Group 16 Nucleophiles. Oxygen

A commonly used nucleophile has been water. Although initial attack affords a hydroxy-carbene derivative, ready cleavage of the Ca—Cp bond resulting from formal keto-enol tautomerism occurs to give either the acyl or the metal carbonyl (usually cationic) and the corresponding organic fragment (Equation 1.13)  

A study of the hydration of HC=CPh in the presence of mer, trows-RuCl2(PPh3) NPr(CH2CH2PPh2)2 showed the intermediacy of vinylidene, aquo(alkynyl), acyl and benzyl(carbonyl) complexes [282]. 

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The structural features especially the very polar nature of the carbonyl group point clearly to the kind of chemistry we will see for aldehydes and ketones in this chapter The partially positive carbon of C=0 has carbocation character and is electrophilic The planar arrangement of its bonds make this carbon relatively uncrowded and susceptible to attack by nucleophiles Oxygen is partially negative and weakly basic... 

SECTION 8.8. ACYLATION OF NUCLEOPHILIC OXYGEN AND NITROGEN GROUPS... 

The synthesis of a large number of y-pyrones and y-pyranols from enamines has been brought about through the use of a wide variety of bifunctional molecules. These molecules include phenolic aldehydes (126,127), phenolic Mannich bases (128), ketal esters (129), and diketene (120-132). All of these molecules have an electrophilic carbonyl group and a nucleophilic oxygen center in relative 1,4 positions. This is illustrated by the reaction between salicylaldehyde (101) and the morpholine enamine of cyclohexanone to give pyranol 102 in a quantitative yield (127). 

In the following transformations, the nucleophilic oxygen of the sulfoxide group plays a... 

Oxidation is the first step for producing molecules with a very wide range of functional groups because oxygenated compounds are precursors to many other products. For example, alcohols may be converted to ethers, esters, alkenes, and, via nucleophilic substitution, to halogenated or amine products. Ketones and aldehydes may be used in condensation reactions to form new C-C double bonds, epoxides may be ring opened to form diols and polymers, and, finally, carboxylic acids are routinely converted to esters, amides, acid chlorides and acid anhydrides. Oxidation reactions are some of the largest scale industrial processes in synthetic chemistry, and the production of alcohols, ketones, aldehydes, epoxides and carboxylic acids is performed on a mammoth scale. For example, world production of ethylene oxide is estimated at 58 million tonnes, 2 million tonnes of adipic acid are made, mainly as a precursor in the synthesis of nylons, and 8 million tonnes of terephthalic acid are produced each year, mainly for the production of polyethylene terephthalate) [1]. 

Sulfation in most aspects is very similar to phosphorylation, except that sulfation is not involved in intracellular signal transduction, but in other forms of signaling. The mechanism of sulfation is similar to that of phosphorylation as a general base from the enzyme active site that deprotonates the hydroxyl groups of tyrosine residues. The nucleophilic oxygen then attacks the /3-position, in contrast to the 7-position in phosphorylation, and releases adenosine 3, 5 -diphosphate. 

The replacement of a substituent on an aromatic ring by a nucleophile is termed arylation. This chapter considers the replacement by nucleophilic oxygen, sulfur, nitrogen, and carbon of aromatic fluorine atoms, which are often activated by electron-withdrawing groups. 

The mechanism shown below involves the nucleophilic oxygen of the alcohol making use of one of its lone pairs of electrons to form a bond to a proton to yield a charged intermediate (Step 1). When the oxygen gets protonated, the molecule has a much better leaving group because water can be ejected as a neutral molecule. 

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