Daxin Gao

An Efficient Synthesis of Ring‐Opening Metathesis Monomers and Polymers Containing Carbazole and Other Electron‐Rich Moieties

An efficient synthesis of ring-opening metathesis monomers and polymers containing ionizable moieties such as N-carbazolyl, 2-dibenzofuranyl, 2-dibenzothiophenyl, and 4-anisyl functionalities has been developed, using cation radical Diels-Alder cycloaddition chemistry to generate the appropriate norbornene-type monomers.

N-(trans-1-Propenyl)carbazole: an excellent dienophile for cation radical Diels–Alder cycloadditions

Abstract The efficient, regiospecific, and periselective Diels–Alder cycloadditions of N -( trans -1-propenyl)carbazole, a highly electron rich dienophile, to both acyclic and cyclic conjugated dienes is found to be induced by tris(4-bromophenyl)aminium hexachloroantimonate.

Cation radical chain cycloaddition polymerization: a fundamentally new polymerization mechanism

Cation radical chain cycloaddition polymerization, a fundamentally new addition polymerization method involving cation radical intermediates in each propagation step, is described and demonstrated. The cycloaddition reactions of appropriately constituted difunctional monomers, catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate in dichloromethane solvent at 0 °C, is shown to afford polymers having average molecular weights of up to 150 000. Both cyclobutanation and Diels–Alder polymers were obtained in this way. The surprising efficiency of these polymerization reactions is believed to be the result of rapid intramolecular hole transfer from the site at which the hole is originally generated in the cycloaddition step to a reactive, terminal alkene moiety. Consequently, chain propagation is much more efficient than in the cycloadditions of corresponding monofunctional compounds, which necessarily involve intermolecular hole transfer. Copyright

The Cation Radical Chain Cycloaddition Polymerization ofN,3-Bis(trans-1-propenyl)carbazole: The Critical Importance of Intramolecular Hole Transfer in Cation Radical Cycloaddition Polymerization

The synthesis and polymerization of N,3-[bis(trans-1-2)]carbazole (1) is reported. Using either the stable cation radical salt tris(4-2)aminium hexachloroantimonate (2 +.) or anodic oxidation to initiate the reaction, novel cycloaddition polymers are obtained in which the intermonomer linkages are of the cyclobutane, and to some extent of the Diels−Alder, type. A novel cation radical chain mechanism is proposed for the reaction, and extensive support for this mechanism is presented. The greatly enhanced reactivity of difunctional, as opposed to monofunctional, substrates in cation radical cycloadditions is dramatically highlighted by a comparison of the cycloaddition reactivity (rapid polymerization) of 1 versus N-propenylcarbazole (inefficient cyclodimerization) under electrochemical oxidation conditions. The sharply enhanced reactivity of 1 is attributed to the availability of intramolecular hole transfer in the bifunctional but not the monofunctional case.

Approaching a possible stepwise/concerted mechanistic crossover point in the cation radical cycloadditions of cis- and trans-anethole

The Diels–Alder cycloadditions of the cis- and trans-anethole cation radicals to cyclopenta-1,3-diene have carefully been examined and are found to produce sharply different stereochemical results. The cis-anethole cation radical adds via a distinctly stepwise mechanism, yielding comparable amounts of all four diastereoisomeric Diels–Alder adducts. In contrast, the trans-anethole cation radical yields only trans adducts, with the endo isomer predominating. None of the cis adducts could be detected, and the percentage of cis adducts formed, if any, is significantly less than 0.1%. This result stands in contrast to the additions of aryl cis- and trans-propenyl ethers to the same diene, in which both geometric alkene isomers yield all four adducts in comparable amounts. Consequently, if trans-anethole also reacts by a stepwise mechanism, the rate of cyclization in the intermediate distonic cation radical must be of the order of 1000 times the rate of bond rotation. Alternatively, although it is less likely, the trans cation radical could react via a concerted reaction path.

The mechanism of the prototype cation radical cycloaddition reaction: the cyclodimerization of N-vinylcarbazole

The mechanism of the first cation radical cycloaddition reaction to be discovered, the cyclodimerization of N-vinylcarbazole, has now been studied by means of a stereochemical criterion. The cyclodimerization of N-(cis-2-deuteriovinyl)carbazole is observed to be non-stereospecific, yielding a cyclodimer in which approximately 80% of the deuterium atoms are cis to the carbazolyl group and 20% are trans to the carbazolyl group. This result stands in contrast to the stereospecific cyclodimerization of trans- and cis-anethole and other stereospecific cation radical cyclobutanation reactions which have been studied more recently. The cyclodimerization of N-vinylcarbazole must therefore occur via a stepwise path, involving an intermediate distonic cation radical, as originally proposed by the Ledwith group. Further, the gauche conformation of the distonic cation radical is implied to be preferred over the anti form.

Diels–Alder cycloadditions of the N-vinylcarbazole radical cation

The first radical cation Diels–Alder reactions of N-vinylcarbazole, the substrate for which radical cation cycloaddition (cyclodimerization) was first observed more than 30 years ago, have now been observed. The additions of N-vinylcarbazole to both cyclopenta-1,3-diene and cyclohexa-1,3-diene, catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate, have been observed. Further, a two step mechanism for these cycloadditions has been established through the use of stereospecifically labelled substrate ((Z)-N-(2-deuteriovinyl)carbazole).