Investigations in the field of carbene-boron chemistry

Untersuchungen zur Chemie von Bor-Carben-Addukten

Diploma Thesis , 2003 , 76 Pages , Grade: 1

Chemistry - Materials Chemistry

From a long time ago attempts have been made to isolate carbenes. A big motivation behind the search for a stable carbene was the fact, that oxidation state II is well known for the late members of group 14, germanium, tin and lead. For lead +II is even the most stable oxidation state. Therefore it should be possible to produce a compound containing a carbon in oxidation state II, which is stable enough to be detected and possibly isolated and characterized. Additionally carbenes may be useful as building blocks in organic syntheses. They form complexes with a wide variety of main group elements and transition metals in both high and low oxidiation states. Many of these complexes are highly efficient homogeneous catalysts. Carbenes are defined as compounds possessing a divalent carbon in their structure. This carbon is bound to two adjacent groups by covalent bonds. It has two nonbonding electrons which may have parallel (singlet state) or antiparallel spins (triplet state). The preferred state depends on the relative energies of both states. If both orbitals are degenerate, the triplet state is favorable. Otherwise both electrons will occupy the orbital lower in energy with antiparallel spins. The simplest example of a carbene is methylene.
The area of carbene boron chemistry is a relatively new area of research. Like most other fields of carbene research it has been revived by the discovery of stable carbenes by Arduengo in 1991. Until then only a few neutral borane adducts with electroneutral carbon bases were known. Most carbon bases are electron deficient on the carbon and therefore electrophiles. However, a nucleophile center is needed to bind to an electron deficient acceptor like borane, especially because boron is not able to provide any pi-backdonation like transition metal carbene complexes, as it lacks free electron pairs.
The new nucleophile imidazole-2-ylides make neutral carbon borane adducts easily accessible. In 1993 Kuhn et al. found that borane adducts of these carbenes can be produced in high yields by allowing the carbene to react with BH3·Me2S complex. Other examples of boron adducts with nucleophilic carbenes are adducts with boron trifluoride and trimethoxyborate.
Carbene boron adducts in which boron bears a single carbene substituent are easily accessible. Adducts with two or more carbene ligands on boron remain unknown.

Inhaltsverzeichnis (Table of Contents)

• 1. Introduction
• 1.1. What are carbenes?
• 1.1.1. Definitions
• 1.1.2. A short history of carbene research
• 1.1.3. Characteristics affecting the stability of carbenes
• 2. Attempted synthesis of a Tris(imidazol-2-yllidene)-borane adduct
• 2.1. Introduction
• 2.2. Results and Discussion
• 2.2.1. General method to prepare imidazol-2-ylidenes from the corresponding imidazolium salts
• 2.2.2. Reaction of 1,3-dimethylimidazol-2-ylidene 9 with boranethf complex at a 3:1 ratio
• 2.2.3. Attempt at the addition of 1,3-Dimethylimidazoliumchloride 25 to 2-borane-1,3-dimethylimidazolin 24 under elimination of hydrogen
• 2.2.4. Reaction of 1,3-dimethylimidazol-2-ylidene 9 with trimethyl borate
• 2.2.5. Reaction of 1,3-dimethylimidazol-2-ylidene 9 with boron trichloride BCl3
• 2.2.6. Attempt to exchange dimethylamine against the 1,3-dimethylimidazolium ion 25 at tris(dimethylamino)borane
• 3. Synthesis and characterization of imidazolium borohydrides
• 3.1. Introduction
• 3.2. Results and Discussion
• 3.2.1. Preparation of 1,3-dimethylimidazolium borohydride 35
• 3.2.2. X-ray crystal structure analysis of 35
• 3.2.3. Preparation of 1,3,4,5-tetramethylimidazolium borohydride 37
• 3.3. Comparison of 1H-NMR shifts of 1,3-dimethylimidazolium salts and adducts of 1,3-dimethylimidazol-2-ylidenes with boron compounds
• 4. Reactions of 1,3-dialkyl- and 1,3-diarylimidazolinium chlorides with borane and sodium borohydride
• 4.1. Introduction
• 4.2. Results and Discussion
• 4.2.2. Attempt to the preparation of 1,3-dialkyl- and 1,3-diarylimidazolinium borohydrides
• 4.2.3. Reaction of 1,3-bis-(tert-butyl)imidazolinium chloride 38c with sodium hydride
• 4.2.4. Reaction of 1,3-dimesitylimidazolinium chloride 38a with sodium hydride, followed by boranethf complex
• 5. Experiments towards the hydrogenation of imidazolium-boron adducts
• 5.1. Introduction
• 5.1.2. Apparatus
• 5.2. Results and Discussion
• 5.2.1. Preparation of 1,3,4,5-tetramethylimidazol-2-ylidene borane adduct 19
• 5.2.2. Preparation of 2-borane-1,3-dimethyl-4,5-dichloro-imidazoin adduct 42
• 5.2.3. Reactions of imidazol-2-ylidene boron adducts with H2 at 900/1500 psi
• 5.2.4. Heating experiments with 35 and 37 to the reversibility of eq. 37
• 6. Conclusions and outlook
• 7. Experimental Part
• 7.1. General
• 7.1.1. NMR spectroscopy
• 7.1.2. Melting Points
• 7.1.3. Single-crystal X-Ray structure analysis
• 7.1.4. Starting materials
• 7.2. Description of the experiments
• 7.3. Handling of chemicals and waste disposal
• 8. Appendix
• 8.1. List of numbered compounds
• 8.2. List of abbreviations
• 8.3. Crystallographic data and parameters of the X-ray structure determination of 35
• 9. References

Zielsetzung und Themenschwerpunkte (Objectives and Key Themes)

This dissertation investigates the synthesis and characterization of novel boron-carbene adducts, aiming to explore the possibility of forming di- and trimeric complexes. The primary focus is on the reaction of 1,3-dimethylimidazol-2-ylidene and 1,3,4,5-tetramethylimidazol-2-ylidene with various boron compounds, including borane, trimethyl borate, and boron trichloride. The dissertation explores the following key themes:

• The reactivity of nucleophilic carbenes with electron-deficient boron compounds.
• The synthesis and characterization of new borane adducts and imidazolium borohydrides.
• The influence of substituents and counterions on the stability and reactivity of carbene boron compounds.
• The potential for hydrogenation of imidazolium-boron adducts, particularly in the context of hydrogen storage.
• The exploration of alternative synthetic pathways to access di- and trimeric carbene boron adducts.

Zusammenfassung der Kapitel (Chapter Summaries)

Chapter 1: Introduction

This chapter defines carbenes and discusses their historical discovery and development. It highlights the factors that influence their stability and reactivity, focusing on the role of substituents and ring systems in promoting the singlet configuration.

Chapter 2: Attempted Synthesis of a Tris(imidazol-2-yllidene)-Borane Adduct

This chapter details the various attempts to synthesize a trimeric boron-carbene adduct using 1,3-dimethylimidazol-2-ylidene as the carbene precursor. Despite various experimental approaches, only monomeric adducts were isolated, suggesting that boron's electron-accepting capacity limits the formation of higher-order complexes.

Chapter 3: Synthesis and Characterization of Imidazolium Borohydrides

This chapter reports the serendipitous discovery and subsequent synthesis of two new imidazolium borohydrides, 1,3-dimethylimidazolium borohydride (35) and 1,3,4,5-tetramethylimidazolium borohydride (37). X-ray analysis of 35 reveals interesting hydrogen bonding interactions between the borohydride anions and the imidazolium cations, suggesting a layered crystal structure.

Chapter 4: Reactions of 1,3-Dialkyl- and 1,3-Diarylimidazolinium Chlorides with Borane and Sodium Borohydride

This chapter examines the reactivity of saturated imidazolinium chlorides with borane and sodium borohydride. The results show that both reagents preferentially reduce the imidazolinium salts to the corresponding imidazolidines, suggesting that the formation of imidazolinium borohydrides and imidazolin-2-ylidene borane adducts is unfavorable.

Chapter 5: Experiments Towards the Hydrogenation of Imidazolium-Boron Adducts

This chapter investigates the potential for hydrogenation of imidazolium-boron adducts, particularly the borane adducts 19, 24, and 42. Experiments using high pressures of dihydrogen in DMSO-d6 reveal a small degree of conversion to the corresponding imidazolium borohydrides, suggesting that DMSO may play a catalytic role in the hydrogenation process.

Schlüsselwörter (Keywords)

This dissertation focuses on the field of carbene boron chemistry, exploring the synthesis, characterization, and reactivity of novel carbene boron adducts. Key terms include nucleophilic carbenes, borane adducts, imidazolium borohydrides, hydrogenation, and hydrogen storage. The dissertation also delves into the influence of substituents and counterions on the stability and reactivity of these compounds, using specific examples like 1,3-dimethylimidazol-2-ylidene and 1,3,4,5-tetramethylimidazol-2-ylidene as carbene precursors.

• College: Technical University of Braunschweig
• Grade: 1
• Author: Dipl.-Chem. Oliver Steinhof (Author)
• Publication Year: 2003
• Pages: 76
• Catalog Number: V186376
• ISBN (eBook): 9783656997702
• ISBN (Book): 9783656997818
• Language: English
• Tags: investigations, untersuchungen, chemie, bor-carben-addukten