Donor synthons reactions

If boranes (K. Utimoto, 1973 H.C. Brown, 1975, 1980 A. Pelter, 1979) are used as donor synthons for the alkylation of a, -unsatarated carbonyl compounds, no enolate anion is formed, and the, 8-position of the C=C bond is the only reaction site. 

The reactions described so far can be considered as alkylation, alkenylation, or alkynylation reactions. In principle all polar reactions in syntheses, which produce monofunctional carbon compounds, proceed in the same way a carbanion reacts with an electropositive carbon atom, and the activating groups (e.g. metals, boron, phosphorus) of the carbanion are lost in the work-up procedures. We now turn to reactions, in which the hetero atoms of both the acceptor and donor synthons are kept in a difunctional reaction produa. 

Heterolytic retrosynthetic disconnection of a carbon-carbon bond in a molecule breaks the TM into an acceptor synthon, a carbocation, and a donor synthon, a carbanion. In a formal sense, the reverse reaction — the formation of a C-C bond — then involves the union of an electrophilic acceptor synthon and a nucleophilic donor synthon. Tables 1.1 and 1.2 show some important acceptor and donor synthons and their synthetic equivalents. "... 

An important method for construction of functionalized 3-alkyl substituents involves introduction of a nucleophilic carbon synthon by displacement of an a-substituent. This corresponds to formation of a benzylic bond but the ability of the indole ring to act as an electron donor strongly influences the reaction pattern. Under many conditions displacement takes place by an elimination-addition sequence[l]. Substituents that are normally poor leaving groups, e.g. alkoxy or dialkylamino, exhibit a convenient level of reactivity. Conversely, the 3-(halomethyl)indoles are too reactive to be synthetically useful unless stabilized by a ring EW substituent. 3-(Dimethylaminomethyl)indoles (gramine derivatives) prepared by Mannich reactions or the derived quaternary salts are often the preferred starting material for the nucleophilic substitution reactions. 

Piers has exploited these 2-ynoate stannylcupration reactions in the preparation of donor and dipolar synthons (Scheme 3.14) [64]. Stereoselective stannylcupration followed by deconjugation provides a stereocontrolled route to vinyl 1,3-dipolar... 

The simple addition reaction in Scheme 19 illustrates how the notation is used. Ester (1) can be dissected into synthons (2), (3) and (4). Synthons for radical precursors (pro-radicals) possess radical sites ( ) A reagent that is an appropriate radical precursor for the cyclohexyl radical, such as cyclohexyl iodide, is the actual equivalent of synthon (2). By nature, alkene acceptors have one site that reacts with a radical ( ) and one adjacent radical site ( ) that is created upon addition of a radical. Ethyl acrylate is a reagent that is equivalent to synthon (3). Atom or group donors are represented as sites that react with radicals ( ) Tributyltin hydride is a reagent equivalent of (4). In practice, such analysis will usually focus on carbon-carbon bond forming reactions and the atom transfer step may be omitted in the notation for simplicity. 

Acyl transfer to alcohols and amines is related mechanistically to ester hydrolysis but yields a complementary set of products that are useful in their own right and as chiral synthons for the preparation of more complex materials. Such transformations can be difficult to achieve in water, however, because the solvent, which is present in vast excess, can participate directly in the reaction as a reactant. Enzyme-like specificity is thus required to favor the bimolecular reaction between alcohol and ester and prevent spontaneous hydrolysis of the acyl donor. 

Numerous adducts, mostly of type MX4L2 (X = Cl, Br, I), are known with nitrogen, phosphorus, arsenic, oxygen, and sulfur donors. They are synthesized by reaction of solid tetrahalides with the ligand or by reduction of pentahalides with the ligand, mainly bidentate or heterocyclic N-donors or phosphines. NbCLt(THF)2 (29) is of particular value since the lability of the THF molecules makes it a useful synthon in Nb chemistry. 

---