Donor synthons

The formal carbanions and carbocations used as units in synthesis are called donor synthons and acceptor synthons. They are derived from reagents with functional groups. 

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. 

All odd numbered acceptor synthons (such as a1 and a3) and even numbered donor synthons (such as d2 and d4) have unnatural polarity. 

You ve now met a variety of synthons and it s useful to be able to classify them as donor or acceptor synthons, We call a negatively polarized synthon a donor synthon and give it the symbol d . Positively polarized synthons are called acceptor synthons and are given the symbol a . 

Notice that the acceptor synthons have odd numbers the donor synthon has an even number donor and acceptor properties alternate along the chain as we move away from a carbonyl group. This natural reactivity5 of carbonyl compounds explains why we find it easy to discuss ways of making 1,3- and 1,5-difunctionalized compounds, because they arise from a1 + d2 and from a3 + d2. Reagents corresponding to synthons like d1 or a2 are rarer, and therefore compounds with 1,2- or 1,4- related functional groups require special consideration retrosynthetically,... 

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. "... 

You will perhaps realise from these particular examples that the d2 synthon will be represented in real life by an enolate or its equivalent and the a3 synthon by an a, 3-unsaturated carbonyl compound, both displaying the natural polarity of these fragments. It is generally true that even numbered donor synthons (d2, d4, etc.) and odd numbered acceptor synthons (a1, a3, etc.) have natural polarity while the odd numbered donor synthons (d1, d3, etc.) and the even numbered acceptor synthons (a2, a4, etc.) have unnatural polarity or umpolung. This makes the numbering of such synthons a useful quick check on the type of reagent likely to be needed. 

Physical properties indicate that the dipolar contribution (1) is the major one and this structure actually accounts for the nucleophilic behavior of the terminal carbon of isonitriles. However, in terms of radical chemistry, the more interesting form is the divalent one (2). This clearly shows that the isonitrile group does not behave toward radical species like a vicinal radical acceptor/radical donor synthon, that is, like an usual unsaturated bond. It instead reacts like a geminal acceptor/donor synthon [2], where an incoming radical attacks the same carbon atom that will be the new radical center in the resulting imidoyl intermediate 3 (Scheme 1). Actually, isonitriles can serve as very efficient radical traps and this chapter reviews the structural, mechanistic, and synthetic studies carried out in this field, with outstanding results, in the last three decades. 

Nucleophiles. See Donor synthons Nucleotides oligonucleotide syntheses, 215-224, 341-343... 

---