Synthetic Strategy

SYNTHETIC STRATEGY

The object of the game in organic synthesis is to assemble a particular molecule, the target, which has the correct carbon skeleton and the proper array of functional groups distributed on that skeleton. There are two general strategies for accomplishing this synthetic challenge. The first is to start with a molecule hav-ing the desired carbon skeleton and then manipulate the functionality on that skeleton to that of the desired compound. The previous chapter as well as many texts deals with functional group preparations and thus provides methods for the installation and/or manipulation of functional groups on the skeleton.

The second general synthetic strategy is to assemble the proper carbon skeleton and then adjust the functionality in the resulting molecule to that of the target. Obviously assembling the carbon skeleton from smaller building blocks is a more versatile and convergent approach because the carbon skeleton of the target may not be available (or at least not readily available). Moreover, once the synthetic sequence needed to assemble the carbon skeleton has been developed, it can potentially be adapted to produce a series of structurally related molecules. This is particularly useful when such a series of compounds is needed to test for particular physical, chemical, or biological properties.

To assemble needed carbon skeletons from smaller units, it is absolutely crucial to be able to form carbon–carbon bonds between them. Consequently carbon–carbon bond-forming reactions are among the most important organic reactions. Since any particular carbon–carbon bond is merely a covalent link between two skeletal fragments, synthesis of a target such as R₃C–CR’₃ can be accomplished by forming a carbon–carbon bond between the two skeletal frag-ments R₃C and CR’₃. Since the bond to be made contains two electrons that are shared, there are three modes by which these electrons can be distributed between the two fragments to be joined.

There are two ionic modes of bond formation where one carbon fragment is nucle-ophilic (e⁻ rich) and the other is electrophilic (e⁻ poor). There is one free-radical mode in which each fragment has a single unpaired electron which becomes shared upon bond formation. To understand carbon–carbon bond-forming reac-tions that take place by ionic or two-electron processes, we need to know what species or compounds can serve as carbon-centered nucleophiles and what species or compounds can serve as carbon-centered electrophiles. (Free-radical reactions will be considered later.)