Eike Hupe

Formal enantioselective Michael addition with umpolung of reactivity.

By a sequence comprising asymmetric hydroboration, boron-zinc exchange, and transmetalation with copper, allylations, alkynylations, and allenylations of unsaturated ketals or acetals can be performed with good enantioselectivities (see scheme, (-)-IpcBH2 =(-)-isopinocampheylborane, TMS=Me3 Si). The products correspond to a formal Michael addition with inversion of polarity.

Diastereoselective synthesis and reactions of diorganozinc reagents obtained after hydroborations with 9-BBN-H, thexylborane and catecholborane

Abstract The scope of organoboranes can be greatly enhanced by a boron–zinc exchange reaction, providing organozincs that react with a broad range of electrophiles. Diastereomerically enriched diorganozincs can be obtained after hydroborations with 9-BBN-H, thexylborane and catecholborane.

Functionalized main-group organometallics for organic synthesis*

Highly functionalized Grignard-reagents can be easily obtained by an iodinemagnesim exchange reaction and further reacted with various electrophiles. Via a BZn exchange reaction, a formal Michael addition with umpolung of the reactivity can be achieved. Chiral phosphine ligands can be easily synthesized by a 2.3 sigmatropic rearrangement and used successfully in asymmetric synthesis.

Unsymmetrical functionalization of 1,3-cyclohexadienes: Palladium-catalyzed stereoselective 1,4-acyloxy-alkoxylation

Abstract Palladium-catalyzed reaction of 1,3-cyclohexadiene with alcohols and acetic or benzoic acid under mild acidic conditions gives trans -1-acetoxy- or trans -1-benzyloxy-4-alkoxy-2-cyclohexenes, respectively, with high regio- and stereo-selectivity. The unsymmtrical 1,4-acyloxy-alkoxy products are obtained by fine tuning the reaction conditions in a narrow pH range.

Selective transformations mediated by main-group organometallics

Main-group organometallics are useful intermediates for the chemoselective and diastereoselective C-C bond formation. The boron-zinc exchange is a unique way for preparing chiral secondary alkylzinc reagents which are configurationally stable over a wide temperature scale. Coupled with the thermal rearrangement of tertiary organoboranes, a broad range of open-chain and cyclic polyfunctional molecules have been prepared. In addition, several examples of a diastereoselective remote C-H activation have been studied. The second part shows that the main-group metallic alkoxides such as t-BuOK can be used for the performance of catalytic reactions such as a new indole synthesis.

One-pot chemoselective functionalization of arylsilanes via cascade metal–metal exchange reactions

A cascade-transmetallation sequence allows a one-pot conversion of arylsilanes into functionalized arylcoppers; they react with various types of electrophiles (allylic halides, acid chlorides, bromoalkynes); bimetallic zinc–copper-reagents can be prepared by this method either directly or in a sequential fashion.

Synthesis and reaction of secondary and primary diorganozinc reagents using a boron–zinc exchange reaction

Abstract Applications of the boron–zinc exchange reaction to make new carboncarbon bonds are reviewed. Functionalized chiral secondary alkylzinc reagents can be prepared by this exchange reaction and allows to perform formal enantioselective Michael-additions with umpolung of reactivity. The scope of substrate controlled diastereoselective hydroborations can be considerably enhanced by this methodology. A chemoselective approach to difunctionalized arylsilanes is also described.