Dynamic stability of carbonyl ylides

The non- and fluorine-substituted singlet carbonyl ylides are studied by using ab initio MCSCF calculations. The thermodynamic stability of the carbonyl ylides and the intramolecular stability to isomerization or fragmentation reaction coordinates is demonstrated in terms of the topological structure of the ab initio potential energy surfaces. The allylic resonance is found to be dynamically unstable, considering out-of-plane vibrational mode. The instability is studied by the symmetries of the low-lying excitations out of the MCSCF wave function.     

Carbon-carbon bond-forming reaction of cyclic sulfonium ylides stabilized by a carbonyl group

Rhodium(II)-catalyzed decomposition of diazoketones 1 and 5 bearing a cyclic dithioacetal, in the presence of aldehyde and ClTi(Oi-Pr)(3), afforded both or one of the C=C-bonded products, i.e., ring-enlarged enone 2 and ring-transformed thiophenone 3, that were formed between aldehyde and intermediate bicyclosulfonium ylide. The stereochemistry of the exocyclic C=C bond in the products was exclusively Z. The sulfonium atom that transiently composed the ylide was incorporated into products, but no oxirane was formed.     

Sequential trapping of dichlorocarbene and dichlorocarbonyl ylides

The thermal decomposition of phenyl (bromodichloromethyl)-mercury in the presence of aryl aldehydes and dimethyl acetylenedicarboxylate leads to the formation of dimethyl 2-halo-5-arylfuran-3, 4-dicarboxylates by selective trapping of the intermediate dihalocarbonyl ylides.     

Dipolar cycloaddition of rhodium-generated carbonyl ylides with p-quinones

[reaction: see text] The dipolar cycloaddition of carbonyl ylides generated by the rhodium-catalyzed decomposition of delta- and epsilon-carbonyl-alpha-diazoketones with p-quinones leads to both C=O and C=C addition products. The product ratio is solvent- and catalyst-dependent and has been optimized to favor formation of either product. The C=C addition products of naphthoquinones are used in the assembly of structures hybridizing the illudin and anthraquinone anticancer agents.     

Cycloaddition chemistry of carbonyl ylides for alkaloid synthesis

As highlighted in this mini-review, a growing area of interest in organic synthesis involves the use of substituted carbonyl ylides as 1,3-dipoles for the preparation of alkaloidal natural products. Cascade reactions proceeding by an intramolecular 1,3-dipolar cycloaddition of carbonyl ylides are of particular interest to the synthetic organic community because of the increase in molecular complexity involved and the high isolated yields.     

Reactions of cyclic sulfur ylides with some carbonyl compounds

The reaction of a six‐membered sulfonium ylide 5 with aldehydes or ketones afforded the oxirane derivatives 6a–d as a mixture of cis and trans isomers in excellent yields. In addition, the same reactions, using five‐ or six‐membered cyclic oxosulfonium ylides 7 and 11 , gave the corresponding oxirane derivatives in good yields. Moreover, the reaction of 11 with two equimolar amounts of base and 4‐hexen‐3‐one afforded the cyclooctene oxide derivative 16 with high stereoselectivity in 59% yield via a sequential Michael–Michael‐type addition of the ylide and the resulting enolate ion followed by an intramolecular Corey–Chaykovsky reaction. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:216–222, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10022     

Intramolecular cycloaddition of carbonyl ylides generated from α-diazo ketones

Two cases of intramolecular cycloaddition of carbonyl ylides, formed from the α-diazo-ketones1 and8, to a C=C bond within the molecule are described. The structures of the products4a and10 have been established from chemical and spectroscopic evidence and by single crystal X-ray crystallographic analysis of6a and10.     

Intramolecular nucleophilic carbonyl trapping of alpha-ketenyl radicals by an amino group

Free-radical carbonylation of omega-alkynylamines with tributyltin hydride gives a mixture of alpha-methylene lactams and alpha-stannylmethylene lactams. Nucleophilic addition of an internal amino group to the carbonyl group of alpha-ketenyl radicals is proposed as the cyclization step. The subsequent unusual 1,4-H shift from the resulting 1-hydroxyallyl radical, followed by elimination of the beta-tributyltin radical leads to the formation of alpha-methylene lactams.     

Diastereoselectively switchable enantioselective trapping of carbamate ammonium ylides with imines

The diastereoselectively switchable enantioselective trapping of protic carbamate ammonium ylides with imines is reported. The intriguing Rh(2)(OAc)(4) and chiral Br?nsted acid cocatalyzed three-component Mannich-type reaction of a diazo compound, a carbamate, and an imine provides rapid and efficient access to both syn- and anti-α-substituted α,β-diamino acid derivatives with a high level control of chemo-, diastereo-, and enantioselectivity.     

Dipole-LUMO/dipolarophile-HOMO controlled asymmetric cycloadditions of carbonyl ylides catalyzed by chiral Lewis acids

We have found the first successful example of reverse-electron-demand dipole-LUMO/dipolarophile-HOMO controlled cycloaddition reactions between carbonyl ylides, which were generated from o-methoxycarbonyl-alpha-diazoacetophenone and their acyl derivatives as precursors, and vinyl ether derivatives with high levels of asymmetric induction (97-77% ee) using chiral 2,6-(oxazolinyl)pyridine-Eu(III) or binaphthyldiimine-Ni(II) complexes as chiral Lewis acid catalysts.     

Poly(phenylacetylene)s bearing a peptide pendant: helical conformational changes of the polymer backbone stimulated by the pendant conformational change

Optically active, cis-transoid poly(phenylacetylene) derivatives bearing a poly(gamma-benzyl-L-glutamate) [poly(PBGAm)] or poly(L-glutamic acid) [poly(PGAm)] chain as the pendant were prepared by polymerisation of the corresponding macromonomer with a rhodium catalyst followed by hydrolysis of the pendant ester groups. Their conformational changes in solution, induced by a helix-coil transition of the pendant polypeptides, were investigated using circular dichroism (CD) and absorption spectroscopies. A series of macromonomers with a different peptide chain lengths was synthesised by the polymerisation of the N-carboxyanhydride of gamma-benzyl-L-glutamate with a phenylacetylene bearing an alanine residue as the initiator. The obtained macromonomers (PBGAm) were further polymerised with a rhodium catalyst in N,N-dimethylformamide (DMF) to yield novel poly(phenylacetylene)s [poly(PBGAm)] with a poly(gamma-benzyl-L-glutamate) pendant. The poly(PBGAm) exhibited an induced circular dichroism (ICD) in the UV/Vis region of the polymer backbone in dimethyl sulfoxide (DMSO), probably due to the prevailing one-handed helix formation. The Cotton effect signs of a DMSO solution of the poly(PBGAm) were inverted and accompanied by a visible colour change in the presence of an increasing amount of chloroform or DMF containing lithium chloride. The results suggest that poly(PBGAm) may undergo a conformational change such as a helix-helix transition with a different helical pitch responding to a change in the alpha-helix content of the poly(gamma-benzyl-L-glutamate) pendant. Moreover, a water-soluble poly(PGAm) also showed a similar, but dramatic change in its helical conformation with a visible colour change stimulated by a helix-coil transition of the pendant poly(L-glutamic acid) chains by changing the pH in water.     

Cobalt cluster-containing carbonyl ylides for catalytic,three-component assembly of oxygen heterocycles

[reaction: see text] A dicobalt hexacarbonyl (Co(2)(CO)(6)) cluster is essential for the unusually broad dipolarophile scope and for the sense and degree of diastereoselection in a catalytic, three-component synthesis of tetrahydrofurans and dihydrofurans. Likely involving a new class of carbonyl ylide, these cycloadditions are stereospecific with respect to the dipolarophile and exhibit high diastereoselectivity and regioselectivity in most cases. Differentiation of all four positions of the tetrahydrofuran can thus be accomplished in a triply convergent manner.     

Exciton trapping at heterojunctions in polymer blends

Optoelectronic devices made from semiconductor polymers often employ partially phase-separated binary polymer blends with "distributed heterojunctions" in the polymer film, and the migration of bulk excitons towards these heterojunctions crucially influences the device performance. Here, we investigate exciton migration in blend films of two polyfluorene derivatives. Localized exciplex states form in electron-hole capture at the heterojunction between the two polymers and these can be thermally excited to transfer to bulk excitons. Rapid radiative emission from these excitons can then allow efficient light-emitting diode operation. We show here that when these excitons migrate to another heterojunction site within their lifetime they are re-trapped at the interface and again form exciplex states or dissociate completely. We demonstrate that in polymer blend light-emitting diodes this can reduce the exciton population by more than 54% and can strongly influence the emission spectrum. We then analyze exciton re-trapping in detail using time-resolved photoluminescence spectroscopy on blends with different morphologies and find that for nanometer-scale phases exciton emission is completely suppressed. We show that the data agree well with a simple kinetic model which confirms the importance of the blend morphology for the exciton trapping efficiency.     

Effect of carrier concentration on the facilitated oxygen transport in a polymer membrane

Facilitated transport of oxygen in a solid Polymer membrane containing cobaltporphyrin (CoP) which carries oxygen specifically and reversibly, leads to permselectivity of oxygen against nitrogen in the membrane. The increase in concentration of the CoP carrier is expected to enhance the oxygen transport. The membranes of poly(octylmethacrylate-co-vinylimidazole) containing 0.8 ～ 10 wt% CoP were prepared, and the effects of the CoP-concentration on the transport and the diffusion constants of oxygen are studied. Although the induction period before the steady state of oxygen permeation was prolonged with the CoP concentration in the polymer membrane, the glass transition temperature (T_g) of the membrane was increased and the diffusion constants of oxygen were decreased with the CoP concentration to yield unexpected reduction of the oxygen permeation in the highly CoP loaded membrane.