Eta3-silapropargyl/alkynylsilyl molybdenum complexes: synthesis, structure, and reactivity toward methanol.

Novel eta(3)-silapropargyl/alkynylsilyl complexes Cp*(CO)(2)Mo(eta(3)-Ph(2)SiCCR) (3a, R = (t)Bu; 3b, R = (i)Pr) were synthesized by the reactions of Cp*(CO)(2)(MeCN)MoMe (1) with alkynylsilanes HPh(2)SiC[triple bond]CR. The structures of 3a and 3b were fully characterized by NMR spectroscopy and X-ray crystallography. The reaction of 3a with MeOH at room temperature gave the stable four-membered metallacycle Cp*(CO)(2)MoC(=CH(t)Bu)SiPh(2)OMe (5a), while the corresponding reaction of 3b led to the formation of the eta(3)-allyl complex Cp*(CO)(2)Mo{eta(3)-(MeOPh(2)Si)HCCHCMe(2)} (7) via Cp*(CO)(2)MoC(=CH(i)Pr)SiPh(2)OMe (5b).

Use of Sm(III)-{1,2-propanediamine-N,N,N',N'-tetra(alpha,alpha-dideuterioacetate)} complex for NMR determination of absolute configuration of each alpha-amino acid in peptide hydrolysate mixtures.

(1)H NMR analyses of individual alpha-amino acids in their mixture were simultaneously conducted in the presence of Sm-(pdta-d(8)) in water: high regularity, promising for direct simultaneous determination of absolute configurations of each alpha-amino acids in peptide hydrolysate mixtures, was observed between absolute configuration and the induced shifts.

Ortho-Vinylation Reaction of Phenols with Ethyne.

Phenols were vinylated at the ortho-position with ethyne in the presence of SnCl(4)-Bu(3)N reagent. The reaction was applicable to phenols possessing either electron-donating or electron-withdrawing groups. 2,6-Divinylphenols were synthesized under modified conditions. A reaction mechanism involving carbostannylation of alkynyltin and phenoxytin was discussed.

Confirmation by IR of the preferred conformations of CFTA esters in solution: a highly reliable criterion for the stereochemistry assignment of chiral alcohols.

Direct confirmation of the preferred conformation of diastereomeric esters derived from a chiral secondary alcohol and a chiral derivatizing agent in solution, which is crucial for reliable NMR-based assignment of absolute stereochemistry of the alcohol, has been attained for the first time by examination of IR spectra of the CFTA esters.

The origin of an unusually large 19F chemical shift difference between the diastereomeric alpha-cyano-alpha-fluoro-p-tolylacetic acid (CFTA) esters of 3',4',5,7-tetra-O-methylepicatechin.

During a study on the relationship between the (19)F chemical shift difference for the diastereomeric alpha-cyano-alpha-fluoro-p-tolylacetic acid (CFTA) esters of chiral secondary alcohols and the absolute configurations of the alcohols, an unusually large 19F chemical shift difference has been observed for the CFTA esters of 3',4',5,7-tetra-O-methylepicatechin. This large chemical shift difference has been reproduced by ab initio calculations of molecular geometry and GIAO calculations of 19F chemical shifts on the stable conformations of the diastereomeric alpha-cyano-alpha-fluorophenylacetic acid (CFPA) esters of cis-flavan-3-ol as model systems. The origin of the large chemical shift difference has been further investigated using NBO analysis. This revealed that the interaction between pi(C=C) of the aryl group in the chiral auxiliary and sigma*(C--F) is a very important determinant of the 19F chemical shifts and this interaction depends on the torsion angle between the plane of aryl group and the C--F bond.

Ab initio molecular orbitals study of the conformational preference in alpha-cyano-alpha-fluorophenylacetic acid ester.

The origin of conformational preference in alpha-cyano-alpha-fluorophenylacetic acid (CFPA) methyl ester that is a model system of alpha-cyano-alpha-fluoro-p-tolylacetic acid (CFTA) esters was theoretically investigated by means of DFT and MP2 calculations. Two stable conformations having the C-F bond syn and anti to the C=O bond, respectively, were obtained for CFPA methyl ester. A small energy difference (0.9 kcal mol-1 at the MP2(fc)/6-31++G(d,p)) was found between the two conformations. From the molecular orbital analysis based on the Natural Bond Orbital analysis and supported by calculations using the Orbital Deletion Procedure technique, we found that sigma-(sigma*+pi*)(C=O) and sigma-sigma*(Ph) and pi(Ph)-sigma* hyperconjugations are the main factors responsible for the conformational preference. The role of the fluorine atom on the stereogenic center was also clarified.

Largest discrete supramolecular architecture based on a lanthanide ion: [lanthanum(III)18{N,N'-bis(2-pyridylmethyl)-N,N'-1,2-ethanediylbis(D-alaninate)}18(H2O)36]18+.

The largest discrete octadecameric architecture based on a lanthanide ion has been found in the crystal structure of a La{N,N'-bis(2-pyridylmethyl)-N,N'-1,2-ethanediylbis(D-alaninate)} complex.

eta1:eta2-Alkynyl-bridged W-Si complexes: formation, structure, and reaction with acetone.

Reactions of (eta5-C5Me4R)(CO)2(MeCN)WMe (R = Me, Et) with HPh2SiCCtBu gave the novel alkynyl-bridged W-Si complexes, (eta5-C5Me4R)(CO)2W(mu-eta1:eta2-CCtBu)(SiPh2) (R = Me, Et), whose alkynyl ligands bridge the tungsten and silicon atoms in an eta1:eta2-coordination mode. The structures of these complexes were fully characterized, including X-ray crystallography. Treatment of (eta5-C5Me5)(CO)2W(mu-eta1:eta2-CCtBu)(SiPh2) with acetone resulted in acetone insertion into the silicon-alkynyl linkage followed by intramolecular C-H activation of the tBu group to give the chelate-type alkyl-alkene complex, (eta5-C5Me5)(CO)2W(eta1:eta2-CH2CMe2C=CHSiPh2OCMe2).

Novel eta 3 -1-silaallyl tungsten complexes via Si-H bond activation of hydrovinylsilanes: structure and reactivity toward methanol.

Reactions of cis-Cp*(CO)2W(MeCN)Me (1) with HSiMe2(CH=CR2) (R = H, Me) afford the novel eta3-1-silaallyl complexes Cp*(CO)2W(eta3-Me2SiCHCR2) [R = H (2), Me (3)] accompanied by liberation of MeCN and CH4 via thermal Si-H bond activation. eta3-Coordination and exo conformation of the 1-silaallyl ligand in 3 are shown by X-ray crystal analysis, which reveals the partial double bond character of the Si-C bond (1.800(4) A) in the silaallyl moiety. Complexes 2 and 3 show extremely high reactivity toward MeOH to give the hydrido-(methoxysilyl)alkene complex trans-Cp*(CO)2WH(eta2-MeOMe2SiCH=CH2) (4) and the four-membered metallacycle Cp*(CO)2WCH(CHMe2)SiMe2OMe (6), respectively.