NMR Spectroscopic Monitoring Research Articles

Experimental Observation of a Terminal Borylene-Dinitrogen Adduct via Cleavage of a 1,2,3,4,5-Diboratriazoline.

While azides do not react with simple alkenes except under harsh conditions, a diboron alkene analogue, the doubly cyclic alkyl(amino)carbene (CAAC)-stabilized dicyanodiborene 1, reacts spontaneously with organic azides (7-10 equiv.) at room temperature to yield two equivalents of stable CAAC-stabilized imino(cyano)boranes (2-R). NMR-spectroscopic monitoring of the reaction mixtures shows the initial formation of a 1:1 mixture of 2-R and a relatively long-lived intermediate (Int), which in the presence of excess azide is converted into a second equivalent of 2-R. In the absence of excess azide, however, Int decomposes to 3, the product of an intramolecular C‒H activation by a putative dicoordinate borylene intermediate "(CAAC)B(CN)". Mechanistic insights from trapping experiments, NMR-spectroscopic and high-resolution mass spectrometry data, as well as DFT computations reveal that Int is the terminal borylene end-on-dinitrogen adduct [(CAAC)B(CN)(η1-N2)]. The formation of the iminoboranes 2-R from diborene 1 and RN3 proceeds via an azide-diborene Huisgen-type [3+2] cycloaddition reaction, followed by a retro-[3+2] cycloaddition, yielding 2-R and [(CAAC)B(CN)(η1-N2)]. The latter then undergoes either N2 extrusion and intramolecular C‒H activationto generate 3, or a Staudinger-type reaction with a second equivalent of azide to generate a second equivalent of the iminoborane 2-R.

Gold‐Catalyzed Synthesis of Ortho‐Quinone Methide Analogues as Reactive Synthetic Precursors

Abstract2‐Exomethylene indolin‐3‐ones are useful synthetic intermediates with a unique indole scaffold. However, their synthesis has not yet been reported owing to their reactivity and instability. In this study, a gold(I)‐catalyzed method for generating 2‐exomethylene indolin‐3‐one derivatives via the 5‐exo‐dig‐selective cyclization of 2‐(2‐amino)phenyl alkynyl ketones with an alkyne terminus was developed. The subsequent pseudo‐dimerization of the generated 2‐exomethylene indolin‐3‐ones and their [4+2] cyclization reactions with 1,1‐disubstituted alkenes afforded spiropseudoindoxyl and pyranoindole derivatives, respectively, in 21% to quantitative yields. Density functional theory calculations of the gold‐catalyzed cyclization provided insights into the 5‐exo‐dig selectivity. Furthermore, the reactive 2‐exomethylene indolin‐3‐one was isolated for the first time based on the results from 1H NMR spectroscopic monitoring. The isolated product was applied to various transformations. Our synthesis method promotes the development of bioactive agents and functional materials with complex cyclic structures.

Efficient additive-free formic acid dehydrogenation with a NNN-ruthenium complex.

A new ruthenium complex containing a pyridylidene amine-based NNN ligand was developed as a catalyst precursor for formic acid dehydrogenation, which, as a rare example, does not require basic additives to display high activity (TOF ∼10 000 h-1). Conveniently, the complex is air-stable, but sensitive to light. Mechanistic investigations using UV-vis and NMR spectroscopic monitoring correlated with gas evolution profiles indicate rapid and reversible protonation of the central nitrogen of the NNN ligand as key step of catalyst activation, followed by an associative step for formic acid dehydrogenation.

Resolving the Mechanistic Complexity in Triarylborane-Induced Conjugate Additions

Previous studies have shown that the catalytically active species in the Lewis acid-catalyzed addition reactions of allyl silanes or silyl enol ethers to carbonyl compounds or Michael acceptors are often silylium-carbonyl adducts rather than the adducts of the carbonyl group with the Lewis acid used for the induction of the reaction. Indirect evidence for such catalyst variations has so far been derived from double-label crossover experiments and comparisons of absolute reaction rates. We have now performed a detailed investigation on the kinetics and mechanism of the triarylborane-initiated conjugate addition reactions of allylsilanes and silyl enol ethers to α,β-unsaturated carbonyl compounds. NMR spectroscopic monitoring of such reactions gave rise to sigmoidal kinetic profiles, allowing us to directly follow the change of the catalytically active Lewis acid from triarylboranes in the induction period to silylium ions during the main part of the reaction. Crossover experiments and the isolation of four- and five-membered cyclic intramolecular trapping products provided further insight into the mechanism. DFT calculations of various mechanistic variants and kinetic modeling elucidated the operation of a complex reaction network, which rationalizes the experimental observations.

Iron(0) tricarbonyl η4-1-azadiene complexes and their catalytic performance in the hydroboration of ketones, aldehydes and aldiminesviaa non-iron hydride pathway

Six iron(0) tricarbonyl complexes (1a-f) with a η4-1-azadiene moiety were prepared and their performance in the hydroboration of unsaturated organic compounds was investigated. All the complexes exhibit catalytic activity towards hydroboration of ketones, aldehydes and aldimines with pinacolborane (HBpin) as a hydride source to lead to secondary alcohols, primary alcohols, and secondary amines, respectively, after hydrolysis of the hydroboration products. Of the iron(0) tricarbonyl complexes, complex 1e is the most robust one and was employed throughout the catalytic investigation. Its preference towards the three types of substrates is as follows: aldimines > aldehydes ≫ ketones. In total, 24 substrates were examined for the catalytic hydroboration reactivity and generally, isolation yields ranging from 40% to 95% were achieved. Mechanistic investigation suggests that the catalytic hydroboration of the substrates proceeds via intramolecular hydride transfer without going through an Fe-H intermediate. As indicated by 1H NMR spectroscopic monitoring, the substrates and the borane agent bind to the iron centre and the imine N atom, respectively, which facilitates the hydride transfer by activating the B-H bond and polarizing the double bond of the substrates.

Understanding the Early Stages of Nickel Sulfide Nanocluster Growth: Isolation of Ni3 , Ni4 , Ni5 , and Ni8 Intermediates.

Addition of sub-stoichiometric quantities of PEt3 and diphenyl disulfide to a solution of [Ni(1,5-cod)2 ] generates a mixture of [Ni3 (SPh)4 (PEt3 )3 ] (1), unreacted [Ni(1,5-cod)2 ], and [(1,5-cod)Ni(PEt3 )2 ], according to 1 H and 31 P{1 H} NMR spectroscopic monitoring of the in situ reaction mixture. On standing, complex 1 converts into [Ni4 (S)(Ph)(SPh)3 (PEt3 )3 ] (2), via formal addition of a "Ni(0)" equivalent, coupled with a CS oxidative addition step, which simultaneously generates the Ni-bound phenyl ligand and the μ3 -sulfide ligand. Upon gentle heating, complex 2 converts into a mixture of [Ni5 (S)2 (SPh)2 (PEt3 )5 ] (3) and [Ni8 (S)5 (PEt3 )7 ] (4), via further addition of "Ni(0)" equivalents, in combination with a series of C-S oxidative addition and CCreductive elimination steps, which serve to convert thiophenolate ligands into sulfide ligands and biphenyl. The presence of 1-4 in the reaction mixture is confirmed by their independent syntheses and subsequent spectroscopic characterization. Overall, this work provides an unprecedented level of detail of the early stages of Ni nanocluster growth and highlights the fundamental reaction steps (i.e., metal atom addition, CS oxidative addition, and CC reductive elimination) that are required to grow an individual cluster.

Dynamic dye emission ON/OFF systems by a furan moiety exchange protocol

Four triphenylamine-based dyes were synthesized by fluorescence turn on reactions. Optical behaviours, molecular arrangements, donor-to-acceptor charge transfer and dipole interactions of these functional dyes were investigated by UV–vis absorption and fluorescence spectroscopy, single-crystal X-ray diffraction and electrochemical cyclic voltammetry. The irreversible isomerization of itaconimide dye leads to an irreversible emission switch ON to OFF. Reversible Diels-Alder reaction of these dyes lead to a reversible emission switch OFF/ON. These luminescent dyes demonstrate dynamic dye molecular features by furan moiety exchanges to form energy-minimized and optimized dye molecular structures. In the dynamic molecular system, α-position furan-substituted dye was converted into more stable β-position furan-substituted dye according to 1H NMR spectroscopic monitoring.

Reactions of Arylsulfonate Electrophiles with NMe4F: Mechanistic Insight, Reactivity, and Scope.

This paper describes a detailed study of the deoxyfluorination of aryl fluorosulfonates with tetramethylammonium fluoride (NMe4F) and ultimately identifies other sulfonate electrophiles that participate in this transformation. 19F NMR spectroscopic monitoring of the deoxyfluorination of aryl fluorosulfonates revealed the rapid formation of diaryl sulfates under the reaction conditions. These intermediates can proceed to fluorinated products; however, diaryl sulfate derivatives bearing electron-donating substituents react very slowly with NMe4F. Based on these findings, aryl triflate and aryl nonaflate derivatives were explored, since these cannot react to form diaryl sulfates. Aryl triflates were found to be particularly effective electrophiles for deoxyfluorination with NMe4F, and certain derivatives (i.e., those bearing electron-neutral/donating substituents) afforded higher yields than their aryl fluorosulfonate counterparts. Computational studies implicate a similar mechanism for deoxyfluorination of all the sulfonate electrophiles.

Mechanistic Study of Arylsilane Oxidation through 19F NMR Spectroscopy.

The mechanism of the oxidation of arylsilanes to phenols has been investigated using 19F NMR spectroscopy. The formation of silanols in these reactions results from a rapid background equilibrium between silanol and alkoxysilane; the relative rates of reaction of these species was evaluated by modeling of concentration profiles obtained through 19F NMR spectroscopic reaction monitoring. Combining these results with a study of initial rates of phenol formation, and of substituent electronic effects, a mechanistic picture involving rapid and reversible formation of a pentavalent peroxide ate complex, prior to rate-limiting aryl migration, has evolved.

Limits of Vinylidene Fluoride RAFT Polymerization

The investigations reported in this article probe the behavior of the RAFT polymerization of vinylidene fluoride (VDF) when degrees of polymerization higher than 50 are targeted: they demonstrate that higher molar mass PVDF (11 000 g mol–1) can indeed be prepared by RAFT polymerization, but only at rather low monomer conversions (<33%). This study more carefully examines the behavior of the reputedly nonreactive −CF2CH−XA chain ends (where XA designates the xanthate group) formed by inverse VDF addition and known to accumulate in the reaction medium during the polymerization. A combination of 1H and 19F NMR spectroscopic monitoring and comprehensive DFT calculations of the various exchange and propagation reactions at work explains the unexpected behavior of this polymerization. The present study disproves entirely the generally accepted belief that −CF2CH2–XA-terminated PVDF chains are “dead” and shows how these chains are reactivated, albeit slowly, throughout the polymerization. This activation occurs prevalently and counterintuitively through degenerative exchange by the minority PVDF–CF2CH2• radicals. The resulting kinetic scheme rationalizes the experimentally observed absence, after conversion of all the dormant chains into the less reactive −CF2CH2–XA end-group, of the longer polymer chains expected from a free radical polymerization mechanism.

Silyl–Thioether Multidentate Ligands – Synthesis of RhIII Complexes via RhI/RhIII Mixed‐Valent and Cyclooctenyl Intermediates

AbstractThe preparation and characterization of a dimeric RhIII complex containing two SSi ligands on each rhodium fragment is reported. {Rh[SiMe2(o‐C6H4SMe)]2Cl}2 (1) was obtained by reaction of thioether–silane SiMe2H(o‐C6H4SMe) (L1) with [Rh(cod)Cl]2. Mixed‐valent RhI/RhIII complex {(cod)Rh(μ‐Cl)2Rh[SiMe2(o‐C6H4SMe)]2} (2), which is an intermediate in the formation of 1, was isolated. Furthermore, 1H NMR spectroscopic monitoring revealed the formation of cyclooctenyl intermediates. The tridentate pincer proligand, SiMeH(o‐C6H4SMe)2 (L2), reacts with [Rh(cod)Cl]2 to yield a robust cyclooctenyl RhIII complex, {Rh(η3‐cyclooctenyl)[SiMe(o‐C6H4SMe)2]Cl} (3). Complexes 1–3 were characterized by X‐ray crystallography.

2'-O-[(4-CF3-triazol-1-yl)methyl] Uridine - A Sensitive (19)F NMR Sensor for the Detection of RNA Secondary Structures.

A sensitive uridine-derived sensor (viz., 2'-O-[(4-CF3-triazol-1-yl)methyl]uridine, 1) for (19)F NMR spectroscopic monitoring of RNA secondary structures is described. The applicability of 1 is demonstrated by monitoring the thermal denaturation of the following double and triple helical RNA models: (1) a miR 215 hairpin, (2) a poly U-A*U triple helix RNA (bearing two C-G*C(H+) interrupts), and (3) a polyadenylated nuclear-nuclear retention element complex. In these RNA models, the (19)F NMR shift of the 2'-O-(CF3-triazolylmethyl) group shows high sensitivity to secondary structural arrangements. Moreover, 1 favors the desired N-conformation, and its effect on both RNA duplex and triplex stabilities is marginal.

Homo- and heteronuclear compounds with a symmetrical bis-hydrazone ligand: synthesis, structural studies, and luminescent properties.

Nine new coordination compounds have been synthesized by the reaction of salts of bivalent metal ions (a=Zn(II) , b=Cu(II) , c=Ni(II) , d=Co(II) ) with the bis(benzoylhydrazone) derivative of 4,6-diacetylresorcinol (H4 L). Three kinds of complexes have been obtained: homodinuclear compounds [M2 (H2 L)2 ]⋅nH2 O (1 a, 1 b, 1 c, and 1 d), homotetranuclear compounds [M4 (L)2 ]⋅n(solv) (2 a and 2 c), and heterotetranuclear compounds [Zn2 M2 (L)2 ]⋅n(solv) (2 ab, 2 ac, and 2 ad). The structures of the free ligand H4 L⋅2 DMSO and its complexes [Zn2 (H2 L)2 (DMSO)2 ] (1 a*), [Zn4 (L)2 (DMSO)6 ] (2 a*), and [Zn0.45 Cu3.55 (L)2 (DMSO)6 ]⋅2 DMSO (2 ab*) were elucidated by single-crystal X-ray diffraction. The ligand shows luminescence properties and its fluorimetric behavior towards M(II) metals (M=Zn, Cu, Ni and Co) has been studied. Furthermore, the solid-state luminescence properties of the ligand and compounds have been determined at room temperature. (1) H NMR spectroscopic monitoring of the reaction of H4 L with Zn(II) showed the deprotonation sequence of the OH/NH groups upon metal coordination. Heteronuclear reactions have also been monitored by using ESI-MS and spectrofluorimetric techniques.

Rhenium–Germanium Triple Bonds: Syntheses and Reactions of the Germylidyne Complexes mer‐[X2(PMe3)3ReGeR] (X=Cl, I, H; R=m‐terphenyl)

A general approach to the first compounds that contain rhenium-germanium triple and double bonds is reported. Heating [ReCl(PMe3)5] (1) with the arylgermanium(II) chloride GeCl(C6H3-2,6-Trip2) (2; Trip=2,4,6-triisopropylphenyl) results in the germylidyne complex mer-[Cl2 (PMe3)3Re≡Ge-C6H3-2,6-Trip2] (4) upon PMe3 elimination. An equilibrium that is dependent on the PMe3 concentration exists between complexes 1 and 4. Removal of the volatile PMe3 shifts the equilibrium towards complex 4, whereas treatment of 4 with an excess of PMe3 gives a 1:1 mixture of 1 and the PMe3 adduct of 2, GeCl(C6H3-2,6-Trip2)(PMe3) (2-PMe3). Adduct 2-PMe3 can be selectively obtained by addition of PMe3 to chlorogermylidene 2. The NMR spectroscopic data for 2-PMe3 indicate an equilibrium between 2-PMe3 and its dissociation products, 2 and PMe3 , which is shifted far towards the adduct site at ambient temperature. NMR spectroscopic monitoring of the reaction of complex 1 with 2 and the reaction of complex 4 with PMe3 revealed the formation of two key intermediates, which were identified to be the chlorogermylidene complexes cis/trans-[Cl(PMe3)4 Re=Ge(Cl)C6H3-2,6-Trip2] (cis/trans-3) by using NMR spectroscopy. Labile chlorogermylidene complexes cis/trans-3 can be also generated from trans-[Cl(PMe3)4 Re≡Ge-C6H3-2,6-Trip2]BPh4 (9) and (nBu4N)Cl at low temperature, and decompose at ambient temperature to give a mixture of complexes 1 and 4. Complex 4 reacts with LiI to give the diiodido derivative mer-[I2(PMe3)3Re≡Ge-C6H3-2,6-Trip2] (5), which undergoes a metathetical iodide/hydride exchange with Na(BEt3H) to give the dihydrido germylidyne complex mer-[H2(PMe3)3Re≡Ge-C6H3-2,6-Trip2] (6). Carbonylation of 4 induces a chloride migration from rhenium to the germanium atom to afford the chlorogermylidene complex mer-[Cl(CO)(PMe3)3Re=Ge(Cl)C6H3-2,6-Trip2] (7). Similarly, MeNC converts complex 4 into the methylisocyanide analogue mer-[Cl(MeNC)(PMe3)3Re=Ge(Cl)C6H3-2,6-Trip2] (8). Chloride abstraction from 4 by NaBPh4 in the presence of PMe3 gives the cationic germylidyne complex trans-[Cl(PMe3)4 Re≡Ge-C6H3-2,6-Trip2]BPh4 (9). Heating complex 4 with cis-[Mo(PMe3)4(N2)2] induces a germylidyne ligand transfer from rhenium to molybdenum to afford the germylidyne complex trans-[Cl(PMe3)4Mo≡Ge-C6H3-2,6-Trip2] (10). All new compounds were fully characterized and their molecular structures studied by X-ray crystallography, which led to the first experimentally determined Re-Ge triple- and double-bond lengths.

2,2-Disubstituted 4-Acylthio-3-oxobutyl Groups as Esterase- and Thermolabile Protecting Groups of Phosphodiesters

Five different 2,2-disubstituted 4-acylthio-3-oxobutyl groups have been introduced as esterase-labile phosphodiester protecting groups that additionally are thermolabile. The phosphotriesters 1-3 were prepared to determine the rate of the enzymatic and nonenzymatic removal of such groups at 37 °C and pH 7.5 by HPLC-ESI-MS. Additionally, (1)H NMR spectroscopic monitoring was used for structural characterization of the intermediates and products. When treated with hog liver esterase, these groups were removed by enzymatic deacylation followed by rapid chemical cyclization to 4,4-disubstituted dihydrothiophen-3(2H)-one. The rate of the enzymatic deprotection could be tuned by the nature of the 4-acylthio substituent, the benzoyl group and acetyl groups being removed 50 and 5 times as fast as the pivaloyl group. No alkylation of glutathione could be observed upon the enzymatic deprotection. The half-life for the nonenzymatic deprotection varied from 0.57 to 35 h depending on the electronegativity of the 2-substituents and the size of the acylthio group. The acyl group evidently migrates from the sulfur atom to C3-gem-diol obtained by hydration of the keto group and the exposed mercapto group attacks on C1 resulting in departure of the protecting group as 4,4-disubstituted 3-acyloxy-4,5-dihydrothiophene with concomitant release of the desired phosphodiester.

Organothorium-Catalyzed Hydroalkoxylation/Cyclization of Alkynyl Alcohols. Scope, Mechanism, and Ancillary Ligand Effects

Organothorium complexes bearing amide or alkyl proligands are active toward the highly selective hydroalkoxylation/cyclization of alkynyl alcohols. Substrates include primary and secondary alcohols, as well as terminal and internal alkynes. Catalysts with strongly binding ligation such as pentamethylcyclopentadienyl (Cp* = C5Me5) or “constrained geometry catalysts” (CGC = Me2Si(η5-Me4C5)(tBuN)) remain soluble throughout the reaction, with the more sterically open (CGC)Th(NMe2)2 (1) exhibiting higher activity than Cp*2Th(CH2TMS)2 (2). The use of precatalyst [(Me3Si)2N]2Th[κ2-(N,C)-CH2Si(CH3)2N(SiMe3)] (3) leads to precipitation upon the addition of alcohol substrates, although catalytic activity is retained. The substrate scope for 1 includes primary and secondary alcohols as well as terminal and internal alkynes. In situ1H NMR spectroscopic monitoring indicates that the rate law is zero-order in [substrate] and first-order in [catalyst]. The rates of primary alcohols and terminal alkynes are significantly more rapid than their more sterically hindered counterparts, suggesting that steric demands dominate the hydroalkoxylation/cyclization transition state. Turnover frequencies as high as 49 h–1 at 60 °C are observed, producing exclusively the exo-methylene products. For internal alkyne substrates, alkenes with E-orientation are formed with complete selectivity. Activation parameters ΔH‡ = 27.9(0.4) kcal/mol, ΔS‡ = −3.0(1.1) eu, and Ea = 28.6(0.4) kcal/mol are largely in accord with observations for other f-element-mediated insertive hydroelementation processes, and an ROH/ROD kinetic isotope effect of 0.97(0.02) is observed. The reactivity patterns, kinetics, and activation parameters are consistent with a pathway proceeding via turnover-limiting alkyne insertion into the Th–O bond, with subsequent, rapid Th–C protonolysis, regenerating the initial Th–OR species.

An Approach to 1,3,4-Dioxaphospholane Complexes through an Acid-Induced Ring Expansion of an Oxaphosphirane Complex: The Problem of Construction and Deconstruction of O,P-Heterocycles

Treatment of oxaphosphirane complex 1, triflic acid (TfOH), and various aldehydes yielded 1,3,4-dioxaphospholane complexes 5a,b-7a,b after deprotonation with NEt(3). In addition to NMR spectroscopy, IR spectroscopy, and MS data, the X-ray structures of complexes 5a and 7a were determined. (31)P NMR spectroscopic monitoring and DFT calculations provided insight into the reaction course and revealed the transient TfOH 1,3,4-dioxaphospholanium association complex TfOH-5a,b and/or TfOH-5a,b' as key reactive intermediates. Furthermore, it was observed that the five-membered ring system was cleaved upon warming and yielded side-on (E,Z)-methylenephosphonium complexes 8a,b if deprotonation did not occur at low temperature. Overall, a novel temperature- and acid-dependent construction and deconstruction process of the 1,3,4-dioxaphospholane ring system is described.

New insights into addition reactions of dialkylzinc reagents to trifluoromethyl ketones: Structural authentication of a β-hydride elimination product containing a tetranuclear zinc chain

A systematic study of the stoichiometric alkylation reactions of 2,2,2-trifluoroacetophenone 1 with [ZnR(2)(TMEDA)] (R= Me, Et, (t)Bu, CH(2)SiMe(3); TMEDA= N,N,N',N'-tetramethylethylenediamine) monitored by (1)H and (19)F NMR spectroscopy is presented. For R = Me, Et the alkylation products alkyl(alkoxides) [(TMEDA)Zn(R){OC(CF(3))(R)Ph}] (R = Me, 2: Et, 3) are obtained as the single products of the reaction. When the steric bulk of the dialkylzinc reagent is increased the alkylation reaction is inhibited. Thus, for R = (t)Bu, the reduction product [(TMEDA)Zn((t)Bu){OC(CF(3))(H)Ph}] is obtained as a result of beta-hydride elimination from one of the (t)Bu groups of the organometallic reagent. (1)H NMR spectroscopic monitoring of the reaction allowed the detection of isobutene as a side product of this reduction process. For the highly sterically demanding group R = CH(2)SiMe(3) which lacks hydrogen atoms at the beta position, no reaction is observed even under refluxing conditions. Two important intermediates from these reactions have been structurally elucidated: [(TMEDA)Zn(Me){OC(CF(3))(Me)Ph}] (2) which could be involved in the previously reported alkylation reaction of trifluoromethyl ketones by ZnR(2) catalysed by TMEDA and unprecedented tetranuclear [((t)Bu)(2)Zn(4){OC(CF(3))(H)Ph}(6)] (5) resulting from the reduction of 1 when reacted with (t)Bu(2)Zn, which displays a rare Zn...Zn...Zn...Zn linear chain arrangement for a zinc alkyl(alkoxide).

Mechanistic Insights on the Copolymerization of Polar Vinyl Monomers with Neutral Ni(II) Catalysts

The Ni(II) complexes [(N--O)Ni(H)(PMe(3))] (1) and [(N--O)Ni(CH(2)CH(3))(dmso)] (3) (NwedgeO = kappa(2)-{(2,6-(3,5-(F(3)C)(2)C(6)H(3))(2)C(6)H(3))-N=C(H)-(3,5-I(2)-2-O-C(6)H(2))}) were found to be well-defined model compounds to study the reactivity of polymerization active neutral Ni(II) species toward polar vinyl monomers. Methyl acrylate (MA) insertion into the Ni(II)-hydride bond of 1 was monitored at T >or= -40 degrees C by NMR spectroscopy. 2,1-Insertion yields the functionalized Ni(II) alkyl complex [(N--O)Ni(C(alpha)H(CH(3))C(beta)(O)OCH(3))(PMe(3))] (4). Low-temperature 2D ROESY data indicate a weak Ni(II)...O=C(beta) interaction in 4. This is supported by ab initio calculations at the gradient-corrected DFT (BP86/LACPV*) level of theory. Exposure of 1 to equal amounts of MA and ethylene afforded 4 and the Ni(II) ethyl complex 7 in a 9:1 ratio, which indicates that MA and ethylene effectively compete with each other for coordination and insertion. NMR spectroscopic monitoring revealed that 4 is stable in the absence of residual 1 at low temperatures but is subject to rapid bimolecular elimination of the functionalized alkyl moiety in the presence of free Ni(II) hydride species even at T = -40 degrees C. Isomerization into the 1,2-MA-insertion product was observed at T = 25 degrees C but occurred slowly compared to decomposition, which occurs at 0 degrees C by reaction of 4 with Ni(II) hydride formed by beta-elimination from 4 itself at this temperature. Exposure of the higher Ni(II) alkyl complex 3 to MA in the presence of excess ethylene results in the immediate formation of methyl pentanoate as the ultimate decomposition product. Functionalized Ni(II) alkyl species formed from 2,1-insertion of MA into the metal-carbon bond of higher Ni(II) alkyls are subject to rapid hydrolysis in the presence of trace amounts of water in the reaction medium, which contrasts the stability of nonfunctionalized Ni(II) alkyls toward water. Exposure of 1 to vinyl acetate (VA) affords the kinetic 1,2-insertion product [(NwedgeO)Ni(CH(2)CH(2)OC(gamma')(O)CH(3))(PMe(3))] (5) at temperatures T >or= -10 degrees C, which rearranges into the thermodynamically favored 2,1-insertion product [(N--O)Ni(CH(CH(3))OC(O)CH(3))(PMe(3))] (6). NMR data and ab initio calculations suggest a Ni(II)...O=C(gamma') interaction in 6. 5 decomposes via beta-acetate elimination to yield ethylene and Ni(II) acetate species. Notably, VA does not undergo observable nickel-carbon bond insertion with 3, but reacted with Ni(II) hydride species in equilibrium with 3 to yield 5* which is subject to rapid decomposition via beta-acetate elimination.

Zn Pyropheophorbide a: A β‐Face Selective Nicotine Receptor

AbstractNicotine binds to a semisynthetic chlorophyll a derivative, Zn pyropheophorbide a, in a receptor‐like manner to form a 1:1 complex with β‐face diastereomeric configuration. The geometry of the complex structure was evidenced by 1H NMR, ROESY and DOSY spectroscopic experiments combined with molecular modelling DFT B3LYP calculations at the 6‐31G* level. The observed binding between the chlorin and nicotine takes place through a “two‐point binding” mechanism, in which the nicotine N‐pyridyl moiety coordinates to the chlorin zinc atom, whilst the nicotine N‐methyl pyrrolidine unit forms an ion pair with the chlorin acid group. The binding mechanism was further confirmed by association constants (Ka) obtained from titration experiments in CDCl3 with 1H NMR spectroscopic monitoring (8.0 × 105 M–1) and in toluene with UV/Vis (6.1 × 104 M–1) monitoring. The corresponding binding constants obtained for nicotine with one‐point binder, Zn pyropheophorbide a methyl ester, were 4.8 × 105 and 2.3 × 104 M–1, respectively. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)