Advancing the chemistry of phosphole
Pursuing a PhD is of course not solely about gaining the degree. It is also about the valuable experiences, both academic and otherwise. In these four years, a number of supportive people have made this long expedition an exciting and worthwhile one. The first and foremost of these people is my supervisor, Prof. Franҫois Mathey, to whom I would like to convey my deepest gratitude. He has always been very patient in providing guidance, and his optimism has made these four years truly enjoyable
... d fruitful. So again, Prof. Mathey, thank you so much, for being so caring and understanding. Secondly, I would like to express my utmost thankfulness my mentor during my undergraduate years, Dr Chiang Minyi. Her critical training, and expert guidance, equipped me very well with the refined skills needed for research in this field. I would also like to show appreciations to my fellow lab mates, Feny, Jeanette for their help, discussions and most importantly, friendship. Not forgetting the internship student and undergraduates who have helped me along the way. I would also like to specially acknowledge Dr Li Yongxin and Dr Rakesh Ganguly, for their help in providing and deciphering the crystallographic data, Dr Lu Yunpeng, for his help with the computational studies and all other support staffs from the CBC office, NMR, CBC general store, MS and teaching laboratory for generally making my PhD life slightly easier. I would also like to thank Nanyang Technological University for the financial support. Last but not least, I wish to take this opportunity to thank my family and my boyfriend, Binayak, for their love and unconditional support. i ABSTRACT We have studied the influence of two types of unsaturated substituents on the αposition of the weakly aromatic phospholes. They include the vinyl group and the Fischer carbenes. The presence of this vinylic group has induced linear π-conjugation with the diene unit of phosphole. This has in turn activated the dienic system resulting in a unique chemistry that is rarely (or never) observed in the other 5-membered heteroarene derivatives. Synthesis of the desired 2-vinylphosphole is via 2-formylphosphole which is prepared from 2lithiophosphole. Then, it is complexed to W(CO) 5 and characterised by X-ray analysis. Both LIST OF SCHEMES Scheme 1.1 Synthesis of 1,2,5-trisubstituted phosphole by 1,3-diynes with primary phosphines 3 Scheme 1.2 Synthesis of 1,2,3,4,5-pentaphenylphosphole by 2 different pathways 4 Scheme 1.3 Preparation of 1,2,3,4,5-pentasubstituted phospholes from zirconacyclopentadiene 5 Scheme 1.4 Dehydrohalogenation of halo-3-phospholenium salts 6 Scheme 1.5 Proposed mechanism of dehydrohalogenation of halo-3phospholene salt by base 6 Scheme 1.6 Phosphole synthesis via 1,1-carboboration of bis(alkynyl)phosphines 8 Scheme 1.7 Isomers of parent phosphole 3 11 Scheme 1.8 Formation of 1-phosphanorbornadiene 10 12 Scheme 1.9 (a) Suzuki-Miyaura cross coupling and (b) Transfer hydrogen of ketones with ligand 20 16 Scheme 1.10 Asymmetric allylic alkylation using chiral ligand 21 16 Scheme 1.11 Reactivity of pyrrole and phosphole towards electrophile 20 Scheme 1.12 Formation of zirconacyclopentadiene from zirconacyclopropene 21 Scheme 1.13 Formation of 2,5-(heteroaryl)phospholes 21 Scheme 1.14 Twofold of [1,5]-sigmatropic shift 22 Scheme 1.15 Friedel-Crafts acetylation on phosphole with bulky P-substituent 23 Scheme 1.16 Functionalization of phosphole sulphide 24 Scheme 1.17 Functionalization of phosphole-borane 25 Scheme 1.18 Series of 2-functionalized phospholes synthesized from 2lithiophosphole 26 Scheme 1.19 2,5-di(functionalized) phospholes from 2,5-dibromophosphole 27 Scheme 2.1 Synthesis of 2-(1-propenyl)phosphole 32 40 Scheme 2.2 Synthesis of 2-aryl-5-styrylphospholes 40 Scheme 2.3 [4+2]-cycloaddition of σ 3 and σ 4 -phospholes 41 Scheme 2.4 Diels-Alder reaction of phosphole complexes with DMAD 42 Scheme 2.5 [4+2]-cycloaddition of 1-chlorophospholes with maleic acid derivatives 43 vii Scheme 2.6 Intra-and extra-annular cycloaddition of 5-membered αvinylheteroarenes 44 Scheme 2.7 Diels-Alder reaction of 2-vinylfuran with DMAD 44 Scheme 2.8 Extra-annular Diels-Alder reaction 45 Scheme 2.9 Cycloaddition of 2-vinylpyrroles with DMAD 45 Scheme 2.10 Diels-Alder reaction of 2vinylpyrrole derivatives with maleic anhydride 46 Scheme 2.11 Cycloaddition of 2-vinylthiophene with maleic anhydride 46 Scheme 2.12 Cycloaddition of 2-vinylthiophene with DMAD 47 Scheme 2.13 Synthesis of 2-vinylphosphole 50 and 2-vinylphosphole tungsten complex 51 50 Scheme 2.14 Diels-Alder reaction of compound 50 with N-phenylmaleimide 53 Scheme 2.15 Diels-Alder cycloaddition of complex 51 with DMAD 54 Scheme 2.16 [1+2] cycloaddition of terminal phosphinidene with vinylphosphole complex 51 56 Scheme 3.1 Fischer method (a) and Semmelhack-Hegedus (b) route to Group VI metal carbene complexes 71 Scheme 3.2 Reaction between (Ph)(OMe)C-Cr(CO) 5 with alkenes 73 Scheme 3.3 Mechanisms of metal-carbenes complexes with electron-rich alkenes (a) and electron-poor alkenes (b) 74 Scheme 3.4 Cyclopropanation of 1-hexyne with heteroaryl Fischer carbene complexes 75 Scheme 3.5 Effect of metal on the cyclization reactions 76 Scheme 3.6 Reaction between Alkoxy(2-furyl and 2-thiophene)carbene complexes and alkynes under different condition 76 Scheme 3.7 General product obtained between (OMe)(C 5 H 6 N)C-Cr(CO) 5 with alkynes 77 Scheme 3.8 Unexpected benzannulation of complex 56 with diethylacetylene 78 Scheme 3.9 Annulation with heteroalkyne 78 Scheme 3.10 Four competitive pathways in annulation with alkynes 79 Scheme 3.11 Formation of carbenes 64 and 65 by Fischer method 84 Scheme 3.12 Sulfurization of compound 64 87 Scheme 3.13 Oxidation by DMSO (a) and reaction with water (b) 89 viii Scheme 3.14 Reaction of 65 with styrene 91 Scheme 3.15 Proposed mechanism for the formation of 70 93 Scheme 3.16 Annulation of complex 65 with alkynes 94 Scheme 3.17 Proposed mechanism for the formation of 74 95 Scheme 3.18 Proposed mechanism for the formation of 75 95 ix 2 Chapter 1 Introduction to phosphole chemistry act as eta-5 ligands due to their aromaticity. 23 Subsequently, a range of phosphametallocenes 6 and 7 were synthesized, 24,25 and pioneering research on the applications of such complexes in catalysis has started. 26 In brief, many recent metal catalysed reactions that phospholes participated in (such as hydroformylation, copolymerization and polymerization with ethylene, etc.) have been summarized. 19,21 One of the new and intriguing applications of phospholes is their ability to be used as the building block for electro-optic substances. The intrinsically low aromaticity of phospholes implies a polarizable endocyclic diene. This was demonstrated by including acetylenic substituents on the α-carbons of the phosphole, leading to the absorption and emission at relatively long wavelengths. 27 Moreover, the reactive P-centers allow chemical modifications which can alter the optical and electro-chemical properties of phosphole-based π-conjugated materials. These appealing properties place phosphole markedly above other commonly used aromatic organic synthons such as pyrrole, thiophene, fluorene, or benzene as the building blocks for electro-optical materials. It is clear that phospholes deserve our attention for more novel advances. Thus, the contents of this thesis concentrate on functionalizing monophospholes with vinylic groups on the α-carbon. We envisaged that the α-vinylic substituent on phospholes would create a significant alteration in phospholes chemistry. In the near future, such vinylic-substituted phopholes may also serve as the building blocks for highly π-conjugated phospholes polymers.