Sourisak Sinbandhit

Heteroleptic Tin(II) Initiators for the Ring-Opening (Co)Polymerization of Lactide and Trimethylene Carbonate: Mechanistic Insights from Experiments and Computations

The tin(II) complexes {LO(x)}Sn(X) ({LO(x)}(-) =aminophenolate ancillary) containing amido (1-4), chloro (5), or lactyl (6) coligands (X) promote the ring-opening polymerization (ROP) of cyclic esters. Complex 6, which models the first insertion of L-lactide, initiates the living ROP of L-LA on its own, but the amido derivatives 1-4 require the addition of alcohol to do so. Upon addition of one to ten equivalents of iPrOH, precatalysts 1-4 promote the ROP of trimethylene carbonate (TMC); yet, hardly any activity is observed if tert-butyl (R)-lactate is used instead of iPrOH. Strong inhibition of the reactivity of TMC is also detected for the simultaneous copolymerization of L-LA and TMC, or for the block copolymerization of TMC after that of L-LA. Experimental and computational data for the {LO(x)}Sn(OR)complexes (OR=lactyl or lactidyl) replicating the active species during the tin(II)-mediated ROP of L-LA demonstrate that the formation of a five-membered chelate is largely favored over that of an eight-membered one, and that it constitutes the resting state of the catalyst during this (co)polymerization. Comprehensive DFT calculations show that, out of the four possible monomer insertion sequences during simultaneous copolymerization of L-LA and TMC: 1) TMC then TMC, 2) TMC then L-LA, 3) L-LA then L-LA, and 4) L-LA then TMC, the first three are possible. By contrast, insertion of L-LA followed by that of TMC (i.e., insertion sequence 4) is endothermic by +1.1 kcal mol(-1), which compares unfavorably with consecutive insertions of two L-LA units (i.e., insertion sequence 3) (-10.2 kcal mol(-1)). The copolymerization of L-LA and TMC thus proceeds under thermodynamic control.

Electrochemical Synthesis and Chemistry of Chiral 1-Cyanotetrahydroisoquinolines. An Approach to the Asymmetric Syntheses of the Alkaloid (−)-Crispine A and Its Natural (+)-Antipode

The stereoselective convergent total syntheses of both enantiomers of the tetrahydroisoquinoline (THIQ) alkaloid crispine A are described. The THIQ precursors (-)-6 (90:10 dr) and (-)-11 (85:15 dr) were prepared from the alkylation-reduction sequence of a common α-amino nitrile (+)-4 derivative that has been conveniently prepared by anodic cyanation. Elaboration of the pyrrolidine ring of the title compound was cleanly achieved by two efficient ring closures methods involving (a) the displacement of a halogen atom and (b) the formation of a cyclic iminium cation to afford (-)-crispine A in 90% and 85% yields, respectively. A crystallization of enantioenriched (-)-crispine A (90:10 er) with 1 equiv of (-)-DBTA afforded the tartrate salt (-)-14 (≥98:2 dr) in 81% yield. The absolute S configuration of (-)-crispine A was simply deduced from examination of the X-ray data of tartrate salt (-)-14. Likewise, the natural (+)-crispine A was prepared in seven workup steps in an overall 30% yield, and reciprocal crystallization with (+)-DBTA afforded the enantiomeric tartrate salt (+)-14 in a ≥98:2 dr. Both enantiomers of crispine A were liberated from their respective DBTA salts in ≥98:2 ers which were determined by proton and carbon NMR spectroscopy, utilizing (R)-(+)-tert-butylphenylphosphinothioic acid (+)-15 as chiral solvating agent.

A new route to 2-oxazolines, bis-oxazolines, and 2-imidazoline-5-ones from imidates using solvent-free cycloadditions: Synthesis, chemical properties, and PM3 MO calculations

Abstract The 1,3-dipolar cycloadditions between imidate 1, derived from dimethylaminomalonate and aldehydes 2(a-f), phthalaldehyde 2g, isophthalaldehyde 2h. 4-chlorophenylisocyanate 9a, ethoxycarbonylisothiocyanate 9d as dipolarophiles proceeds regioselectively in good yield using solvent-free conditions. Synthesis of ortho- and meta-bis-(2′-oxazoline-5′-yl)benzenes 6g, 6h are also reported. The regiochemistry and the reactivity in these cycloaddition reactions are rationalized by PM3 MO calculations. The stereoselective demethoxycarbonylation of dimethyl 2-oxazoline-4,4-dicarboxylate 4a is described. The mild hydrolysis of 5-(2-formylphenyl)-2-oxazoline 4g leads to the formation of a new 3,4-dihydroisoquinoline 8h in quantitative yield.

Addition d'hydrazone aux oléfines en milieu acide: cycloaddition polaire cationique [3+ + 2]

Abstract The stereospecificity and the orientation observed for the addition reaction of acetaldehyde hydrazone with (E)- and (Z)-alkenes in acidic media are compatible with a concerted process of the polar [3 + + 2] cycloaddition type.

Stable divalent germanium, tin and lead amino(ether)-phenolate monomeric complexes: structural features, inclusion heterobimetallic complexes, and ROP catalysis

Stable germanium(II) and lead(II) amido complexes {LO(i)}M(N(SiMe3)2) (M = Ge(II), Pb(II)) bearing amino(ether)phenolate ligands are readily available using the proteo-ligands {LO(i)}H of general formula 2-CH2NR2-4,6-tBu2-C6H2OH (i = 1, NR2 = N((CH2)2OCH3)2; i = 2, NR2 = NEt2; i = 3, NR2 = aza-15-crown-5) and M(N(SiMe3)2)2 precursors. The molecular structures of these germylenes and plumbylenes, as well as those of {LO(3)}GeCl, {LO(3)}SnCl and of the congeneric {LO(4)}Sn(II)(N(SiMe3)2) where NR2 = aza-12-crown-4, have been determined crystallographically. All complexes are monomeric, with 3-coordinate metal centres. The phenolate systematically acts as a N^O(phenolate) bidentate ligand, with no interactions between the metal and the O(side-arm) atoms in these cases (for {LO(1)}(-), {LO(3)}(-) and {LO(4)}(-)) where they could potentially arise. For each family, the lone pair of electrons essentially features ns(2) character, and there is little, if any, hybridization of the valence orbitals. Heterobimetallic complexes {LO(3)}M(N(SiMe3)2)·LiOTf, where the Li(+) cation sits inside the tethered crown-ether, were prepared by reaction of {LO(3)}M(N(SiMe3)2) and LiOTf (M = Ge(II), Sn(II)). The inclusion of Li(+) (featuring a close contact with the triflate anion) in the macrocycle bears no influence on the coordination sphere of the divalent tetrel element. In association with iPrOH, the amido germylenes, stannylenes and plumbylenes catalyse the controlled polymerisation of L- and racemic lactide. The activity increases linearly according to Ge(II) ≪ Sn(II) ≪ Pb(II). The simple germylenes generate very sluggish catalysts, but the activity is significantly boosted if the heterobimetallic complex {LO(3)}Ge(N(SiMe3)2)·LiOTf is used instead. On the other hand, with 10-25 equiv. of iPrOH, the plumbylenes afford highly active binary catalysts, converting 1000 or 5000 equiv. of monomer at 60 °C within 3 or 45 min, respectively, in a controlled fashion.

Regio- and Diastereoselective Synthesis of α-Cyanoamines by Anodic Oxidation of 6-Membered α-Silylamines

The electrochemical cyanation of N-benzyl-substituted cyclic six-membered α-silylamines including tetrahydroquinoline and piperidine derivatives was studied. The results of these investigations demonstrate that α-silylamines are valuable precursors for the preparation of the corresponding α-cyanoamines. The TMS group α to the N atom not only governs the chemoselectivity of the iminium formation through a preferential desilylation reaction under the experimental conditions (i.e., amine cation radical desilylation versus deprotonation), it also lowers the oxidation potential of tertiary amines compared to their non-TMS counterparts. Both the stereoselectivity and regioselectivity of the cyanide addition were investigated with 3-methylpiperidine as the model compound. The formation of a single cisdiastereoisomer in which the 2-cyano group is axial and the 5-methyl group is equatorial, indicates that the addition of the cyanide anion onto the iminium species is under stereoelectronic control. In addition, the redox reaction involving the intermediate nitrogen-centered cation radical and the cyanide anions played no role, because the α-silyl radical has such a short lifetime.

Lobarin from the Sumatran lichen, Stereocaulon halei☆

The diphenyl ether, lobarin (1) (syn. lobariol carboxylic acid) related to lobaric acid was isolated for the first time as a natural product along with five known compounds from Stereocaulon halei, a fruticose lichen collected in Indonesia. The structure of lobarin was elucidated by spectroscopic data analysis and its most stable conformers were determined by molecular mechanic dynamic calculations. A marked superoxide anion scavenging was found for compound 1 while no cytotoxicity on the B16 murine melanoma and HaCaT human keratinocyte cell lines was observed.

2-aza-1,3-dienes with electron-releasing substituents at the 1,3 positions. Reagents for the construction of pyridine and pyrimidine derivatives

Abstract New 2-aza-1,3-dienes bearing 1 and 3-donor substituents are prepared from N-thioacylacetamidines through deprotonation of N-ylidene acetamidinium iodides. The 2-aza-3-(dimethylamino)-1-(methylthio)-1-phenylbutadiene (3) is trapped in situ by the residual precursor salt acting as a heterodienophile to give the pyrimidine 5. Substituted 2-aza-1-(dimethylamino)-3-(methylthio) analogues react readily with a variety of electron-deficient dienophiles to yield pyridine or pyrimidine derivatives. The stereochemistry of the hetero Diels-Alder reaction in the cases of dimethyl fumarate and acrylonitrile has been assigned by X-ray diffraction analyses of the resulting tetrahydropyridines and corresponds to an exo selectivity. The number and nature of cycloadducts in the cases of dimethyl acetylenedicarboxylate and phenyl isothiocyanate depend on C-4 substitution. The results obtained from the C-4 unsubstituted azabutadiene 8 are explained by an allylic rearrangement involving the 1,3-migration of dimethylamino group in the primary [4+2] adduct.

Homodimetallic iron(II) hydrazones: syntheses, spectroscopic, electrochemical, and theoretical investigations. X-Ray crystal structure of both syn- and anti- rotamers of [(η5-Cp)Fe(η6-C6H5)–NHNC(Me)–(η5-C5H4) Fe(η5-Cp)]+PF6−

Two homogeneous series of homodimetallic iron(II) hydrazone complexes of general formula [CpFe(η6-p-RC6H4)–NHNCH–(η5-C5H4)FeCp]+PF6− (Cp=η5-C5H5): R=H, [5]+PF6−; Me, [6]+PF6−; MeO, [7]+PF6−; Cl, [8]+PF6−; and [CpFe(η6-p-RC6H4)–NHNCMe–(η5-C5H4)FeCp]+PF6−: R=H, [9]+PF6−; Me, [10]+PF6−; MeO, [11]+PF6−, have been prepared. These hydrazones were stereoselectively obtained as their trans isomers about the NC double bond, by reaction of the corresponding organometallic hydrazines [CpFe(η6-p-RC6H4NHNH2)]+PF6−, R=H, [1]+PF6−; Me, [2]+PF6−; MeO, [3]+PF6−; and Cl, [4]+PF6−, with formylferrocene, CpFe(η5-C5H4–CHO), and acetylferrocene, CpFe(η5-C5H4–CO-Me), respectively. All the new compounds were characterized by elemental analysis and IR, UV-Vis and 1H NMR spectroscopies. For compound [9]+PF6−, the activation energy ΔG# for the hindered rotations of the mixed sandwich [CpFe(η6-C6H5–)]+ (41.8±0.9 and 41.2±3.7 kJ mol−1) and the ferrocenyl unit (40.0±3.2 kJ mol−1) about the –NH–NCMe– hydrazone backbone have been determined by variable temperature 1H NMR spectroscopic studies. The electrochemical behavior of the dinuclear hydrazones was explored by cyclic voltammetry, and features the role of the cationic [CpFe(η6-arene)]+ moiety as an electron acceptor. The crystalline and molecular structure of [9]+PF6− was determined by single crystal X-ray crystallographic analysis. The structure shows the presence of both syn and anti rotamers in the asymmetric unit. The analysis of the electronic structure of [9]+ through DFT calculations indicate a strong perturbation of the metal centers by the hydrazone bridge as well as some metal-metal interaction through the bridge.

Syntheses of deuterated sandwich- and piano-stool organo-iron complexes and polyalkylbenzenes

Abstract Deuteration of cationic iron sandwich complexes of the FeCp(arene) + family has been achieved by use of 1 M NaOD in D 2 O. The deuterated complexes are the starting point for the syntheses of various organo-iron and aromatic compounds.