Bicyclic (Alkyl)(amino)carbenes (BICAACs): Stable Carbenes More Ambiphilic than CAACs

Article abstract of DOI:10.1021/jacs.7b04640

A straightforward strategy allows for the synthesis of storable bicyclic (alkyl)(amino)carbenes (BICAACs), which feature enhanced σ-donating and -accepting properties compared to monocyclic (alkyl)(amino)carbenes (CAACs). Due to the bicyclo[2.2.2]octane skeleton, the steric environment around the carbene center is different from that of CAACs and similar to that observed in classical N-heterocyclic carbenes. The different electronic properties of BICAACs as compared to CAACs allow for ligand exchange reactions not only at a metal center, but also at main group elements.

Full text of DOI:10.1021/jacs.7b04640

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Bicyclic (Alkyl)(amino)carbenes (BICAACs):
Stable Carbenes more Ambiphilic than CAACs
Eder Tomás-Mendivil, Max M. Hansmann, Cory M. Weinstein,
Rodolphe Jazzar, mohand melaimi, and Guy Bertrand
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 25 May 2017
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Bicyclic (Alkyl)(amino)carbenes (BICAACs): Stable Carbenes more
Ambiphilic than CAACs
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Eder Tomás-Mendivil,a,b Max M. Hansmann,a Cory M. Weinstein,a Rodolphe Jazzar,a Mohand Melai-
mi,a Guy Bertrand*a
a UCSD-CNRS Joint Research Chemistry Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of
California San Diego, La Jolla, California 92093-0358, United States of America.
b Department of Applied Chemistry, Faculty of Chemistry, University of the Basque Country (UPV-EHU), Donostia-San
Sebastián, 20018 Gipuzkoa, Spain.
Supporting Information Placeholder
enyl functionality remained untouched. With this in mind, we
envisaged that replacement of the 2-methyl-2-propenyl group by a
of storable bicyclic (alkyl)(amino)carbenes (BICAACs), which
ABSTRACT: A straightforward strategy allows for the synthesis
saturated R substituent, the acid promoted rearrangement would
involve the endocyclic double bond of the cyclohexene, and
would lead to a bicyclic iminium salt via a 6-endo cyclization.
feature enhanced σ-donating and π-accepting properties compared
to monocyclic (alkyl)(amino)carbenes (CAACs). Due to the bicy-
clo[2.2.2]octane skeleton, the steric environment around the car-
bene center is different from that of CAACs, and similar to that
observed in classical NHCs. The different electronic properties of
BICAACs as compared to CAACs allow for ligand exchange
reactions not only at a metal center, but also at main group ele-
ments.
Subsequent deprotonation would generate
a
cyclic (al-
kyl)(amino)carbene (CAAC) with the bicyclo[2.2.2]octane
framework that we named BICAACs.
Scheme 1. Inspiration for the synthesis of BICAACs.
C
C
Previous work
Dipp
N
Dipp
N
N
O
In 2005, our group reported the synthesis of stable cyclic (al-
C
,
C
kyl)(amino)carbenes (CAACs)1 2 which result from the replace-
CAAC A
ment of one of the two amino substituents of classical NHCs3 by a
quaternary carbon atom. This modification increases both the
nucleophilicity and electrophilicity of the carbene center,4 thereby
allowing CAACs to outperform NHCs for the stabilization of
R
C
1
R
cis/trans
95/5
Dipp
N
C
Dipp
This work
C
,
paramagnetic species,5 6 and the activation of small molecules and
BICAAC
enthalpically strong bonds.7 Additionally, CAAC-metal bonds are
stronger than in NHC complexes, which allowed for promoting
difficult transition metal catalyzed reactions under very demand-
ing conditions.8 Herein we report the preparation of stable bicyclic
(alkyl)(amino)carbenes (BICAACs) and show that this modifica-
tion of the CAAC skeleton leads to a geometry similar to that of
NHCs. Moreover, this novel family of carbenes displays enhanced
σ-donating and π-accepting properties compared to those of
CAACs, and thus NHCs. We present computational and experi-
mental evidence for this claim. Of particular interest we found
that ligand exchange between BICAACs and CAACs occurred
not only at a transition metal center, but also at a main group
element.
Condensation of 2,6-diisopropylaniline and trivertal 1 under
acidic conditions quantitatively led to imine 2 (Scheme 2). Depro-
tonation of 2 with n-butyl lithium followed by alkylation with
iodomethane diastereoselectively afforded 3a [i.e. the facial selec-
tivity is controlled by the methyl group being in equatorial posi-
tion (see SI for further details)]. The 6-endo cyclization of 3a took
place in a sealed flask at 120 ºC in the presence of excess HCl in
diethyl ether. Anion exchange from chloride to tetrafluoroborate
in aqueous medium gave iminium salt 4a. Lastly, deprotonation
with KHMDS generated free BICAAC 5a as a racemic mixture of
a single diastereomer in moderate overall yield. The overall syn-
thetic route is also compatible with the introduction of a second-
ary R group as shown with the isopropyl BICAAC 5b. Carbenes
5a,b can indefinitely be stored in the solid state at room tempera-
ture under inert atmosphere, without any decomposition. The 13C
NMR spectra of both 5a and 5b show a singlet at 334.4 and 336.0
ppm, respectively, for the carbene carbon nucleus, which is slight-
ly deshielded compared to that of A (320 ppm). Single-crystals of
5a and 5b were obtained from a pentane solution at -40 ºC, and X-
ray diffraction studies confirmed the diastereoselectivity of the
alkylation step. The C1-N1 [5a: 1.343(6), 5b: 1.3154(15) Å] and
C1-C2 [5a: 1.523(7), 5b: 1.5304(15) Å] distances are in the range
of those observed in CAACs, and the
The synthetic strategy to access BICAACs is derived from that
used for the preparation of the spirocyclic CAAC A9 (Scheme 1).
In our previous work, starting from the commercially available
2,4-dimethyl-3-cyclohexene carboxaldehyde (trivertal) (cis/trans
isomers, racemate) 1, a common fragrance and flavor material
produced in bulk quantities,10 we prepared in two steps the corre-
sponding alkenyl-aldimine as a single diastereomer. Under acidic
conditions, an intramolecular 5-exo cyclization reaction led to the
corresponding five-membered ring iminium salt (commercially
available), which can be deprotonated to give the free stable
carbene A. In this process, the C-C double bond of the cyclohex-
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Scheme 2. Synthesis of BICAACs 5a,b.
1
NH2
2
3
4
5
6
7
O
1) nBuLi, Et2O
-78 ˚C to rt
2 h
NDipp
NDipp
R
R
R
BF4
1) HCl, Et2O
120 ˚C, 16 h
AcOH (cat.)
KHMDS, Et2O
N
N
MgSO4, CH2Cl2, rt
16 h
2) RX, Et2O
-78 ˚C to rt
16 h
2) NaBF4 (aq.)
-78 ˚C to rt
1 h
Dipp
Dipp
1
2
4a (R = Me)
4b (R = iPr)
5a (R = Me)
5b (R = iPr)
3a (R = Me)
3b (R = iPr)
8
9
To compare the electronic properties of BICAACs 5 with
CAACs, the cis-[(5a)Rh(CO)2Cl] complex 8a was prepared as
shown in Scheme 4. The IR spectrum of 8a in dichloromethane
solution shows two absorptions for C-O bond stretching at 1990
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and 2074 cm-1, (νav = 2032 cm-1), suggesting that BICAAC 5a
CO
is an overall better donor than classic CAAC ligands (νav
=
CO
2036 cm-1).11 To differentiate between the σ-donation and the π-
accepting ability of the carbene, the phosphinidene adduct 9a was
prepared by reacting 5a with (PhP)5.12 The 31P NMR signal ap-
peared at +90 ppm, substantially downfield shifted compared to
anagolous adducts of monocyclic CAACs (+56 to +69 ppm), and
even diamidocarbenes (DACs) (+ 83 ppm), one of the most elec-
trophilic carbenes known so far.13 To verify the surprising elec-
trophilic character of 5a, the selenium adduct 10a was also syn-
thesized.14 As expected, the 77Se NMR of 10a shows a signal at
lower field (+645 ppm) than CAACs (+492 ppm)15 but surprising-
Figure 1. Solid-state structures of 5a (left) and 5b (right) viewed
from two different orientations with ellipsoids at 50% level of
probability. One enantiomer is shown. H-atoms are omitted for
clarity. Selected bond lengths [Å] and angles [°]. 5a: C1-N1
1.343(6), C1-C2 1.523(7), N1-C1-C2 108.3(4), C10-N1-C1-C2
175.883, N1-C1-C2-C22 178.986. 5b: C1-N1 1.3154(15), C1-C2
1.5304(15), N1-C1-C2 109.17(9), C10-N1-C1-C2 176.218, N1-
C1-C2-C22 178.575.
ly at higher field than DACs (+847 ppm),14b, which means that
16
N1-C1-C2 angle [5a: 108.3(4), 5b: 109.17(9) º] is significantly
wider [CAACs: 106.54(18) º]. Very importantly, C10, N1, C1, C2
and C22 are coplanar with the lone pair, leading to a geometry
similar to that observed in classical NHCs, but different to that of
CAACs such as A.
the electrophilicity scales based on phosphinidene and selenium
adducts do not always correlate. Nevertheless, these data clearly
suggest that BICAACs have higher π-accepting ability than
CAACs, and when combined with the TEP values, it can be con-
cluded that BICAACs are also more σ-donating.
The reaction of (SMe2)CuBr with methyl- and isopropyl-
BICAACs 5a and 5b nicely illustrates the steric tunability of these
carbenes (Scheme 3). With the less hindered carbene 5a, a mix-
ture of meso and racemic homoleptic [bis(BICAAC)Cu][CuBr2]
6a was obtained. In contrast, with the more sterically demanding
carbene 5b, the heteroleptic (5b)CuBr 7b was selectively formed.
The molecular structures of 6a and 7b were unambiguously con-
firmed by single crystal X-ray diffraction studies (Figure 2).
Scheme 4. Synthesis of rhodium complex, phosphinidene
and selenium-adducts of 5a.
Cl
OC
OC
Rh
1) [RhCl(COD)]2 (0.5 eq.)
THF, rt
N
2) CO, CH2Cl2, rt
Dipp
8a
(44% yield)
P
Ph
0.1 eq. (PhP)5
THF, rt
Scheme 3. Synthesis of copper complexes 6a and 7b.
N
N
Dipp
Dipp
9a
(36% yield)
CuBr2
Dipp
5a
N
Se
R = Me
2 eq. Se
Et2O, rt
Cu
R
N
N
Dipp
6a
CuBr(SMe2)
Et2O, rt, 16 h
Dipp
10a
(70% yield)
N
Dipp
iPr
R = iPr
To confirm these conclusions, we performed DFT calculations at
the B3LYP/def2-TZVPP level of theory. Indeed, we found that
the HOMO and LUMO of the model BICAAC are higher and
lower in energy, respectively, than those of CAACs (Figure 3).
This is further supported by a significant decrease of the singlet-
triplet gap from 49.2 kcal/mol (CAAC) to 45.7 kcal/mol
(BICAAC).17 The increased σ-donation and π-acceptance of BI-
CAACs compared to CAACs might be correlated to the wider
Br Cu
5a (R = Me)
5b (R = iPr)
N
7b
Dipp
carbene bond angle (110.2 º vs 106.9 º, values from DFT).12a,
18
Figure 2. Solid-state structures of 6a (left) and 7b (right) with ellipsoids
at 50% level of probability. One enantiomer is shown. H-atoms and
[CuBr2] in 6a are omitted for clarity. Selected bond lengths [Å] and angles
[°]. 6a: C1-Cu1 1.919(4), N1-C1-C2 111.5(4), C1-Cu1-C1’ 180. 7b: C1-
Cu1 1.902(3), Cu1-Br1 2.2351(6), N1-C1-C2 112.3(3), C1-Cu1-Br1
174.64(10).
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Journal of the American Chemical Society
ocyclic CAACs, and of course NHCs as shown by exchange
1
2
3
reactions at metal centers but also main group elements. Of par-
ticular importance, BICAACs can be stored in the solid state at
room temperature over a year.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/xxx. CCDC 1548015
(5a), CCDC 1548016 (5b), CCDC 1548017 (6a), CCDC 1548018
(7b), CCDC 1548019 (10a) and CCDC 1548020 (13) contain the
crystallographic data.
AUTHOR INFORMATION
Corresponding Author
*guybertrand@ucsd.edu
Notes
The authors declare no competing financial interests.
Figure 3. Frontier molecular orbital comparison between CAAC
and BICAAC at the B3LYP/def2-TZVPP level.
ACKNOWLEDGMENTS
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Thanks are due to the DOE (DE-SC0009376) for financial sup-
port, to the Basque Government for a Doctoral Research Staff
Improvement Program scholarship (ETM), to the Alexander von
Humboldt foundation for a Feodor-Lynen scholarship (MMH),
and to the Keck Foundation for funding the KeckII computer
center. Dr. Anthony Mrse and Dr. Milan Gembicky are greatly
acknowledged for their assistance with 77Se NMR experiments
and X-ray diffraction, respectively, and Dr. Zoraida Freixa for her
support (ETM).
Based on these results, one can expect that BICAACs would lead
to even stronger metal-carbon bonds than CAACs. As a proof of
principle, we reacted 5a with one equivalent of (CAAC)AuPh
o
complex 11, and after 40 h at 60 C, we reached an equilibrium
containing a 65/35 ratio of (BICAAC)AuPh complex 12 (along
with free CAAC) and (CAAC)AuPh 11 (along with 5a) (Scheme
5). Carbene exchange at a metal center is a very classical reaction,
but we wondered if a similar phenomenon could also occur at
main group elements.19 Indeed, the reaction between the CAAC-
Se adduct 13 and BICAAC 5a reached an equilibrium after 16 h
at 60 ºC; a 70/30 ratio of 10a and 13 was observed together with
the corresponding amount of the free carbenes. Lastly, the bromo-
iminium 14 cleanly reacted at room temperature with 5a quantita-
tively affording the corresponding BICAAC-Br adduct 15 along
with the free CAAC.
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2
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N
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Dipp
N
Dipp
5a
5a
+
+
+
N
N
N
Dipp
Dipp
Dipp
60 ˚C, 40 h
C6D6
11
13
Au
Ph
Au
Ph
12
3
+
For thematic issues and books on NHCs, see: (a) Rovis, T.; Nolan, S. P.
N
N
N
N
Dipp
Dipp
Dipp
60 ˚C, 16 h
C6D6
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Effective Tools for Organometallic Synthesis; Wiley-VCH: Weinheim,
2014.
Se
Se
10a
5a
+
+
Dipp
Br
rt, 0.5 h
C6D6
Br 14
Br Br
15
4
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In summary, a straightforward strategy allows for the synthesis of
bicyclic (alkyl)(amino)carbenes (BICAACs) from abundant and
cheap trivertal. The steric bulk around the carbene center is easily
tunable, and the bicyclo[2.2.2]octane skeleton confers to BI-
CAACs a fan-like geometry which is different than that of
CAACs, but similar to that observed in classical NHCs. These
novel carbenes are more electrophilic and nucleophilic than mon-
3
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17
The nitrogen substitution (Me vs Dipp) has little influence on the calcu-
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7
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lated S/T gap at the B3LYP-D3BJ/def2-TZVP level of theory [49.4
kcal/mol (Dipp-CAAC) and 45.3 kcal/mol (Dipp-BICAAC) instead of
49.2 kcal/mol (Me-CAAC) and 45.7 kcal/mol (Me-BICAAC).
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(a) Hirai, K.; Itoh, T.; Tomioka, H. Chem. Rev. 2009, 109, 3275. (b)
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We have recently reported that CAACs can displace triphenylphosphine
19
from a phosphinidene-PPh3 adduct. Hansmann, M. M.; Bertrand, G. J.
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7
(a) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand,
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See for exemples: (a) Lavallo, V.; Frey, G. D.; Donnadieu, B.; Soleil-
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9
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Lavallo, V.; Canac, Y.; Dehope, A.; Donnadieu, B.; Bertrand, G.
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(a) Back, O.; Henry-Ellinger, M.; Martin, C. D.; Martin, D.; Bertrand,
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Hudnall, T. W.; Bielawski, C. W. J. Am. Chem. Soc. 2009, 131, 16039. (c)
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