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-YLIDENE)-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 borane thf 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
- 2.2.6. 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 'H-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 borane.thf complex
- 5. EXPERIMENTS TOWARDS THE HYDROGENATION OF IMIDAZOLIUM-BORON ADDUCTS
- 5.1. INTRODUCTION
- 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
- 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
Zielsetzung und Themenschwerpunkte (Objectives and Key Themes)
This diploma thesis investigates the synthesis and characterization of novel compounds in the field of carbene-boron chemistry. The main objective is to explore the reactivity of different imidazolium salts and their corresponding carbenes with various boron compounds to create new imidazolium boron adducts. The experimental work focuses on synthesis attempts, characterization through various methods (NMR, X-ray crystallography), and reaction behavior.
- Synthesis of imidazolium borohydrides
- Reactions of imidazolium salts with boranes
- Characterisation of novel carbene-boron adducts
- Investigation of the reactivity of carbene-boron adducts
- Hydrogenation reactions of imidazolium-boron adducts
Zusammenfassung der Kapitel (Chapter Summaries)
1. INTRODUCTION: This introductory chapter provides a foundational understanding of carbenes, encompassing their definitions, historical research context, and crucial stability-influencing characteristics. It sets the stage for the experimental work by clearly defining the subject matter and its significance within the broader field of organometallic chemistry. The background provided is vital for understanding the rationale behind the subsequent synthetic attempts and characterization efforts detailed in later chapters.
2. ATTEMPTED SYNTHESIS OF A TRIS(IMIDAZOL-2-YLIDENE)-BORANE ADDUCT: This chapter details various attempts to synthesize a tris(imidazol-2-ylidene)-borane adduct. The discussion encompasses different reaction pathways explored, including the use of borane-THF complexes and trimethyl borate as boron sources. Analysis of reaction outcomes, including spectroscopic data and reasons for unsuccessful synthesis routes, is provided. The significance lies in illustrating the challenges involved in the synthesis of this specific type of carbene-boron adduct, highlighting the complexities of controlling reaction pathways and the steric effects involved.
3. SYNTHESIS AND CHARACTERIZATION OF IMIDAZOLIUM BOROHYDRIDES: This chapter focuses on the successful synthesis and detailed characterization of 1,3-dimethylimidazolium borohydride and 1,3,4,5-tetramethylimidazolium borohydride. The synthesis procedures are described, and the characterization involves 'H-NMR spectroscopy and X-ray crystallography. The comparison of NMR shifts between imidazolium salts and their corresponding carbene-boron adducts provides valuable insights into the electronic structure and bonding interactions. The success in synthesizing these stable compounds demonstrates a successful approach in creating novel carbene-boron structures, paving the way for further investigation into their properties and potential applications.
4. REACTIONS OF 1,3-DIALKYL- AND 1,3-DIARYLIMIDAZOLINIUM CHLORIDES WITH BORANE AND SODIUM BOROHYDRIDE: This chapter explores the reactivity of various 1,3-dialkyl- and 1,3-diarylimidazolinium chlorides with borane and sodium borohydride. The attempts to prepare the corresponding borohydrides are described, along with the results and interpretation of the observed reactions, including unexpected side reactions. This chapter expands on the investigation of carbene-boron adducts by using different starting materials and exploring variations in the alkyl/aryl substituents on the imidazolium ring, offering a broader perspective on the reactivity and stability of such compounds. Specific examples of reactions and analysis of their outcome contribute to a better understanding of the influence of different substituents on the overall reaction success and product formation.
5. EXPERIMENTS TOWARDS THE HYDROGENATION OF IMIDAZOLIUM-BORON ADDUCTS: This chapter delves into experiments aimed at hydrogenating different imidazolium-boron adducts under high-pressure conditions. Preparation methods for specific adducts are described, followed by a detailed analysis of the reactions performed with dihydrogen under varying conditions of pressure and reaction time. The results contribute to understanding the stability and reactivity of the synthesized compounds under these conditions, providing valuable information about their potential applications and further synthetic possibilities.
Schlüsselwörter (Keywords)
Carbene, boron, imidazolium, borohydride, synthesis, characterization, NMR spectroscopy, X-ray crystallography, hydrogenation, adduct, reactivity, stability.
Frequently Asked Questions: Diploma Thesis on Carbene-Boron Chemistry
What is the main topic of this diploma thesis?
This diploma thesis investigates the synthesis and characterization of novel compounds in the field of carbene-boron chemistry. The focus is on the reactivity of different imidazolium salts and their corresponding carbenes with various boron compounds to create new imidazolium boron adducts.
What are the key objectives of the research?
The main objectives include the synthesis of imidazolium borohydrides, investigation of reactions between imidazolium salts and boranes, characterization of novel carbene-boron adducts, examination of the reactivity of these adducts, and exploring hydrogenation reactions of imidazolium-boron adducts.
What types of carbenes are studied in this thesis?
The thesis primarily focuses on imidazolylidene carbenes, specifically 1,3-dimethylimidazol-2-ylidene and related derivatives with variations in alkyl and aryl substituents on the imidazole ring.
What boron compounds are used in the synthesis?
The research utilizes various boron compounds including borane-THF complexes, trimethyl borate, and sodium borohydride.
What methods are used to characterize the synthesized compounds?
The characterization techniques employed include NMR spectroscopy (specifically ¹H-NMR) and single-crystal X-ray crystallography.
What are the key findings regarding the synthesis of tris(imidazol-2-ylidene)-borane adducts?
The attempts to synthesize a tris(imidazol-2-ylidene)-borane adduct using various approaches (including borane-THF complexes and trimethyl borate) proved unsuccessful. The thesis details the challenges encountered and analyzes the reasons for the failed synthesis attempts.
What successful syntheses are reported in the thesis?
The thesis successfully synthesizes 1,3-dimethylimidazolium borohydride and 1,3,4,5-tetramethylimidazolium borohydride. The synthesis procedures and characterization data are thoroughly described.
How are the NMR shifts used in the analysis?
A comparison of ¹H-NMR shifts between imidazolium salts and their corresponding carbene-boron adducts provides insights into the electronic structure and bonding interactions within the newly formed compounds.
What are the results of the reactions of 1,3-dialkyl- and 1,3-diarylimidazolinium chlorides with borane and sodium borohydride?
This section explores the reactivity of various 1,3-dialkyl- and 1,3-diarylimidazolinium chlorides with borane and sodium borohydride. The attempts to prepare the corresponding borohydrides are described, along with an analysis of unexpected side reactions and the influence of substituents on reaction outcomes.
What are the findings regarding the hydrogenation of imidazolium-boron adducts?
The thesis investigates the hydrogenation of different imidazolium-boron adducts under high-pressure conditions. The preparation methods for specific adducts are described, along with a detailed analysis of the reactions performed under varying pressures and reaction times.
What are the key words associated with this research?
Key words include carbene, boron, imidazolium, borohydride, synthesis, characterization, NMR spectroscopy, X-ray crystallography, hydrogenation, adduct, reactivity, and stability.
Where can I find the complete experimental details?
The complete experimental details, including general procedures, specific reaction descriptions, and handling of chemicals and waste disposal are presented in a dedicated "Experimental Part" section within the thesis.
- Quote paper
- Dipl.-Chem. Oliver Steinhof (Author), 2003, Investigations in the field of carbene-boron chemistry, Munich, GRIN Verlag, https://www.grin.com/document/186376
