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Synthesis and Application of an Electronically Chiral Mimic of Cpfe

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Synthesis and Application of an Electronically Chiral Mimic of Cpfe D¶epartement de Chimie, Section Chimie Organique Universit¶ede Fribourg (Suisse) Synthesis and Application of an Electronically Chiral Mimic of CpFe THESE Pr¶esent¶ee`ala Facult¶edes Sciences de l'Universit¶ede Fribourg (Suisse) pour l'obtention du grade de Doctor rerum naturalium par CHRISTOPHE ALLEMANN de RÄuttenen(SO) Th`esen± 1369 Imprimerie Saint-Paul, Fribourg 2002 ii Accept¶eepar la Facult¶edes Sciences de l'Universit¶ede Fribourg (Suisse) sur la proposition du Prof. Titus A. Jenny, du Dr. Roger L. Snowden (Firmenich, Gen`eve,Suisse) et du Prof. Carl-Whilhelm SchlÄapfer,pr¶esidentdu jury. Fribourg, le 07 F¶evrier2002, iii A ma m`ere A ma famille iv Remerciements Je tiens `aexprimer ma reconnaissance envers le professeur Titus Jenny pour m'avoir donn¶ela possibilit¶ed'e®ectuer cette th`esesous sa direction. Je le remercie pour son soutien, pour sa totale disponibilit¶e,et pour ses nombreux conseils qui ont transform¶ece travail en une passionnante recherche. Je remercie le Dr. Roger L. Snowden pour avoir accept¶ede lire ce m¶emoire et pour s'^etreengag¶ecomme expert lors de l'examen de soutenance. Je le remercie aussi pour ses renseignements, ainsi que pour avoir propos¶ed'analyser les ¶echantillonsd'Ambrox°R au sein de son groupe de recherche. A cet ¶egard, j'exprime ma gratitude `aM. Christian Vial, qui s'est occup¶ede ces analyses (GC et GC chirales). J'adresse des remerciements particuliers `aF¶elixFehr, Fredy Nydegger et Inge MÄullerpour les mesures RMN et les spectres de masses. J'exprime ma sympa- thie `aAndreas Saxer pour avoir e®ectu¶equelques analyses GC chirale d'¶echantil- lons d'Ambrox°R . Les professeurs Claude Daul, Philippe Renaud, Carl-Whilhelm Schlaepfer et Franzpeter Emmeneger recoivent ma sympathie pour avoir r¶epondu `ames ques- tions. Je remercie aussi le Prof. Michael Allan pour m'avoir pr^et¶esa pr¶ecieuse bonbonne de CO et le Priv. Doc. Dr. Norbert Engel pour les recherches de litt¶eratureet les dicussions sympathiques que nous avons eues. Pour la lecture et la correction de ce document, j'adresse mes remerciements `a Bassam Alameddine. Pour son excellent travail de travaux pratiques avanc¶es, durant lequel il a synth¶etis¶el'homofarnesol, je remercie Thomas Bark. vi Christophe Eggerstwyler, Manuel Raemy, Bassam Alameddine, Thomas Sieber, Vroni Huber, David Zeller, Jacques Raemy et Patrizio Aspari ont toute ma sym- pathie pour leur aide, leurs conseils et la bonne ambiance de travail `alaquelle ils ont contribu¶e. Mes vifs remerciements vont aussi `atous les collaborateurs et amis de la Setion de Chimie Organique et du D¶epartement de Chimie pour leur soutien moral et scienti¯que. Pour l'accueil au sein de l'Institut de Chimie Organique et pour la place d'assistant qu'il m'a o®ert, je dis ma gratitude au Prof. Albert Gossauer. Merci au personnel du service central (L. Rouiller, N. Chassot, P. Rime et H. Favre) et `aMonsieur Crottet pour les multiples services rendus. Je remercie ¯nalement le Fonds National Suisse pour la Recherche Scienti¯que pour le soutien ¯nancier. R¶esum¶e Le domaine des compos¶esorganom¶etalliquesoptiquement actifs est plus que jamais un champ d'activit¶eimportant. Dans ce domaine, le complexe (-)-(1S)- dicarbonyliode[´5-(2,8,8-trim¶ethylbicyclo[3.2.1]oct-3,6-di`ene)2-yle]fer (1), ap- pel¶e 2-Me-BOD-Fe(CO)2I, se pro¯le comme une nouvelle ¶etape dans le d¶eve- loppement de complexes cyclopentadi`enyleschiraux apparent¶es. L'¶etudede la r¶eactivit¶edu complexe 1 a d¶emontr¶eque le syst`eme ´5 discontinu ainsi que sa chiralit¶einh¶erentele di®¶erencientdes complexes apparent¶escomme 5 FpI (´ -(C5H5)Fe(CO)2I). Plus stable que FpI, le seul moyen de l'activer de mani`ereutile est l'utilisation de sels d'argent, par exemple AgBF4, qui g¶en`erent un complexe cationique insatur¶e,capable de compl¶etersa sph`erede coordina- 1) AgBF , CO 4 Nuc 2) Nuc- (CO)2FeI (CO)3Fe 1 AgBF4 OH O homofarnesol (-)-Ambrox tion soit en acceptant un ligand fortement ¶electrodonneur (par ex. CO) soit en complexant une double liaison. Dans le premier cas, le complexe tricarbonyl¶e cationique ainsi obtenu est stable et peut subir une attaque r¶egios¶electived'un nucl¶eophileformant ainsi un complexe di`enique,qui, apr`esd¶ecomplexation, fournit d'int¶eressantesmol¶eculesorganiques. Dans le deuxi`emecas, la faible viii + acidit¶ede Lewis de 2-Me-BOD-Fe(CO)2 initie une cyclisation tandem cas- cade si la double liaison appartient `aun poly`ene. Nous nous sommes partic- uli`erementint¶eress¶es`ala cyclisation de l'homofarnesol en Ambrox°R , un impor- tant parfum. Malgr¶eune faible induction asym¶etriqueet un rendement plut^ot modeste (42%), la diast¶er¶eos¶el`ectivit¶es'av`eretr`esbonne (dr =» 80%). Une analyse du processus est pr¶esent¶eeet des hypoth`esesconcernant le m¶ecanisme sont ¶emises. A¯n d'optimiser et de varier les applications d¶ecritespr¶ec¶edemment,plusieurs d¶eriv¶esde 2-Me-BOD-Fe(CO)2I ont ¶et¶esynth¶etis¶esen substituant le groupe m¶ethylest¶er¶eog¶eniqueavec des groupes ¶electrodonneurs et des groupes ¶electro- attracteurs. De plus, comme le rendement de la synth`esede 2-Me-BOD- Fe(CO)2I chute dramatiquement lors de l'augmentation de l'¶echellede sa synth`ese,le contr^olede tous les param`etresconcevables ainsi qu'une inves- tigation IR en temps r¶eelnous ont aid¶es`aam¶eliorer et `amieux comprendre sa synth`ese. Summary The ¯eld of new optically active and reactive organometallic species is of tremendous interest. Within this area, (-)-(1S)-dicarbonyliodide[´5-(2,8,8-tri- methylbicyclo[3.2.1]oct-3,6-diene)2-yl] iron (1), called 2-Me-BOD-Fe(CO)2I, emerges as a new stage in the development of chiral cyclopentadienyl-like com- plexes. Studying the reactivity of 1 has shown that both its non-contiguous ´5-system and its inherent chirality di®erentiate it from related complexes like FpI (´5- (C5H5)Fe(CO)2I). Of higher stability than FpI, the only means to activate it in a controlled manner is the use of silver salts, like AgBF4, which generates an unsaturated cationic complex able then to complete its coordination sphere either with a strong electron donating ligand (e.g. CO), or by complexing a double bond. In the ¯rst case, the resulting stable tricarbonyl cationic complex 1) AgBF , CO 4 Nuc 2) Nuc- (CO)2FeI (CO)3Fe 1 AgBF4 OH O homofarnesol (-)-Ambrox x can undergo a regiospeci¯c attack of a nucleophile forming thereby a diene complex which, after decomplexation, yields interesting organic molecules. In + the second case, the weak Lewis acidity of 2-Me-BOD-Fe(CO)2 can initiate a tandem cascade cyclization if the double bond belongs to a polyene. We have focused our attention on the cyclization of homofarnesol into Ambrox°R , a fra- grance of importance. Although the asymmetric induction is very weak and the yield modest (42%), the diastereoselectivity is very good (dr =» 80%). An anal- ysis of the process is presented and hypotheses about the mechanism are drawn. In order to optimize, and to vary the applications described above, a series of derivatives of 2-Me-BOD-Fe(CO)2I have been synthesized by substituting the stereogenic methyl group with electron donating as well as with electon withdrawing groups. Moreoever, as the yield of 2-Me-BOD-Fe(CO)2I synthe- sis has dramatically decreased during the scale-up of its synthesis, control of all conceivable parameters and a real-time IR investigation helped us to improve its synthesis and to better understand its mechanism. Contents List of Tables xv List of Figures xix List of abbreviations xx I Theoretical Part 1 1 Introduction 3 2 Iron carbonyls in synthesis 7 2.1 Iron: an omnipresent element . 7 2.1.1 Abundance and importance . 7 2.1.2 From ores to carbonyls . 8 2.2 Complexation methods [9, 10] .................... 10 2.3 The cyclopentadienyl unit in iron carbonyl complexes . 11 2.3.1 Introduction and focal orientation . 11 2.3.2 Precursors of CpFeL2 ................... 12 2.3.3 FpI . 13 2.3.4 New classes of ligands derived from Cp . 18 2.4 Conclusion . 19 3 Tandem reactions 21 3.1 De¯nition [54, 55] ........................... 21 3.2 Tandem cascade cyclization . 22 xii CONTENTS 4 Ambrox°R 25 4.1 Ambergris: origin and characteristics [69] . 25 4.2 Ambergris: historical background [69] . 26 4.3 Ambergris: a chemical insight . 28 4.4 Racemic Ambrox°R : a chemical insight [73] . 29 4.5 (-)-Ambrox°R : a chemical insight [73] . 31 4.5.1 Ambrox°R from a tricyclic building block . 32 4.5.2 Ambrox°R from bicyclic building blocks . 33 4.5.3 Ambrox°R from monocyclic starting building blocks . 39 4.5.4 Ambrox°R from acyclic building blocks . 40 5 Homofarnesol 45 II Results and discussion 49 6 2-Me-BOD-Fe(CO)2I: synthesis and reactivity 51 6.1 Synthesis and large scale optimization . 51 6.2 Properties and reactivity . 60 6.2.1 Photochemical reactivity . 61 6.2.2 Non-photochemical process . 62 7 Tandem cascade cationic cyclization of homofarnesol 65 7.1 Unique behaviour of the 2-Me-BOD-Fe(CO)2 complex . 66 7.2 2-Me-BOD-Fe(CO)2: catalyst or promotor ? . 69 7.3 Reaction conditions and cyclization e±ciency . 71 7.3.1 Solvent and temperature: influence . 71 7.3.2 Additives influence . 72 7.3.3 Role of protecting groups . 72 7.4 Stereochemistry . 74 7.4.1 General considerations .
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