Lewis Acidic Compounds Research Articles

New Bis(amino)cyclodiphosph(III)azane Complexes of Aluminum

AbstractThe reaction of the bis(amino)cyclodiphosph(III)azane cis‐{(tBuNH)2(PNtBu)2} with AlEt3, AlBrEt2, AlBr2Et, and AlBr3 was investigated. The less Lewis acidic compound AlEt3 forms the adduct cis‐[(tBuNH)2(PNtBu){P(·AlEt3)NtBu}], in which the aluminum atom is exclusively coordinated to one phosphorus atom. No ethane elimination was observed. By using the more Lewis acidic compounds AlBr2Et and AlBr3 the N‐coordinated complexes cis‐[(tBuNH)(PNtBu)(tBuN·AlBr2Et){P(H)NtBu}] and cis‐[(tBuNH)(PNtBu)(tBuN·AlBr3){P(H)NtBu}] were obtained, in which the aluminum atoms are N‐coordinated by a tBuN=PH unit. Upon coordination of the aluminum compounds, a concomitant 1,2‐H shift from the nitrogen atom onto the phosphorus atom within the cis‐{(tBuNH)2(PNtBu)2} ligand was observed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Stepwise Oxygenation of Pinacolborane by a Rhodiumperoxo Complex: Detection of an Intermediate Metal Borate and Perborate

Acting as go-between: A rhodium borate and perborate have been identified as intermediates in the rhodium-mediated oxygenation of pinacolborane (HBpin; see scheme). The rhodium(III)–peroxo complex reacts with the Lewis acidic boron compound HBpin by oxygen transfer from the rhodium center to HBpin to give a rhodium(I) species. The reaction sequence might play a crucial role in the homocoupling of boronic acid.

N- VersusP-Coordination in Bis(amino)cyclodiphosph(III)azane Complexes of Aluminum

The reaction of the bis(amino)cyclodiphosph(III)azane, cis-{(tBuNH)(2)(PNtBu)(2)}, with AlMe(3), AlClMe(2), AlCl(2)Me, and AlCl(3) is reported. The less Lewis acidic compound AlMe(3) forms the adduct cis-[(tBuNH)(2)(PNtBu){P.(AlMe(3))NtBu}] (1), in which the aluminum atom is exclusively coordinated to one phosphorus atom. At elevated temperatures AlMe(3) undergoes migratory exchange between the two phosphorus atoms, but no methane elimination is observed. By using the more Lewis acidic compound AlClMe(2) the P-coordinated compound cis-[(tBuNH)(2)(PNtBu){P(AlClMe(2))NtBu}] (2) can be obtained at low temperatures. Compound 2 rearranges irreversibly to a product in which the AlClMe(2) group is coordinated by one exo-cyclic nitrogen atom. A concomitant 1,2-H shift from this nitrogen atom onto the phosphorus atom is observed. The N-coordinated rearrangement product slowly decomposes via a P-N bond cleavage in solution. Reaction of the even more Lewis acidic compounds AlCl(2)Me and AlCl(3) finally led to stable adducts, cis-[(tBuNH)(PNtBu)(tBuNAlCl(2)Me){P(H)NtBu}] (3), and cis-[(tBuNH)(PNtBu)(tBuNAlCl(3)){P(H)NtBu}] (4), in which the aluminum atoms are N-coordinated by a tBuN=PH unit.

Condensation reaction of formaldehyde and methyl formate catalyzed by a composite catalyst system

The condensation reaction of formaldehyde and methyl formate to form methyl glycolate and methyl methoxy acetate catalyzed by p-toluenesulfonic acid and different Lewis acid compounds has been investigated. The composite catalytic system consisting of p-toluenesulfonic acid and NiX 2 (X = Cl, Br, I), especially NiI 2, exhibited a high catalytic performance for the condensation reaction, the total yield of MG and MMAc was up to 72.37%.

Neutral and Cationic Heteroscorpionate Aluminum Complexes: Synthesis, Structure, and Ring-Opening Polymerization of ε-Caprolactone

Treatment of 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenol (bpzmp-H) as tridentate ligand with aluminum trialkyls affords the corresponding heteroscorpionate aluminum complexes (bpzmp)AlR2 (R = Me (1); R = Et (2); R = iBu (3)) in high yield. In the solid state, complexes 1−3 were isolated as racemic mixtures in which each stereoisomer adopts a tetrahedral structure with the bpzmp ligand κ2-coordinated to the metal via the phenoxy group and the imino nitrogen of one of pyrazolyl rings. The investigation of the solution structure of 1−3 by means of VT 1H NMR spectroscopy revealed fluxional exchange between coordinated and noncoordinated pyrazolyl rings, producing interconversion between the two enantiomers. The activation enthalpy (ΔH#) for racemization processes (10.1 ± 0.2 kcal·mol-1 (1); 11.6 ±0.3 kcal·mol-1 (2); 14.1 ± 0.2 kcal·mol-1 (3)) was dependent on steric hindrance at the aluminum center. Reaction of 1 or 2 with B(C6F5)3 proceeds through net alkyl abstraction, forming the expected cationic aluminum complexes [(bpzmp)AlR]+[RB(C6F5)3]- (R = Me (4); R = Et (5)), in which the bpzmp fragment acts as a tridentate ligand. The ionization reaction of 3 with Lewis acidic compounds (B(C6F5)3 or [Ph3C][B(C6F5)4]) proceeds by net β-H abstraction from the isobutyl group, forming [(bpzmp)AliBu]+[HB(C6F5)3]- (6) along with isobutene; in contrast, reaction with the Brönsted acid [HNMe2Ph][B(C6F5)4] proceeds by protonolysis of the isobutyl group. Complex 4 is active in ring-opening polymerization of ε-caprolactone (ε-CL), producing high-molecular-weight polymers. The 1H NMR monitoring of the reaction between 4 and ε-CL in 1:1 molar ratio showed that the initiation involves monomer insertion into the Al−O bond of the bpzmp fragment, forming the intermediate [(bpzphe)AlMe]+[MeB(C6F5)4]- (bpzphe = 2,4-di-tert-butyl-6-(bis(3,5-dimethylpyrazol-1-yl)methyl)phenyl 6-hydroxyhexanoate (8). The growth of the polymer chain occurs through continuous insertions of the monomer in the Al−alkoxide bond.

Development in the Green Synthesis of Cyclic Carbonate from Carbon Dioxide Using Ionic Liquids

AbstractFor Abstract see ChemInform Abstract in Full Text.

Reaction of η 2-enone and enal-platinum(0) complexes with Lewis acidic compounds

Reaction of η 2-enone and enal-platinum(0) complexes Pt(CH 2CHCOR)(PPh 3) 2 (R=H, Me) with Lewis acidic compounds BX 3 (X=F, C 6F 5) afforded adducts formed by coordination of boron to oxygen of the carbonyl group. The X-ray structure determination of the adduct formed from B(C 6F 5) 3 and η 2-methylvinylketone complex showed no strong interaction between Pt and carbonyl carbon. In contrast to the inability of the palladium(0) η 2-enone complexes to form any Me 3Al adduct, η 2-cyclohexenoneplatinum(0) complex formed an isolable adduct with Me 3Al, the structure of which was also confirmed by X-ray analysis. The NMR spectral parameters (Pt–C, Pt–P and P–P coupling constants) of these adducts were compared with those of the original η 2-enone or enal-platinum(0) complexes as well as the ordinary η 3-allylplatinum cation [Pt(PPh 3) 2(MeCHCHCH 2)] +.

Dealkylation with boron bromide chelates

Potential two-point Lewis acid compounds can be formed by combining the salen( t Bu)H 2 ( N, N′-alkylenenebis(3,5-di-tertbutyl(2-hydroxy)benzylidenimine) class of ligands and its derivatives with boron tribromide. In the present study compounds having the formula, L(BBr 2) 2 (L=salen( t Bu) ( 1), salhen( t Bu) ( 2), L([ p tolyl]BBr) (L=salen( t Bu) ( 3), salpen( t Bu) ( 4), salben( t Bu) ( 5), salhen( t Bu) ( 6), and t Bu-sal( t Bu)BBr 2 ( 7) were prepared. These compounds are active towards the dealkylation of alkyl phosphates and are catalytic when stoichiometric amounts of BBr 3 and trimethylphosphate are introduced to the chelate ligand. All of the compounds were characterized by Mp, elemental analysis, 1H- and 11B-NMR, IR, MS and in the case of 2 by X-ray crystallography.

Group 13 trihalide complexes of 9-fluorenone: a comparison of methods for assigning relative Lewis acidity

The 9-fluorenone complexes MX 3(9-fluorenone) [M=B, X=Cl ( 1); M=Al, X=Cl, Br ( 2), I ( 4), M=Ga; X=Cl ( 5), Br ( 6), I ( 7) and AlBr 3(9-fluorenone) 2 ( 3) have been prepared and characterized by NMR, IR and UV–vis spectroscopy and X-ray crystallography ( 1, 2, 3, 5 and 6). Complexes of MgCl 2 ( 8), HgCl 2 ( 9), ZrCl 4 ( 10) and SnCl 4 ( 11) were characterized by IR and UV–vis spectroscopy, while the unusual mixed vanadium(IV,V) compound, VO(Cl)(9-fluorenone) 2(H 2O)(μ-O)(VOCl 2) ( 12) has been structurally characterized. Solution ligand dissociation energies for MX 3(9-fluorenone) were determined by variable temperature 1H-NMR spectroscopy. For all the compounds, Δ S D is large and positive, as would be expected from a dissociative process. A correlation of the results for the five structurally characterized compounds MX 3(9-fluorenone) (M=B, X=Cl; M=Al, X=Cl, Br; M=Ga, X=Cl, Br) was used to examine the suitability of the following parameters to measuring Lewis acidity: IR ν CO, UV λ max, and 13C-NMR δ CO, K eq at 298 K, Δ H, Δ G, and a variety of structural parameters. Parameters determined from X-ray crystallography appear to be controlled by inter-ligand repulsion or the ionic radii of the metal atom. The change in the ν CO band upon coordination provides a reasonable ordering for a specific metal (i.e. AlI 3≈AlBr 3>AlCl 3), but does differentiate simply between 1:1 and 1:2 complexes. Unexplainably, the shift in the δ CO in the 13C-NMR appears to be the complete opposite of other trends. It appears important to differentiate, the ability of a Lewis acidic compound to bind a Lewis base from the effect the metal compound has upon that Lewis base once coordinated. In this regard, the K eq at 298 K and Δ G would appear to be the best measures of the ability of a Lewis acid to bind a specific Lewis base, while the Δ λ max for the 9- fluorenone complexes provides a good indication as to the effect of a Lewis acid on a particular Lewis base.

Phosphate-Binding Characteristics and Selectivity Studies of Bifunctional Organotin Carriers

The selective recognition of the orthophosphate anion by a series of bifunctional Lewis acidic organotin compounds is investigated. The binding affinity of these carriers to anions, as measured by NMR titrations in CH2Cl2 obeyed the potentiometric selectivity order phosphate > sulfate > perchlorate, and provided the corresponding complex-formation constants. More accurate calculations of these values were obtained by the segmented-sandwich-membrane method performed directly inside the liquid polymeric membrane. These carriers were also studied potentiometrically in polymeric liquid membranes. The results indicate that (PhBr2Sn)2CH2 is 2 to 5 orders of magnitude more selective towards phosphate over other oxoanions. These results set the ground for the development of a new series of highly selective anions carriers with a wide range of possible applications.

Hydrogen Transfer Hydrozirconation of Alkenes withiBuZrCp2Cl Catalyzed by Lewis-Acidic Metal Compounds Containing Al, Zn, Si, Ag, and Pd

The hydrozirconation reaction of monosubstituted alkenes with iBuZrCp2Cl can be significantly accelerated by catalytic amounts of various Lewis acidic metal compounds, most notably AlCl3, Me3SiI, and Pd complexes, such as Li2PdCl4 and Cl2Pd(PPh3)2.

Reactions of [ReN(Cl)(Me 2PhP) 2(HEt 2tcb)] with Lewis Acids. Synthesis, Characterization and Structures of [Re(NBBr 3)Br 2(Me 2PhP) 3], [Re(NGaCl 3)Cl(Me 2PhP) 2(H 2Et 2tcb)][GaCl 4] and [Re{NB(C 6F 5) 3}Cl(Me 2PhP) 2(HEt 2tcb)] (H 2Et 2tcb= N,N-diethylthiocarbamoylbenzamidine)

Reactions of [ReN(Cl)(Me 2PhP) 2(HEt 2tcb)] (HEt 2tcb −= N,N-diethylthiocarbamoylbenzamidinate, Me 2PhP=dimethylphenylphosphine) with Lewis acidic compounds such as BBr 3, (C 6F 5) 3B or gallium(III) chloride yield nitrogen-bridged binuclear complexes with covalent bonds between the nitrido ligand and boron or gallium. The reactions go along with activation of the transition metal centre which can lead to ligand re-arrangements and reactions with solvent molecules. The reaction with BBr 3 results in cleavage of the bonds to the chelating ligand. [Re(NBBr 3)Br 2(Me 2PhP) 3] was isolated as the only product with a nitrido bridge. The bis-chelate [ReN(HEt 2tcb) 2] was formed as a side-product. Upon formation of a nitrido bridge to GaCl 3 the first co-ordination sphere of rhenium is retained. The co-ordinated thiocarbamoylbenzamidinate, however, undergoes protonation and is bonded as a neutral, bidentate ligand in the product [Re(NGaCl 3)Cl(Me 2PhP) 2(H 2Et 2tcb)][GaCl 4]. In [Re{NB(C 6F 5) 3}Cl(Me 2PhP) 2(HEt 2tcb)], which can be obtained in good yield from the reaction of [ReN(Cl)(Me 2PhP) 2(HEt 2tcb)] with (C 6F 5) 3B, the co-ordination environment of the metal remains essentially unchanged. X-ray structural studies on the products suggest that the strong structural trans influence of the terminal nitrido ligand in the starting complex decreases significantly as a consequence of the formation of the N–B/Ga bonds. The rhenium–nitrogen multiple bond distances, however, remain almost unchanged.

Polyolefin/polystyrene in situ compatibilization using Friedel–Crafts alkylation

Polyolefin/polystyrene (PS) blends are difficult to compatibilize using in situ reactive compatibilization techniques, because neither of these polymers has any functional groups that one can use in the formation of a copolymer from these polymer components. In this study, the Friedel–Crafts alkylation was realized in a polyethylene/PS (PE/PS) melt blend, which resulted in improved compatibility between PE and PS. A number of Lewis acid compounds were tested as catalysts, among which the AlCl3 was the most efficient. It was found in this study that the presence of a cocatalyst, such as a cationically polymerizable monomer or a halogenated alkane, significantly enhances the formation of PE-g-PS copolymer. The effects of blending parameters, such as temperature and blending time, on the in situ copolymer formation were investigated. The mechanical properties of compatibilized PE/PS blends were improved considerably. Such an in situ compatibilization technique has potential in the recycling of mixed polymer wastes. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1385–1393, 1997

Polyolefin/polystyrenein situ compatibilization using Friedel-Crafts alkylation

Polyolefin/polystyrene (PS) blends are difficult to compatibilize using in situ reactive compatibilization techniques, because neither of these polymers has any functional groups that one can use in the formation of a copolymer from these polymer components. In this study, the Friedel–Crafts alkylation was realized in a polyethylene/PS (PE/PS) melt blend, which resulted in improved compatibility between PE and PS. A number of Lewis acid compounds were tested as catalysts, among which the AlCl3 was the most efficient. It was found in this study that the presence of a cocatalyst, such as a cationically polymerizable monomer or a halogenated alkane, significantly enhances the formation of PE-g-PS copolymer. The effects of blending parameters, such as temperature and blending time, on the in situ copolymer formation were investigated. The mechanical properties of compatibilized PE/PS blends were improved considerably. Such an in situ compatibilization technique has potential in the recycling of mixed polymer wastes. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 1385–1393, 1997

Polaron mechanism in the thermal stabilization of poly(vinyl chloride). Part II: action of metal chlorides

By polaron theory, it is shown that the mechanism of dehydrochlorination (DHC) of PVC is determined by the interaction between the conjugated double bonds in the polymer and the metal or organometallic chlorides. Lewis acid compounds, such as metal chloride, oxidize the π electrons of conjugated polyene sequences (CjPS) into polarons, which leads to accelerated chain DHC. By contrast, organotinchlorides can significantly delay DHC. The difference can be explained by the effect of the alkyl chains in the organotin compounds. It has been demonstrated that, by controlling the cis configuration of CjPS existing initially or developing in the polymer during the heat treatment, and by lowering the Lewis acidity of the organotin chlorides, the alkyl chains attached to the central metal atom contribute their stabilization effect by inhibiting the proliferation of polarons or by favoring the readdition reaction of HCl to conjugated double bonds and the cyclization of the polymer chain.

Poly(vinyl chloride) dehydrochlorination via the polaron mechanism

ESR studies show that the paramagnetic species detected during the dehydrochlorination of PVC are associated with the π-electrons, i.e. the solitons or polarons, but not with the conventional active radicals (resulting from σ-bond scission). The interaction of HCl or Lewis acid compounds or their combination with conjugated unsaturated systems (model compounds with two to eleven conjugated double bonds or dehydrochlorinated PVC) substantially supports the polaron mechanism for the thermal degradation of poly(vinyl chloride) (Tran & Guyot, Polym. Deg. Stab., 32 (1991) 93–103).

Macrocycles containing tin. Tin-119 NMR studies of chloride binding by Lewis acidic tin compounds

Abstract : This report updates and corrects information in Technical Report No. 6 (September 15, 1985). New data includes concentration effects on tin-119 chemical shifts, measurements of chemical shifts of dibutyltin dichloride (5) plus chloride ion at low concentrations, direct determination of relative complexing strengths of acyclic model 5 against macrocyclic hosts and modified calculations of host-guest binding energies in acetonitrile. Three macrocyclic Lewis acidic hosts containing two symmetrically disposed dichlorostanna moieties (1-3, ring sizes 18, 22, 26 atoms, respectively) have been prepared as has an acyclic di-tin model, 5,5-16,16-tetrachloro-5,16-distannadocosane (4). The complexation of 1-4 and dibutyltin dichloride (5) with chloride ion in acetonitrile was studied by tin-119 NMR spectroscopy. Rapid exchange of chloride between tin atoms occurred at all ratios of chloride to host studied. An iterative computer program was used to provide estimates of the binding constants of 1, 2, 4 and 5 with chloride. Competition experiments were performed wherein two hosts competed for limited chloride ion, and the tin-119 NMR spectra of the mixtures were analyzed to determine the relative amount of chloride bound by each host and hence the ratio of the binding constants for the two hosts. Good agreement between the relative binding constants found in the latter studies and the ratios of the absolute binding constants found in the former studies was found.

Silicon-directed selective gamma substitution of an α,β-unsaturated ester

Ethyl 3-methyl-2-trimethylsilyl-3-butenoate ( 3 ) undergoes reaction selectively at carbon 4, upon treatment with a Lewis acid and carbonyl compounds, acetals/ketals, acid chlorides, and chloromethyl phenyl sulfide. This overall conversion represents a highly selective gamma substitution on an α,β-unsaturated ester.