Heteroatomic nucleophiles carbon/oxygen additions

In addition to protons, alkyl and acyl groups, silyl groups are sufficiently electropositive to initiate carbocationic polymerizations. Silicon is more electropositive than carbon, and therefore reacts with many nucleophiles, especially those based on oxygen. The resulting silicon-heteroatom bonds are relatively labile trimethylsilyl is thus often referred to as a bulky... 

An addition reaction is formally the reverse of an elimination reaction. We will see both electrophilic and nucleophilic additions in this section. There are few new generic classes of reactants to consider, and there are three new routes, but these are just the reverse of the elimination routes just covered. The new generic classes are discussed in much more detail in Chapter 5 and 6, but need to be introduced here. A carbon-carbon double bond can range from electrophilic to nucleophilic depending on what is attached to it (Fig. 4.33). Another way to make a double bond electrophilic is to replace one of its carbon atoms with an electronegative heteroatom like oxygen, C=0, a carbonyl. 

When acid derivative 2 reacts with sulfuric acid, the oxygen atom is the base and the conjugate acid product of this acid-base reaction is oxocarbenium ion 3, which is resonance stabilized. When 2 is an acid chloride, anhydride, ester, or amide, a heteroatom is attached to the positive carbon in 3. As in Chapter 18 (Section 18.1), the acid-base reaction of the carbonyl unit in 2 to give 3 facilitates reactions with nucleophiles. The reaction of intermediate 3 with a nucleophile ( Y) gives tetrahedral intermediate 4 contrary to acyl addition, reaction 4 contains an X group that can function as a leaving group. Loss of X leads to the final product of this reaction 5. If the nucleophile ( Y) is hydroxide, compormd 5 is the carboxylic acid (X = OH). If the nucleophile Y is an alcohol, the product 5 is an ester, and if Y is an amine, the product 5 is an amide. This first reaction is therefore the acid-catalyzed acyl substitution reaction of acid derivatives. 

CO is very seasitive to nucleophilic attack when coordinated to metal sites of low n basicity. On such a site, the CO carbon is positively charged because L-to-M a donation is not compensated by M-to-L back donation, and the CO rr orbitals are open to attack by the nucieophile. Nucleophilic lithium reagents convert a number of metal carbonyls to the corresponding anionic acyls. The net negative charge now makes the acyl liable to electrophilic addition to the acyl oxygen to give the Fischer (heteroatom-stabilized) carbene complex, 8.1. ... 

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