Oxygen nucleophiles, 184 Table

Neutral sulphur and oxygen nucleophiles of similar structure react with carbonyl groups at similar rates (Jensen and Jencks, 1979) the position of the transition state for thiolactonization is therefore expected to be similar. The comparisons of EM possible for three compounds in Table 4 show that the EMk/EMeq ratios are somewhat smaller for thiolactonization, by factors of 3 and 9 for the two compounds where all four EM s can be estimated. 

Palhdium-CatalYzed Allylic Alkylation of Sulfur and Oxygen Nucleophiles 235 Table 2.1.4.14 Pd(0)/BPA-catalyzed asymmetric synthesis of allylic thioesters. 

The reactions in this chapter begin with the addition of a nucleophile to the carbon of a carbonyl group and an electrophile, usually a proton, to the oxygen. Under basic conditions the nucleophile adds first, whereas the proton adds first under acidic conditions. Depending on the nature of the nucleophile, the reaction may stop at this stage or proceed further. Figure 18.7 summarizes the mechanisms followed by the various nucleophiles. Table 18.2 lists the nucleophiles and the products that result from their reactions with aldehydes and ketones. 

The significance of the equation is that performance of the 5 2 reaction depends both on nucleophile and on the carbon electrophile. We can make a reaction go better by changing either. If we want to displace I from Mel by an oxygen nucleophile we might consider using any of those in Table 17.3. 

This observation is very significant. It is not only the concentration of the nucleophile that doesn t matter—its reactivity doesn t matter either We are wasting our time adding NaOll to this reaction—water will do just as well. All the oxygen nucleophiles in Table 17.3 react at the some rate with f-BuBr though they react at. very different rates with Mel. 

Elsewhere in this chapter we have used oxygen nucleophilic processes to illustrate stereoselectivity (Sections II.B and II.C), coelectrophiles (Section I.A), etc. and shall not repeat these here. The literature of oxygen nucleophile attacks on alkynes includes trends in substituent effects but the data are often qualitative and usually scattered. Numerous examples indicate that the reactions of alkoxides with C2H2 are relatively slow and that most substituents facilitate the addition (Tables 13 and 14) ... 

Table 13. Rate data for the addition of oxygen nucleophiles to alkynes...

The precursor compound, phosphornitrilic chloride trimer, reacts with nitrogen and oxygen nucleophiles to form stable products [10]. Reaction of the trimeric phosphazene chloride with nitrogen nucleophiles, such as 3-aminopropyltriethoxysilane, facilitates the introduction of the silane component in a single step resulting in the modified compounds 13 and 14 in yields of about 90 % (Eq. 5 Table 3)... 

Within a Period or row of the Periodic Table, the better nucleophile has the lower atomic number. This means that nitrogen nucleophiles such as ammonia and amines are better nucleophiles than the corresponding oxygen nucleophiles, water and alcohols. In fact, water and alcohols generally react too slowly to be of use, and often it is necessary to use their anionic forms HO or RO . 

Nucleophiles can be classified according to the kind of atom that forms a new covalent bond. For example, the hydroxide ion in eq. 6.1 is an oxygen nucleophile. In the product, a new carbon-oxygen bond is formed. The most common nucleophiles are oxygen, nitrogen, sulfur, halogen, and carbon nucleophiles. Table 6.1 shows some examples of nucleophiles and the products that they form when they react with an alkyl halide. 

The second item in Table 9.7 is similar to the first in that, again, the chloride of the acid chloride is replaced by an oxygen nucleophile. It would be presumed that the ethyl alcohol (ethanol, CH3CH2OH) simply adds to the carbon of the carbonyl (sufficient solubility is expected) with loss of chloride (CT). However, pyridine (azabenzene, C5H5N) is present, and it is clear that, in addition to removing the... 

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