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Development of New Synthetic Routes to Organoboronates by Catalytic Allylic Substitution and C-H Bond Functionalization

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Development of New Synthetic Routes to Organoboronates by Catalytic Allylic Substitution and C-H Bond Functionalization Development of New Synthetic Routes to Organoboronates by Catalytic Allylic Substitution and C-H bond Functionalization Vilhelm Olsson ©Vilhelm Olsson, Stockholm 2009 ISBN 978-91-7155-899-2 Printed in Sweden Edita Sverige AB, Stockholm 2009 Distributor: Department of Organic Chemistry ii To my love and family. iii iv Abstract This thesis describes the development of new catalytic methods for the synthesis and application of organometallic reagents, mainly focusing on allylboronic acid derivatives. Thus, palladium pincer-complex ca- talysis has been applied for extending the scope of palladium- catalyzed borylation reactions in the synthesis of regio- and stereode- fined functionalized allylboronic acid derivatives. These novel allyl- boronic acids were also employed as substrates in palladium catalyzed regioselective coupling reactions with iodobenzenes. We have also developed a new one-pot sequence based on preparation of allyl- and vinylboronates via catalytic carbon-hydrogen bond activa- tion/borylation reactions. The synthetic scope of the reaction as well as mechanistic studies on the borylation process are presented. Fi- nally, the synthesis of new chiral palladium pincer-complexes is de- scribed. These species were employed as catalysts in asymmetric elec- trophilic allyation of imines. v vi List of Publications This thesis is based on the following papers, which will be referred to by Roman numerals: I. S. Sebelius, V. J. Olsson and K. J. Szabó: Palladium Pincer- complex Catalyzed Substitution of Vinyl Cyclopropanes, Vinyl Aziridines and Allyl Acetates with Tetrahydroxydiboron. An Effi- cient Route to Functionalized Allylboronic Acids and Potassium Trifluoro(allyl)borates. J. Am. Chem. Soc. 2005, 127, 10478. II. V. J. Olsson, S. Sebelius, N. Selander and K. J. Szabó: Direct Bo- ronation of Allyl Alcohols with Diboronic Acid using Palladium Pincer-Complex Catalysis. A Remarkably Facile Allylic Dis- placement of the Hydroxy Group Under Mild Reaction Condi- tions. J. Am. Chem. Soc. 2006, 128, 4588. III. S. Sebelius, V. J. Olsson, O. A. Wallner and K. J. Szabó: Palla- dium-Catalyzed Coupling of Allylboronic Acids with Iodoben- zenes. Selective Formation of the Branched Allylic Product in the Absence of Directing Groups. J. Am. Chem. Soc. 2006, 128, 8150. IV. V. J. Olsson and K. J. Szabó: Selective One-Pot Carbon-Carbon Bond Formation by Catalytic Boronation of Unactivated Cycloal- kenes and Subsequent Coupling. Angew. Chem. Int. Ed. 2007, 46, 6891. V. V. J. Olsson and K. J. Szabó: Synthesis of Allylsilanes and Dienylsilanes by a One-Pot Catalytic C−H Borylation Suzuki−Miyaura Coupling Sequence. Org. Lett. 2008, 10, 3129. VI. V. J. Olsson and K. J. Szabó: Functionalization of Unactivated Alkenes through Iridium-Catalyzed Borylation of Carbon- Hydrogen Bonds. Mechanism and Synthetic Applications. Manu- script. 2009. vii VII. O. A. Wallner, V. J. Olsson, L. Eriksson and K. J. Szabó: Synthe- sis of New Chiral Pincer-complex Catalysts for Asymmetric Ally- lation of Sulfonimines. Inorg. Chim. Acta. 2006, 359, 1767. Reprints were made with kind permission from the publishers. viii Contents Abstract .........................................................................................................v List of Publications .................................................................................... vii Abbreviations............................................................................................... xi 1. Introduction .............................................................................................1 1.1 Allyl metal reagents in organic synthesis........................................................1 1.2 Reactivity of allylpalladium complexes ............................................................3 1.3 Palladium pincer-complexes...............................................................................4 1.4 Synthesis of allylboronates ................................................................................5 1.5 Synthesis of organoboronates via metal-catalyzed carbon-hydrogen bond activation.............................................................................................................7 1.6 Major objectives of this thesis ...........................................................................9 2. Selective synthesis and transformation of allylboronic acids via palladium-catalyzed allylic substitution reactions (Papers I-III) ....10 2.1 Palladium pincer complex catalyzed substitution of allylic substrates ...11 2.2 Reactivity and selectivity ..................................................................................12 2.3 Synthetic scope of the reaction.......................................................................13 2.4 Stability of the allylboronic acid derivatives.................................................13 2.5 Employment of allylic alcohols as substrates in the boronation reactions .......................................................................................................................................14 2.6 Regio- and stereoselectivity of the catalytic process .................................16 2.7 Substituent effects on the reactivity of the allylic alcohols.......................16 2.8 Proposed mechanism of the boronation reaction........................................17 2.9 Investigations on the activation of allyl alcohols.........................................17 2.10 Comparisons to other palladium catalysts..................................................19 2.11 Palladium catalyzed regioselective allylation of aryl iodides with functionalized allylboronic acids .............................................................................20 2.12 Regioselectivity of the coupling reaction ....................................................22 2.13 Comparison of the regioselectivity to common allylic substitution reactions ......................................................................................................................23 2.13 Proposed mechanism of the coupling reaction ..........................................24 ix 3. In situ generation of allylboronates and vinylboronates via selective catalytic carbon-hydrogen bond borylation of alkenes (Paper IV-VI)..............................................................................................26 3.1 One-pot regio- and stereoselective CH-functionalization of unactivated alkenes.........................................................................................................................27 3.2 Mechanism of the iridium-catalyzed C-H borylation...................................32 3.3 Synthetic relevance of the C-H activation-based borylation of alkenes.40 4. Development of novel chiral palladium pincer-complex for asymmetric allylation of sulfonimines (Paper VII) .............................42 4.1 Synthesis of BINOL- and TADDOL-based chiral pincer-complexes .........43 4.2 Structural and catalytic properties of the synthesized complexes ..........44 5. General conclusions and outlook.......................................................46 Acknowledgements ...................................................................................47 References ..................................................................................................48 x Abbreviations The abbreviations are used in agreement with standards of the subject. .1 Only non-standard and unconventional ones that appear in the thesis are listed here. BINOL 1,1’-Bi-2-naphtol Bs Benzenesulfonyl Cond Conditions COD 1,4-Cyclooctadiene Cy Cyclohexyl Equiv Equivalents Ln Unspecified number of ligands Py Pyridine Pin Pinacol TMEDA Tetramethylethylenediamine 1 Org. Lett. 2009, 11, 24A-22A. xi xii 1. Introduction Transition metal complexes are extensively employed as catalysts in modern preparative organic chemistry, natural product synthesis and manufacturing of drugs and specialized chemicals.1-16 This thesis is mainly focused on the development of new innovative palladium and iridium catalyzed methods for the synthesis of allylboronate deriva- tives. The main attention has been directed to finding catalytic reac- tions affording functionalized allylboronates with broad synthetic scope and high regio- and stereoselectivity. 1.1 Allyl metal reagents in organic synthesis Allylic organometallic compounds (1) are extensively used as reagents for the organic transformation17-31 of aldehydes (2) affording homoal- lylic alcohols (3) (Scheme 1). This reaction is also a synthetic alterna- tive to the aldol reaction31 since the homoallyl alcohol products can easily be converted to the corresponding aldol 4. Moreover, the alkene group may readily be converted to other functionalities such as alde- hydes, β−lactones, or epoxides (Scheme 11).32-33 Scheme 1. Synthetic scope of the allylmetal-aldehyde condensation.33 Selectivity of the allylation reactions. Allylboron reagents have be- come one of most widely applied class of allylic substrates because of 1 their high stereoselectivity in the reaction with aldehydes.17-44 The high diastereoselectivity can be explained by reaction via a so called “type I” mechanism,45 (Scheme 2) which features a coordination bond between the carbonyl oxygen and the boron atom in a compact six- membered chair-like transition state (5a). This mechanism contrasts with the reaction of allylsilanes and allylstannanes, which proceed by a type II mechanism (Scheme 2) via an open transition structure (5b) and generally requires external Lewis acid activators.
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