o-Quinodimethanes: Efficient Intermediates in Organic Synthesis Jose ´ L. Segura and Nazario Martı ´n* Departamento de Quı ´mica Orga ´nica, Facultad de Quı ´mica, Universidad Complutense, E-28040 Madrid, Spain Received February 11, 1999 (Revised Manuscript Received April 16, 1999) Contents I. Introduction 3199 II. Structure and Properties of o-Quinodimethanes 3200 III. Generation of o-Quinodimethanes 3203 A. Thermal Generation of Substituted Carbocyclic o-Quinodimethanes 3203 1. Thermolysis of Benzocyclobutenes 3203 2. 1,4-Elimination of R,R′-Substituted o-Xylenes 3204 3. From Benzo-Fused Heterocyclic Compounds 3205 B. Photochemical Generation of Substituted Carbocyclic o-Quinodimethanes 3207 C. Electrochemical Generation of Substituted Carbocyclic o-Quinodimethanes 3209 D. Generation of o-Quinodimethane Heteroanalogues 3210 IV. o-Quinodimethanes as Synthetic Intermediates 3213 A. o-Quinodimethanes Reactivity 3213 B. o-Quinodimethane as a Diene in Diels-Alder Reactions 3214 V. Synthesis of Natural Products from o-Quinodimethanes 3215 VI. o-Quinodimethanes in Materials Chemistry 3217 A. o-Quinodimethanes as Precursors of π-Extended Systems 3217 1. Synthesis of Polyacene Derivatives 3217 2. π-Extended Electron-Donor and Electron-Acceptor Systems 3218 B. Cycloadditions of o-Quinodimethane to Fullerenes: A Versatile Approach to Stable Modified Fullerenes 3219 1. Cycloadditions of Masked oQDMs and Related Structures to C 60 3221 2. o-Quinodimethanes as Useful Tools in the Selective Functionalization of [60]- Fullerene 3222 3. Diels-Alder Adducts of [60]Fullerene and Heterocyclic o-Quinodimethanes 3224 4. o-Quinodimethanes as Precursors for Electroactive C 60 -Based Dyads 3228 5. o-Quinodimethanes in the Chemistry of Higher Fullerenes 3230 VII. o-Quinodimethanes in Polymer Chemistry 3231 A. Polymerization of Benzocyclobutene Monomers Containing the Benzocyclobutene Functionality as the Only Reactive Species 3231 B. Polymerization of Benzocyclobutene Monomers Containing Sites of Unsaturation 3233 C. Polymeric Materials with Pendant Benzocyclobutene Groups 3234 D. Polymerization of Bisbenzocyclobutenes with Molecules Containing Two Sites of Reactive Unsaturation 3235 E. Polymerization of o-Quinodimethanes Generated from Precursors Different from Benzocyclobutene 3235 F. Fullerenes and Polymer Chemistry 3236 VIII. o-Quinodimethanes as Nitric Oxide Cheletropic Traps (NOCT) 3238 IX. Summary and Perspectives 3241 X. Acknowledgments 3242 XI. References and Notes 3242 I. Introduction The elusive intermediate o-quinodimethane (oQDM), also named o-xylylene, has attracted much attention of both theoretical and synthetic chemists over the past 30 years. As cis-dienes, oQDMs have a remark- able Diels-Alder reactivity and are often used as building blocks in the syntheses of cyclic organic compounds by inter- or intramolecular [4 + 2] trap- ping. In 1957, Cava and co-workers first suggested the participation of an o-quinodimethane 2 as a reaction intermediate in the conversion of R,R,R′,R′- tetrabromo-o-xylene (1) to trans-1,2-dibromobenzo- cyclobutene 3 (Scheme 1), 1 and this was later proven by trapping experiments with a number of cyclic dienophiles. Since then, many methods for the generation of oQDM 2 and oQDM heteroanalogues 3 have been de- veloped and their characterization, structure, and chemical reactivity have been thoroughly studied. Thus, during the past decade an explosive amount of activity in the area of the o-quinodimethane chemistry has been developed from their interest in a variety of fields ranging from the syntheses of steroids and other natural products, 4 their applica- Scheme 1 3199 Chem. Rev. 1999, 99, 3199-3246 10.1021/cr990011e CCC: $35.00 © 1999 American Chemical Society Published on Web 10/16/1999