small cartel of metals capable of directly metalating a NHC.     

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.     

Structurally Defined Potassium‐Mediated Zincation of Pyridine and 4‐R‐Substituted Pyridines (R=Et,iPr, tBu,Ph, and Me2N) by Using Dialkyl–TMP–Zincate Bases

Two potassium–dialkyl–TMP–zincate bases [(pmdeta)K(μ‐Et)(μ‐tmp)Zn(Et)] ( 1 ) (PMDETA=N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine, TMP=2,2,6,6‐tetramethylpiperidide), and [(pmdeta)K(μ‐nBu)(μ‐tmp)Zn(nBu)] ( 2 ), have been synthesized by a simple co‐complexation procedure. Treatment of 1 with a series of substituted 4‐R‐pyridines (R=Me₂N, H, Et, iPr, tBu, and Ph) gave 2‐zincated products of the general formula [{2‐Zn(Et)₂‐μ‐4‐R‐C₅H₃N}₂ ? 2{K(pmdeta)}] ( 3 – 8 , respectively) in isolated crystalline yields of 53, 16, 7, 23, 67, and 51 %, respectively; the treatment of 2 with 4‐tBu‐pyridine gave [{2‐Zn(nBu)₂‐μ‐4‐tBu‐C₅H₃N}₂ ? 2{K(pmdeta)}] ( 9 ) in an isolated crystalline yield of 58 %. Single‐crystal X‐ray crystallographic and NMR spectroscopic characterization of 3 – 9 revealed a novel structural motif consisting of a dianionic dihydroanthracene‐like tricyclic ring system with a central diazadicarbadizinca (ZnCN)₂ ring, face‐capped on either side by PMDETA‐wrapped K⁺ cations. All the new metalated pyridine complexes share this dimeric arrangement. As determined by NMR spectroscopic investigations of the reaction filtrates, those solutions producing 3 , 7 , 8 , and 9 appear to be essentially clean reactions, in contrast to those producing 4 , 5 , and 6 , which also contain laterally zincated coproducts. In all of these metalation reactions, the potassium–zincate base acts as an amido transfer agent with a subsequent ligand‐exchange mechanism (amido replacing alkyl) inhibited by the coordinative saturation, and thus, low Lewis acidity of the 4‐coordinate Zn centers in these dimeric molecules. Studies on analogous trialkyl–zincate reagents in the absence and presence of stoichiometric or substoichiometric amounts of TMP(H) established the importance of Zn? N bonds for efficient zincation.     

A representation of chemical transformations by labeled graphs

A transitional labeling of a graphG is an assignment of one of the elements of the set (1, -1, 0) to each vertex and edge ofG so that each edge labeled 0 is incident only with vertices labeled 0, and no edge labeled 1 (respectively, -1) is incident with a vertex labeled -1 (respectively, 1). Chemical transformations can be represented by graphs possessing a transitional labeling. The positive (negative) graph of a transitional labelingt of a graphG is the subgraph ofG consisting of the nonnegative (nonpositive) elements ofG. The linking graph oft is the subgraph consisting of the zero elements ofG. A maximum common subgraph of two given graphsG ₁ andG ₂ is a graphF isomorphic to a common subgraph ofG ₁ andG ₂ such that the sum of the number of vertices and number of edges ofF is maximum. A transitional labelingt of a graphG is a transform if there exists an extensiont oft to a supergraphG of G such that the linking graph oft is a maximum common subgraph of the positive and negative graphs oft. Transforms are used to model chemical reaction pathways. Transforms and related concepts are studied in this paper. A characterization of transforms is also given.Dedicated to Professor Frank Harary on the occasion of his 70th birthday Research supported in part by Office of Naval Research Contract N00014-91-J-1060.     

Exploiting Deep Eutectic Solvents and Organolithium Reagent Partnerships: Chemoselective Ultrafast Addition to Imines and Quinolines Under Aerobic Ambient Temperature Conditions

Shattering the long‐held dogma that organolithium chemistry needs to be performed under inert atmospheres in toxic organic solvents, chemoselective addition of organolithium reagents to non‐activated imines and quinolines has been accomplished in green, biorenewable deep eutectic solvents (DESs) at room temperature and in the presence of air, establishing a novel and sustainable access to amines. Improving on existing methods, this approach proceeds in the absence of additives; occurs without competitive enolization, reduction or coupling processes; and reactions were completed in seconds. Comparing RLi reactivities in DESs with those observed in pure glycerol or THF suggests a kinetic anionic activation of the alkylating reagents occurs, favoring nucleophilic addition over competitive hydrolysis.     

Alkali-Metal-Alkoxide Powered Zincation of Fluoroarenes Employing Zinc Bis-Amide Zn(TMP)2

Reminiscent of Lochmann-Schlosser superbase recipes, the addition of two molar equivalents of KOtBu to Zn(TMP)₂ (TMP=2,2,6,6-tetramethylpiperidide) transforms this mild zinc bis-amide base to a powerful metalating agent able to perform facile regioselective zincation of a wide range of sensitive fluoroarenes. Structural authentication of the intermediates post Zn−H exchange demonstrates activation of both TMP groups to form a range of higher order bis-aryl potassium zincates, isolable as solids and further functionalized in electrophilic interception reactions. Studies assessing the role of KOtBu reveal that the first equivalent undergoes co-complexation with Zn(TMP)₂, enabling kinetic activation of the amide groups; whereas the second equivalent stabilizes the metalated intermediate preventing ligand redistribution. Showcasing its metalating power, this bimetallic KOtBu/Zn(TMP)₂ partnership, can effect zincation of toluene and benzene at room temperature.     

SO2-promoted catalytic N2O removal over iron zeolites

SO(2) promotes the rate of N(2)O removal over Fe-zeolites more efficiently than other reducing agents (NH(3)) and promoters (NO); the effect of SO(2) as selective reductant is independent of the framework type and composition, preparation method, and iron content, suggesting that its occurrence is not determined by a highly specific iron speciation.     

Activation of a 1,10-phenanthroline ligand on a rhenium tricarbonyl complex

The reactions of complex [Re(PPh(2))(CO)(3)(phen)] with either methyl acrylate or methyl propiolate afford products in which the phen ligand has been activated.