Addition reactions continued intermolecular

Intermolecular Michael reactions continue to be developed. Karl Anker Jorgensen of Aarhus University, Denmark, has found (Angew. Chem. Ini. Ed. 2004,43, 1272) that the organocatalyst 3 mediates the addition of 2 to 1 with high enantiomeric excess. What is more, under the reaction conditions the intial Michael addition is followed by an aldol condensation, to give 4 as essentially a single diastereomer. 

The experimental evidence for the second hypothesis was the observed increase of the cracking rate of alkanes after addition of small amounts of alkenes to the feed. Both early theories assumed the continuation of the cracking reaction by intermolecular transfer of the charge from the products to fresh starting molecules, that is like an ionic chain mechanism with the catalyst acting only as an initiator. The problem was further clouded by the fact that two types of acid centres exist on the surface, the Br0n-... 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

The reaction is thought to proceed by co-ordination of the alkene with the organopalladium(II) species, followed by carbopalladation. Subsequent p-hydride elimination regenerates an alkene and releases palladium(II). This is reduced (reductive elimination) to palladium(O) in the presence of a base, to allow further oxidative addition and continuation of the cycle (1.211). The carbopalladation and p-hydride elimination steps occur syn selectively. Excellent regioselectivity, even for intermolecular reactions, is often observed, with the palladium normally adding to the internal position of terminal alkenes (except when the alkene substituent is electron-rich as in enamines or enol derivatives), thereby leading to linear substitution products. 

In continuation of the aforementioned reaction, Hiroya and coworkers used cop-per(II) acetate for the synthesis of indoles 2-943 in reasonable yields from the corresponding ethynylanilines 2-941 by a domino intermolecular Michael addition/cop-per-assisted nucleophilic tosylate displacement reaction via 2-942 (Scheme 2.211) [482],... 

Less basic malonic ester anions may be employed for the twofold alkylation of dibromides. Cyclic 1,1-dicarboxylic esters are formed, if the reaction is executed in an appropriate manner. In the synthesis of cyclobutane diester A the undesired open-chain tetraester B was always a side product (J.A. Cason, 1949), the malonic ester and its monoalkylation product were always only partially ionized. Alkylation was therefore slow and intermolecular reactions of mono-alkyl intermediates with excess malonic ester prevailed. If the malonic ester was dissolved in ethanol containing sodium ethoxide, and 1,3-dibromopropane as well as more sodium ethoxide were added slowly to the solution, 63% of A and only 7% of B were isolated. The latter operations kept the malonic ester and its monoalkylated product in the ionic form, and the dibromide concentration low, so that the intramolecular reaction was favored against intermolecular reactions. The continuous addition of base during the reaction kept the ethoxide concentration low, which helped to prevent decomposition of the bromide by this nucleophile. 

The chiral molecular receptor (35) has been used to effect enantioselective cyclization of the enone (36). The complex of (36) and (35) undergoes energy transfer from the ketonic acceptor to (36) and results in its conversion into the cyclobutanes (37) and (38) in a total yield of 21%. Bach et aV have continued their investigations of enantioselective additions mediated by the chiral lactam hosts (39). The present reactions involve intra and intermolecular additions of quinolone systems (40) at -60°C in toluene as solvent. The irradiation affords the cycloadducts (41) and (42). As can be seen, the ee of the products is high and the chemical yields are also good. An extension of the work to intermolecular reactions of the quinolone (43) was also reported. The additions of the alkenes... 

Although examples of intramolecular radical additions and radical cyclizations continue to dominate the literature, there are a few examples of intermolecular additions mainly to the C-2 or C-3 positions of the indole ring. Early examples utilized hydrogen peroxide and FeS04-7H20, oxidative free-radical reaction conditions (Mn(III), Fe(II) and Ce(IV)) [7], or photolysis [8], but required a large excess of reagents and suffered from low product yields. 

To this day, work continues to establish a general catalyst for the hydroamina-tion of alkynes, the most thermodynamically favorable addition of an N-H bond across a C-C multiple bond. While catalysts from across the periodic table have been reported for this transformation intramolecularly, the past decade has seen favorable development of the intermolecular variant of this reaction. 

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