Extrait
1
Technische Universität
Carolo-Wilhelmina zu Braunschweig
in Zusammenarbeit mit dem Department of Chemistry,
University of Alabama, Tuscaloosa
Investigations in the field of carbene-
boron chemistry
Diplomarbeit
im
Studiengang Chemie
von
Oliver Steinhof
2
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.
3
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.
4
1. INTRODUCTION ... 6
1.1. What are carbenes? ... 6
1.1.1. Definitions... 6
1.1.2. A short history of carbene research ... 9
1.1.3. Characteristics affecting the stability of carbenes ... 12
2. ATTEMPTED SYNTHESIS OF A TRIS(IMIDAZOL-2-YLIDENE)-BORANE ADDUCT ... 14
2.1. I
NTRODUCTION
... 14
2.2. R
ESULTS AND
D
ISCUSSION
... 17
2.2.1. General method to prepare imidazol-2-ylidenes from the corresponding imidazolium salts ... 18
2.2.2. Reaction of 1,3-dimethylimidazol-2-ylidene 9 with borane thf complex at a 3:1 ratio ... 19
2.2.3. Attempt at the addition of 1,3-Dimethylimidazoliumchloride 25 to 2-borane-1,3-dimethylimidazolin
24 under elimination of hydrogen... 21
2.2.4. Reaction of 1,3-dimethylimidazol-2-ylidene 9 with trimethyl borate... 22
2.2.6. Attempt to exchange dimethylamine against the 1,3-dimethylimidazolium ion 25 at
tris(dimethylamino)borane ... 28
3. SYNTHESIS AND CHARACTERIZATION OF IMIDAZOLIUM BOROHYDRIDES... 29
3.1. I
NTRODUCTION
... 29
3.2. R
ESULTS AND DISCUSSION
... 30
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. C
OMPARISON OF
1
H-NMR
SHIFTS OF
1,3-
DIMETHYLIMIDAZOLIUM SALTS AND ADDUCTS OF
1,3-
DIMETHYLIMIDAZOL
-2-
YLIDENES WITH BORON COMPOUNDS
... 35
4. REACTIONS OF 1,3-DIALKYL- AND 1,3-DIARYLIMIDAZOLINIUM CHLORIDES WITH
BORANE AND SODIUM BOROHYDRIDE ... 38
4.1. I
NTRODUCTION
... 38
4.2. R
ESULTS AND DISCUSSION
... 39
4.2.2. Attempt to the preparation of 1,3-dialkyl- and 1,3-diarylimidazolinium borohydrides ... 39
4.2.3. Reaction of 1,3-bis-(tert-butyl)imidazolinium chloride 38c with sodium hydride ... 41
4.2.4. Reaction of 1,3-dimesitylimidazolinium chloride 38a with sodium hydride, followed by borane thf
complex ... 42
5. EXPERIMENTS TOWARDS THE HYDROGENATION OF IMIDAZOLIUM-BORON ADDUCTS. 43
5.1. I
NTRODUCTION
... 43
5.1.2. Apparatus... 44
5.2. R
ESULTS AND DISCUSSION
... 45
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 ... 47
5.2.4. Heating experiments with 35 and 37 to the reversibility of eq. 37... 49
6. CONCLUSIONS AND OUTLOOK... 50
7. EXPERIMENTAL PART... 54
7.1. G
ENERAL
... 54
7.1.1. NMR spectroscopy ... 54
7.1.2. Melting Points... 54
7.1.3. Single-crystal X-Ray structure analysis ... 54
7.1.4. Starting materials... 54
7.2. D
ESCRIPTION OF THE EXPERIMENTS
... 55
7.2.1. Synthesis of 1,3-dimethylimidazol-2-ylidene 9, reaction of 1,3-dimethylimidazolium chloride 25 with
sodium hydride... 55
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... 55
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 ... 55
5
7.2.4. Attempt at the addition of 1,3-dimethylimidazoliumchloride 25 to 2-borane-1,3-dimethylimidazolin
adduct 24 under elimination of hydrogen ... 56
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... 56
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 ... 56
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... 57
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... 57
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. ... 57
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 ... 58
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... 58
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 ... 59
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 ... 59
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 ... 60
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... 60
7.2.20. Synthesis of 1,3-dimesitylimidazolin-2-ylidene 40a, Reaction of 1,3-dimesitylimidazolinium
chloride 38a with sodium hydride... 60
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 ... 60
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... 61
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... 61
7.2.24. Preparation of 1,3-dimethyl-4,5-dichloroimidazolium tetrafluoroborate 44, reaction of N-methyl-
4,5-dichloroimidazole 43 with trimethyloxonium tetrafluoroborate... 61
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 ... 62
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 ... 62
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 ... 62
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 ... 62
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 ... 63
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 ... 63
7.3. Handling of chemicals and waste disposal ... 63
8. APPENDIX ... 64
8.1. L
IST OF NUMBERED COMPOUNDS
... 64
8.2. L
IST OF ABBREVIATIONS
... 70
8.3. C
RYSTALLOGRAPHIC DATA AND PARAMETERS OF THE
X-
RAY STRUCTURE DETERMINATION OF
35 ... 71
9. REFERENCES ... 76
1. Introduction
6
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.
H
C
H
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).
C
O
C
O
R
N
C
R
N
C
(1)
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.
R
C
R
R
C
R
2
3
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 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
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
1. Introduction
7
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
F
C
F
5
4
H
C
H
1. Introduction
8
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:
C
N
N
R
R
R
C
R
N
2
C
R
R
R
C
R
N
2
CHX
R
R
R
C
R
C
C
R
R
R
C
R
C
R
R
R
C
R
R'HgX
N
N
R
R
O
R
C
R
O
BH
Hg
R'
X
R'
R'
N
N
R
R
H
X
-
R'
R'
N
N
R
R
R'
R'
N
N
R
R
S
+
Photolysis or Thermolysis
+
Photolysis
+
Strong base
+
+
Thermolysis
Photolysis
X
-
+
Diazoalkanes
Diazirines
Epoxides
Halides
Halomercury
compounds
1
2
3
4
5
Entry #
Reaction
Strong base
R'
R'
N
N
R
R
BH
+
X
-
+
2 K
+
K
2
S
1. Introduction
9
1.1.2. A short history of carbene research
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 .
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
-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
O
Et
O
O
O
Et
Et
+
NaOEt
O
Et
O
Et
O
Et
O
Et
+
EtOH
O
O
Et
O
Et
O
Et
O
Et
O
O
ONa
Et
Et
H
2
O
-(EtO)
2
CHCO
2
Na
2
O
Et
O
Et
O
Et
O
Et
Et
O
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 well-
known ethylacetate.
1. Introduction
10
In 1960 Schmeisser claimed isolation of dichlorocarbene, using procedure analogous to that he
used for the synthesis of silicon dibromide some time earlier.
10
n Si
+
n SiBr
4
[SiBr
2
]
2n
C
+
CCl
4
2
Cl
Cl
1300 °C
high vacuum
high vacuum
1300 °C
(2)
(3)
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.
Cl
Cl
+
Cl
Cl
(4)
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
-elimination of chloroform from 1,3-diphenyl-
trichloromethylimidazolidine and postulated the formation of a carbene in equilibrium with the
corresponding dimer (eq. 5).
N
N
Ph
Ph
H
CCl
3
- CHCl
3
N
N
Ph
Ph
N
N
Ph
Ph
N
N
Ph
Ph
(5)
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 time
11
.
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
1. Introduction
11
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
W(CO)
6
LiR
Et
2
O
W
CO
OC
CO
OC
CO
C
R
OLi
[(CH
3
)
3
O]BF
4
W
CO
OC
CO
OC
CO
C
R
OCH
3
(6)
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
Fe(CO)
5
N
N
Ph
Ph
N
N
Ph
Ph
N
N
Ph
Ph
(7)
- CO
Fe(CO)
4
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
N
N
R
R
H
Cl
-
NaOMe
MeOH
- NaCl
N
N
R
R
S
8
MeOH
N
N
R
R
S
(8)
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
1. Introduction
12
N
N
H
Cl
-
DMSO (cat)
(9)
NaH
N
N
H
2
+
NaCl
+
6
7
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 reported
17,18
. Nature's thiazole carbene 8
from vitamine B
1
gives an example for this.
N
S
8
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 -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).
1. Introduction
13
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 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.
N
N
9
N
N
10
N
N
11
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 C
4
and C
5
, a factor that is responsible
for its ultimate stability.
22
N
N
Cl
Cl
12
2. Attempted synthesis of a Tris(imidazol-2-ylidene)-borane adduct
14
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
-backdonation like transition
metal carbene complexes, as it lacks free electron pairs.
O
C
BH
3
N
C
BH
3
R
P
C
BH
3
13
14
15
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 BH
3
Me
2
S complex. Compounds 17 and 19 are colorless solids
which can be easily recrystallized and melt at 92° resp. 138° C without decomposition.
N
N
Et
Et
BH
3
·Me
2
S
N
N
Et
Et
BH
3
N
N
BH
3
·Me
2
S
N
N
BH
3
17
19
16
18
(10)
(11)
Other examples of boron adducts with nucleophilic carbenes are adducts with boron trifluoride
21
24
and trimethoxyborate 22
25
2. Attempted synthesis of a Tris(imidazol-2-ylidene)-borane adduct
15
N
N
R
R
BF
3
·Et
2
O
N
N
R
R
BF
3
N
N
B(OMe)
3
N
N
B(OMe)
3
21
22
R
R
R
R
20
9
(12)
(13)
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 documented
26
. In view of the abundance of trialkylboranes, a compound like 23
appears to be a reasonable synthetic target.
B
B
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.
Fin de l'extrait de 76 pages
- Citation du texte
- Dipl.-Chem. Oliver Steinhof (Auteur), 2003, Investigations in the field of carbene-boron chemistry, Munich, GRIN Verlag, https://www.grin.com/document/186376
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