Investigations in the field of carbene-boron chemistry

Untersuchungen zur Chemie von Carbene-Bor-Addukten


Diploma Thesis, 2003

76 Pages, Grade: 1,0


Excerpt

Content

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 30
3.2.2. X-ray crystal structure analysis of 35 31
3.2.3. Preparation of 1,3,4,5-tetramethylimidazolium borohydride 37 35
3.3. COMPARISON OF 1 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
5.1.2. Apparatus
5.2. RESULTS AND DISCUSSION
5.2.1. Preparation of 1,3,4,5-tetramethylimidazol-2-ylidene borane adduct 19 45
5.2.2. Preparation of 2-borane-1,3-dimethyl-4,5-dichloro-imidazoin adduct 42 45
5.2.3. Reactions of imidazol-2-ylidene boron adducts with H 2 at 900/1500 psi
5.2.4. Heating experiments with 35 and 37 to the reversibility of eq. 37 49

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
7.2.1. Synthesis of 1,3-dimethylimidazol-2-ylidene 9 , reaction of 1,3-dimethylimidazolium chloride 25 with sodium hydride
7.2.2. Attempt at the synthesis of a tris-(1,3-dimethylimidazol-2-ylidene)borane adduct 23a , reaction of 1,3-dimethylimidazol-2-ylidene 9 with borane · thf complex at a 3:1 ratio
7.2.3. Synthesis of 2-borane-1,3-dimethylimidazolin 24 , reaction of 1,3-dimethylimidazol-2-ylidene 9 with borane · thf complex at a 1:1 ratio
7.2.4. Attempt at the addition of 1,3-dimethylimidazoliumchloride 25 to 2-borane-1,3-dimethylimidazolin adduct 24 under elimination of hydrogen
7.2.5. Attempt at the synthesis of a tris-(1,3-dimethylimidazol-2-ylidene)methylborate adduct 23c , reaction of 1,3-dimethylimidazolium chloride 25 with trimethyl borate at a 3:1 ratio in the presence of potassium tert-butoxide
7.2.6. Synthesis of 1,3-dimethylimidazol-2-ylidene trimethylborat adduct 22 , reaction of 1,3- dimethylimidazol-2-ylidene 9 with trimethyl borate at a 1:1 ratio
7.2.7. Towards a tris(1,3-dimethylimidazol-2-ylidene)boronmonochloride adduct 23d , reaction of 1,3- dimethylimidazol-2-ylidene 9 with boron trichloride at a 3:1 ratio
7.2.8. Synthesis of bis-(1,3-dimethylimidazol-2-ylidene)silver(I)chloride complex 27 , reaction 1,3- dimethylimidazolium chloride 25 with silver(I)oxide
7.2.9. Synthesis of the bis-(1,3-dimethylimidazol-2-ylidene)silver(I)nitrate complex 28 , reaction bis-(1,3- dimethylimidazol-2-ylidene)silver(I)chloride complex 27 with silver nitrate
7.2.10. Synthesis of the bis-(1,3-dimethylimidazol-2-ylidene)silver(I-tetrafluoroborate complex 29 , reaction of bis-(1,3-dimethylimidazol-2-ylidene)silver(I)chloride complex 27 with silver tetrafluoroborate .57
7.2.13. Towards a bis-(1,3-dimethylimidazolium)-dichloroboron chloride complex 32a , reaction of bis- (1,3-dimethylimidazol-2-ylidene)silver(I)nitrate complex 28 with boron trichloride
7.2.14. Towards a bis-(1,3-dimethylimidazolium)-dichloroboron tetrafluoroborate complex 32b , reaction of bis-(1,3-dimethylimidazol-2-ylidene)silver(I)-tetrafluoroborate complex 29 with boron trichloride
7.2.15. Reaction of tris(dimethylamino)borane with 1,3-dimethylimidazolium chloride 25 59
7.2.16. Preparation of 1,3-dimethylimidazolium borohydride 35 from 1,3-dimethylimidazolium chloride 25 and sodium borohydride
7.2.17. Preparation of Preparation of 1,3,4,5-tetramethylimidazolium borohydride 37 from 1,3,4,5- tetramethylimidazolium chloride 36 and sodium borohydride
7.2.18. Attempt to the preparation of 1,3-bis-(p-tolyl)imidazolinium borohydride, reaction of 1,3-bis-(p- tolyl)imidazolinium chloride 38b with sodium borohydride
7.2.19. Attempt at the preparation of 1,3-bis-(tert-butylimidazolin)-2-ylidene 40c , reaction of 1,3-bis-(tert- butylimidazolinium) chloride 38c with sodium hydride
7.2.20. Synthesis of 1,3-dimesitylimidazolin-2-ylidene 40a , Reaction of 1,3-dimesitylimidazolinium chloride 38a with sodium hydride
7.2.21. Attempt at the synthesis of a 1,3-dimesitylimidazolin-2-ylidene borane adduct, reaction of 1,3- dimesitylimidazolin-2-ylidene 40a with borane · thf complex at a 1:1 ratio
7.2.22. Synthesis of 1,3,4,5-tetramethylimidazol-2-ylidene 18 , reaction of 1,3,4,5-tetramethylimidazolium chloride 36 with sodium hydride
7.2.23. Preparation of 2-borane-1,3,4,5-tetramethylimidazolin 19 , reaction of 1,3,4,5-tetramethylimidazol- 2-ylidene 18 with borane · thf complex
7.2.24. Preparation of 1,3-dimethyl-4,5-dichloroimidazolium tetrafluoroborate 44 , reaction of N-methyl- 4,5-dichloroimidazole 43 with trimethyloxonium tetrafluoroborate
7.2.25. Preparation of 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene 45 , reaction of 1,3-dimethyl-4,5- dichloroimidazolium tetrafluoroborate 44 with sodium hydride
7.2.26. Preparation of 2-borane-1,3-dimethyl-4,5-dichloroimidazolin 42 , reaction of 1,3-dimethyl-4,5- dichloroimidazol-2-ylidene 45 with borane · thf complex
7.2.27. Reaction of 1,3-dimethylimidazol-2-ylidene borane adduct 24 with dihydrogen at 1500 psi in DMSO-d 6 for 0.5 hours
7.2.28. Reaction of 2-borane-1,3,4,5-tetramethylimidazolin 19 with dihydrogen at 900 psi in DMSO-d 6 for 16 hours
7.2.29. Attempt at the reaction of 2-borane-1,3-dimethyl-4,5-dichloroimidazolin 42 with dihydrogen at 1500 psi in DMSO-d 6 for 48 h
7.2.30. Attempt at the reaction of 1,3-dimethylimidazol-2-ylidene trimethylborat adduct 22 with dihydrogen at 1500 psi in DMSO-d 6 for 30 h
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

Danksagung

Mein besonderer Dank gilt Herrn Prof. Dr. A. J. Arduengo III vom Department of Chemistry der University of Alabama, Tuscaloosa, USA für die Möglichkeit, Forschung an einem aktuellen Themengebiet zu betreiben. Durch viele fruchtbare Diskussionen und Anregungen hat er sehr zum Gelingen dieser Arbeit beigetragen.

Den Herren Prof. Dr. W.-W. du Mont und Prof. Dr. M. Fild vom Institut für Anorganische und Analytische Chemie der Technischen Universität Braunschweig und den Mitarbeiterinnen des Fachbereichs 3 der Technischen Universität Braunschweig möchte ich für die Möglichkeit danken, den praktischen Teil meiner Arbeit bei Herrn Prof. Dr. A. J. Arduengo III in den USA durchführen zu können.

Herrn Dr. W. Marshall und der Firma duPont de Neumours & Co, Inc. in Wilmington, Delaware, USA danke ich für die Anfertigung der Röntgenstrukturanalyse.

Für Hilfestellung bei der Aufnahme der NMR-Analysen danke ich Dr. K. Belmore vom Department of Chemistry der University of Alabama.

Außerdem danke ich allen Mitgliedern des Arbeitskreises von Prof. Dr. A. J. Arduengo III für ihre Hilfsbereitschaft und die interessanten Diskussionen.

1. Introduction

1.1. What are carbenes?

1.1.1. Definitions

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 1.

illustration not visible in this excerpt

For the classic examples of oxidation state II carbon like carbon monoxide or isonitriles the term ‘carbene’ is not appropriate, because they are only monocoordinated carbon species. (eq. 1).

illustration not visible in this excerpt

Its two nonbinding valence electrons can possess parallel spins occupying two orbitals resulting in the triplet carbene 2. Or they can possess antiparallel spins with both electrons paired in the same orbital resulting in the singlet carbene 3. The preferred configuration depends on the substituents R on the carbon center.

illustration not visible in this excerpt

In case of singlet carbenes, the carbon is thought to be sp2 -hybridized. The electron pair occupies one sp2 -orbital. One would expect an RCR-angle of 120°, but X-ray crystallography found an angle of 102° for a imidazol-2-ylidene1 and 105° for a stable acyclic diaminocarbene21. An explanation for this phenomenon is the increased s-orbital character used to stabilize the lone pair. The remaining bond pairs are forced to take on more p-orbital character and therefore come closer to 90°.

The carbon in a triplet carbene is formally sp-hybridized and the unpaired electrons occupy two p-orbitals. One would expect a linear configuration. An angle of 135°, calculated by MO- calculations, has been proven by ESR-spectroscopy. According to theoretical calculations the triplet configuration is thought to be about 14 kcal·mol-1 higher in energy than the singlet configuration2. That explains the differences in stability between carbenes of both kinds. The most stable triplet carbene has a half-life of 19 minutes3 (it was stabilized mainly by steric protection). The half-life of the most stable singlet carbene is almost umlimited22. Triplet and open shell singlet carbenes may be considered diradicals and react accordingly. Closed shell singlet carbenes may react as strong electrophiles or strong nucleophiles, depending on whether their chemistry is dominated by the nucleophilic lone pair or the valence shell deficiency at carbon. This in turn depends on the substituents at the carbene center. Electron donors like oxygen, nitrogen and halogens will make the singlet state more favourable, because they can provide additional electron density to the valence shell deficient carbon.

Methylene 4 and difluorocarbene 5 typical examples for triplet and singlet carbenes.4

illustration not visible in this excerpt

Typical examples of reactions which can be used to generate carbene as highly reactive intermediates can be found in Table 1.4

Table 1 : Reactions towards carbene intermediates:

illustration not visible in this excerpt

1.1.2. A short history of carbene research

The history of carbenes has been extensively reviewed elsewhere5 a-d. The following chapter gives a short summary.

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 detect and possibly isolate and characterize it. Additionally carbenes may be useful as building blocks in organic syntheses and 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 .

In 1835 Dumas and Péligot tried to prepare methylene 1 by dehydration of methanol by sulfuric acid or phosphorus pentoxide. They regarded methanol as adduct of methylene and water.6 Later Butlerov produced ethylene from the reaction of methyl iodide with copper and made the suggestion, that methylene acted as an intermediate. Geuther in 1862 made a nowadays well established proposal, that dehydrohalogenationation of chloroform in presence of a strong base forms dichloromethylene as an intermediate (see Table 1, entry 4).7

The second period of carbene research began around 1900, when Nef proposed his “General Methylene Theory”, which suggested that all substitution reactions proceeded via methylene and methylene-like intermediates by the sequence of α-elimination and addition. Staudinger investigated the decomposition of diazo compounds (Table 1, entry 1) and ketenes around 1910.8

In 1926 Scheibler announced the formation of diethoxycarbene from tetraethoxyethylene using the reaction sequence pointed out in Fig. 1.9

illustration not visible in this excerpt

Fig. 1 : Scheibler ’ s reaction sequence towards diethoxycarbene

But this attempt has been proved to be wrong later. Due to the insufficient analytical methods used in 1926 (which relied heavily on the comparison of melting and boiling points) Scheibler was unable to distinguish between his proposed carbene and other compounds of similar boiling point and molecular mass. He expected a compound of lower boiling point than the dimeric tetraethoxyethylene. But his “carbene”, boiling at 77 °C, has later been proved to be the well- known ethylacetate.

In 1960 Schmeisser claimed isolation of dichlorocarbene, using procedure analogous to that he used for the synthesis of silicon dibromide some time earlier.10

illustration not visible in this excerpt

He also reported that the product from eq. 3 produced dichloronorcarane upon reaction with cyclohexene (eq. 4) and formed phosgene when exposed to air, which both would prove its identity as dichlorocarbene.

illustration not visible in this excerpt

But later investigation proved these results to be wrong. The compound he isolated was a mixture of dichloroacetylene and chlorine, eq. 4 could not be confirmed.

In 1960 Wanzlick published his work on his heterocyclic carbene chemistry, mainly based on imidazolidinium derivatives. He started with an α-elimination of chloroform from 1,3-diphenyl- trichloromethylimidazolidine and postulated the formation of a carbene in equilibrium with the corresponding dimer (eq. 5).

illustration not visible in this excerpt

He assumed the existence of an equilibrium, because his osmotic molecular weight determinations gave him an average molecular weight between the monomer and dimer. More advanced methods like Raman spectroscopy were not available to him at that time11. In 1964 Lemal could prove, that Wanzlick’s equilibrium did not exist. He dissolved dimers with different substituents on the nitrogens and crystallized the substances again. If the equilibrium existed, he should get mixed dimers. But he was only able to isolate the unchanged starting materials. These results stopped the quest for stable carbenes for about 25 years.12

The isolation of stable carbenes has been unsuccessful so far, but new metal complexes containing carbene-like (Carbenoides) entities have been discovered by Fischer in 1964 (eq. 6).13

illustration not visible in this excerpt

This complex formally represents the product resulting from an addition between W(CO)5 and a carbene.

Lappert in 1971 used Wanzlicks dimer and iron pentacarbonyl to form a iron carbene complex (eq. 7).14

illustration not visible in this excerpt

In 1988 Arduengo et al. worked at DuPont on the development of a new crosslinker for polymers for water based paints. The most promising group of compounds were imidazolium-2-thiones. A convenient synthesis for these compounds proved to be the deprotonation with a base, followed by reaction with sulphur (eq. 8).15

illustration not visible in this excerpt

The reaction involves a carbene as intermediate, but it was remarkably insensitive to air and moisture. Therefore Arduengo et al. tried to isolate this carbene-intermediate using an imidazolium salt with sterically demanding adamantly substituents on the nitrogens (eq. 9). Catalytic amounts of DMSO were employed in order to generate the dimsyl anion as the active base to deprotonate the imidazolium salt and sodium hydride to regenerate the dimsyl anion. The carbene was easily separated from the other reaction products and could be fully characterized. Its thermal stability is remarkable, as it melts at 240°C without decomposition.1

illustration not visible in this excerpt

1.1.3. Characteristics affecting the stability of carbenes

16 The most important feature affecting the stability of a carbene are the adjacent substituents at the divalent carbon which promote the singlet configuration of a carbene. These substituents have to be able to donate electron density to the carbene carbon, and therefore have to possess a lone pair in their valence shells. Elements from groups 15 to 17 are suitable substituents. Experimental result showed that nitrogen seems to be suited best for this. In fact all stable (bottleable) carbenes isolated so far possess at least one nitrogen bound to the carbene.15 Examples with the second substituent replaced by sulphur and oxygen have been reported17,18. Nature’s thiazole carbene 8 from vitamine B1 gives an example for this.

illustration not visible in this excerpt

In many stable carbenes the divalent carbon is part of a five membered ring containing a C-C double bond. This ring system possesses a delocalized π-system that may play an important role in stabilizing the carbene. Steric hindrance by the substituents at nitrogen can also be used to further enhance the stability of the carbene (eq. 9).

From these features, all of them at first believed to be required for the exceptional stability, only the nitrogens are really essential. For example carbene 9 possesses only methyl groups as substituents on the nitrogens, but it can be easily isolated as a colorless oil stable for several hours at room temperature.16 Diaminocarbenes lacking the double bond in the ring have been isolated as well19, although they are significantly more sensitive towards moisture and air.20 Compound 10 was the first example of this kind. In 1996 the first stable acyclic carbene 11 has been reported21 by Alder et al. finally showing that the substituents on the carbene carbon are the most important characteristic affecting the stability of a carbene.

illustration not visible in this excerpt

However other factors do influence the stability of carbenes. The actual stability of an isolable carbene in terms of half-life (“shelf life”) and tendency to dimerize depends highly on these secondary factors. Compound 12 is the most stable carbene known so far. It is even air stable. It has sterically demanding substituents on the nitrogens, its ring contains a double bond. It additionally has two electron withdrawing chlorine atoms at C4 and C5, a factor that is responsible for its ultimate stability.22

illustration not visible in this excerpt

2. Attempted synthesis of a Tris(imidazol-2-ylidene)-borane adduct

2.1. Introduction

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. 13-15 are some classic examples. Most carbon bases are electron deficient on the carbon and therefore electrophils. However, a nucleophile center is needed to bind to an electron deficient acceptor like borane, especially because boron is not able to provide any π-backdonation like transition metal carbene complexes, as it lacks free electron pairs.

illustration not visible in this excerpt

The new nucleophile imidazole-2-ylides make neutral carbon borane adducts easily accessible. In 1993 Kuhn et al. 23 found that borane adducts of these carbenes can be produced in high yields by allowing the carbene to react with BH3·Me2S complex. Compounds 17 and 19 are colorless solids which can be easily recrystallized and melt at 92° resp. 138° C without decomposition.

illustration not visible in this excerpt

Other examples of boron adducts with nucleophilic carbenes are adducts with boron trifluoride 2124 and trimethoxyborate 2225

illustration not visible in this excerpt

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.

However, trialkylboranes with bulky substituents (e.g. trinorbornylborane, tricyclohexylborane etc.) are well documented26. In view of the abundance of trialkylboranes, a compound like 23 appears to be a reasonable synthetic target.

illustration not visible in this excerpt

Calculations using the software Unichem with the semi empirical method MNDO/AM1 support the possible existence of a 3:1 adduct. The calculated geometry for the dication 23 is illustrated in Fig. 2.

illustration not visible in this excerpt

Fig. 2 : Proposed Borane-Tris-imidazol-2ylidene adduct and calculated structure

The calculated total heat of formation is 478.5 kcal/mol.

The HOMO-LUMO gap in 23 is calculated to be 9.38 eV, suggestive of a stable structure. The 1,3-dimethylimidazole-2-ylidene 9 is chosen as the pedant carbene, because it is the smallest of the known stable carbenes.

2.2. Results and Discussion

A number experimental approaches towards 23 have been made. On the way interesting results have been achieved besides the main goal. These include the synthesis and characterization of a new ionic imidazolium borohydride. Table 2 shows a summary of the reactions employed.

Table 2 : Reactions employed towards the synthesis of a Boron-(Tris-imidazol-2ylidene)-adduct

illustration not visible in this excerpt

2.2.1. General method to prepare imidazol-2-ylidenes from the corresponding imidazolium salts

illustration not visible in this excerpt

All imidazole-2-ylidenes were prepared by deprotonation of the corresponding imidazolium salt (chloride or tetrafluoroborate) with sodium hydride in the presence of catalytic KO t Bu. The reactions have been carried out in tetrahydrofuran, in which most imidazolium salts and the sodium hydride are only moderately soluble. To monitor the proceeding of the reaction, the volume of the generated hydrogen was measured. The reaction time varied between 30 minutes and several hours, depending on factors like solubility of the imidazolium salt and amount of solvent. Acetonitrile has been tested as solvent, but proved to be incompatible, as no product could be isolated.

2.2.2. Reaction of 1,3-dimethylimidazol-2-ylidene 9 with borane · thf complex at a 3:1 ratio ( Table 2, Entry 1 )

illustration not visible in this excerpt

Carbene 9 was generated by deprotonation of 1,3-dimethylimidazolium chloride and isolated. Then 0.33 equivalents of borane·thf complex were added.

The light yellow, crystalline product was identified as compound 24 by1 H,11 B and13 C-NMR spectroscopy. Except for the substituents on the nitrogens, this compound is closely related the imidazole-2-ylidene adducts 17 and 19 reported by Kuhn et al.

No 23a could be identified. Compound 24 is soluble in thf, acetonitrile and slightly soluble in toluene and benzene and melts at 110°C. In the1 H-NMR spectrum (C6D6) a quartet of 1:1:1:1 intensity is observed at = 1.91, resulting from the coupling of the borane protons bound to the11 B nucleus, which has a spin of 3/2. The11 B-NMR spectrum shows a quartet at = -36.4 (standard: BF3·OEt2) with peak intensities of 1:3:3:1. As expected, the coupling constant is the same in both spectra and has a value of1 J BH = 87.5 Hz. Kuhn et al. reported BH-coupling constants of 86.3 Hz in CD2Cl2 for 17 and 19. The observed shifts and coupling constants are also consistent with other BH3 adducts (see Table 3).

Table 3 : 11 B- NMR-shifts and coupling constants of borane adducts 27

illustration not visible in this excerpt

Fig. 3: 1 H- and 11 B-NMR spectra of 24

To confirm eq. 16 and to conduct further experiments compound 24 was synthesized directly by allowing carbene and borane to react in a 1:1 ratio (eq. 17).

illustration not visible in this excerpt

The reaction worked as expected with a yield of 67%. NMR spectroscopy confirmed the identity of the product as 24.

This specific compound has not been published yet. Kuhn introduced the 2-borane-1,3- diethylimidazolin 17 in 1993.

2.2.3. Attempt at the addition of 1,3-Dimethylimidazoliumchloride 25 to 2-borane-1,3-dimethylimidazolin 24 under elimination of hydrogen

illustration not visible in this excerpt

Compound 24, obtained in the previous reaction, was refluxed in acetonitrile with the corresponding imidazolium salt 25 for 3 days. The starting materials were isolated unchanged.

2.2.4. Reaction of 1,3-dimethylimidazol-2-ylidene 9 with trimethyl borate

illustration not visible in this excerpt

Carbene 9 was generated in situ using potassium tert -butoxide in the presence of 0.33 equivalents of trimethyl borate.

As in case of the reaction with borane only the mono adduct was isolated. Compound 22 is a light yellow solid of melting point 95° C. Its solubility is good in thf, moderate in benzene and toluene. In the1 H-NMR spectrum in DMSO-d6 the methoxy-groups resonate at = 2.90 ppm ( = 3.45 ppm in case of free trimethyl borate), the N-methyl groups can be observed at = 3.83 ppm, the olefinic protons at = 7.65 ppm. The values of the integrals of these peaks correspond with structure 22. The shifts are comparable to the spectrum of the 1,3-dimethylimidazolium salts and BF3-adducts, but are shifted about 0.3 ppm towards lower field compared to the borane adduct 24. Reasons for this are probably differences in the polarity of the carbon-boron bond due to differences in electronegativity at the substituents on the boron (hydrogen vs. oxygen/fluorine). More research is necessary to clarify this matter.

[...]

Excerpt out of 76 pages

Details

Title
Investigations in the field of carbene-boron chemistry
Subtitle
Untersuchungen zur Chemie von Carbene-Bor-Addukten
College
Technical University of Braunschweig  (Institut für Anorganische und Analytische Chemie)
Course
TU Braunschweig und University of Alabama
Grade
1,0
Author
Year
2003
Pages
76
Catalog Number
V80000
ISBN (eBook)
9783638808330
ISBN (Book)
9783640862634
File size
1318 KB
Language
English
Notes
Der experimentelle Teil dieser Arbeit wurde in der Arbeitsgruppe von Anthony J Arnduengo III an der University of Alabama durchgeführt. Es werden die klassischen Methoden zur Synthese von Singulett-Carbenen beschrieben, sowie Versuche zu Anlagerung von Borverbindungen. Die dabei entdeckten Imidazolium-Borhydride wurden auf Tauglichkeit als Speicherverbindung für molekularen Wasserstoff hin untersucht.
Tags
Investigations
Quote paper
Dipl-Chem. Oliver Steinhof (Author), 2003, Investigations in the field of carbene-boron chemistry, Munich, GRIN Verlag, https://www.grin.com/document/80000

Comments

  • No comments yet.
Read the ebook
Title: Investigations in the field of carbene-boron chemistry



Upload papers

Your term paper / thesis:

- Publication as eBook and book
- High royalties for the sales
- Completely free - with ISBN
- It only takes five minutes
- Every paper finds readers

Publish now - it's free