Organocatalytic tandem Michael addition reactions: A powerful access to the enantioselective synthesis of functionalized chromenes, thiochromenes and 1,2-dihydroquinolines

Enantioselective organocatalysis has become a field of central importance within asymmetric chemical synthesis and appears to be efficient approach toward the construction of complex chiral molecules from simple achiral materials in one-pot transformations under mild conditions with high stereocontrol. This review addresses the most significant synthetic methods reported on chiral-amine-catalyzed tandem Michael conjugate addition of heteroatom-centered nucleophiles to α,β-unsaturated compounds followed by cyclization reactions for the enantioselective construction of functionalized chiral chromenes, thiochromenes and 1,2-dihydroquinolines in optically enriched forms found in a myriad of bioactive natural products and synthetic compounds

Synthesis and structure of binuclear thioazobenzene pallada-cycles and their reaction with m-chloroperbenzoic acid: formation and structure of dinuclear azophenolates

The cyclopalladation of 1,3-bis(2-thioazobenzene)propane (L<SUP>1</SUP>H<SUB>2</SUB>) and its methyl substituted derivative L<SUP>2</SUP>H<SUB>2</SUB> affords complexes of the type LPd<SUB>2</SUB>Cl<SUP>2</SUP> (where L represents L<SUP>1</SUP> or L<SUP>2</SUP> which have been characterised by spectroscopy and X-ray crystallography. The crystal structure of L<SUP>1</SUP>Pd<SUP>2</SUP>C<SUP>2</SUP> has revealed that each azobenzene fragment along with its thioether sulphur acts in the tridentate (C,N,S) fashion and the fourth coordination position is occupied by a chloride ion. The complexes are thus of type [Pd(C,N,S)Cl]<SUB>2</SUB>, with a Pd Pd contact of 5.420(1) &#197;. The metallated carbon atom exerts a strong trans influence on the Pd---S bond. The reaction of LPd<SUB>2</SUB>Cl<SUB>2</SUB> with am-chloroperbenzoic acid leads to smooth oxygen insertion into both the Pd---C bonds to give excellent yields of dinuclear azophenolato complexes o-LPd<SUB>2</SUB>Cl<SUB>2 </SUB>having the coordination sphere [Pd(O,N,S)Cl]<SUB>2</SUB>. The crystal structure of o-L<SUP>1</SUP>Pd<SUB>2</SUB>Cl<SUB>2</SUB> is similar to that of L<SUP>1</SUP>Pd<SUB>2</SUB>Cl<SUB>2</SUB> but with a somewhat longer Pd...Pd distance of 5.890(1) &#197;. The insertion reaction has a negative entropy of activation, in keeping with an associative transition state for the electrophilic incorporation of oxygen. Reduction of the azophenolato complexes with hydrazine hydrate affords the free azophenols (o-LH<SUB>2</SUB>) in high yields. Thus organometallic route to o-LH<SUB>2</SUB> from LH<SUB>2</SUB> is provided