SUPERELECTROPHILES AND THEIR CHEMISTRY GEORGE A. OLAH DOUGLAS A. KLUMPP WILEY-INTERSCIENCE A John Wiley & Sons, Inc., Publication SUPERELECTROPHILES AND THEIR CHEMISTRY SUPERELECTROPHILES AND THEIR CHEMISTRY GEORGE A. OLAH DOUGLAS A. KLUMPP WILEY-INTERSCIENCE A John Wiley & Sons, Inc., Publication Copyright © 2008 by John Wiley & Sons, Inc. All rights reserved. Published by John Wiley & Sons, Inc., Hoboken, New Jersey. Published simultaneously in Canada. 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QD271.35.E54043 2007 547.2–dc22 2007019597 Printed in the United States of America. 10987654321 CONTENTS Preface vii 1 General Aspects 1 2 Study of Superelectrophiles 17 3 Generating Superelectrophiles 81 4 Gitonic Geminal Superelectrophiles 105 5 Gitonic Vicinal Superelectrophiles 125 6 Gitonic 1,3-Superelectrophiles 187 7 Distonic Superelectrophiles 231 8SigniÞcance and Outlook 283 Index 287 v PREFACE Our book is about the emerging Þeld of “Superelectrophiles and Their Reactions.” It deals Þrst with the differentiation of usual electrophiles from superelectrophiles, which show substantially increased reactivity. Ways to increase electrophilic strength, the classiÞcation into gitionic, vicinal, and distonic superelectrophiles, as well as the differentiation of superelec- trophilic solvation from involvement of de facto dicationic doubly electron deÞcient intermediates are discussed. Methods of study including sub- stituent and solvent effects as well as the role of electrophilic solvation in chemical reactions as studied by kinetic investigations, spectroscopic and gas-phase studies, and theoretical calculations are subsequently reviewed. Subsequently, studied superelectrophilic systems and their reactions are discussed with speciÞc emphasis on involved gitionic, vicinal, and distonic superelectrophiles. A brief consideration of the signiÞcance of superelec- trophilic chemistry and its future outlook concludes this book. Results of substantial experimental and theoretical work of the Þeld accumulated in recent years warrant a comprehensive review and discus- sion. This should be of general use to chemists not only with academic and research Þelds interest but also to advanced students. Because of relevance to potential signiÞcant practical applications (including the pharmaceutical and petrochemical Þelds), industrial chemists should also beneÞt from it. vii viii PREFACE We believe that continuing work will result in much further progress and practical applications. If our book will be of help toward this endeavor, our goal will be achieved. George A. Olah Douglas A. Klumpp 1 GENERAL ASPECTS Electrophiles (i.e., electron-deÞcient species) are of fundamental impor- tance to chemistry. The concept of nucleophiles (lit. “nucleus seeking”) and electrophiles (lit. “electron seeking”) was suggested by Ingold follow- ing similar views implied by Lapworth’s description of “anionoid” and “cationoid” reagents, Robinson’s concepts, and Lewis’s theory of bases (electron donors) and acids (electron acceptors).1 The realization of carbon electrophiles or carbocations dates back to 1901, with the reports of the ionization of triphenylmethyl alcohol in con- centrated sulfuric acid and triphenylmethyl chloride with aluminum and tin chlorides.1b,2 These reactions gave deeply colored solutions, which are now attributed to the formation of the π-conjugatively delocalized triphenylmethyl cation. In later studies by Meerwein, Ingold, Hughes, Whitmore, Roberts, Winstein, Schleyer, and others, using kinetic, stere- ochemical, and varied experimental methods, carbocation electrophiles were recognized as intermediates in reactions. It was Olah who discov- ered in the early 1960s methods to prepare and study long-lived persistent carbocations, for which he received the Nobel Prize in 1994. The topic was well reviewed and there is no need for further discussion here.3 Varied diverse electrophilic reagents, functionalities, and intermediates have been further studied in detail.3 They were reviewed in preceding monographs, which are referred to for the interested reader.3b,d With the advance of our Superelectrophiles and Their Chemistry, by George A. Olah and Douglas A. Klumpp Copyright © 2008 John Wiley & Sons, Inc. 1 2 GENERAL ASPECTS structural and mechanistic understanding, it became clear that electrophilic reactivity is an important driving force in many chemical reactions. Extensive efforts have been made to characterize nucleophile and elec- trophile strengths. Hammett Þrst correlated4a the acidities of substituted benzoic acids (1) O X OH 1 with the structures of the substituent groups and set up his equation as = log k/k o = σ p(wherek o is the rate or equilibrium constant for X H, k is the rate or equilibrium constant for the substituted benzoic acid, p is a constant for the given reaction, and σ (Hammet’s constant) is the value characteristic for the substituent).4 In their linear free-energy studies, Swain and Scott characterized nucleophiles and electrophiles in kinetic experiments by comparing reaction rates according to the equation 1, k /k = sn log x H2O x (1) where s is the parameter characteristic for the electrophile and nx is the parameter characteristic for the nucleophile.5 More recently, Mayr and co-workers have conducted extensive kinetic studies in estimating the electrophilicities and nucleophilicities of a wide variety of reactants (Figure 1).6 Using equation 2, ◦ log k(20 C) = s(N + E) (2) the rate constants k for nucleophile-electrophile reactions may be cal- culated from three parameters (N the nucleophilicity parameter, E the electrophilicity parameter, and s the nucleophile-dependent slope param- eter). By analyzing pseudo-Þrst order rate constants with various types of nucleophiles, the electrophilicities of many cationic and neutral species have been established. These electrophiles include dithiocarbenium ions, iminium ions, cationic organometallic complexes (such as propargyl cations with cobalt carbonyl stabilization, cationic palladium complexes, and others), and quinone methides. When experimentally observed, rate constants k are compared with those values of k predicted from the three parameter equation (eq 2), they are generally accurate to within a factor of 10 to 100, excluding reactions with bulky reagents or multi-centered reac- tions (like SN2). Moreover, the three-parameter equation may be used to GENERAL ASPECTS 3 −25 CO Me 2 No reactions at 20°C, CO2Me −6 −1 −1 MeO kcalcd < 10 L•mol •s tBu −20 N O tBu CN −15 CN −1 −1 Me2N k = 1 L•mol •s − + E 10 (OC)3Fe −5 + 0 + MeO OMe + 5 Diffusion-controlled + reactions, Me C 10 −1 −1 3 kcalcd > 10 L•mol •s 10 −10 −5 0 5 10 15 20 25 N Me OSiMe − 3 O Me Me OSiMe − 3 O N Me NO2 Me Me O Me Figure 1. Estimated reaction rates, k calcd, using nucleophilicity parameter N and elec- trophilicity parameter E. describe numerous types of reactions involving electrophiles in such reac- tions as Michael additions, Mannich aminoalkylations, palladium-cataly- zed allylations, Freidel-Crafts alkylations, and others. In addition to the linear free energy studies discussed, there have been many attempts to estimate the thermodynamic