Anhydrous TEMPO-H: Reactions of a good hydrogen atom donor with low-valent carbon centres

Article abstract of DOI:10.1039/c0ob00999g

In this paper, we report a novel synthesis of anhydrous 1-hydroxy-2,2,6,6-tetramethyl-piperidine (TEMPO-H). An X-ray crystal structure and full characterization of the compound are included. Compared to hydrated TEMPO-H, its anhydrous form exhibits improved stability and a differing chemical reactivity. The reactions of anhydrous TEMPO-H with a variety of low-valent carbon centres are described. For example, anhydrous TEMPO-H was reacted with 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes), an unsaturated NHC. Crystals of [CHNC₆H₂(CH₃)₃] ₂C...HO(NC₅H₆(CH₃) ₄), IMes...TEMPO-H, were isolated and a crystal structure determined. The experimental structure is compared to the results of theoretical calculations on the hydrogen-bonded dimer. Anhydrous TEMPO-H was also reacted with the saturated NHC, 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr), giving the product [CH₂Ni-Pr₂C₆H ₃]₂CH...O(NC₅H₆(CH ₃)₄). In contrast, the reaction of hydrated TEMPO-H with 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene gave small amounts of the hydrolysis product, N-(2,6-diisopropylphenyl)-N-[2-(2,6-diisopropylphenylamino) ethyl]formamide. Finally, anhydrous TEMPO-H was reacted with (triphenylphosphoranylidene)ketene to generate Ph₃PC(H)C(O)O(NC ₅H₆(CH₃)₄). A full characterization of the product, including an X-ray crystal structure, is described.

Full text of DOI:10.1039/c0ob00999g

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PAPER
Anhydrous TEMPO-H: reactions of a good hydrogen atom donor with
low-valent carbon centres†
Nick A. Giffin, Miller Makramalla, Arthur D. Hendsbee, Katherine N. Robertson, Cody Sherren, Cory C. Pye,
Jason D. Masuda* and Jason A. C. Clyburne*
Received 9th November 2010, Accepted 11th February 2011
DOI: 10.1039/c0ob00999g
In this paper, we report a novel synthesis of anhydrous 1-hydroxy-2,2,6,6-tetramethyl-piperidine
(TEMPO-H). An X-ray crystal structure and full characterization of the compound are included.
Compared to hydrated TEMPO-H, its anhydrous form exhibits improved stability and a differing
chemical reactivity. The reactions of anhydrous TEMPO-H with a variety of low-valent carbon centres
are described. For example, anhydrous TEMPO-H was reacted with 1,3-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene (IMes), an unsaturated NHC. Crystals of [CHNC6H2(CH3)3]2C ◊ ◊ ◊ HO-
(NC5H6(CH3)4), IMes ◊ ◊ ◊ TEMPO-H, were isolated and a crystal structure determined. The
experimental structure is compared to the results of theoretical calculations on the
hydrogen-bonded dimer. Anhydrous TEMPO-H was also reacted with the saturated NHC,
1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr), giving the product
[CH2Ni-Pr2C6H3]2CH ◊ ◊ ◊ O(NC5H6(CH3)4). In contrast, the reaction of hydrated TEMPO-H with
1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene gave small amounts of the hydrolysis product,
N-(2,6-diisopropylphenyl)-N-[2-(2,6-diisopropylphenylamino)ethyl]formamide. Finally,
anhydrous TEMPO-H was reacted with (triphenylphosphoranylidene)ketene to generate
Ph3PC(H)C( O)O(NC5H6(CH3)4). A full characterization of the product, including
an X-ray crystal structure, is described.
oxidation reactions, where NHCs and TEMPO/TEMPO-H are
Introduction
important in catalytic cycles.7,8
We have had a long standing interest in the reactivity of low-valent
main group compounds with protons, hydride and the hydrogen
atom.1 We have successfully detected unusual hydrogen bonds in
a variety of instances, and have also observed transient radicals
using EPR and/or muon spectroscopy.2 Recently, we detected
The combination of 1 with NHCs (2) can theoretically give rise
to a number of radical, neutral or ionic alternatives (Scheme 1).
Specifically, 3a represents the radical reaction product derived
from hydrogen atom transfer, 3b is the charge-separated species,
3c is the hydrogen-bonded derivative and 3d can be considered the
a radical derived from the addition of muonium, a hydrogen
O–H insertion product.
atom surrogate, to an N-heterocyclic carbene (NHC),3,4 and also
identified a related radical via the electrochemical reduction of an
imidazolium ion. The intermediate radical was observed using
EPR spectroscopy.5 Here, we report some chemical reactions
of one of the best chemical sources of hydrogen atoms (1-
hydroxy-2,2,6,6-tetramethylpiperidine, TEMPO-H; 1) with sev-
eral molecules possessing low-valent carbon centres, including two
NHCs and (triphenylphosphoranylidene)ketene.6 The reactions
reported herein are relevant to a number of NHC-catalyzed
The Maritimes Centre for Green Chemistry, Department of Chemistry,
Saint Mary’s University, Halifax, NS, B3H 3C3, Canada. E-mail: jason.
Scheme 1 Structural isomers formed by the reaction of TEMPO-H (1)
clyburne@smu.ca; Tel: + 1 902 420 5827, jason.masuda@smu.ca; Tel: +1
with NHCs (2).
902 420 5077
† Electronic supplementary information (ESI) available. CCDC reference
numbers 793072–793075. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0ob00999g
Access to anhydrous TEMPO-H has not been reported in
the literature, but it was clear to us early on that the water in
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TEMPO-H could result in unwanted side reactions (vide infra).
Here, we report the preparation of anhydrous TEMPO-H, show
that its reactivity does indeeddiffer from thatof hydrated TEMPO-
H and report its chemistry with some low-valent carbon centres.
by the three hydrogen bonds (Table 2). There are two O–H ◊ ◊ ◊ O
type hydrogen bonds and one relatively weaker O–H ◊ ◊ ◊ N type
bond holding the trimer together.
The crystal structure of the related, hydrated TEMPO-H was
reported by Mader et al. in 2007.10 This structure also has three
molecules (and one molecule of water) in the asymmetric unit and
is held together in a hydrogen-bonded network (shown in Fig. 2),
Results and discussion
¯
We have been interested in the reaction of protic reagents with a
variety of main group compounds for some time. In general, we
have always observed the formation of diamagnetic materials, so
we decided to explore the addition of a protic reagent, where
addition of the proton could occur, but where there was also
some possibility of hydrogen atom addition resulting in radical
formation. We had considerable experience working with a variety
of reactive main group compounds and began this investigation
from the literature preparation of TEMPO-H, a sensitive solid
isolated as a hydrate.
Noting the high reactivity of low-valent carbon compounds
with water, we prepared anhydrous 1 using a new route, as shown
in Scheme 2. The sodium salt of TEMPO was prepared and
isolated according to the literature.9 It was subsequently treated
with triethylamine hydrochloride, producing 1 in approximately
54% yield.
but it crystallizes in the lower symmetry, triclinic space group P1.
The hydrogen bonds in the crystal structure of hydrated TEMPO-
H (listed in Table 2) involve not only the donor H(O) protons of the
TEMPO-H itself but also those of the solvated water molecules.
Although the authors describe the hydrogen bonds as forming
a chain, the hydrogen bonded unit formed is actually a closed
structure involving a total of six TEMPO-H molecules (two of each
unique type) and two water molecules. The hydrogen bonding of
hydrated TEMPO-H is shown in Fig. 2; the diagram was generated
using data deposited in the Cambridge Structural Database.11 The
two very different hydrogen bonding motifs in TEMPO-H and
hydrated TEMPO-H must arise from the involvement of the water
molecule in the hydrogen bonding pattern of the latter and result in
the two compounds crystallizing in very different crystal systems
and space groups.
Scheme 2 Preparation of anhydrous TEMPO-H.
The full characterization matched the spectroscopic data found
in the literature, with the exception of those data consistent with
the presence of water in the material. The crystal structure of
anhydrous TEMPO-H was determined and is shown in Fig. 1.
Selected bond lengths and angles for the structures presented in
this work are listed in Table 1.
1
3
Fig. 2 TEMPOH· H2O crystal structure showing the [N ◊ ◊ ◊ H–O] and
[O ◊ ◊ ◊ H–O] hydrogen bonding arrangement. The diagram was generated
using data deposited in the Cambridge Structural Database11 and the
Mercury program for visualization.12
The N–O bond length in anhydrous TEMPO-H (this work)
˚
averages 1.459(2) A, similar to the average value reported in
hydrated TEMPO-H (1.456(6) A) by Mader et al.10 These authors
˚
noted that this is substantially longer than the N–O distance
observed in TEMPO13 (1.284(8) A) because the N–O bond in
˚
the radical has partial p character.
Finally, from a synthetic point of view, anhydrous TEMPO-H
is much more stable than hydrated TEMPO-H, and does not as
readily decompose to TEMPO during work-up and purification
by sublimation. Anhydrous TEMPO-H is prepared under an inert
atmosphere, where it can be kept indefinitely in the absence of
Fig. 1 Crystal structure (50% probability ellipsoids) of the anhydrous
TEMPO-H (1) trimer, showing the [N ◊ ◊ ◊ H–O] and [O ◊ ◊ ◊ H–O] hydrogen
bonds.
◦
water; storing it at low temperature (-35 C) impedes its loss by
sublimation. In addition, anhydrous TEMPO-H is highly reactive.
In all of the reactions performed for this study, all the products
1
Anhydrous TEMPO-H crystallizes in the trigonal space group
were diamagnetic species, as indicated by their sharp H and 13C
¯
R3, with three unique molecules in the asymmetric unit. The three
NMR resonances. The spectroscopic data indicates that structures
discrete molecules are held together as a single, closed cyclic unit
of the type 3a were not formed.
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◦
˚
Table 1 Selected structural data (bond lengths [A] and angles [ ]) for the reported compounds
[CH2Ni-Pr2C6H3]2CH ◊ ◊ ◊
OC6H2CH3(C(CH3)3)2]
Anhydrous TEMPO-H
IMes ◊ ◊ ◊ TEMPO-H
Ph3PC(H)C( O)ON(C5H6(CH3)4)
(for one of the three
molecules in the unit cell)
C(1)–N(1)
C(1)–C(7)
C(1)–C(2)
C(1)–C(6)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–N(1)
C(5)–C(9)
C(5)–C(8)
N(1)–O(1)
O(1)–H(1)
1.4884(19)
N(1)–C(1)
N(1)–C(2)
N(1)–C(4)
N(2)–C(1)
N(2)–C(3)
N(2)–C(13)
N(3)–O(1)
N(3)–C(26)
N(3)–C(22)
O(1)–H(1)
C(2)–C(3)
1.3629(17)
1.3905(18)
1.4446(16)
1.3648(16)
1.3913(18)
1.4422(17)
1.4448(14)
1.4895(17)
1.4898(17)
0.96(2)
1.340(2)
1.532(2)
1.523(2)
1.523(2)
N(1)–C(1)
N(1)–C(2)
N(1)–C(4A)
N(1)–C(4B)
N(2)–C(1)
N(2)–C(3)
N(2)–C(16)
C(1)–H(1)
C(2)–C(3)
O(1)–C(28)
C(28)–C(29)
C(28)–C(33)
C(29)–C(30)
C(30)–C(31)
C(31)–C(32)
C(32)–C(33)
C(1)–N(1)–C(2)
C(1)–N(1)–C(4A)
C(1)–N(1)–C(4B)
C(2)–N(1)–C(4A)
C(2)–N(1)–C(4B)
C(1)–N(2)–C(3)
C(1)–N(2)–C(16)
C(3)–N(2)–C(16)
N(1)–C(1)–N(2)
N(1)–C(1)–H(1)
N(2)–C(1)–H(1)
N(1)–C(2)–C(3)
N(2)–C(3)–C(2)
O(1)–C(28)–C(29)
O(1)–C(28)–C(33)
C(29)–C(28)–C(33)
C(28)–C(29)–C(30)
C(29)–C(30)–C(31)
C(30)–C(31)–C(32)
C(31)–C(32)–C(33)
C(28)–C(33)–C(32)
1.309(3)
1.478(3)
1.478(9)
1.399(9)
1.321(3)
1.477(3)
1.466(3)
1.00(3)
P(1)–C(1)
P(1)–C(7)
1.8065(15)
1.8085(14)
1.8086(15)
1.7075(14)
0.966(18)
1.4046(19)
1.3944(16)
1.2315(17)
1.4562(15)
1.4950(19)
1.4955(19)
1.531(2)
1.534(2)
1.536(2)
1.542(2)
1.520(2)
1.520(2)
1.535(2)
1.4932(19)
1.529(2)
1.541(2)
1.4503(16)
0.89(2)
P(1)–C(13)
P(1)–C(19)
C(19)–H(19)
C(19)–C(20)
O(1)–C(20)
O(2)–C(20)
N(1)–O(1)
N(1)–C(21)
N(1)–C(25)
C(21)–C(22)
C(22)–C(23)
C(23)–C(24)
C(24)–C(25)
1.303(3)
1.519(4)
1.437(4)
1.435(4)
1.384(4)
1.394(4)
1.377(4)
1.395(4)
111.1(2)
129.2(12)
126.9(12)
119.6(12)
122.0(12)
109.9(2)
126.2(2)
120.7(2)
112.1(2)
121.3(15)
126.6(15)
101.9(2)
103.0(2)
121.6(2)
121.4(3)
116.9(2)
119.8(2)
123.2(3)
116.8(3)
123.7(3)
119.3(3)
C(22)–C(23)
C(23)–C(24)
C(24)–C(25)
C(25)–C(26)
1.519(2)
1.521(2)
1.541(2)
1.524(2)
N(1)–C(1)–C(7)
N(1)–C(1)–C(2)
C(7)–C(1)–C(2)
N(1)–C(1)–C(6)
C(7)–C(1)–C(6)
C(2)–C(1)–C(6)
C(3)–C(2)–C(1)
C(2)–C(3)–C(4)
C(3)–C(4)–C(5)
N(1)–C(5)–C(9)
N(1)–C(5)–C(4)
C(9)–C(5)–C(4)
N(1)–C(5)–C(8)
C(9)–C(5)–C(8)
C(4)–C(5)–C(8)
O(1)–N(1)–C(1)
O(1)–N(1)–C(5)
C(1)–N(1)–C(5)
N(1)–O(1)–H(1)
106.14(12)
106.37(12)
109.04(13)
115.89(13)
108.06(13)
111.08(13)
112.98(13)
109.08(13)
113.53(13)
106.39(12)
106.12(12)
108.83(13)
116.02(13)
108.30(13)
110.92(13)
107.44(11)
106.68(11)
119.71(12)
105.8(12)
C(1)–N(1)–C(2)
C(1)–N(1)–C(4)
C(2)–N(1)–C(4)
C(1)–N(2)–C(3)
C(1)–N(2)–C(13)
C(3)–N(2)–C(13)
O(1)–N(3)–C(26)
O(1)–N(3)–C(22)
C(26)–N(3)–C(22)
N(3)–O(1)–H(1)
N(1)–C(1)–N(2)
C(3)–C(2)–N(1)
C(2)–C(3)–N(2)
N(3)–C(22)–C(23)
C(24)–C(23)–C(22)
C(23)–C(24)–C(25)
C(24)–C(25)–C(26)
N(3)–C(26)–C(25)
112.64(11)
122.89(11)
124.27(11)
112.43(11)
123.10(11)
124.31(11)
107.30(10)
107.30(10)
118.62(10)
103.3(12)
102.23(11)
106.27(12)
106.43(12)
107.26(11)
112.89(13)
109.04(12)
113.17(12)
107.39(11)
C(1)–P(1)–C(7)
106.02(6)
105.79(7)
105.83(7)
117.21(7)
108.03(7)
113.19(7)
119.5(10)
116.19(11)
123.8(11)
120.89(13)
126.32(13)
112.77(11)
115.58(9)
106.17(10)
106.65(10)
119.32(11)
106.53(12)
112.78(13)
109.16(13)
113.24(13)
106.41(12)
C(1)–P(1)–C(13)
C(7)–P(1)–C(13)
C(19)–P(1)–C(1)
C(19)–P(1)–C(7)
C(19)–P(1)–C(13)
P(1)–C(19)–H(19)
C(20)–C(19)–P(1)
C(20)–C(19)–H(19)
O(1)–C(20)–C(19)
O(2)–C(20)–C(19)
O(2)–C(20)–O(1)
C(20)–O(1)–N(1)
O(1)–N(1)–C(21)
O(1)–N(1)–C(25)
C(21)–N(1)–C(25)
N(1)–C(21)–C(22)
C(23)–C(22)–C(21)
C(22)–C(23)–C(24)
C(23)–C(24)–C(25)
N(1)–C(25)–C(24)
Table 2 Hydrogen bond distances and angles in anhydrous and hydrated TEMPO-H
(DHA) (◦)
Symmetry of acceptor
˚
˚
˚
D–H ◊ ◊ ◊ A
d(D–H)/A
d
H ◊ ◊ ◊ A/A
d
D ◊ ◊ ◊ A/A
TEMPOH
O1–H1 ◊ ◊ ◊ O2
O2–H2 ◊ ◊ ◊ N3
O3–H3 ◊ ◊ ◊ O1
0.89(2)
0.89(2)
0.94(2)
1.89(2)
2.50(2)
1.83(2)
2.7172(16)
3.2953(17)
2.7562(16)
152.1(17)
149.0(18)
169.2(19)
x, y, z
x, y + 1, z
x, y - 1, z
1
3
TEMPOH· H2O
O2–H2 ◊ ◊ ◊ O1
O1–H1 ◊ ◊ ◊ O3
O3–H3 ◊ ◊ ◊ O4
0.89(2)
0.88(2)
0.83(3)
1.94(2)
2.00(2)
1.86(2)
2.830(2)
2.882(2)
2.685(2)
175(2)
173(2)
158(3)
x, y, z
x, y, z
x, y, z
CCDC Refcode11
UDENOH
O4–H58 ◊ ◊ ◊ N1
O4–H59 ◊ ◊ ◊ N3
0.99(3)
0.94(3)
1.922
2.079
2.885
2.970
164.2
158.2
-x, 1 - y, 1 - z
x, y, z
The differing reactivity of hydrated TEMPO-H vs. its anhydrous
form is clearly illustrated by their respective reactions with
the saturated NHC, 1,3-bis(2,6-diisopropylphenyl)imidazolidin-
2-ylidene (SiPr) (Scheme 3). In our hands, the reaction with
hydrated TEMPO-H gave N-(2,6-diisopropylphenyl)-N-[2-(2,6-
diisopropylphenylamino)ethyl]formamide as the hydrolysis prod-
uct. The product was identified by comparison with literature data
previously reported for the same compound by Gu¨nay et al.14 In
their work, Gu¨nay et al. obtained the identical compound from the
reaction of 1,3-bis(4-dimethylaminobenzyl)imidazolinium chlo-
ride with N,C-type palladacyclic acetate in a 2 : 1 ratio in THF
(not their anticipated product). They speculated that the free NHC
or the corresponding carbene dimer must have been generated
initially, followed by insertion into a water H–OH bond to give
the observed hydrolysis product. They report that previous studies
on the reactivity of NHCs showed that the saturated ring reacts
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instantly with water to give formamide derivatives, and that the
hydrolysis necessitates the study of NHC complex formation in
anhydrous media. In our case, the water present in hydrated
TEMPO-H must be the source of the water necessary for hydrolysis
to occur. In contrast, the reaction with anhydrous TEMPO-H gives
a single product (vide infra). We anticipate that the reaction of
hydrated TEMPO-H with IMes should also result in the hydrolysis
of the NHC rather than a reaction with the TEMPO-H substrate,
based on the results recently reported by Hollo´czki et al.15 The
increased stability during preparation, the improved chances of
isolating tractable products from reaction, and the possibility of
differing chemistry make anhydrous TEMPO-H a reagent worthy
of further investigation.
The reaction of 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-
ylidene (IMes), an unsaturated NHC, with anhydrous TEMPO-H
in hexanes resulted in the formation of a clear solution (Scheme 3).
Filtration and crystallisation by slow evaporation of solvent
resulted in the isolation of clear, light yellow crystals.
Fig.
3
Crystal structure (50% probability ellipsoids) of the
IMes ◊ ◊ ◊ TEMPO-H adduct [CHNC6H2(CH3)3]2C ◊ ◊ ◊ HO(NC5H6(CH3)4),
showing the [C ◊ ◊ ◊ H–O] hydrogen bond. The symmetry used to generate
the IMes molecule is -x + 12 , y - 21 , -z +
.
1
2
this is midway between the angles reported for the methanol and
triphenylmethanol complexes, 174 and 166◦, respectively.
Movassaghi and Schmidt16 discuss the N–C–N angle within
the imidazolium ring as being indicative of the degree of proton
transfer (i.e. the strength of the hydrogen bond). Hydrogen bond
formation is expected to relax the N–C–N bond angle of the
carbene toward that found in an imidazolium ion. The N–C–N
bond angle of the TEMPO-H ◊ ◊ ◊ IMes complex is 102.23(11)◦,
which is closer to the observed angle in the free carbene (101.4◦)18
than that in imidazolium chloride (108.6◦).19 The angle observed
here is midway between that found in the methanol complex
(102.53(16)◦) and that found in the triphenylmethanol complex
(101.9(2)◦).
Scheme 3 Reaction of TEMPO-H with an unsaturated NHC.
NMR solution studies of the crystalline product in C6D6
revealed signals for a 1 : 1 complex of TEMPO-H and IMes.
1
Diagnostic peaks were observed at 210.9 ppm in the 13C{ H} NMR
spectrum, consistent with the presence of the NHC carbeneic
carbon centre, and a TEMPO-H OH resonance occurs at 8.38
ppm, very close to the resonance observed for free TEMPO-H
(8.36 ppm) in the same solvent. The analytical data, as well as the
elemental analysis, were in accord with a 1 : 1 complex of 1 with
the NHC. We note that in C6D6 a reaction slowly occurs and we
are unsure of the product.
Ab initio calculations were carried out to investigate the inter-
molecular hydrogen bonding and electronic structure of the com-
plex formed when N,N¢-dimethylimidazol-2-ylidene (“NHC”)
reacts with the TEMPO radical (see the Experimental section
for calculation details). N,N¢-Dimethylimidazol-2-ylidene exists
as a C2v symmetric structure while the free TEMPO radical
exists in a Cs symmetric chair conformation. The calculated value
In an attempt to resolve the connectivity of the components, a
single crystal X-ray structural determination was performed, the
results of which are shown in Fig. 3. The most distinctive feature
of the crystal structure is the O–H ◊ ◊ ◊ C hydrogen bond that links
the TEMPO-H molecule to the IMes ring. Hydrogen atom H1
was refined isotropically and found to still be covalently bonded
to O1 of TEMPO with a slightly elongated O1–H1 bond length
1
2
for the reaction TEMPO + H2 → TEMPO-H is exothermic
(52 kJ mol-1). TEMPO-H exists in one of two possible chair
conformations, with the abstracted hydrogen in either a synclinal
or an anticlinal arrangement with respect to both N–C bonds.
The anticlinal arrangement is 15 kJ mol-1 more stable than the
synclinal arrangement; the boat conformation is not stable with
respect to the other conformations.
˚
of 0.96(2) A. The hydrogen bond is characterized by a H1 ◊ ◊ ◊ C1
˚
˚
distance of 1.91(2) A, an O1 ◊ ◊ ◊ C1 distance of 2.854(2) A and an
O1–H1 ◊ ◊ ◊ C1 angle of 170(2)◦.
There are only two related structures, with similar O–
H ◊ ◊ ◊ C (IMes) hydrogen bonds reported in the literature,
an IMes ◊ ◊ ◊ methanol complex reported by Movassaghi and
Schmidt16 in 2005, and an IMes ◊ ◊ ◊ triphenylmethanol complex
reported by the same group in 2008.17 The length of the hydrogen
“NHC” interacts with TEMPO-H to form a complex that is
33 kJ mol-1 more stable than the reactants, TEMPO-H and free
“NHC”. The N–C–N angle in free “NHC” relaxes a small amount
and goes to 103.0◦ upon complexation. The O–H bond distance
˚
˚
increases from 0.970 A in TEMPO-H to 1.001 A in the complex.
The vibrational frequencies (B3LYP/6-31+G*, unscaled) of the
modes predominantly involving the hydrogen (O–H) undergo
significant changes upon complexation. The stretching frequency
drops from 3746 to 3146 cm-1. The NOH bending mode increases
from 1341 to 1556 cm-1 and the torsion about the N–O bond
increases from 373 to 903 cm-1. These trends are all consistent
with a hydrogen bonding interaction between the carbene lone
˚
bond O ◊ ◊ ◊ C interaction in the current complex, 2.854(2) A, is very
˚
close to that found in the triphenylmethanol adduct, 2.856(3) A,
˚
and slightly longer than that in the methanol complex, 2.832(2) A.
In theTEMPO-H ◊ ◊ ◊ IMes complex, theoxygen(O1) andhydrogen
˚
(H1) atoms sit 0.597(3) and 0.28(2) A, respectively, from the plane
defined by the five atoms of the imidazolylidene ring, allowing
the formation of quite a linear hydrogen bond, 170(2)◦. Again,
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pair and the acidic hydrogen of TEMPO-H, as also observed in
the X-ray crystal structure.
The reaction of anhydrous TEMPO-H with the saturated NHC
1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene20 in hexanes
resulted in the formation of a single product, as indicated by the
spectroscopic data (Scheme 4). The 1H NMR exhibited a singlet at
9.17 ppm, integrating for one hydrogen, and the OH resonance for
TEMPO-H was no longer observed. In the 13C NMR spectrum, the
resonance for the carbeneic NHC carbon was no longer evident
but a signal (163 ppm) corresponding to a new CH fragment,
diagnostic of an imidazolidinium cation, was present. Although no
crystalline product was ever isolated, even after multiple attempts,
we believe the product to be the saturated SIPr ◊ ◊ ◊ TEMPO-H
adduct [CH2Ni-Pr2C6H3]2CH ◊ ◊ ◊ O(NC5H6(CH3)4).
Fig. 4 Crystal structure (50% probability ellipsoids) of the saturated
NHC ◊ ◊ ◊ phenol adduct [CH2Ni-Pr2C6H3]2CH ◊ ◊ ◊ OC6H2CH3(C(CH3)3)2,
showing the [O ◊ ◊ ◊ H–C] hydrogen bond. Hydrogen atoms (except for H1)
and the minor component of all the disordered portions of the molecule
have been removed for clarity. The symmetry used to generate the BHT
molecule is 1 - x, -12 + y, - z.
1
2
Scheme 4 Reaction of TEMPO-H with a saturated NHC.
dimesitylimidazol-2-ylidene was also found to react with 2,6-di-
t-butyl-4-methylphenol to give a protonated salt with unusually
short C–H ◊ ◊ ◊ O hydrogen bonds. In the structure, ion pairs are
associated through strong C–H ◊ ◊ ◊ O intera◦ctions: C1–H1 ◊ ◊ ◊ O1:
To help confirm the nature of the product formed in the above
reaction, we deliberately protonated the saturated NHC 1,3-
bis(2,6-diisopropyl-phenyl)imidazolidin-2-ylidene with 2,6-di-t-
butyl-phenyl-4-methylphenol (BHT)20 and confirmed its structure
by X-ray crystallography. The successful solution and refinement
of the structure showed that an ionic product had been formed
between the two constituents, without solvation and with only
one unique cation and anion in the asymmetric unit. The
orthorhombic space group P212121 (#19) proved most suitable for
refinement. Although the data was collected at 100 K, the structure
was found to be highly disordered. It contains a strong C–H ◊ ◊ ◊ O
˚
˚
D = 2.801(4) A, d = 1.87(3) A, q ◦= 175(2) ; C4–H4 ◊ ◊ ◊ O2: D =
˚
˚
2.842(4) A, d = 1.88(3) A, q = 169(2) . The authors report that these
interactions are significantly shorter than any previously reported
hydrogen bonds between CH donors and O acceptors. The C–
H ◊ ◊ ◊ O hydrogen bond in the current complex is slightly weaker
than these, judging from the distances, but is equally linear. Even in
the disordered structure, it likely offers a significant contribution
to the stability of the crystal.
hydrogen bond that holds the cation and anion together, as shown
A comparison of the solution and solid state structures is
informative, and infrared studies on the materials reported above
are useful. TEMPO-H exhibits two strong infrared resonances at
3411 and 3448 cm-1. These bands disappear in all the reactions
with the carbenes. The reaction product formed from TEMPO-H
and IMes is soluble in hexanes, and in C6D6 solution evidence of a
hydrogen bonded complex is suggested by the observation of O–H
and N–C–N signals in the 1H and 13C NMR spectra, respectively.
The reaction product formed between TEMPO-H and SIPr is
also soluble in hexanes. Evidence of proton transfer is observed in
solution, where C–H and N–C(H)–N signals are observed in the
1H and 13C NMR (C6D6) spectra, respectively. We were unable to
crystallize this product but solid samples exhibit a stretch in the
infrared spectrum at 3378 cm-1, which is suggestive of a hydrogen
bonded complex. Lastly, the product formed from the reaction of
BHT and SIPr is soluble in hexanes, and in C6D6 solution evidence
of the formation of a discrete salt is observed. The anticipated C–
1
in Fig. 4. The C1–H1 ◊ ◊ ◊ O1 (O1 symmetry -x + 1, y - 12 , -z +
)
2
hydrogen bond is characterized by a C–H bond length of 1.00(3)
˚
˚
˚
A, an H ◊ ◊ ◊ O distance of 1.89(3) A, a C ◊ ◊ ◊ O distance of 2.886(3)
◦
A and an almost linear C–H ◊ ◊ ◊ O angle of 175(2) .
The cation maintains a saturated C2–C3 bond with a bond
˚
length of 1.519(4) A and two hydrogen atoms clearly visible
on each carbon centre in the Fourier difference map. This
is in keeping with the structural details reported by Giffin
et al.21 for 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene.
The unprotonated starting material crystallizes in the monoclinic
space group P21/c and this structure is also somewhat disordered.
In the neutral saturated compound, the C2–C3 bond length
˚
is a comparable 1.514(3) A. Protonation at C1 has definite
consequences for the structural parameters of the ring. In the
˚
neutral complex, the C1–N bond lengths are 1.346(2) A (to N1)
◦
˚
and 1.347(2) A (to N2), and the N1–C1–N2 angle is 104.98(11) .
Protonation results in a shortening of the C1–N bonds, 1.309(3)
1
H and N–C(H)–N signals are detected in the H and 13C NMR
˚
˚
A to N1 and 1.321(3) A to N2, with a concurrent increase in the
N1–C1–N2 angle to 112.1(2)◦. The ring in the protonated cation
remains quite planar; using the five ring atoms to define a plane,
spectra, respectively. In addition, an O–H stretch was not observed
in the infrared spectrum.
The difference in the reactivity of TEMPO-H with IMes
vs. SiPr is dramatic. In the former case, a neutral (O–H ◊ ◊ ◊ C
hydrogen bonded) adduct is formed, while in the latter case,
proton transfer occurs and an ionic product is recovered. It is
difficult to deconvolute the various contributions that give rise
˚
the mean deviation from this plane is only 0.060 A, with C3 lying
˚
farthest out of the plane at -0.080(2) A.
A structure closely related to the current one was re-
ported by Cowan et al. in 2002.22 The unsaturated NHC 1,3-
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to such differing reactivity. It is likely that steric factors, and p-
acceptor effects and s-donor abilities of the carbenes, all combine
to produce the results observed experimentally.
The molecule can be thought of in two parts, the TEMPO and
PCCO regions, joined by a OTEMPO–CPCCO bond. The geometry
of the TEMPO fragment can be compared to the structures of
anhydrous TEMPO-H (this work), hydrated TEMPO-H (Mader
et al.10) and the free TEMPO radical (Yonekuta et al.13). The
TEMPO fragment of this molecule has a geometry similar to those
of TEMPO-H and hydrated TEMPO-H, and is quite different
from that of the TEMPO radical. In particular, the N–O bond
To investigate the reactivity of N-heterocyclic carbenes with
TEMPO, one equivalent of TEMPO was allowed to react with 1,3-
bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene. No evidence of
a reaction was observed in the 1H NMR or infrared spectra of the
solids isolated. It has recently been established that much more
reactive transient radicals are required for reactions with NHCs
such as IMes.23 Likewise, reaction mixtures of IMes and TEMPO
in hexane solutions under a pressure of 5 atm of H2 did not result
in any reaction, as monitored by infrared spectroscopy.
˚
length of 1.4562(15) A is similar to those reported in TEMPO-
˚
H (1.459(2) A average, this work) and in hydrated TEMPO-H
(1.456(6) average).10 However, it is quite different from that in
13
˚
TEMPO (1.284(8) A), since the latter bond is shortened by
a significant p contribution. This is also reflected in the C–N–
C (119.32(11)◦) and C–N–O (106.65(10) and 106.17(10)◦) bond
angles in the molecule, which are similar to those in TEMPO-H
and hydrated TEMPO-H, but which are again quite different from
those in the TEMPO radical13 (C–N–C (123.7(6)◦) and C–N–O
(114.9(6) and 116.8(5)◦), where the bond angles are affected by the
p contribution in the NO bond.
Scheme 5 Reaction of Ph3PCCO with TEMPO-H.
The TEMPO part of the molecule is joined to the PCCO
˚
part by an O–C bond that is 1.3944(16) A, slightly shorter than
27
˚
Lastly, we have examined the reactivity of TEMPO-H with
(triphenylphosphoranylidene)ketene (8), a molecule that can be
viewed as a low-valent carbon source.24,25 The C-centre in this
molecule can potentially hydrogen bond,26 and we were unsure
of how it would react with TEMPO-H. The treatment of 8 in
anhydrous CH2Cl2 with one equivalent of 1 (Scheme 5) resulted in
the formation of a clear solution. 31P NMR studies on the reaction
mixture indicated the formation of a single species (19 ppm). After
the solvent was removed, a light pink-coloured solid was obtained.
1H NMR studies in CDCl3 solutions indicated the presence of both
a Ph3P fragment as well as the TEMPO moiety. The solid had a
sharp melting point and its elemental analysis was consistent with
a 1 : 1 addition product between TEMPO-H and 8. Notable from
the IR data was a strong peak at 1644 cm-1, consistent with a
a typical C–O single bond (1.43 A). The slightly decreased
bond length undoubtedly occurs because of the interaction with
the PCCO fragment of the molecule. There are a number of
related PCCO-type complexes reported in the literature, for
example Ph3PCHCOPh28 and Ph3PC(COMe)(COPh).29 In these
complexes, the authors describe the PCCO unit as being best
thought of as a combination of three resonance contributors, the
ylene (P C), the ylide (P+–C-) and the enolate (charge separation
on P+ and O-). The geometries of the complexes they report are
echoed in the PCCO portion of the current molecule.
The geometry around the central P1 atom is nearly tetrahedral.
˚
The P1–C19 bond, with a length of 1.7075(14) A, is shorter than
˚
the other P–Cphenyl bonds (average 1.8079(15) A) in the molecule,
which suggests it possesses partial double bond character. The
˚
C
O moiety in the product. An X-ray structural determination
C20–O2 bond is longer (1.2315(17) A) than an average C
O
˚
27
˚
was performed on the crystals, the results of which are shown in
Fig. 5.
bond length (1.210 A), while the C19–C20 bond (1.4046(19) A)
27
˚
is also longer than a typical C C bond (1.331 A). These bond
lengths support the idea of resonance delocalization occurring
in the molecule. Atoms C19 and C20 have distorted trigonal
planar geometries (P1–C19–C20 = 116.19(11)◦, C19–C20–O1 =
120.89(13)◦ and C19–C20–O2 = 126.32(13)◦), suggesting an sp2-
type hybridization for these carbon atoms, in keeping with the
resonance delocalized model.
H(19), the hydrogen atom in the central portion of the PCCO
fragment, was refined isotropically to give a C19–H19 bond length
˚
of 0.966(18) A. It did not participate in any hydrogen bonding-type
interactions and, in fact, no hydrogen bonds, stacking interactions
or other intermolecular interactions of any importance were
noted.
The atoms in the central region of the molecule, including P1,
C19, C20, O2, O1 and N1, can be used to define a plane. The
˚
mean deviation of these atoms from the plane is only 0.0611 A
˚
and the largest deviation from the plane is -0.1046 A for C19
˚
(H19 lies -0.1285 A from the defined plane). As suggested by
Kalyanasundari et al.,28 this planarity also supports the resonance
delocalization model. It is reflected again in the torsion angles
for this region of the molecule, P1–C19–C20–O1 = 175.35(10)◦,
P1–C19–C20–O2 = -6.3(2)◦, C19–C20–O1–N1 = -10.72(18)◦ and
Fig.
5 Crystal structure (50% probability ellipsoids) of the
TEMPO–C( O)C(H)P(Ph)3 molecule.
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O2–C20–O1–N1 = 170.70(11)◦, all values similar to those reported
by Spencer et al.29 and Kalyanasundari et al.28
the suspension was filtered and the remaining volatiles removed
under reduced pressure, yielding a light grey powder. Yield: 2.91 g
(83.3%).
˚
The non-bonded distance between P1 and O2 is 2.94 A,
significantly less than the sum of the van der Waals radii for
Preparation of anhydrous 1-hydroxy-2,2,6,6-tetramethyl-
piperidine (TEMPO-H). In a 500 mL Schlenk flask 2.91 g
(0.0163 mol) of sodium 2,2,6,6-tetramethyl-piperidine-1-oxide
was added to a suspension of 2.18 g (0.0163 mol) of triethylamine
hydrochloride in 100 mL of dry Et2O. The reaction was allowed to
proceed for 12 h, at which point the solution was filtered through a
30
˚
phosphorus and oxygen (3.3 A). As described by both Spencer
et al.29 and Kalyanasundari et al.,26 this suggests that there is a
strong intramolecular interaction between the P+ and O- charge
centres of the enolate resonance contributor, which in turn leads
to the molecule adopting a cis orientation.
R
vacuum frit with a Celiteꢀ bed to remove excess reagents and any
Conclusions
sodium salts produced. Removal of the Et2O and triethylamine
in vacuo yielded pure anhydrous 1-hydroxy-2,2,6,6-tetramethyl-
piperidine (TEMPO-H). Single crystals were grown at -35 ◦C by
sublimation of the material onto the side of a sealed vial. Yield:
1.39 g (54%). mp 35–36 ◦C. 1H NMR (250 MHz, C6D6) d: 8.36 (s,
In summary, we have reported a reliable preparation for anhydrous
TEMPO-H and fully characterized this product. We have demon-
strated the utility of anhydrous TEMPO-H vs. the hydrated form,
both in terms of greater stability during synthesis and differing
reactivity patterns. We have described the reactions of TEMPO-H
with a number of low-valent C-containing materials. Anhydrous
TEMPO-H is much more stable than its hydrated form, and reacts
in a variety of modes with low-valent carbon sites.
1
1 H, NOH), 2.35 (br m, 18 H, 3JH–H = 7 Hz, CH3, CH2). 13C{ H}
NMR (250 MHz, C6D6) d: 128.7, 128.3, 127.9, 59.1, 39.9, 26.6,
17.6. IR (KBr, cm-1) m: 3478 (m), 3411 (m), 2972 (s), 2934 (s),
1468 (m), 1421 (w), 1380 (m), 1360 (m), 1293 (m), 1262 (m), 1240
(w), 1207 (w), 1183 (m), 1135 (m), 1045 (m), 997 (w), 957 (m), 872
(w), 783 (m), 684 (m), 582 (m), 506 (m). Anal. calc. for C9H19NO:
C, 68.74; H, 12.18; N, 8.91. Found: C, 68.45; H, 12.40; N, 9.08%.
Experimental
General experimental
Synthesis of [CHNC6H2(CH3)3]2C ◊ ◊ ◊ HO(NC5H6(CH3)4)
An argon atmosphere double manifold vacuum line and argon
atmosphere dry box (mBraun Unilab) were used for the manipu-
lation of air- and moisture-sensitive compounds. Ultra-high purity
argon gas was used in all cases. Anhydrous solvents were available
directly using an mBraun MB-SPS solvent purification system.
All melting points were determined using a Barnstead Mel-Temp
apparatus. NMR spectra were obtained using Bruker Avance 500
MHz and/or AC-250 spectrometers. IR spectra were obtained
using a Bruker Vertex 70 infrared spectrometer as either potassium
bromide pellets or as dichloromethane solutions between potas-
sium bromide plates. Elemental analyses were performed using a
Perkin-Elmer CHN analyzer.
NMR spectra were obtained at the Nuclear Magnetic Reso-
nance Research Resource (NMR-3), Dalhousie University, Hali-
fax, Canada. 1H and 13C spectral shifts are reported in relation to
either known residual solvent peaks or TMS, when available. 31P
spectral shifts were taken without internal calibration.
To a solution of 100.0 mg (0.33 mmol) of 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene in hexanes, 51.9 mg
(0.33 mmol) of anhydrous 1-hydroxy-2,2,6,6-tetramethyl-
R
piperidine was added. Filtration through a Celiteꢀ plug followed
by slow evaporation of the remaining solvent yielded X-◦ray
quality crystals. Yield: 82 mg (0.177 mmol, 54%). mp 86–89 C.
1H NMR (250 MHz, C6D6) d: 8.38 (br s, 1 H, NOH), 7.75 (s, 2
H, Ar-H), 7.34 (s, 2 H, Ar-H), 5.33 (s, 2 H, NCH), 3.10 (s, 18
H, Ar–CH3), 2.32 (br s, 6 H, CH2), 1.98 (br s, 12 H, CCH3).
1
13C{ H} NMR (250 MHz, C6D6) d: 210.9, 138.0, 137.1, 135.0,
128.7, 128.3, 127.9, 120.3, 57.5, 39.7, 26.0, 20.7, 19.6. IR (KBr,
cm-1) m: 3124 (m), 2988 (s), 2968 (s), 2920 (s), 2770 (m), 1642 (w),
1609 (w), 1542 (w), 1489 (s), 1444 (s), 1388 (s), 1350 (m), 1293
(w), 1251 (s), 1208 (w), 1181 (w), 1131 (m), 1084 (m), 1034 (m),
959 (m), 926 (m), 854 (s), 783 (w), 723 (m), 665 (w), 633 (w), 573
(w), 510 (w). Anal. calc. for C30H43N3O, C, 78.05; H, 9.39; N,
9.10. Found C, 77.89; H, 9.45; N, 9.11%.
Synthesis of [CH2Ni-Pr2C6H3]2CH ◊ ◊ ◊ O(NC5H6(CH3)4)
Synthetic methods
To a solution of 100.0 mg (0.255 mmol) of 1,3-bis(2,6-
diiso-propylphenyl)imidazolidin-2-ylidene in hexanes, 40.0 mg
(0.255 mmol) of anhydrous 1-hydroxy-2,2,6,6-tetramethyl-
piperidine was added. No crystalline product was isolated after
multiple attempts; the solid was characterized by spectroscopic
methods. Yield: 122 mg (0.223 mmol, 87%). mp 112–115 C. H
NMR (250 MHz, C6D6) d: 9.17 (s, 1 H, NCH), 8.10–7.89 (m, 6
Preparation of anhydrous 1-hydroxy-2,2,6,6-tetramethyl-piperidine
(TEMPO-H) in two steps
Preparation of sodium 2,2,6,6-tetramethyl-piperidine-1-oxide.
In a 300 mL sealable Schlenk-type reaction vessel, 0.46 g (20 mmol)
of Na metal was heated until molten. Through slow rotation of
the vessel as the metal cooled, a sodium mirror was deposited
on the flask wall and allowed to harden via slow cooling to
ambient temperature. 3.12 g (20 mmol) of commercially available
2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) and 100 mL of
dry hexanes were added and the mixture brought to reflux at 70 ◦C
for 12 h. The solution underwent an observable change overnight
from the characteristic deep red of TEMPO to a brown solution
with a light precipitate. After reduction of the solvent by 50%,
◦
1
H, Ar–H), 4.80 (t, 2 H, 3JH–H = 7 Hz, NCH2), 4.57 (t, 2 H, 3JH–H
=
3
7 Hz, N¢C¢H2), 4.32 (sept, 1 H, JH–H = 7 Hz, CH(CH3)2), 4.13
(sept, 2 H, 3JH–H = 7 Hz, C¢H(CH3)2), 3.96 (sept, 1 H, 3JH–H = 7 Hz,
C¢¢H(CH3)2), 2.19 (br m, 18 H, (CH3)2CCH2CH2CH2C(CH3)2,
3
3
2.00 (d, 12 H, JH–H = 7 Hz, CHCH3), 1.89 (d, 12 H, JH–H = 7
Hz, C¢HCH3). 13C{ H} NMR (250 MHz, C6D6) d: 163.1, 147.6,
1
142.1, 136.0, 128.7, 127.3, 127.9, 124.2. 123.8, 123.5, 49.5, 48.7,
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28.1, 27.7, 25.0, 24.1, 23.1. IR (KBr, cm-1) m: 3378 (w), 3068 (w),
2964 (s), 2930 (m), 2869 (m), 1660 (s), 1624 (m), 1463 (s), 1411 (m),
1384 (m), 1362 (m), 1301 (m), 1262 (m), 1236 (m), 1191 (m), 1097
(w), 1055 (w), 993 (w), 868 (w), 845 (m), 756 (m), 578 (w), 510 (w).
Anal. calc. for C36H57N3O, C, 78.92; H, 10.49; N, 7.67. Found, C,
78.15; H, 10.24; N, 7.29%.
The solution was stirred and slow evaporation of the sol-
vent yielded light pink, X-ray◦quality crystals. Yield: 403 mg
(0.87 mmol, 87%). mp. 163–166 C. 1H NMR (500 MHz, CD2Cl2)
d: 7.68 (m, 6 H, Ar-H), 7.55 (m, 3 H, Ar-H), 7.47 (m, 6 H, Ar-H),
3.61 (br s, 1 H, CH), 1.58 (m, 5 H, CH2), 1.38 (m, 1 H, CH2), 1.24
(m, 12 H, CH3). 13C-NMR (500 MHz, CD2Cl2) d: 133.2, 131.9,
128.8, 128.0, 60.0, 40.0, 32.6, 20.6, 17.4. 31P (500 MHz, CD2Cl2)
d: 19.4. IR (NaCl salt plate, cm-1): m: 3027 (s), 2975 (s), 2924 (s),
2871 (s), 2102 (w), 1944 (w), 1860 (w), 1806 (w), 1644 (m), 1605 (s,
Synthesis of [CH2N i-Pr2C6H3]2CH ◊ ◊ ◊ OC6H2CH3(C(CH3)3)2
To
a solution of 100.0 mg (0.26 mmol) of 1,3-bis(2,6-
C
O), 1494 (s), 1461 (m), 1439 (m), 1356 (m), 1333 (m), 1265 (s),
diisopropylphenyl)-imidazolidin-2-ylidene in hexanes, 57.3 mg
(0.26 mmol) of butylated hydroxytoluene (BHT) was added. After
1183 (w), 1107 (m), 1085 (m), 1030 (w), 932 (w), 893 (s), 732 (s),
696 (s), 552 (w), 520 (w).
R
filtration of the solution through a Celiteꢀ plug, slow evaporation
of solvent yielded X-ray quality◦yellow crystals. Yield: 127 mg
(0.208 mmol, 80%). mp 134–136 C. 1H NMR (250 MHz, C6D6)
d: 9.17 (s, 1 H, NCH), 8.25–8.00 (m, 8 H, Ar-H), 4.81 (t, 2 H,
Theoretical calculations
Calculations were performed using Gaussian 0331 and a stepping
stone approach, in which the geometries at the levels HF/STO-3G,
HF/3-21G, HF/6-31G*, HF/6-31+G* and B3LYP/6-31+G*
were sequentially optimized using default specifications. After
each level, a frequency calculation was performed to verify the
nature of the stationary point. Z-matrix coordinates constrained
to the appropriate symmetry were used for efficiency, as any
problems would manifest themselves by an imaginary mode
orthogonal to the spanned Z-matrix space. The Hessian was also
evaluated at the starting STO-3G geometry to aid convergence.
Only the B3LYP/6-31+G* results are reported.
3
3JH–H = 7 Hz, NCH2), 4.63 (t, 2 H, JH–H = 7 Hz, N¢C¢H2), 4.23
(sept, 4 H, 3JH–H = 7 Hz, CH(CH3)2), 3.19 (s, 3 H, Ar–CH3), 2.33 (s,
1
18 H, C(CH3)3), 1.89–2.30 (m, 24 H, CH(CH3)2). 13C{ H} NMR
(250 MHz, C6D6) d: 163.1, 151.7, 146.9, 135.6, 128.7, 128.3, 127.9,
125.4, 123.6, 53.3, 33.9, 30.1, 28.6, 25.0, 24.1, 23.3, 21.1. IR (KBr,
cm-1) m: 2961 (s), 2717 (w), 2617 (w), 2370 (w), 1658 (w), 1606 (m),
1583 (s), 1462 (s), 1421 (s), 1386 (m), 1311 (m), 1262 (m), 1192 (w),
1101 (w), 1051 (w), 1018 (w), 991 (w), 802 (m), 754 (m), 579 (w).
Anal. calc. for C42H62N2O C, 82.57; H, 10.23; N, 4.59. Found C,
81.98; H, 10.39; N, 4.23%.
Synthesis of Ph3PC(H)C(=O)O(NC5H6(CH3)4)
General data collection and refinement details
To a solution of 302 mg (1 mmol) (triphenylphosphoranyli-
dene)ketene in anhydrous dichloromethane, 157 mg (1 mmol) of
anhydrous 1-hydroxyl-2,2,6,6-tetramethyl-piperidine was added.
A summary of crystallographic data for the studied compounds
is shown in Table 3. Data was collected on a Bruker Smart
diffractometer running APEX232 software, with Mo-Ka radiation
Table 3 Crystallographic data for the reported compounds
Anhydrous
TEMPO-H
Unsaturated IMes ◊ ◊ ◊
TEMPO-H
Saturated SIPr ◊ ◊ ◊
BHT
Ph3PC(H)C(=O)ON-
(C5H6(CH3)4)
Identification code
Empirical formula
Formula weight
T/K
C9 H19NO
157.25
100(2)
C30H43N3O
461.67
100(2)
0.23 ¥ 0.19 ¥ 0.18
Monoclinic
P21/n
10.989(2)
15.262(3)
16.648(4)
90
94.250(3)
90
2784.5(10)
4
1.101
0.067
C42H62N2O
610.94
100(2)
0.30 ¥ 0.21 ¥ 0.20
Orthorhombic
P212121
10.2181(7)
19.3153(14)
19.5555(14)
90
90
90
3859.6(5)
4
1.051
C29H34NO2P
459.54
296(2)
0.40 ¥ 0.37 ¥ 0.22
Monoclinic
P21/n
11.9030(13)
12.1631(13)
18.100(2)
90
93.6420(10)
90
2615.1(5)
4
1.167
0.130
Size/mm3
0.31 ¥ 0.29 ¥ 0.27
Crystal system
Trigonal
¯
Space group
R3
˚
a/A
18.7761(16)
18.7761(16)
43.329(4)
90
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
120
13229(2)
54
1.066
0.068
21413
3
˚
Volume/A
Z
Density (calculated)/Mg m-3
Absorption coefficient/mm-1
Reflections collected
0.061
42693
22240
20480
Independent reflections
Rint
5195
0.0471
0.7459 and 0.6486
5195/0/322
1.096
0.0420
0.0861
0.0781
0.0993
6707
0.0392
0.7458 and 0.6662
6707/0/311
1.025
0.0475
0.1140
0.0695
0.1271
4462
0.1297
0.9877 and 0.9819
4462/393/549
0.956
0.0438
0.0945
0.0675
0.1026
6326
0.0492
0.7457 and 0.6680
6326/0/30
1.020
0.0419
0.1017
0.0591
0.1133
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2s(I)] R1
Final R indices [I > 2s(I)] wR2
R indices (all data) R1
R indices (all data) wR2
-3
˚
Largest differential peak and hole/e A
0.200 and -0.199
0.273 and -0.247
0.204 and -0.158
0.448 and -0.286
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(0.71073 A) and w scans. A temperature of 100(2) K was used for
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Acknowledgements
Funding was provided by the Natural Sciences and Engineering
Council of Canada (NSERC) through the Discovery Grants
Program to J. D. M. and J. A. C. C. J. A. C. C. acknowledges
support from the Canada Research Chairs Program, the Canadian
Foundation for Innovation (CFI), and the Nova Scotia Research
and Innovation Trust (NSRIT). J. D. M. acknowledges support
from the CFI and NSRIT. We are also grateful to the Nuclear
Magnetic Resonance Research Resource (NMR-3) at Dalhousie
University for NMR data acquisition.
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3680 | Org. Biomol. Chem., 2011, 9, 3672–3680
This journal is
The Royal Society of Chemistry 2011
©