Thermochemistry of amineborane adduct formation of pyridine and picoline isomers

The enthalpy of reaction of pyridine, 2-, 3-, and 4-picoline with BH₃·THF and the enthalpy of solution of the same amines in THF were determined by reaction-solution calorimetry. From these data, the enthalpies of formation of the corresponding amineborane adducts in solution of THF were also determined. The results can be explained by considering the steric and inductive effects of the methyl group on the pyridine ring and the basicity of amines. The enthalpy of formation of the adducts in solution of THF correlates well with the available literature values of pK _a of amines also determined in THF, and the influence of the solvent on the basicity features of studied amines is verified.     

Stereochemistry of the Addition of Organometallic Reagents to Methyl 2,3-O-Isopropylidene-β-D-ribo-pentodialdo-1,4-furanose

Methyl 2, 3-O- isopropylidene-β-D-ribo-pen tod ialdo-1,4-furanoside, upon reaction with either methyl lithium or methyImagnesium iodide/ gave a ca. 2-3:1 mixture of β-D-allo to α-L-talo adducts. Reaction with 2-lithio-l,3-d ithiane gave much improved stereoselectivity, in line with either the Cram or Felkin model, to give the dithianyl adducts in a ratio of 97:3 of β-D-allo to α-talo isomers.     

AIPO4 Supported Zinc Borohydride as a Novel Reagent for the Hydration of Aromatic Alkenes and Alkynes

Hydration of aromatic alkenes (styrene, α-methylstyrene and E-stilbene) and alkynes (phenyl and diphenylacetylene) has been achieved by the reaction of the corresponding alkenes or alkynes on zinc borohydride combined with AIPO₄ in DME. Except in the case of α-methylstyrene, Zn(BH₄)₂/AIPO₄ provides a more efficient and selective catalytic system than the combination with SiO₂ or Al₂O₃.     

α,α-Difluoro-α-phenylsulfanylmethyl carbanion equivalent: a novel gem-difluoromethylenation of carbonyl compounds

α,α-Difluoro-α-phenylsulfanyl-α-trimethylsilylmethane (PhSCF₂SiMe₃) has been demonstrated as an α,α-difluoro-α-phenylsulfanylmethyl carbanion equivalent. gem-Difluorophenylsulfanylmethylation of carbonyl compounds has been successfully achieved by using PhSCF₂SiMe₃ in the presence of TBAF in THF. The adducts have been converted to the corresponding gem-difluoroalkenes by a novel pyrolytic and/or FVP elimination of the β-hydroxy-α-phenylsufinyl derivatives under reduced pressure.     

“Spontaneous” Ambient Temperature Dehydrocoupling of Aromatic Amine–Boranes

The dehydrocoupling/dehydrogenation behavior of primary arylamine–borane adducts ArNH₂ ? BH₃ ( 3 a – c ; Ar= a : Ph, b : p‐MeOC₆H₄, c : p‐CF₃C₆H₄) has been studied in detail both in solution at ambient temperature as well as in the solid state at ambient or elevated temperatures. The presence of a metal catalyst was found to be unnecessary for the release of H₂. From reactions of 3 a , b in concentrated solutions in THF at 22 °C over 24 h cyclotriborazanes (ArNH‐BH₂)₃ ( 7 a , b ) were isolated as THF adducts, 7 a , b? THF, or solvent‐free 7 a , which could not be obtained via heating of 3 a – c in the melt. The μ‐(anilino)diborane [H₂B(μ‐PhNH)(μ‐H)BH₂] ( 4 a ) was observed in the reaction of 3 a with BH₃?THF and was characterized in situ. The reaction of 3 a with PhNH₂ ( 2 a ) was found to provide a new, convenient method for the preparation of dianilinoborane (PhNH)₂BH ( 5 a ), which has potential generality. This observation, together with further studies of reactions of 4 a , 5 a , and 7 a , b , provided insight into the mechanism of the catalyst‐free ambient temperature dehydrocoupling of 3 a – c in solution. For example, the reaction of 4 a with 5 a yields 6 a and 7 a . It was found that borazines (ArN‐BH)₃ ( 6 a – c ) are not simply formed via dehydrogenation of cyclotriborazanes 7 a – c in solution. The transformation of 7 a to 6 a is slowly induced by 5 a and proceeds via regeneration of 3 a . The adducts 3 a – c also underwent rapid dehydrocoupling in the solid state at elevated temperatures and even very slowly at ambient temperature. From aniline–borane derivative 3 c , the linear iminoborane oligomer (p‐CF₃C₆H₄)N[BH‐NH(p‐CF₃C₆H₄)]₂ ( 11 ) was obtained. The single‐crystal X‐ray structures of 3 a – c , 5 a , 7 a , 7 b? THF, and 11 are discussed.     

Lanthanide borohydride complexes with an aryloxide ligand: Synthesis, structural characterization and polymerization activity

Reactions of LnCl₃, NaBH₄ and ArONa (Ar = C₆H₂-t-Bu₃-2,4,6) in a molar ratio of 1:3:1 in THF afforded the aryloxide lanthanide borohydrides of (ArO)Ln(BH₄)₂(THF)₂ (Ln = Yb (1), Er (2)). They were characterized by elemental analysis, infrared spectrum and X-ray crystallography. The two complexes are neutral and isostructural. The lanthanide atom is nine-coordinated by an aryloxide ligand, two borohydride ligands and two THF ligands in a trigonal bipyramidal geometry. Both of the BH₄ ligands in each monomeric complex are η³-coordinated. These complexes displayed moderate high catalytic activities for the polymerization of methyl methacrylate. The polymerization temperature had great influence on the catalysis. At about 0 °C, the catalysts showed the polymerization activity best.     

gem-Difluoromethylation of α- and γ-ketoesters: preparation of gem-difluorinated α-hydroxyesters and γ-butyrolactones

PhSCF₂SiMe₃ has been demonstrated as difluoromethyl carbanion synthon (⁻CF₂H). It reacts chemoselectively with α- and γ-ketoesters at the keto group in the presence of a catalytic amount of TBAF in THF to give the corresponding α-hydroxy ester adducts as well as γ-gem-difluorophenylsulfanylmethylated-γ-butyrolactones in good yields. Reductive cleavage of the phenylsulfanyl group of these products employing Bu₃SnH/AIBN gives the corresponding gem-difluoromethylated α-hydroxyesters and γ-butyrolactones in good yields.     

Enantio- and diastereoselective construction of vicinal quaternary and tertiary carbon centers by catalytic Michael reaction of α-substituted β-keto esters to cyclic enones

A catalytic enantio- and diastereoselective Michael reaction was achieved to construct vicinal quaternary and tertiary carbon centers in one step. Using 5 mol % of La(O-i-Pr)₃ and 10 mol % of a new N-linked-BINOL type ligand, the reaction of α-substituted β-keto esters to cyclic enones provided the corresponding Michael adducts in up to quantitative yield with a diastereomeric ratio up to 86/14 and enantiomeric excess up to 86% for the major isomer. An alternative catalyst preparation method using La(OTf)₃ instead of La(O-i-Pr)₃ was also examined.     

C-F?M interaction in anionic α-fluorovinyl rhenium oxycarbene complexes and their β-fluoroenolate analogs

The C-F?M⁺ interaction in anionic σ-(α-fluorovinyl)rhenium oxycarbene complexes, [RCFCFReC(O)R′(CO)₄]M (1-6), M = Na, Li, K is studied by ¹⁹F NMR in THF and Et₂O. The coordination of α-F to M⁺ results in an upfield shift of the corresponding ¹⁹F NMR signal and a decrease of ¹J_CF. The maximum shift is found for the Li salt of complex 4 in Et₂O (Δδ_Fα = 36.4 ppm), in which case a ⁷Li-¹⁹F spin-spin coupling is also observed (J_LiF = 40 Hz). The ΔE of C-F?M⁺ interaction and its effect on ¹⁹F shielding was further studied by DFT calculations using β-fluoroenolates as models, which confirmed a strong impact of CF-bond environment on the coordination ability of fluorine in these F,O-chelates. A compound with a β-fluoroenolate backbone but without rhenium, o-(α-fluorovinyl)phenolate 12, was prepared and studied by ¹⁹F NMR, and similarly showed indications of C-F?M⁺ interaction in THF solution. It is concluded that the donor ability of fluorine in the studied system is enhanced because of the conjugation of α-fluorovinyl group with the enolate π-system and back donation from the transition metal.     

Open metallocenes XI. A pentadienyl compound of heavier rare earth element [2,4-(CH3)2-η5-C5H5]3Tb·1/2THF

The reaction of TbCl₃ with K[2,4-(CH₃)₂ C₅H₅] at 0°C in THF followed by crystallization at ?90°C led to a pale yellow hexagonal prismatic crystalline product [2,4-[CH₃)₂-η⁵-C₅H₅]₃-Tb·1/ 2THF , which is highly sensitive to air and water and rapidly efflorescent at ambient temperature. The single crystal X-ray diffraction data of the compound have been collected at low temperature (-60°C) and the crystal structure has been solved by heavy atom method. It belongs to triclinic system, space group P1 with lattice parameters α=8.477 Å , b=12.583 Å , c=12.858 Å , α=118.08°, β=91.38°, γ=108.75°, V=1120.36 ų and Z=2. Least-squares refinement converged to a final value R=0.043. The compound possesses an idealized C_3h symmetry with three 2,4dimethylpentadienyl ligands bound to the central terbium atom in a pentahapto mode (η⁵). Each unit cell contains two molecules of [2,4-(CH₃)₂-η⁵-C₅H₅]₃Tb and one molecule of solvent THF, of which the role in the lattice has been discussed in detail.     

[Na · Triglyme]2[S(BH3)4]: Ein Salz des neuen Anions Tetrakis(boran)sulfat(2—) Kristallstruktur und theoretische Untersuchung der Molekülstruktur

[Na · Triglyme]₂[S(BH₃)₄]: a Salt of the New Anion Tetrakis(borane)sulfate(2? ). Crystal Structure and Theoretical Investigation of the Structure Na[H₃B-m?₂-S(B₂H₅)] 1 is produced by the reaction between NaSH and THF · BH₃, under dehydrogenation. 1 is also formed as the first ¹¹B-NMR-spectroscopically detectable reaction product by the reaction between anhydrous Na₂S and THF · BH₃. Adducts of BH₃ with the S^2? ion are not detectable in THF. The anion [S(BH₃)₄]^2? can however be obtained, by the addition of NaBH₄ to 1 in diglyme or triglyme respectively: [Na — Triglyme]₂[S(BH₃)₄] 2. 2 crystallizes in the monoclinic space group P2₁/n (Nr. 14). Structural data of 1 and 2 have been calculated by SCF methods. The anion of 2 may be viewed either as an adduct of B₂H₆ with S^2?, or as a bridge substituted thia derivative of B₂H₇^?; furthermore the anion of 2 is isoelectronic and isostructural with the SO ion.     

Synthesis and Structural Characterization of New Divanada‐ and Diniobaboranes Containing Chalcogen Atoms

The reaction of [Cp_nMCl_4?x] (M=V: n=2, x=2; M=Nb: n=1, x=0; Cp=η⁵‐C₅H₅) with LiBH₄ ? THF followed by thermolysis in the presence of dichalcogenide ligands E₂R₂ (E=S, Te; R=2,6‐(tBu)₂‐C₆H₂OH, Ph) and 2‐mercaptobenzothiazole (C₇H₅NS₂) yielded dimetallaheteroboranes [{CpV(μ‐TePh)}₂(μ₃‐Te)BH ? thf] ( 1 ), [(CpV)₂(BH₃S)₂] ( 2 ), [(CpNb)₂B₄H₁₀S] ( 3 ), [(CpNb)₂B₄H₁₁S(tBu)₂C₆H₂OH] ( 4 ), and [(CpNb)₂B₄H₁₁TePh] ( 5 ). In cluster 1 , the V₂BTe atoms define a tetrahedral framework in which the boron atom is linked to a THF molecule. Compound 2 can be described as a dimetallathiaborane that is built from two edge‐fused V₂BS tetrahedron clusters. Cluster 3 can be considered as an edge‐fused cluster in which a trigonal‐bipyramidal unit (Nb₂B₂S) has been fused with a tetrahedral core (Nb₂B₂) by means of a common Nb₂ edge. In addition, thermolysis of an in‐situ‐generated intermediate that was produced from the reaction of [Cp₂VCl₂] and LiBH₄ ? THF with excess BH₃ ? THF yielded oxavanadaborane [(CpV)₂B₃H₈(μ₃‐OEt)] ( 6 ) and divanadaborane cluster [(CpV)₂B₅H₁₁] ( 7 ). Cluster 7 exhibits a nido geometry with C_2v symmetry and it is isostructural with [(Cp*M)₂B₅H_9+n] (M=Cr, Mo, and W, n=0; M=Ta, n=2; Cp*=η⁵‐C₅Me₅). All of these new compounds have been characterized by ¹H NMR, ¹¹B NMR, and ¹³C NMR spectroscopy and elemental analysis and the structural types were established unequivocally by crystallographic analysis of compounds  1 – 4 , 6 , and 7 .     

Metallboranate und Boranatometallate. 14. Chlorid-tetrahydridoborate des Calciums und Strontiums

Metal Tetrahydridoborates and Tetrahydridoborato Metalates. 14. Chloro Tetrahydridoborates of calcium and Strontium The action of hydrogen chloride on E(BH₄)₂ (E ? Ca, Se) in a 1:1 mole ratio in tetrahydrofuran yields the title compounds isolated as their tetrahydrofuran adducts. No intermediates were detected by ¹¹B n.m.r. spectroscopy in the reaction of methanol with Ca(BH₄)₂. The final product is Ca[B(OCH₃)₄]₂ · THF.     

Mono-Glycosylated 3-N-Alkylcatechols: Direct Synthesis from Glycosylacetates, 1H Nmr Analysis and Conformational Studies

ABSTRACT

The direct coupling of 3-n-alkyl catechols to the acetate or trichloroacetimidate derivatives of β-D- or α-D-glycosides (glucose, galactose, xylose, mannose and maltose) catalyzed by BF₃Ot₂ has been studied. β-Glycosides with an equatorial acetate group at position 2 formed exclusively β adducts with yields of 60–80%. α-Glycosides with an equatorial acetate group at position 2 formed β adducts, while β-glycosides with an axial acetate group formed α adducts when activated as trichloroacetimidates, with yields of 70–85%. This was applied to the coupling of 3-n-alkylcatechols of increasing chain length (up to C15) to sugar derivatives. The coupling position of glycosides on the catechol was determined either by differential NOE experiments and by the regioselective synthesis of 1-(O-β-D-glucopyranosyl)-3-pentadecylcatechol, a water soluble analogue of the poison ivy skin allergen. ¹H NMR of acetylated and deprotected compounds were investigated and the conformational preferences of the C₆ side chain determined using molecular modeling.     

Beiträge zur Chemie des Phosphors. 240 [1]. Zum reaktiven Verhalten von Diphosphan-boran,P2H4 · BH3

Contributions to the Chemistry of Phosphorus. 240. On the Reactive Behaviour of Diphosphane-borane, P₂H₄ · BH₃ Under mild temperature conditions, the thermal decomposition of diphosphane-borane ( 1 ) gives rise to the formation of phosphane-borane, PH₃ · BH₃, and triphosphane-2-borane, PH₂? PH(BH₃)? PH₂ ( 2 ). In the presence of diphosphane-1,2-bis(borane), triphosphane-1,3-bis(borane), BH₃? PH₂? PH? PH₂? BH₃ ( 3 ), is formed additionally. The thermolysis product at room temperature is a polymeric solid of varying composition which contains phosphorus, boron, and hydrogen. Compound 1 reacts with metalating agents such as n-BuLi, LiBH₄, and NaBH₄ to furnish the borane-trihydrogendiphosphide ion, [PH₂? PH? BH₃]^?, which immediately disproportionates to give the corresponding mono-and triphosphane derivatives. In the presence of an excess of THF-borane and in the case of a 1 : 1 molar ratio of 1 : NaBH₄, the disproportionation does not occur and the new diphosphide derivative sodium-1,1,2-tris(borane)-1,2,2-trihydrogendiphosphide, Na[(BH₃)₂PH? PH₂BH₃] ( 4 ) can be obtained. The action of additional NaBH₄ yields the diphosphide dianion with four coordinated BH₃ groups.     

The Synthesis of Substituted α, β-Butenolides from γ-Hydroxy-α, β-Acetylenic Esters

Abstract

The α, β-butenolide ring system is found in a number of physiologically important natural products² and there has been recent interest in the development of methods of synthesis of compounds of this type.³ It is well known that α, β-unsubstituted butenolides may be prepared by catalytic hydrogenation of γ-hydroxy acetylenic acids.^4,5b Recently, an excellent route of γ-hydroxy acetylenic esters which involves the addition of the lithium acetylide salts of propiolic esters to aldehydes⁵ and ketones⁶ has become available. We have carried out the addition of ethyl lithiopropiolate (1) to cyclohexanone (2) and 4-t-butylcyclohexanone (3) and wish to report the conversion of these adducts into corresponding β-methyl or β-methyl-α-allyl-α, β-butenolides.     

Z-Selective intramolecular Horner-Wadsworth-Emmons reaction for the synthesis of macrocyclic alkenes

The Z-selective intramolecular Horner-Wadsworth-Emmons reaction of the substrates 7-12 (RO)₂P(O)CHR′CO₂Et (R′ = (CH₂)_nCHO) (R = Ph or o-tBuC₆H₄) gives the 13-18-membered cyclic alkenes selectively (up to Z:E = 97:3) in good yields using NaH in THF under high dilution conditions.     

Reaction of N-Tosylated Imines with α,β-Acetylenic Ketones via Organozinc Species

A new procedure for stereoselective synthesis of trisubstituted alkenes by an one-pot, three-component reaction of N-tosylated imines with α,β-acetylenic ketones and organozinc species CF₃COOZnEt was described. This reaction system provides β-branched Morita–Baylis–Hillman adducts with excellent yields in absence of any catalyst.     

Reversible coordinative chain transfer polymerization of styrene by rare earth borohydrides,chlorides/dialkylmagnesium systems

The ability of various rare earth borohydride and chloride complexes/n‐butylethylmagnesium systems to operate styrene chain transfer polymerization in mild conditions has been assessed. Thirteen precatalysts have been considered: the rare earth trisborohydrides Ln(BH₄)₃(THF)_x (x = 3, Ln = Nd (1), La (2), Sm (3), x = 2, Ln = Y (4), Sc (5)), the rare earth chlorides LnCl₃(THF)_x (x = 3, Ln = Nd (6), La (7), Sm (8), Y (9), x = 2, Ln = Sc (10)), the mixed La(BH₄)₂Cl(THF)_2.6 (11) and the half‐lanthanidocenes Cp*Ln(BH₄)₂(THF)₂ (Ln = Nd (12), La (13)). Six systems were found to be active precatalysts for the polymerization of styrene. 1 , 2 , and 11 led to an efficient transmetalation of the growing polystyrene chain with the simultaneous occurrence of βH elimination, whereas 7 , 12 , and 13 led to catalyzed chain growth behavior. It is noteworthy that the catalyzed chain growth obtained with 12 and 13 occurs with significant stereoselectivity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 802–814, 2010     

Bis(phosphinimino)methanide Borohydride Complexes of the Rare‐Earth Elements as Initiators for the Ring‐Opening Polymerization of ε‐Caprolactone: Combined Experimental and Computational Investigations

Rare‐earth‐metal borohydrides are known to be efficient catalysts for the polymerization of apolar and polar monomers. The bis‐borohydrides [{CH(PPh₂NSiMe₃)₂}La(BH₄)₂(THF)] and [{CH(PPh₂NSiMe₃)₂}Ln(BH₄)₂] (Ln=Y, Lu) have been synthesized by two different synthetic routes. The lanthanum and the lutetium complexes were prepared from [Ln(BH₄)₃(THF)₃] and K{CH(PPh₂NSiMe₃)₂}, whereas the yttrium analogue was obtained from in situ prepared [{CH(PPh₂NSiMe₃)₂}YCl₂]₂ and NaBH₄. All new compounds were characterized by standard analytical/spectroscopic techniques, and the solid‐state structures were established by single‐crystal X‐ray diffraction. The ring‐opening polymerization (ROP) of ε‐caprolactone initiated by [{CH(PPh₂NSiMe₃)₂}La(BH₄)₂(THF)] and [{CH(PPh₂NSiMe₃)₂}Ln(BH₄)₂] (Ln=Y, Lu) was studied. At 0 °C the molar mass distributions determined were the narrowest values (M?_w/M?_n=1.06–1.11) ever obtained for the ROP of ε‐caprolactone initiated by rare‐earth‐metal borohydride species. DFT investigations of the reaction mechanism indicate that this type of complex reacts in an unprecedented manner with the first B? H activation being achieved within two steps. This particularity has been attributed to the metallic fragment based on the natural bond order analysis.