Nudeophilic center

Meanwhile, there are many substrates of FMO already known, including tertiary and secondary amines, hydroxyamines, thiols, sulfides, thiocar-bamides, and thioamides (for a review, see Ziegler, 1991). Compounds accepted by this mono-oxygenase have one chemical properly in common All are soft nucleophiles readily oxidized by per-adds. Apparently, the nudeophilic centers are oxidized by an ionic r er than a radical mechanism in an active site requiring more than one point of contact (Ziegler, 1991). 

In the following we have broadly divided the available C(l) nucleophiles according to structure and reactivity. C (1 )-Nudeophiles based on metallation at an sp3-center are categorized in terms of the presence and, as appropriate, the nature of a hetero atom (O or N) substituent at C(2). C(l)-Lithiated glycals correspond to nucleophilic sp2-centers, and variations on this theme (concerning the nature of the metal component) are covered in depth. 

Besides direct nucleophilic attack onto the acceptor groups an activated diene may also undergo 1,4- or 1,6-addition in the latter case capture of the ambident enolate with a soft electrophile can take place at two different positions. Hence the nucleophilic addition can result in the formation of three regioisomeric alkenes, which may in addition be formed as /2 isomers. Moreover depending on the nature of nudeophile and electrophile, the addition products may contain one or two stereogenic centers, and, as a further complication basic conditions may give rise to the isomerization of the initially formed y-unsaturated carbonyl compounds (and other acceptor-substituted alkenes of this type) to the thermodynamically more stable conjugated isomer (Eq. 4.1). 

This four-centered pathway requires a large amount of energy, since the covalent Me Cu bond (55 kcal mol [93]) must be deaved. A neutral RCu spedes is therefore not a reactive nudeophile. 

Phenylseleno- and methylseleno-alkyllithiums usually exhibit a closely related reactivity towards carbonyl compounds whether the reaction is performed in THF or in ether. As expected, the nudeophilicity of such species often decreases by increasing the substitution around the ct anionic center (6 and 8 Scheme 112), but interestingly they are often far more nucleophilic than the corresponding alkyl-lithiums. However, in some rare cases employing particularly hindered reaction partners there is a significant difference of reactivity between phenyl- and methyl-selenoalkyllithiums when the reactions are performed in THF, since the former reagents are much less nucleophilic than the latter (compare 11 and 12 in Scheme 113 and 17a-17f in Scheme 114). As general trends, a-methylselenoalkyllithiums are more nucleophilic in ether than in THF (compare 15 in Scheme 113.17d and 17e in Scheme 114 and 24 in Scheme 116). 

Several metal carbonyl dusters (e.g. [Ru3(CO)i2] and [Ir4(CO),2]) have been investigated in solution as possible catalysts for the water gas shift reaction. [120] A plausible common mechanism has been proposed Nudeophilic attack by H2O or OH on an electrophilic metal center of the duster to form an unstable carbo-hydroxy metal complex, which is then decarboxylated to give a metal hydride from which H2 is eliminated. Hiis reaction is one of the relatively few for which there is good evidence for catalysis by dusters themselves, rather than fragments or metal aggregates formed from them. 

The Grignard reagent also functions as a good nucleophile in nucleophilic addition reactions of the carbonyl group. The carbonyl group has electrophilic character at its carbon atom (due to resonance), and a good nudeophile seeks out this center for addition. 

Silacyclobutanes play an important role in the formation of block copolymers. For example, the relatively low-nucleophilic anion of poly(ethylene oxide) cannot initiate the polymerization of vinyl monomers, which require more nucleophilic carboanionic initiators. Srlacydobutane may serve as the nudeophile-transfer agent, which transforms a weaker nudeo-philic center into a more nudeoptulic one ( anionic pump. Scheme 15). This makes possible the copolymerization of heterocydics with vinyl monomers. ... 

---