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Monday, June 13, 2005
Douglass Taber
University of Delaware

Best Synthetic Methods: C-C Bond Formation

Carbon-carbon bond formation is basic to organic synthesis. Progress is made by developing new transformations, but it also is made by developing more practical and scalable procedures for already-known transformations

Ketone methylenation is usually effected with Ph3P=CH2, scarcely an atom-efficient process. Tu-Hsin Yan of National Chung-Hsing University reports (Org. Lett. 2004, 6, 4961. ) that reduction of CH2Cl2 with Mg powder in the presence of TICl4 leads to a reagent that efficiently homologates even easily-enolizable ketones such as 1. The reagent also converts esters such as 3 to alkenes, albeit more slowly.

Carbon monoxide is an inexpensive feedstock. David J. Cole-Hamilton of St. Andrews University has found (Chem. Comm. 2004, 1720. ) Pd catalysts that effect isomerization of internal alkenes. The transient less-stable terminal alkene is selectively homologated to the corresponding ester. Remarkably, conditions can be tuned such that the alkene 5 can be converted to either 6 or 7.

Carbonylation with CO in the presence of H2 leads to aldehydes. Bernhard Breit of Albert-Ludwigs-Universität, Freiburg, has found (Chem. Comm. 2004, 114. ) that conversion of an allylic alcohol to the o-diphenylphosphanylbenzoate 8 allows highly diastereoselective and regioselective Rh-mediated one-carbon homologation.

Nucleophilic organometallic reagents bearing functional groups are important intermediates in organic synthesis. David M. Hodgson of the University of Oxford has optimized (Org. Lett. 2004, 6, 4187. ) the metalation of a terminal epoxide, using s-BuLi and a designed diamine, DBB. The resulting anion adds efficiently to aldehydes, amides and Bu3SnCl.

Paul Knochel of the Universität München has also been developing functionalized organometallics, using Grignard exchange on aromatic halides. He has now (Angew. Chem. Int. Ed. 2004, 43, 3333. ) extended his earlier work on aryl iodides to the less expensive aryl bromides such as 12. Note that the organometallic reagent so produced will couple to even an unactivated primary iodide. Professor Knochel has also shown (Org. Lett. 2004, 6, 4215. ) that alkenyl iodides such as 14 exchange and couple under these conditions.

D. F. Taber, Org. Chem. Highlights 2005, June 13.