Conjugate acceptors

Polarized LUMO of Ethenes Substituted with a Conjugate Acceptor... 

Here we are intentionally restricting from our discussion bond fixation of the type found in cross-conjugated acceptor-donor substituted benzenes. Fw a discussion of these effects, see Baldridge, K. K. Siegel, J. S. J. Am. Chem. Soc. 1992,114,9583. 

The synthesis of highly substituted tetrahydrofurans by means of a multicomponent reaction based on this palladium-mediated cyclization process was reported by Balme and co-workers [94]. In this reaction, formation of enolate 103 by the initial 1,4-addition of an allylic alcohol to the conjugate acceptor 102 is followed by a... 

EO response can be introduced into polymers containing the donor-conjugation-acceptor moieties by applying a very strong electric field while the polymer is heated to its Tg. The field-induced order is subsequently frozen in by cooling the polymer films well below Tg. The achievable second-order susceptibility... 

Scheme 9.7 Addition of the acyl anion equivalent 26 to a conjugate acceptor 27.

Thus, the dominance of d-jr or d-n conjugation acceptor effects in R3MRjr and R3MX compounds is more typical for M = Si and probably Ge, and the a-n or a-n conjugation donor effect dominates for M = Pb. 

The Michael adduct can be the precursor of several cyclizations giving rise to new tandem sequences. This has been mainly due to the mechanistic aspects of the process itself and to the synthetic potential of the resultant products. A new stereoselective synthesis of pyrrolo[2,l-c/]isoindol-5-ones has been described. It consisted of a sequential Michael addition to the in situ generated anion of methyl IV-phthaloylalaninate 289, onto a series of conjugate acceptors. Cyclization of the resultant anion intermediate by condensation with one of the carbonyl imido groups gave the desired products 291 in good yields as single isomers in only one step (Scheme 89). 

The new methodology is particularly effective for the conjugate alkylation to a,/3-unsaturated aldehydes, which, among various conjugate acceptors, are prone to be more susceptible to 1,2 addition with a number of nucleophiles than a,/3-unsaturated ketones, esters, and amides, as exemplified in Sch. 97 [135]. 

Heteroatom-Carbon Double Bonds , Heteroatom-Carbon Triple Bonds Heterocumulenes Conjugate Acceptors Triply Bonded and Allenic Conjugate Acceptors... 

These two electron sinks are the least common of the six that make up the set of multiple-bonded electron sinks with no attached leaving group. They behave very similarly to the conjugate acceptors previously discussed. The electronegative C-ewg group replaces the electronegative Y in accepting the electron flow from the nucleophile. 

Conjugate acceptors C=C-ewg Enones C=C C=0, Conjugated esters C=C-COOR... 

Answer We have two problems translating the typed formula into a recognizable structure and doing a correct classification. The first compound is a carboxylic acid, which should tip us off to put it in the acids, the H-L or H-A class. The next compound is an aldehyde conjugated to a pi bond. Since an aldehyde is a polarized multiple bond and also an ewg, we would put this compound in the conjugate acceptor class, C=C-ewg. 

Simple pi bonds are not good enough nucleophiles to react with conjugate acceptors. It is basically a case of little push and little pull, so no electron flow occurs. If the pi bond is made more nucleophilic by adding a pi donor (making it an allylic source), then the reaction is similar to the one above. Alternatively, a Brpnsted acid or a Lewis acid could be added to improve the electron sink few examples of this reaction are known. 

Aromatic rings are usually not good enough nucleophiles to react with conjugate acceptors. If a Brpnsted acid or a Lewis acid is added to improve the electron sink, then electrophilic aromatic substitution can occur. 

A conjugate acceptor system with two 6+ sites can be attacked at either site. The thermodynamic product will be the most stable of the possible products. A Af/ calculation shows that the thermodynamic product is the 1,4 or conjugate addition product, primarily because of the greater bond strength of the C=0 bond than the C=C bond. Any equilibrium would produce the conjugate addition product. 

Ambident Sink 1,2 versus 1,4 Addition Decision to Conjugate Acceptors... 

---