An N,N′-diamidocarbene: Studies in C-H insertion, reversible carbonylation, and transition-metal coordination chemistry

Article abstract of DOI:10.1021/ja907481w

(Chemical Equation Presented) The ring-opening polymerization of a mixture of enantiomerically pure but different monomers using an yttrium complex as initiator proceeds readily at room temperature to give the corresponding highly alternating polyester.

Full text of DOI:10.1021/ja907481w

Published on Web 10/20/2009
An N,N′-Diamidocarbene: Studies in C-H Insertion, Reversible Carbonylation,
and Transition-Metal Coordination Chemistry
Todd W. Hudnall and Christopher W. Bielawski*
Department of Chemistry and Biochemistry, The UniVersity of Texas at Austin, Austin, Texas 78712
Received September 3, 2009; E-mail: bielawski@cm.utexas.edu
Scheme 1a
Considerable controversy has surrounded the carbenoid character
of N-heterocyclic carbenes (NHCs) since their isolation nearly two
decades ago by Arduengo.1 Indeed, these remarkable compounds,
as well as other diaminocarbenes (DACs), are termed “ylidenes”
according to IUPAC nomenclature because of their ylidic reactiv-
ity.2 Traditional carbenes, such as methylene, typically facilitate
insertions into unactivated C-H bonds,3 activate H2,4 cyclopro-
a Conditions: (i) (1) dimethylmalonyl dichloride, toluene, 25 °C, 1 h;
(2) TMS ·OTf, 25 °C, 1 h. (ii) NaHMDS, C6H6, 25 °C, 30 min. Dipp )
2,6-diisopropylphenyl.
panate olefins,5 and form ketenes upon exposure to CO.6 In contrast,
DACs do not facilitate the aforementioned chemical reactions. This
disparity is primarily due to the π-donating nature of the amino
substituents bonded to the carbene atom, which, in addition to
stabilizing the divalent carbon nucleus, renders it nucleophilic.
Moreover, DACs do not display reactivities typical of Fischer-type
carbenes despite their high propensity to metalate,2 presumably
because of a lack of electrophilic character.7-9 In fact, as recently
documented by Bertrand,9 DACs are not sufficiently electrophilic
7
to activate H2 or fix CO to form ketenes.8
We sought to alter this paradigm and increase the electrophilic
nature of DACs by shepherding nitrogen lone pair donation away
from the empty p orbital on the carbon nucleus. We envisioned an
N,N′-diamidocarbene, where significant electron density would be
localized on the low-lying π* orbitals of carbonyls attached directly
to the nitrogen atoms of a DAC. As detailed below, we discovered
that such a carbene is not only electrophilic but, surprisingly,
displays familiar nucleophilic characteristics as well.10
As shown in Scheme 1, the pyrimidinium salt [1H][OTf] was
prepared by treatment of N,N′-bis(2,6-diisopropylphenyl)-N-tri-
methylsilylformamidine11 with dimethylmalonyl dichloride in
toluene. Immediate precipitation of the desired compound was
observed upon addition of trimethylsilyl triflate (TMS ·OTf), which
facilitated isolation of the product in nearly quantitative yield.12,13
Single crystals suitable for X-ray diffraction analysis were obtained
by cooling a concentrated solution of [1H][OTf] in 1:1 (v/v) CH2Cl2/
toluene to -40 °C (Figure 1). Remarkably, [1H]+ exhibited minimal
amide character in the solid state, presumably because of donation
of the nitrogen lone pairs into the formally empty p orbital on the
positively charged carbon center rather than to the R-carbonyl
moieties. The carbonyl bond lengths [C4-O1 ) 1.188(4) Å,
C6-O2 ) 1.183(4) Å] as well as the N1-C6 and N2-C4 distances
[1.456(5) and 1.447(5) Å, respectively] deviate substantially from
those of prototypical amides such as formamide [C-O ) 1.193 Å,
N-CO ) 1.349 Å (calcd)].14 This assignment was confirmed by
FT-IR spectroscopy, which revealed that [1H]+ exhibits a carbonyl
stretching frequency (νCO) at 1760 cm-1 (CH2Cl2). These structural
features, which are strikingly similar to those observed by Stoltz
for 2-quinuclidone,14,15 suggested to us that [1H]+ contains
functional groups that are more consistent with R-ketoimines than
amides.
Figure 1. ORTEP representation of [1H]+ in [1H][OTf] (drawn as 50%
thermal ellipsoids; the triflate counteranion and all of the H atoms except
for H2A have been omitted for clarity). Pertinent structural metrics are
provided in the text.
resonance of the carbene nucleus was observed at 278.4 ppm, a
value that is significantly downfield relative to those displayed by
other six-membered NHCs.11,16 Indeed, 1 exhibits one of the most
downfield-shifted carbene resonances of any NHC featuring two
donor substituents10 and deficient of main-group heteroatoms.17 As
expected, the carbonyl stretching frequency of 1 was shifted to lower
energy (νCO ) 1714 cm-1 in CH2Cl2) with respect to the analogous
energy exhibited by [1H]+, reflecting the loss of cationic character
upon deprotonation.
Upon the successful synthesis of 1, subsequent efforts were
directed toward elucidating its structure in the solid state. To our
surprise, X-ray diffraction analysis of crystals obtained by cooling
a concentrated 1:1 (v/v) toluene/hexane solution of the carbene to
-40 °C revealed the structure of the C-H insertion product 2
(Figure 2 and Scheme 2). Unlike [1H]+, the NCO fragments in 2
were found to be characteristic of prototypical amides with short
N-C bond lengths [N1-C2 ) 1.373(2) Å, N2-C4 ) 1.376(2) Å]
and long C-O distances [C2-O1 ) 1.216(2) Å, C4-O2 )
1.216(2) Å]. Likewise, 2 exhibited νCO values of 1690 and 1662
Treatment of [1H][OTf] with sodium hexamethyldisilazide
(NaHMDS) in C6D6 afforded the corresponding N,N′-diamidocar-
bene 1 (Scheme 1), as determined by NMR spectroscopy. The 13
C
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10.1021/ja907481w CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 16039–16041 16039

C O M M U N I C A T I O N S
Figure 4. ORTEP representation of 3 in 3·NaHMDS (drawn as 50%
thermal ellipsoids; for clarity, H atoms have been omitted and the Dipp
substituents drawn as thin lines). Pertinent structural metrics are provided
in the text.
Figure 2. ORTEP representation of 2 (drawn as 50% thermal ellipsoids;
H atoms except for H1A have been omitted for clarity). Pertinent structural
metrics are provided in the text.
Scheme 2a
substrates employed in such reactions are relatively acidic. Fur-
thermore, the formation of 2 occurs under mild conditions24 and
involves a C-H insertion reaction at a tertiary alkyl group.25 In a
broader sense, NHCs akin to 1 are expected to find applications in
C-H activation chemistry, particularly as organic alternatives to
commonly employed metal-catalyzed processes.26
The formation of 2 is characteristic of transformations typically
exhibited by traditional (unstabilized) carbenes, indicating that 1
displays a certain degree of electrophilic character.7,9,10 Upon the
observation of this unprecedented reactivity, subsequent efforts were
focused on further investigation of the electrophilic character of 1.
Although DACs were previously reported to be incapable of affixing
CO,7-9 an instant color change from pale-yellow to purple (λmax
) 510 nm) was observed upon bubbling CO into a toluene-d8
solution of 1. A new compound that exhibited all of the signals
corresponding to diamidoketene 3 (see Scheme 2) was observed
by H NMR spectroscopy, albeit in a 2:1 1/3 ratio. Regardless,
the ketene CO resonance (250.5 ppm) was shifted significantly
downfield relative to that for CO (184.5 ppm), and the carbene
CCO resonance (98.85 ppm) was drastically upfield-shifted relative
to the carbene atom signal for 1 (278.3 ppm).28 Additionally, a
sharp band in the IR spectrum of this purple solution centered at
2100 cm-1 was also observed, consistent with the formation of a
ketene.7 Subsequent heating of this solution resulted in an
instantaneous loss of color, presumably due to expulsion of CO;
in contrast, cooling the same solution resulted in a more intense
purple color. The thermally reversible CO fixation process was
confirmed using variable temperature (VT) NMR spectroscopy (see
the SI), and from the van’t Hoff plot shown in the right panel of
Figure 3, the thermodynamic parameters for the carbonylation of
1 were determined to be Keq ) 2.62 M-1 at 30 °C, ∆H° ) -35.33
kJ mol-1, and ∆S° ) -109.5 J mol-1 K-1. Although the reversible
coordination of CO to metals has been well-documented,29 to the
best of our knowledge this is the first example of a reversible
carbonylation process involving an organic substrate. Furthermore,
these results expand the short list of noncatalyzed, reversible CdC
bond-forming processes.1c,30
a Conditions: (i) C6H6, 50 °C, 30 h; (ii) 100 psi CO(g), C7D8, 25 °C;
(iii) [Ir(COD)Cl]2, C6D6, 25 °C, 12 h; (iV) 1 atm CO(g), CH2Cl2, 25 °C,
2 h. R ) Dipp. Isolated yields are indicated.
27
1
Figure 3. (left) Plot of ln [1] ([1]0 ) 1.6 × 10-1 M) vs time in C6D6 in the
presence of (circles) 1.0, (squares) 3.0, and (triangles) 10 equiv of Na+ at
50 °C. The Na+ was derived from NaHMDS. (right) Plot of ln Keq vs 1/T
for the reaction 1 + CO h 3 using the data summarized in Table S2 in the
SI. The equation for the best-fit line is y ) 4249.3x - 13.171 (R2 ) 0.98).
cm-1 (CH2Cl2), which are at significantly lower frequencies than
the analogous signals observed for [1H]+.
An investigation of the kinetics of this reaction revealed that
the formation of 2 was first-order in carbene concentration and
proceeded with a first-order rate constant k ) 3.6 × 10-3 min-1 at
50 °C [see the Supporting Information (SI)]. Additionally, the rate
of this reaction did not depend on the concentration of NaHMDS,
effectively eliminating the possibility of a catalyzed process (Figure
3, left).18 While it has been previously reported that stabilized
singlet carbenes facilitate insertion into C-H bonds,18-23 the
Cooling a saturated solution of 3 in toluene-d8 to -80 °C resulted
in the formation of purple crystals of 3·NaHMDS, as determined
by X-ray crystallography (Figure 4). The CdCdO fragment in the
solid-state structure of 3 clearly indicated the formation of a ketene31
with C1-C2 and C2-O3 distances of 1.302(9) and 1.182(8) Å,
respectively. Additionally, the C1-C2-O3 angle was nearly linear
[177.8(9)°], as expected for a ketene functional group. The NCO
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C O M M U N I C A T I O N S
(2) (a) Bourissou, D.; Guerret, O.; Gabba¨ı, F. P.; Bertrand, G. Chem. ReV.
moieties were similar to those observed in 2, demonstrating a large
degree of amide character with relatively short C-N bonds
[1.355(7)-1.369(7) Å] and long C-O distances [1.220(7)-1.214(7)
Å].
¨
2000, 100, 39. (b) Ofele, K.; Tosh, E.; Taubmann, C.; Herrmann, W. A.
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Having demonstrated that 1 exhibits reactivity characteristic of
electrophilic carbenes, our efforts shifted toward probing its intrinsic
nucleophilicity. Treating a freshly prepared solution of 1 in C6D6
with [Ir(COD)Cl]2 (COD ) 1,5-cyclooctadiene) led to the formation
of the metal complex 1-[Ir(COD)Cl] (Scheme 2).32 Notably, the
carbene’s 13C NMR resonance was found at 231.3 ppm (C6D6),
which is significantly downfield-shifted relative to those for other
NHC complexes reported in the literature (179.6-208.2 ppm).33
This unusual chemical shift was attributed to reduced electron
density at the carbene nucleus due to delocalization between the
nitrogen atoms of 1 and the adjoining carbonyl groups. In agreement
with this assessment, the complex exhibited νCO ) 1736 and 1711
cm-1 (CH2Cl2) and the acyl carbons resonated at 171.4 ppm (C6D6),
indicative of carbonyls with bond orders that were effectively
hybrids of those for ketones and amides.
The electronic structure of 1 was also expected to result in a
significantly reduced ligand donicity compared with other NHCs.
To investigate this, 1-[Ir(CO)2Cl] was prepared by sparging a
CH2Cl2 solution of 1-[Ir(COD)Cl] with CO. Salient spectroscopic
features of the product included a 13C NMR resonance at 224.1
ppm (C6D6), which was assigned to the carbene nucleus. This
chemical shift was shifted downfield relative to those for other
NHC-[Ir(CO)2Cl] complexes reported in the literature (176.2-204.9
ppm),33 reflecting the electron-deficient nature of 1. The IR
spectrum of 1-[Ir(CO)2Cl] exhibited two strong νCO signals at 2074
and 1991 cm-1 (CH2Cl2), which were attributed to the trans and
cis CO ligands on the metal center, respectively, as well as a νCO
of 1736 cm-1 for the NCO groups. By Nolan’s method,33a the
Tolman Electronic Parameter34 for 1 was calculated to be 2057
cm-1, a value similar to that obtained for analogous metal
complexes containing tricyclohexylphosphine.33 Hence, while 1
retains the nucleophilic reactivity typical of traditional NHCs,35
the carbonyl groups adjacent to the nitrogen atoms significantly
attenuate the ligand’s donicity.
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(10) Similar reactivities have been observed with cyclic alkyl amino carbenes
(CAACs). See: (a) Zeng, X.; Frey, G. D.; Kinjo, R.; Donnadieu, B.;
Bertrand, G. J. Am. Chem. Soc. 2009, 131, 8690. (b) Lavallo, V.; Canac,
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Chem. Soc. 2008, 130, 11286.
(13) In the presence of water and other coordinating solvents, including
tetrahydrofuran, [1]OTf rapidly decomposes and therefore should be
manipulated under an inert atmosphere. This reactivity is a testament to
the highly electrophilic nature of this cation.
(14) Tani, K.; Stoltz, B. M. Nature 2006, 441, 731.
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four-membered NHCs reported by Bertrand17 and Grubbs.11 These results
may be explained by the small ring systems in these compounds, which
result in relatively low steric encumbrances at the carbene centers. In
contrast, we attribute the chemical shift of the analogous signal observed
in 1 to its electron-withdrawing carbonyl moieties.
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DAC, see: Lloyd-Jones, G. C.; Alder, R. W.; Owen-Smith, G. J. J.
Chem.sEur. J. 2006, 12, 5361.
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(24) Previous accounts of C-H insertion reactions with phosphinocarbenes were
reported to occur under relatively extreme conditions (300 °C, 10-2 mmHg)
(see refs 2, 22, and 23).
In conclusion, the electrophilic character of N,N′-diamidocarbene
1, imparted by the strongly π-withdrawing carbonyl moieties linked
to the nitrogen atoms of a DAC, has effectively opened a new vista
in NHC reactivity, including an ability to insert into a tertiary C-H
bond and the discovery of an unprecedented reversible carbonylation
reaction. However, like other NHCs, 1 is also sufficiently nucleo-
philic to coordinate to transition metals. We believe these results
will enable the rational design of novel classes of transition-metal
complexes and enrich many stoichiometric, metal-mediated, and
organocatalyzed reactions involving NHCs.2,36,37
(25) For monoaminocarbenes known to activate aryl C-H bonds, see ref 20.
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Oxgaard, J.; Goddard, W. A. Angew. Chem., Int. Ed. 2004, 43, 4626.
(27) Pressurizing an NMR tube containing 1 dissolved in toluene-d8 (0.16 M)
with 100 psi CO resulted in a greater conversion to 3 (1/3 ) 45:55).
(28) These results are in good agreement with those for previously reported
cyclic alkyl amino ketenes: 277.96 ppm (CO), 81.40 ppm (CCO) (see ref
7).
(29) (a) Darensbourg, D. J. AdV. Organomet. Chem. 1982, 21, 113. (b) Li, J.;
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(31) It has previously been reported that 1,3-bis(1-adamantyl)imidazol-2-ylidene
reacts with CO to afford the respective diaminoketene (see: Lyashchuk,
S. N.; Skrypnik, Y. G. Tetrahedron Lett. 1994, 35, 5271. ). However, the
results have been disputed (see ref 8).
Acknowledgment. We are grateful to the NSF (CHE-0645563),
the Welch Foundation (F-1621), and the Sloan Foundation for their
generous financial support. We thank Dr. V. M. Lynch for assistance
with the X-ray diffraction analyses.
(32) The solid-state structure of 1-[Ir(COD)Cl] was determined (see the SI).
(33) (a) Kelly, R. A. I.; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.;
Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P.
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E.; Sanau´, M.; Peris, E. Organometallics 2006, 25, 3713. (c) Herrmann,
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Supporting Information Available: Synthetic details, VT NMR
data, and crystallographic data for [1H]OTf, 2, 3·NaHMDS, and
1-[Ir(COD)Cl] (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
(35) Addition of CS2 to 1 afforded the expected zwitterionic adduct (see the
SI).
(36) Monfette, S.; Fogg, D. E. Chem. ReV. 2009, 109, 3783.
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