Using first-principles molecular dynamics, we have simulated reactions that can be induced by mechanical stress in a polymer. We have stretched a small piece of poly(ethylene glycol) (PEG) in water at finite temperature. Both the molecule and the solvent were described quantum mechanically on an equal level. The formation of ions was observed, which corresponds to a heterolytic bond cleavage. We were able to monitor the motion of the electrons during the reactions. Our simulations show that the electron transfer and the breaking of the bond occur almost simultaneously and that both processes are initiated by the approach of a solvent molecule toward the destabilized bond.
Consecutive reactions [1] have gained increasing importance and interest since efficient syntheses inherently claim to generate a maximum of structural complexity in few steps and in good yields from simple starting materials and with high chemo-, regio-, and stereoselectivity. In such reactions, the directly preceding step forms the functionality necessary for the following transformation. Particularly with regard to multicomponent reactions, the development of novel cascade reactions for the in-situ-generation of reactive functional groups is an intensively pursued goal. Ideally, all these processes occur in a consecutive fashion, occasionally by successive addition of reagents, without the isolation of intermediates and in the sense of a ªone-pot synthesisº. A model for these consecutive processes are multicomponent condensations [2] as well as palladium-catalyzed cascade reactions, [3] which have proved particularly useful in many cases owing to the mild reaction conditions and the pronounced tolerance for functional groups.In the course of our work on the chemistry of (arene)Cr-(CO) 3 complexes with conjugated side chains, [4] we now have found that the usually reliable Sonogashira coupling [5] of chloroarene complexes 1 with 1-aryl prop-2-ynoles 2 does not furnish the expected alkyne coupling products, the propargyl alcohols 3, but that the isomeric aryl complexed chalcones 4 [6] are formed in good yields (Scheme 1). Scheme 1. Synthesis of chromium carbonyl complexed chalcones by means of a coupling ± isomerization sequence.
We study with an ab initio molecular dynamics method the bond-breaking and bond-forming processes in chemical reactions. To obtain reactive trajectories, we use a newly developed method based on the optimization of a suitably defined action. The Hellmann-Feynman forces, which are needed to optimize the action, are calculated within density-functional theory. We contrast a concerted [4+2] cycloaddition of cyclopentadiene and ethylene with the nonconcerted [2+2] cycloaddition of two ethylene molecules. We find that the duration of the bond-breaking and bond-forming processes due to the nuclear motion is ∼100 fs. Moreover the electronic delocalization, as well as the HOMO-LUMO energy gap during the two reactions, allows us to distinguish clearly between the concerted and the nonconcerted mechanism.
An Unexpected Coupling-Isomerization Sequence as an Entry to Novel Three-Component-Pyrazoline Syntheses.-Sonogashira coupling of terminal arylpropargyl alcohols (II) with sufficiently electron-poor sp 2 -hybridized halogen compounds (I) results in the unexpected formation of trans-configured enones (III) (10 examples) in good yields. This unusual coupling-isomerization sequence enables the development of a novel one-pot synthesis of 3,5-disubstituted 2-pyrazolines [cf. (VI), 4 examples] via Michael addition-cyclocondensation of hydrazine to in situ formed α,β-unsaturated carbonyl compounds. -(MUELLER, THOMAS J. J.; ANSORGE, MARKUS;
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