Kinetics and thermodynamics of H. transfer from (eta5-C5R5)Cr(CO)3H (R = Ph,Me, H) to methyl methacrylate and styrene

The rates of H/D exchange have been measured between (a) the activated olefins methyl methacrylate-d(5) and styrene-d(8), and (b) the Cr hydrides (eta(5)-C(5)Ph(5))Cr(CO)(3)H (2a), (eta(5)-C(5)Me(5))Cr(CO)(3)H (2b), and (eta(5)-C(5)H(5))Cr(CO)(3)H (2c). With a large excess of the deuterated olefin the first exchange goes to completion before subsequent exchanges begin, at a rate first order in olefin and in hydride. (Hydrogenation is insignificant except with styrene and CpCr(CO)(3)H; in most cases, the radicals arising from the first H. transfer are too hindered to abstract another H. .) Statistical corrections give the rate constants k(reinit) for H. transfer to the olefin from the hydride. With MMA, k(reinit) decreases substantially as the steric bulk of the hydride increases; with styrene, the steric bulk of the hydride has little effect. At longer times, the reaction of MMA or styrene with 2a gives the corresponding metalloradical 1a as termination depletes the concentration of the methyl isobutyryl radical 3 or the alpha-methylbenzyl radical 4; computer simulation of [1a] as f(t) gives an estimate of k(tr), the rate constant for H. transfer from 3 or 4 back to Cr. These rate constants imply a DeltaG (50 degrees C) of +11 kcal/mol for H. transfer from 2a to MMA, and a DeltaG (50 degrees C) of +10 kcal/mol for H. transfer from 2a to styrene. The CH(3)CN pK(a) of 2a, 11.7, implies a BDE for its Cr-H bond of 59.6 kcal/mol, and DFT calculations give 58.2 kcal/mol for the Cr-H bond in 2c. In combination the kinetic DeltaG values, the experimental BDE for 2a, and the calculated DeltaS values for H. transfer imply a C-H BDE of 45.6 kcal/mol for the methyl isobutyryl radical 3 (close to the DFT-calculated 49.5 kcal/mol), and a C-H BDE of 47.9 kcal/mol for the alpha-methylbenzyl radical 4 (close to the DFT-calculated 49.9 kcal/mol). A solvent cage model suggests 46.1 kcal/mol as the C-H BDE for the chain-carrying radical in MMA polymerization.     

Heterotricyclodecane VIII. (−)-(1S, 3R, 6R, 8R)-2, 7-Dioxaisotwistan und (−)-(1R, 3R, 6R, 8R)-2, 7-Dioxa-twistan; Synthese und Bestimmung der absoluten Konfiguration

A synthesis and the determination of the absolute configuration of (?)-(1S, 3R′ 6R, 8R)-2, 7-dioxa-isotwistane ( 13 ) and (?)-(1R, 3R, 6R, 8R)-2, 7-dioxa-twistane ( 14 ) is described. The results for 14 are compared with those for carboeyclic (+)-twistane ( 2 ) of known chirality.     

Convergent synthesis of (2R,3R,8R,9R)-N-Boc-ADDA

The convergent synthesis of N-Boc-(2R,3R,8R,9R,4E,6E)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldecadenoic acid (enantio-N-Boc-ADDA) is reported. Our flexible approach takes advantage of highly efficient non-aldol aldol and cross-metathesis methodologies.     

Asymmetric synthesis of (S,R)- and (R,R)-methiin stereoisomers

Abstract

An asymmetric synthesis of (+)- and (–)-methiine (S-methyl-(R)-cysteine sulfoxide) diastereomers has been developed. These natural sulfur compounds were isolated from a variety of Brassica vegetables. As the starting compound, (R)-cysteine was used, which was methylated to form (R)-S-methylcysteine. Then the oxidation of S-methylcysteine with tert-butyl hydroperoxide catalyzed by the chiral tetra(isopropylate)titanium/(S)- or (R)-Binol complex led to the formation of (1?R,2S)-(+)- or (1?R,2R)-(–)-methiin stereomers.     

Kinetics and enthalpy change in decomposition of Ni(NCS)2(3-R-pyridine)4 complexes (R=ethyl,Cl, Br,CN, NH2)

Isothermal TG and DSC measurements were used to study the effect of the pyridine substituent (3-R) on the kinetics and enthalpy change in the thermal decomposition of Ni(NCS)₂(3-R-py)₄ complexes.The found activation energies (E) of the interface advance decreased with the increase in volume of the substituent, allowing the assumption of a dissociative activation mechanism in the thermal decomposition reaction.The relations between (E) and H and the occurrence of the kinetic compensation effect InA=f(E) are discussed.Part XX in the series Heterogeneous reactions of solid Ni(II) complexes.     

Synthesis, structure and antitumor activity of a water-soluble platinum complex, (1R,3R,4R,5R)-(-)-quinato(1R,2R-cyclohexanediamine)platinum (II).

The reaction of dihydroxo(1R,2R-cyclohexanediamine)platinum(II) with (-)-quinic acid gave a water soluble complex, (-)-quinato(1R,2R-cyclohexanediamine)platinum(II). The crystal structure of the complex was determined by X-ray analysis. The data indicate a chelation of the alpha-hydroxycarboxylic acid part of quinic acid to platinum(II). The complex shows moderate antitumor activity against murine leukemia L1210 at high doses (T/C x 100 = 179% at a dose of 200 mg/kg).     

Studies on the transformation of nitrosugars into iminosugars III: synthesis of (2R,3R,4R,5R,6R)-2-(hydroxymethyl)azepane-3,4,5,6-tetraol and (2R,3R,4R,5R,6S)-2-(hydroxymethyl)azepane-3,4,5,6-tetraol

A divergent synthesis of the two novel polyhydroxylated azepanes (2R,3R,4R,5R,6R)-2-(hydroxymethyl)azepane-3,4,5,6-tetraol and (2R,3R,4R,5R,6S)-2-(hydroxymethyl)azepane-3,4,5,6-tetraol from d-mannose is described. The method involves a Henry reaction between dimethyl-tert-butylsilyl 2,3-O-isopropylidene-α-d-lyxo-pentodialdo-1,4-furanoside and 2-nitroethanol followed by a reductive ring closure of the resulting epimeric nitro aldols. Glycosidase inhibition tests showed that (2R,3R,4R,5R,6S)-2-(hydroxymethyl)azepane-3,4,5,6-tetraol exhibits a weak but selective inhibition against α-l-fucosides.     

Stoffwechselprodukte von Mikroorganismen. 237. Mitteilung. (2S, 3R, 4R, 6R)-2,3,4-Trihydroxy-6-methylcyclohexanon aus zwei Actinomyceten-Stämmen

(2S, 3R, 4R, 6R)-2,3,4-Trihydroxy-6-methylcyclohexanone from Two Strains of Actinomycetes A tetrazolium-blue positive compound was isolated from two strains of acinomycetes. Its constitution and relative configuration 1 were determined by spectroscopic methods, and the absolute configuration by degradation to (+)-(R)-methylsuccinic acid.     

Novel,short, stereospecific synthesis of lyxo-(2R,3R,4R)-phytosphingosine and erythro-(2R,3S)-sphingosine

Lyxo-phytosphingosine and erythro-sphingosine have been elaborated from a common intermediate. The key step in the reaction sequence involves stereo- and regiospecific functionalization of an olefin by intramolecular nucleophilic sulfinyl group participation.     

Crystal structure of (S,S)-diphenylethanediammonium (R,R)-tartrate

Summary The crystal structure of (–)-diphenylethanediammonium-(R,R)-tartrate was determined. From this structure determination, the (S,S) configuration was assigned to the (–)-diphenylethanediamine. The asymmetric unit of the crystal structure contains two units of the title compound plus one molecule of ethanol and one water molecule, which form an intricate network of 19 hydrogen bonds.

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Die Kristallstruktur von (S,S)-Diphenylethandiammonium-(R,R)-tartrat

Zusammenfassung Es wurde die Kristallstruktur von (–)-Diphenylethandiammonium-(R,R)-tartrat bestimmt. Aus dieser Strukturbestimmung ergab sich die Zuordnung der (S,S)-Konfiguration zum (–)-Diphenylethandiamin. Die asymmetrische Einheit der Kristallstruktur besteht aus zwei Formeleinheiten der Titelverbindung sowie einem Molekül Ethanol und einem Wassermolekül, welche ein komplexes Netzwerk von insgesamt 19 Wasserstoffbrücken bilden.

     

Total syntheses of beta-carboline alkaloids, (R)-(-)-pyridindolol K1, (R)-(-)-pyridindolol K2, and (R)-(-)-pyridindolol.

The total syntheses of beta-carboline alkaloids, (R)-(-)-pyridindolols (1, 5, and 6) are described. The two key steps involved are (1) a thermal electrocyclic reaction of the 3-alkenylindole-2-aldoxime 10 and (2) a thermal cyclization of 3-alkynylindole-2-aldoxime 11 to construct the beta-carboline N-oxides 8, which upon heating with acetic anhydride and sequential treatment with trifluoromethanesulfonic anhydride gave the triflates 18. The Stille coupling reaction of 18 with vinylstannane, followed by cleavage of MOM ether, afforded the 1-ethenyl-3-hydroxymethyl-beta-carboline (7a). Subsequent acetylation of 7a yielded the acetate 7b, which was subjected to the Sharpless asymmetric 1,2-dihydroxylation by AD-mix-beta to produce (R)-(-)-pyridindolol K2 (6). Selective acetylation of 6 was effected by Ac(2)O and collidine to form (R)-(-)-pyridindolol K1 (5). By contrast, hydrolysis of 6 provided (R)-(-)-pyridindolol (1).