Electronegative elements attack

You see in Chapter 10 that aldehydes and ketones contain a carbonyl group attached to carbon or hydrogen atoms. In the case of carboxylic acids and their derivatives, a carbonyl group is attached to an electronegative element such as oxygen, chlorine, or nitrogen. The presence of these elements tends to increase the 5+ charge on the carbonyl carbon, which makes the carbon atom more susceptible to nucleophilic attack. 

There is one further complication, but this time easily resolved, well understood, and supported by high-level calculations. When the electronegative element coordinates to a metal that can simultaneously coordinate to, and activate, the carbonyl group, the conformation will be that of a ring 5.90. The attack from the less hindered side, opposite to the group R, is then relatively easily predicted, and is the opposite of that predicted from the Felkin-Anh rule. It has long been known as chelation control. 

Proton transfers between electronegative elements, and especially between oxygen and nitrogen acids and bases, are extremely fast and diffusion controlled (18). In fact, acids with completely diffusion-controlled proton transfer reactions are defined as normal acids. In this case, the proton transfer occurs before the rate-determining step of the reaction. In contrast, when carbon atoms are the basic centers, proton transfers usually take place in a slow, rate-determining step. In such reactions, protonation is considered as the electrophilic attack of the... 

The main differences between using an iron cation and Pd(ll) to activate an olefin are (a) Pd(ll) reactions are often catalytic processes (b) the C-Pd a bond that results from nucleophilic attack on the coordinated olefin is reactive and undergoes either decomposition or further transformations, usually with reduction of the Pd species to Pd(0). There are numerous examples of arylation reactions of Pd(ll)-coordinated olefins, but only a few simple alkylation reactions. Pd-catalyzed reactions are usually more important for forming C-X bonds where X is an electronegative element (Section IV). Some examples of typical Pd-catalyzed C-C bond-forming reactions-are given in Eqs.(ll)-(15)(Heck, 1968, 1969), Eq. (16) (Heck, 1972), and Eq. (17) (Brown, 1974) (see also S. Tsuji, 1969 J. Tsuji, 1972 Bird, 1972, Henry, 1973). 

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