Carbon-Carbon Bond Formation

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Carbon-Carbon Bond Formation

The number of generally useful and well tested reactions for effecting carbon-carbon bond formation, ideally in a regio and stereospecific fashion, is relatively small, compared with reactions used to modify functional groups. Many of the most commonly used of these have been described in other chapters of this text, as summarized in the following list:

1.   Friedel-Crafts alkylation and acylation.

2.   Diels-Alder cycloaddition.

3.   Addition of organometallic reagents to aldehydes, ketones & carboxylic acid derivatives.

4.   Conjugate addition reactions.

5.   Alkylation of acetylide anions.

6.   Wittig and other ylide reactions.

7.   Alkylation of enolate anions.

8.   Claisen and aldol condensations.

With the exception of Friedel-Crafts alkylation, these reactions all give products having one or more functional groups at or adjacent to the bonding sites. As a result, subsequent functional group introduction or modification may be carried out in a relatively straightforward manner. It should also be noted that more than half these reactions involve carbonyl reactants. The "classic" reactions listed above are major components in the array of methods available to chemists for the construction of complex carbon structures. Moreover, in the half century since Woodward's reserpine synthesis was carried out, this "toolkit" has been expanded to include an assortment of new, tolerant and selective carbon-carbon bond forming reactions. These include:

9. Addition of carbon radicals to multiple bonds.

10. Photochemical cycloaddition and rearrangement reactions.

11. Coupling reactions effected by transition metal catalysts.

12. Reactions of transition metal alkylidene complexes, especially olefin methathesis.

In addition to development of these and other new reactions, many classic procedures have been modified and enhanced in scope by:

  1. Control of regio and stereo-selectivity in the formation of reactive intermediates.
  2. Charge inversion in polar reactants, Umpolung.
  3. Application of tandem, domino or cascading reaction sequences.