Activating groups addition

The one-carbon addition we used in frames 98 and 101 is all right if we just want to add an activating group to a readily available ketone, but is not otherwise good synthetic practice ... 

Completely ah initio predictions can be more accurate than any experimental result currently available. This is only true of properties that depend on the behavior of isolated molecules. Colligative properties, which are due to the interaction between molecules, can be computed more reliably with methods based on thermodynamics, statistical mechanics, structure-activity relationships, or completely empirical group additivity methods. 

Aldol Addition and Related Reactions. Procedures that involve the formation and subsequent reaction of anions derived from active methylene compounds constitute a very important and synthetically useful class of organic reactions. Perhaps the most common are those reactions in which the anion, usually called an enolate, is formed by removal of a proton from the carbon atom alpha to the carbonyl group. Addition of this enolate to another carbonyl of an aldehyde or ketone, followed by protonation, constitutes aldol addition, for example... 

Hydrogen cyanide adds to an olefinic double bond most readily when an adjacent activating group is present in the molecule, eg, carbonyl or cyano groups. In these cases, a Michael addition proceeds readily under basic catalysis, as with acrylonitrile (qv) to yield succinonitnle [110-61-2], C4H4N2, iu high yield (13). Formation of acrylonitrile by addition across the acetylenic bond can be accompHshed under catalytic conditions (see Acetylene-DERIVED chemicals). 

Both terminal and nonterminal acetylenes have been used. Activating groups oL to the acetylenic bond have included sulfone (131-135), sulfoxide (134), ester (28,133-139), and ketone (134,140). Whether adduct 183 Is designated as cis or trans depends on the investigators and the particular compound. If the addition reaction is carried out in aprotic solvents, the major isomer is 183 formed by cis addition (135,138,139). For example, the addition of aziridine to dimethyl acetylenedicarboxylate (182, X, Y = CO2CH3) in dimethyl sulfoxide (135) gave 75 % of a mixture containing 95 % of the Chester 185. Collapse of the intermediate zwitterion intermediate 186... 

Read ions of Heterocyclic Enamines with a,p-Unsaturated Compounds Enamines react readily with compounds containing a double bone activated by electronegative groups. Addition of acrolein to 1-methyl-2 ethylidenepyrrolidine, followed by dehydrogenation, leads to 1,7-dimethyl indole (133) (Scheme 9) (215). 

Chiral oxazolines developed by Albert I. Meyers and coworkers have been employed as activating groups and/or chiral auxiliaries in nucleophilic addition and substitution reactions that lead to the asymmetric construction of carbon-carbon bonds. For example, metalation of chiral oxazoline 1 followed by alkylation and hydrolysis affords enantioenriched carboxylic acid 2. Enantioenriched dihydronaphthalenes are produced via addition of alkyllithium reagents to 1-naphthyloxazoline 3 followed by alkylation of the resulting anion with an alkyl halide to give 4, which is subjected to reductive cleavage of the oxazoline moiety to yield aldehyde 5. Chiral oxazolines have also found numerous applications as ligands in asymmetric catalysis these applications have been recently reviewed, and are not discussed in this chapter. ... 

The first use of chiral oxazolines as activating groups for nucleophilic additions to arenes was described by Meyers in 1984. " Reaction of naphthyloxazoline 3 with phenyllithium followed by alkylation of the resulting anion with iodomethane afforded dihydronaphthalene 10 in 99% yield as an 83 17 mixture of separable diastereomers. Reductive cleavage of 10 by sequential treatment with methyl fluorosulfonate, NaBKi, and aqueous oxalic acid afforded the corresponding enantiopure aldehyde 11 in 88% yield. 

As noted previously, conjugate addition of a nucleophile to the j3 carbon of an cr,/3-unsaturated aldehyde or ketone leads to an enolate ion intermediate, which is protonated on the a carbon to give the saturated product (Figure 19.16). The net effect is addition of the nucleophile to the C=C bond, with the carbonyl group itself unchanged. In fact, of course, the carbonyl group is crucial to the success of the reaction. The C=C bond would not be activated for addition, and no reaction would occur, without the carbonyl group. 

Not only are there substrates for which the treatment is poor, but it also fails with very powerful electrophiles this is why it is necessary to postulate the encounter complex mentioned on page 680. For example, relative rates of nitration of p-xylene, 1,2,4-trimethylbenzene, and 1,2,3,5-tetramethylbenzene were 1.0, 3.7, and 6.4, though the extra methyl groups should enhance the rates much more (p-xylene itself reacted 295 times faster than benzene). The explanation is that with powerful electrophiles the reaction rate is so rapid (reaction taking place at virtually every encounter between an electrophile and substrate molecule) that the presence of additional activating groups can no longer increase the rate. ... 

Activation methods can be divided into two groups. Activation by addition of selected metals (a few wt%), mainly transition metals, e.g., fine powders of Fe, Ni, Co, Cr, Pt, Pd, etc. ", or chlorides of these metals when these are reducible to the metal by hydrogen during presintering. The mechanism of activation is not understood (surface tension, surface diffusion, etc.) but is related to the electronic structure of the metal additive. Activation by carbon is also effective. Alternatively, activation utilizes powders in a specially activated state, e.g., very fine (submicronic) powders. ... 

The first disconnection must be of the acetal to reveal two identical 1,5-relationships (23). Disconnection of these would require a change of oxidation level to, say (24), and the addition of activating groups (25). 

Control will be needed for the Michael addition, and it proved necessary to protect one carbonyl of (14) as an acetal and add an activating group to the other to give (16), There is no ambiguity in either of these steps as protection of one carbonyl also deactivates the other (Chapter T5) and (15) can enolise on one side only. Removal of the acetal from (17), cyclisation, and decarboxylation can all be accomplished in one step. Synthesis ... 

The symmetry of (32) suggests addition of an activating group followed by 1,3-dicarbonyl disconnection to symmetrical (33). This is a 1,6-dicarbonyl compound but reconnection gives (34) with no obvious disconnections. An alternative is to work back to readily available (37) by FGA and chain extension. 

Reduction of the aromatic amine (15) is the usual source of (14), and reductive amination of (16) gives (15). There are many published routes to (15) of which addition of an activating group (17) is probably easiest on a large scale. You may also have considered using nitro compound (18) or epoxide (19). 

A more recent study, which measured three established markers of free-radical activity in addition to serum ascorbic acid in two groups of elderly diabetic patients (with and without retinopathy), found no significant differences in any of the markers between patients and age-matched controls despite significant depletion of ascorbic acid in patients with diabetes, especially those with retinopathy (Sinclair et al., 1992). These rather paradoxical findings suggest the existence of a complex interrelationship between the levels of individual antioxidant molecules in cells and tissues. 

More generally, many combinations of EWG substituents can serve as the anion-stabilizing and alkene-activating groups. Conjugate addition has the potential to form a bond a to one group and (3 to the other to form a a,y-disubstituted system. 

---