Diploma Thesis, 2003
76 Pages, Grade: 1
Die vorliegende Arbeit wurde von Mai 2003 bis November 2003 am Institut für Anorganische und Analytische Chemie an der Technischen Universität Carolo-Wilhelmina zu Braunschweig unter der Betreuung von Herrn Prof. W.-W. du Mont durchgeführt. Die praktischen Arbeiten wurden im Arbeitskreis von Herrn Prof. Dr. A. J. Arduengo III am Department of Chemistry an der University of Alabama in Tuscaloosa durchgeführt.
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.
Für Hilfestellung bei der Aufnahme der NMR-Analysen danke ich Dr. K. Belmore vom Department of Chemistry der University of Alabama.
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.
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).
R N C R N C (1) O C C O
substituents R on the carbon center.
In case of singlet carbenes, the carbon is thought to be sp 2 -hybridized. The electron pair occupies one sp 2 -orbital. One would expect an RCR-angle of 120°, but X-ray crystallography found an angle of 102° for a imidazol-2-ylidene 1 and 105° for a stable acyclic diaminocarbene 21 . 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 MOcalculations, has been proven by ESR-spectroscopy. According to theoretical calculations the triplet configuration is thought to be about 14 kcalmol -1 higher in energy than the singlet configuration 2 . That explains the differences in stability between carbenes of both kinds. The most stable triplet carbene has a half-life of 19 minutes 3 (it was stabilized mainly by steric protection). The half-life of the most stable singlet carbene is almost umlimited 22 . Triplet and
Table 1: Reactions towards carbene intermediates:
The history of carbenes has been extensively reviewed elsewhere 5a-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 .
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
+ EtOH Et O E t O Et
2 O Et
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 wellknown ethylacetate.
1300 °C n Si + n SiBr 4 [SiBr 2 ] 2n (2) high vacuum
(3) CCl 4 + C Cl
identity as dichlorocarbene.
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-diphenyltrichloromethylimidazolidine and postulated the formation of a carbene in equilibrium with the corresponding dimer (eq. 5).
Ph Ph Ph Ph
- CHCl 3
N N N N
Ph Ph Ph Ph
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
CO (6) W(CO) 6 CO C C R O L i R O C H 3
Ph Ph Ph
N N N
(7) Fe(CO) 4
- CO N N N
Ph Ph Ph
- NaCl -
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
(9) H 2 + + NaCl
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 reported 17,18 . Nature’s thiazole carbene 8 from vitamine B 1 gives an example for this.
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).
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 well 19 , 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 reported 21 by Alder et al. finally showing that the substituents on the carbene carbon are the most important characteristic affecting the stability of a carbene.
10 9 11
has sterically demanding substituents on the nitrogens, its ring contains a double bond. It additionally has two electron withdrawing chlorine atoms at C 4 and C 5 , a factor that is responsible for its ultimate stability. 22
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.
O BH 3
13 14 15
which can be easily recrystallized and melt at 92° resp. 138° C without decomposition.
(10) BH 3
(11) BH 3
R (12) BF 3
R R R
(13) B(OMe) 3
appears to be a reasonable synthetic target.
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.
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
- 2 H 2 N 2 MeO -
- 3x HNMe 2 N
- R = H, Cl or Me ; X = Cl - or BF 4
2 H - (16) 3
Then 0.33 equivalents of boranethf complex were added.
The light yellow, crystalline product was identified as compound 24 by 1 H, 11 B and 13 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 the 1 H-NMR spectrum (C 6 D 6 ) a quartet of 1:1:1:1 intensity is observed at = 1.91, resulting from the coupling of the borane protons bound to the B nucleus, which has a spin of 3/2. The 11 B-NMR spectrum shows a quartet at = -36.4
same in both spectra and has a value of 1 J BH = 87.5 Hz. Kuhn et al. reported BH-coupling constants of 86.3 Hz in CD 2 Cl 2 for 17 and 19. The observed shifts and coupling constants are also consistent with other BH 3 adducts (see Table 3).
Table 3: 11 B- NMR-shifts and coupling constants of borane adducts 27
2-Borane-1,3-Dimethylimidazolin 24 2 -36.4 87.5* 2-Borane-1,3,4,5-Tetramethylimidazolin 19 -35.75 87.5* 3 NH 3 BH 3 4 -23.8 93.9 PH 3 BH 3 5 -42.7 103 6 BH 3 CO -52 105 BH 3 THF 7 29 -1.1 fast exchange 8 B 2 H 6 +16-18 125-135 9 30 BH 3 +57.1 fast exchange
* in C6D6
1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50
-40 -30 -20 -10 0 10 20
This specific compound has not been published yet. Kuhn introduced the 2-borane-1,3- diethylimidazolin 17 in 1993.
under elimination of hydrogen
2+ 2 Cl - (18)
- 2 H 2
24 25 23b
(19) 3 2 MeO -
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 the 1 H-NMR spectrum in DMSO-d 6 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 BF 3 -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.
But the isolated compound was not pure. The boron NMR shows additional signals at = 3.4 ppm and = -25.0 ppm.
Their origin cannot be determined clearly, but the first might be due to contamination with 1,3- dimethylimidazolium tetramethylborate, the latter might originate from imidazolium trimethylmonohydrogenborate [(MeO) 3 HB - ]. 27 These assumptions are supported by a small resonance at = 9.11 ppm in the proton NMR-spectrum. In case of other imidazolium salts, the
acidic proton resonates in this part of the spectrum ( = 9.26 ppm in case of 1,3- dimethylimidazoliumchloride).
2 Cl - (20) 3
27 A precursor for compound 23d, which might react more specifically with boron trichloride was needed. Compound 27 has been produced following published procedures. 31 The reaction proceeded smoothly, affording pure 27 in 89% yield. In the 1 H-NMR both the N-methyl groups and the olefinic protons resonate at higher field than in the starting material. As the solubility of 27 in common solvents is not very good, the counterion was exchanged against nitrate and tetrafluoroborate by simple precipitation of AgCl using AgNO 3 and AgBF 4 . The solubility of 28 and 29 is good in dichloromethane and acetonitrile.
- - 2 AgCl
- 2 AgCl Ag
- N N BF 4
In the 1 H-NMR spectra all three salts show different shifts. This confirms that an exchange of ions took place.
The monomeric 1,3-dimethylimidazol-2-ylidene silver(I)chloride complex 30 would give a better yield of 23d upon reaction with BCl 3 . Ramnial et al. 32 published the successful preparation of 1,3- dimesityl-2-ylidene silver(I)chloride 31 in 2003. However, the synthesis of the 1,3-dimethylderivative failed.
N N - - BF 4 Cl
+ AcOH Ag
N N - - BF 4 BF 4
identical with the dimeric silver complexes 27-29, except for possessing acetate as the counterion. The differences in size between the methyl- and the mesityl-substituents are obviously crucial for the products obtained.
N Cl - N
- (20) Cl
- NO 3
The reaction was carried out in both toluene and dichloromethane. In both cases the major product (about 80%) was a 1,3-dimethylimidazolium salt. The identity of the counterion cannot be confirmed by means of NMR-spectroscopy, but the shifts observable in the 1 H-NMR in DMSO-d 6 are shifted upfield to those of the corresponding chloride 25. As the counterion of the starting molecule 28 is nitrate, structure 33 seems to be a valid assumption. The source of protons used to form the salt cannot be determined, but is most likely adventitious water.
Examination Thesis, 171 Pages
Examination Thesis, 149 Pages
Diploma Thesis, 122 Pages
Diploma Thesis, 146 Pages
Diploma Thesis, 80 Pages
Diploma Thesis, 82 Pages
Diploma Thesis, 140 Pages
Diploma Thesis, 107 Pages
Diploma Thesis, 77 Pages
Diploma Thesis, 217 Pages
Diploma Thesis, 130 Pages
Diploma Thesis, 133 Pages
Diploma Thesis, 276 Pages
Examination Thesis, 91 Pages
Diploma Thesis, 161 Pages
Diploma Thesis, 102 Pages
GRIN Publishing, located in Munich, Germany, has specialized since its foundation in 1998 in the publication of academic ebooks and books. The publishing website GRIN.com offer students, graduates and university professors the ideal platform for the presentation of scientific papers, such as research projects, theses, dissertations, and academic essays to a wide audience.
Free Publication of your term paper, essay, interpretation, bachelor's thesis, master's thesis, dissertation or textbook - upload now!