Radical addition-fragmentation chemistry in polymer synthesis

Recei Ved January 7, 2000 Over the past 30 years, enantioselective catalysis has become one of the most important frontiers in exploratory organic synthetic research. During this time, remarkable advances have been made in the development of organometallic asymmetric catalysts that in turn have provided a wealth of enantioselective oxidation, reduction, π-bond activation, and Lewis acid-catalyzed processes. 1 Surprisingly, however, relatively few asymmetric transformations have been reported which employ organic molecules as reaction catalysts, 2 despite the widespread availability of organic chemicals in enantiopure form and the accordant potential for academic, industrial, and economic benefit. Herein, we introduce a new strategy for organocatalysis that we expect will be amenable to a range of asymmetric transformations. In this context, we document the first highly enantioselective organocatalytic Diels -Alder reaction. 3

New Strategies for Organic Catalysis: The First Highly Enantioselective Organocatalytic Diels−Alder Reaction

The legacy of Gilbert Newton Lewis (1875-1946) pervades the lexicon of chemical bonding and reactivity. The power of his concept of donor-acceptor bonding is evident in the eponymous foundations of electron-pair acceptors (Lewis acids) and donors (Lewis bases). Lewis recognized that acids are not restricted to those substances that contain hydrogen (Bronsted acids), and helped overthrow the "modern cult of the proton". His discovery ushered in the use of Lewis acids as reagents and catalysts for organic reactions. However, in recent years, the recognition that Lewis bases can also serve in this capacity has grown enormously. Most importantly, it has become increasingly apparent that the behavior of Lewis bases as agents for promoting chemical reactions is not merely as an electronic complement of the cognate Lewis acids: in fact Lewis bases are capable of enhancing both the electrophilic and nucleophilic character of molecules to which they are bound. This diversity of behavior leads to a remarkable versatility for the catalysis of reactions by Lewis bases.

Lewis Base Catalysis in Organic Synthesis

The past decade of organic chemistry may be charcterized as a period of violent development of new synthetic methods. This was accompained by a systematization of the analysis of synthetic problems (synthetic strategy). The planning of the synthesis of an organic target molecule is greatly facilitated by distinguishing between reagents X(C)n … with normal reactivity (acceptor properties at C1,3,5…, donor properties at X, C2,4…) and with reactivity umpolung (acceptor properties at X, C2,4…, donor properties at C1,3,5…). In this context, reactivity umpolung turned out to be useful as a heuristic principle, as a classification scheme, and as an aid for locating so-called strategic bonds (synthon, transform, and antithesis according to E. J. Corey). There are six principal methods of umpolung: 1 2n-oxidation, heteroatom exchange and modification, homologation and its reversal, the cyclopropane “trick”, use of acetylenes, and redox reactions; under certain circumstances none of these techniques is necessary in cases where direct umpolung is possible. Throughout the article, normal reactivity is indicated by green print; reactivity umpolung by red print.

Methods of Reactivity Umpolung

The electrophilic/nucleophilic character of a series of captodative (CD) ethylenes involved in polar cycloaddition reactions has been studied using DFT methods at the B3LYP/6-31G(d) level of theory. The transition state structures for the electrophilic/nucleophilic interactions of two CD ethylenes toward a nucleophilically activated ethylene, 2-methylene-1,3-dioxolane, and an electrophilically activated ethylene, 1,1-dicyanoethyelene, have been studied, and their electronic structures have been characterized using both NBO and ELF methods. Analysis of the reactivity indexes of the CD ethylenes explains the reactivity of these species. While the electrophilicity of the molecules accounts for the reactivity toward nucleophiles, it is shown that a simple index chosen for the nucleophilicity, Ν, based on the HOMO energy is useful explaining the reactivity of these CD ethylenes toward electrophiles.

Understanding the reactivity of captodative ethylenes in polar cycloaddition reactions. A theoretical study.

The captodative effect

Capto‐dative Substituent Effects in Syntheses with Radicals and Radicophiles [New synthetic methods (32)]

Die 100 Jahre alten Wursterschen Salze sind seit langem als Verbindungen mit Radikalkationen erkannt worden. Ihre ungewohnliche Stabilitat beruht teilweise auf capto-dativer (cd) Substitution. Dieses Prinzip wird nun als Faktor der Radikalstabilisierung diskutiert und auf Methinderivate angewendet. cd-Substitution ist fur Synthesezwecke nutzlich: cd-Substituenten am gleichen Kohlenstoffatom ermoglichen selektive Dehydrodimerisierung; Olefine mit geminalen und daher kreuzkonjugierten cd-Substituenten sind „radikophil” und konnen zwei Kohlenstoffradikale addieren; cd-substituierte Olefine sind wirksame Antioxidantien, Polymerisationsinhibitoren und vielversprechende Agentien fur die Kontrolle biologischer Radikalreaktionen. Ganz allgemein konnten viele Reaktionen cd-substituierter Molekule uber Radikale verlaufen.

Capto‐dative Substituenteneffekte bei der Synthese mit Radikalen und Radikophilen

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Heinz G. Viehe

Papers

The captodative effect

Capto‐dative Substituent Effects in Syntheses with Radicals and Radicophiles [New synthetic methods (32)]

Capto‐dative Substituenteneffekte bei der Synthese mit Radikalen und Radikophilen

Substituent Effects in Radical Chemistry

Allenetetramine and Dialkoxydiaminoallene