Heteroatomic nucleophiles palladium catalysis

The same transition metal systems which activate alkenes, alkadienes and alkynes to undergo nucleophilic attack by heteroatom nucleophiles also promote the reaction of carbon nucleophiles with these unsaturated compounds, and most of the chemistry in Scheme 1 in Section 3.1.2 of this volume is also applicable in these systems. However two additional problems which seriously limit the synthetic utility of these reactions are encountered with carbon nucleophiles. Most carbanions arc strong reducing agents, while many electrophilic metals such as palladium(II) are readily reduced. Thus, oxidative coupling of the carbanion, with concomitant reduction of the metal, is often encountered when carbon nucleophiles arc studied. In addition, catalytic cycles invariably require reoxidation of the metal used to activate the alkene [usually palladium(II)]. Since carbanions are more readily oxidized than are the metals used, catalysis of alkene, diene and alkyne alkylation has rarely been achieved. Thus, virtually all of the reactions discussed below require stoichiometric quantities of the transition metal, and are practical only when the ease of the transformation or the value of the product overcomes the inherent cost of using large amounts of often expensive transition metals. 

While the major use for palladium catalysis is to make carbon-carbon bonds, which are difficult to make using conventional reactions, the success of this approach has recently led to its application to forming carbon-heteroatom bonds as well. The Overall result is a nucleophilic substitution at a vinylic or aromatic centre, which would not normally be possible. A range of aromatic amines can be prepared direcdy from the corresponding bromides, iodides, or triflates and the required amine in the presence of palladium(O) and a strong alkoxide base. Similarly, lithium thiolates couple with vinylic triflates to give vinyl sulfides provided lithium chloride is present. 

Such substitutions follow the same mechanistic route as the displacement of halide from 2- and 4-halo-nitrobenzenes, i.e. the nucleophile first adds and then the halide departs. By analogy with the benzenoid situation, the addition is facilitated by (i) the electron-deficiency at a- and y-carbons, further increased by the halogen substituent, and (ii) the ability of the heteroatom to accommodate negative charge in the intermediate thus produced. A comparison of the three possible intermediates makes it immediately plain that this latter is not available for attack at a p-position, and thus p nucleophilic displacements are very much slower - for practical purposes they do not occur (see, however, reactions with palladium catalysis, 4.2)... 

Allylic aeetates or carbonates can undergo nucleophilic substitutions via palla-dium(0)-catalysis (11). In this paper, we report on the extension of this reaction to unsaturated fatty aeids by the preparation of allyl carbonates and acetates of oleic, linoleic, and 10-undecenoic acid and their substitution with carbon- and heteroatom-nucleophiles by palladium(0)-catalysis. In this way, different substituents can be in-trodueed into the alkyl chain of fatty acids. This leads to fatty acid derivatives in which the properties of biologically active compounds may possibly be combined with the amphiphilic property of the fatty acid. 

Use of other nucleophiles in the presence of an oxidant can install other functionality (Scheme 3.55). While N-halosuccinimides are very effective, other systems, such as iodosobenzene diacetate-halide mixtures, or copper(II) halide salts can be employed. The powerful oxidant oxone can also be used in combination with alcohols, to give ethers (Scheme 3.56). Iodine acetate has been used for C-H activation directed by carboxylic acids (Scheme 3.57). The heteroatom may also be supplied intramolecularly (Scheme 3.58). The use of palladium catalysis can also override the inherent regioselectivity of an arene substrate (Scheme 3.59). 

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