Persistent (amino)(silyl)carbenes

Article abstract of DOI:10.1021/ja051109f

(Amino)(silyl)carbenes, prepared by substitution reactions at a carbene center, can survive several days at 0 °C. These species are not push-pull carbenes as their phosphino analogues and therefore are excellent ligands for transition-metal centers. Copyright

Full text of DOI:10.1021/ja051109f

Published on Web 04/27/2005
Persistent (Amino)(Silyl)Carbenes
Yves Canac,† Salvador Conejero,† Bruno Donnadieu,† Wolfgand W. Schoeller,‡ and Guy Bertrand*,†
UCR-CNRS Joint Research Chemistry Laboratory (UMR 2282), Department of Chemistry, UniVersity of California,
RiVerside, California 92521-0403, and Fakulta¨t fu¨r Chemie der UniVersita¨t, D-33615 Bielefeld, Germany
Received February 21, 2005; E-mail: gbertran@mail.ucr.edu
The stable singlet carbenes1 can be classified in three categories.
Push-pull carbenes A are quasilinear. The interaction of the D
substituent lone pair and the W substituent vacant orbital with the
py and px orbitals of the carbene, respectively, gives rise to a
polarized allene-type system; the most representative carbenes of
this type are the (phosphino)(silyl)carbenes2 (Figure 1). Push-push
carbenes B are bent, and the π-donation of the D substituent lone
pairs results in a polarized four-electron three-center π-system; the
archetypical carbenes of this type are the diaminocarbenes.3 Last,
carbenes C represented by (amino)- and (phosphino)(alkyl)carbenes
have only a single electron-active substituent (a strong π-donor)
and are also bent due to the presence of an heteroatom-carbon
double bond.2 On the basis of this classification, (amino)(silyl)-
carbenes should be of type A, and since (phosphino)(silyl)carbenes
have been known for more than 15 years,4 it is rather surprising
that thus far none of these compounds have been described. Here
we report that in fact (amino)(silyl)carbenes are bent and, surpris-
ingly, are not very stable.
Figure 1. Electronic effects of the substituents for push-pull carbenes A,
push-push carbenes B, and carbenes C with a single electron-active group.
Figure 2. B3LYP/6-31g* optimized geometry of 4a. Selected bond
distances (Å) and angles (deg): N-C1 129.3, C1-Si 186.6, N-C1-Si
133.1, C1-N-C2 118.0, C1-N-C3 127.2, C2-N-C3 114.8.
(Phosphino)(silyl)carbenes are usually prepared by photolysis or
thermolysis of the corresponding diazo compounds, whereas
aminocarbenes are classically obtained by deprotonation of iminium
salts. None of these methods can be extrapolated to the synthesis
of (amino)(silyl)carbenes since neither amino-substituted diazo
derivatives nor silyl-substituted iminium salts are known; both of
these compounds are probably unstable. Therefore, we turn our
attention to our recently published synthesis of carbenes by
substitution reactions at a carbene center.5 Treatment of Alder’s
dimer 1a6 with 1 equiv of di(tert-butyl)phenylphosphine, followed
by deprotonation with sodium t-butoxide cleanly led to the carbene
precursor 3a. Then, addition of the lithium salt of t-butyldiphenyl
silane quantitatively afforded (according to multinuclear NMR) the
(amino)(silyl)carbene 4a, along with di-tert-butylphenylphosphine
(Scheme 1).
Carbene 4a has a half-life of about 12 h at 0 °C, but quantitatively
decomposes into the corresponding (E)-imine 5 and propene within
a few minutes at room temperature. This rearrangement is similar
to that observed for the (t-butyl)(di-i-propylamino)carbene.7 In the
hope of increasing the stability of such carbenes, we replaced the
i-propyl at nitrogen by bulkier c-hexyl groups. Carbene 4b is stable
for a few days at 0 °C and has even a half-lifetime of about 1 h at
room temperature. The 13C NMR signals for the carbene carbon of
4a (+377 ppm) and 4b (381 ppm) are shifted downfield by more
than 200 ppm compared to that for (phosphino)(silyl)carbenes. This
is a good indication that carbenes 4 do not belong to the family of
push-pull carbenes A. Indeed, these signals are even at lower field
than those for aryl-, alkyl- and even phosphino-amino carbenes
(300-350 ppm), which are of type C. All attempts to obtain crystals
of carbenes 4 failed. It is noteworthy that, despite numerous
Scheme 1
examples of stable (phosphino)(silyl)carbenes, only one single-
crystal X-ray diffraction study has been reported.8
We performed ab initio calculations at the B3LYP/6-31g* level9
on the experimentally observed carbene 4a (Figure 2) and on related
(amino)(silyl)-4c-f, (amino)(germyl)-6, (amino)(alkyl)-7, and (phos-
phino)(silyl)-8a,b carbenes (Table 1).
The singlet-triplet energy separation for the parent amino-
carbenes 4c, 6, and 7 increases with the electronegativity of the
second carbene substituent as predicted relatively early on for non-
amino carbenes.10 More importantly, the singlet-triplet gap is much
larger for the parent (amino)- (4c: 20.9 kcal/mol) than that for the
(phosphino)(silyl)carbene (8a: 4.2 kcal/mol). This is in line with
the better π-donor ability of the amino group. The values of the
carbene bond angle (4c: 116.8; 8a: 133.7; 8b: 151.1°) coupled
with the carbon-silicon bond lengths (4c: 188.3; 8a: 183.8; 8b:
180.5 pm) indicate a stronger interaction of the silyl group with
the carbene center for the phosphino carbenes 8. In fact, carbenes
4 are much more similar to the (alkyl)(amino)carbenes 7 (type C)
than to the push-pull (phosphino)(silyl)carbenes 8 (type A). In
other words, for 4 there is a very weak, if any, interaction between
the carbene lone pair and the vacant σ*-orbitals at silicon. This is
a further demonstration that the stabilizing effect of an amino group
toward a carbene center is so efficient, that a second electronically
† University of California.
‡ Fakulta¨t fu¨r Chemie der Universita¨t.
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10.1021/ja051109f CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
NHCs (2081 and 1996 cm-1),14 and are very similar to those
observed with the (tert-butyl)(di-i-propylamino)carbene (2070 and
1989 cm-1).7
These results show the usefulness of the substitution route for
the synthesis of original stable carbenes. They demonstrate that
(amino)(silyl)carbenes are not push-pull carbenes as their phos-
phino analogues and therefore are excellent ligands for metal
centers. We are currently investigating cyclic versions of these
species as well as the catalytic properties of the ensuing transition-
metal complexes.
Table 1. Calculated Geometric Parameters and Singlet/Triplet
Energy Separation for Carbenes 4 and 6-8
S/T gap
carbene angle
(deg)
C−Si
carbene
(kcal/mol)
(pm)
t
4a
4c
4d
4e
4f
iPr2N-C-SiPh2 Bu
20.3
20.9
22.3
20.5
22.2
23.4
33.3
4.2
133.1
116.8
126.6
129.8
132.0
112.3
111.0
133.7
151.1
186.6
188.3
186.6
187.0
186.5
H2N-C-SiH3
Me2N-C-SiH3
Me2N-C-SiMe3
iPr2N-C-SiMe3
H2N-C-GeH3
H2N-C-CH3
6
7
8a
H2P-C-SiH3
183.8
180.5
8b12c
(H2N)P-C-SiH3
11.1
Acknowledgment. Financial support by RHODIA is gratefully
acknowledged.
Supporting Information Available: Full experimental and com-
putational details, spectroscopic data (PDF), and X-ray crystallographic
data for 10 and 11 (CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
References
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Figure 3. Thermal ellipsoid diagram (50% probability) of 10 (H atoms
are omitted). Selected bond distances (Å) and angles (deg): N1-C1 1.3163-
(15), C1-Rh1 1.9935(11), C1-Si1 1.9275(12), Rh1-C24 2.1076(13),
Rh1-C31 2.1331(11), Rh1-C27 2.2405(10), Rh1-C28 2.2375(13), N1-
C1-Si1 124.03(8), N1-C1-Rh1 123.67(8), Si1-C1-Rh1 112.30(6).
Scheme 2
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active substituent is useless.7,11 Note that the calculated carbene
bond angle for 4a (133.1°) is rather large, but this is due to the
steric bulk of the substituents, as already observed for (amino)-
t
(alkyl)carbenes (7: 111.0; BuCNiPr2: 120.5°).
Except diaminocarbenes (B), stable carbenes readily react with
isocyanides, and indeed addition of t-BuNC at -40 °C to 4a cleanly
leads to keteneimine 9 (Scheme 2). In contrast to carbenes B and
C, push-pull carbenes A are reluctant to act as ligands for transition
metals.12 Therefore, it was of primary interest to evaluate the ligand
properties of 4. Addition of carbene 4a to [RhCl(cod)]2 in THF at
-78 °C afforded carbene complex 10, which was isolated by
column chromatography in 56% yield (based on dication 2a) as
highly thermally and air-stable single crystals (mp 150-152 °C).
Compared to the known [RhCl(cod)(NHC)] complexes,14 the 13
C
1
NMR signal for the carbene center of 10 (306 ppm, d, JCRh
)
33 Hz) is more deshielded and the Ccarbene-Rh bond length
(1.993 Å)13 is shorter (Figure 3). The carbonyl stretching frequencies
of cis-[RhCl(CO)2(L)] complexes are recognized as an excellent
measure of the σ-donor and π-acceptor properties of the ligand
L.14a,b Complex 11 was readily obtained by treatment of a THF
solution of 10 with CO at room temperature. The substitution of
the cod ligand by the stronger acceptor CO ligands shows the strong
donor capability of carbene 4.14c The carbonyl stretching frequencies
of 11 (2072 and 1989 cm-1) fall between those observed for the
analogous complexes featuring the very basic acyclic bis(diisopro-
pylamino)carbene15 (2057 and 1984 cm-1) and the saturated
(15) Alder, R. W.; Allen, P. R.; Murray, M.; Orpen, A. G. Angew. Chem., Int.
Ed. Engl. 1996, 35, 1121.
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