N,N ′-diamidoketenimines via coupling of isocyanides to an N-heterocyclic carbene

Article abstract of DOI:10.1021/jo100427g

Treatment of an N-heterocyclic carbene that features two amide groups N-bound to the carbene nucleus with various organic isocyanides afforded a new class of ketenimines in yields of up to 96% (isolated). DFT analyses revealed that the carbene exhibits a unique, low-lying LUMO, which may explain the atypical reactivity observed.

Full text of DOI:10.1021/jo100427g

pubs.acs.org/joc
N,N0-Diamidoketenimines via Coupling of
Isocyanides to an N-Heterocyclic Carbene
typically generated from their diazo or protonated precur-
sors, or via ring-cleavage of an appropriate oxazetidine.3
Surprisingly, there are no reported examples of ketenimines
derived from N-heterocyclic carbenes (NHCs), an important
class of structurally and functionally diverse substrates that
have been employed in a wide range of applications.4 This
deficiency may be due to the low electrophilicities inherent to
NHCs, which effectively prevents them from coupling with
nucleophilic isocyanides.
Todd W. Hudnall,† Eric J. Moorhead,† Dmitry G. Gusev,‡
and Christopher W. Bielawski*,†
†Department of Chemistry and Biochemistry, The University
of Texas at Austin, Austin, Texas 78712, and
‡Department of Chemistry, Wilfrid Laurier University,
Waterloo, ON N2L 3C5, Canada
bielawski@cm.utexas.edu
Received March 6, 2010
FIGURE 1. Representative examples of known ketenimines.
Recently, we reported that N,N0-diamidocarbene5 1 dis-
plays an extraordinary degree of electrophilic characteristics.
For example, 1 was found to facilitate a noncatalyzed C-H
insertion reaction at a tertiary hydrocarbon and, remark-
ably, was capable of affixing carbon monoxide (CO) to
afford a N,N0-diamidoketene in a reversible manner.5a,6 On
the basis of this unusual reactivity, we reasoned that 1 should
be capable of reacting with other inherently nucleophilic
substrates, particularly those that are isoelectronic with CO.
Herein, we demonstrate that NHC 1 couples to organic
isocyanides to form N,N0-diamidoketenimines, a previously
unknown class of molecules. In addition, we provide com-
putational data indicating that the unusual reactivity exhib-
ited by 1 is due to a relatively low-lying LUMO centered on
the carbene atom of this molecule.
Treatment of an N-heterocyclic carbene that features two
amidegroupsN-bound tothecarbenenucleuswithvarious
organic isocyanides afforded a new class of ketenimines in
yields of up to 96% (isolated). DFT analyses revealed that
the carbene exhibits a unique, low-lying LUMO, which
may explain the atypical reactivity observed.
Ketenimines are a fascinating class of organic compounds
that possess an alkene cumulated with an imine (i.e.,
R2CdCdN;R0). Because of their unique structure and high
chemical potential, they are of interest in fields that range
from synthetic organic to organometallic chemistry to astro-
chemistry.1 Despite this broad appeal, they remain relatively
unexplored and underutilized, particularly compared to their
isoelectronic ketenes, on account of synthetic limitations.2
Ketenimines (see Figure 1 for representative examples) are
often prepared by coupling isocyanides to suitable carbenes,
As summarized in Scheme 1, treating toluene solutions of
1 (generated in situ from its pyrimidinium precursor,
[1H][OTf]; OTf = triflate) with primary, secondary, tertiary,
or aryl isocyanides (1.0 equiv) afforded the desired N,N0-
diamidoketenimines 2in moderate to excellent yields (52-96%).
Unlike the reaction of 1 with CO, the analogous reactions with
ꢀ
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(1) (a) Schmittel, M.; Steffen, J.-P.; Angel, M. A. W.; Engels, B.;
Lennartz, C.; Hanrath, M. Angew. Chem., Int. Ed. 1998, 37, 1562–1564.
(b) Lovas, F. J.; Hollis, J. M.; Remijan, A. J.; Jewell, R. P. Astrophys. J. 2006,
645, L137–L140. (c) Sole, S.; Gornitzka, H.; Schoeller, W. W.; Bourissou, D.;
Bertrand, G. Science 2001, 292, 1901–1903.
(4) (a) Bourissou, D.; Guerret, O.; Gabbaı, F. P.; Bertrand, G. Chem. Rev.
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(2) (a) For a review of ketenimine chemistry, see: Krow, G. R. Angew.
Chem., Int. Ed. 1971, 10, 435–449. (b) For an account of ketene chemistry,
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2000, 100, 39–91. (b) Ofele, K.; Tosh, E.; Taubmann, C.; Herrmann, W. A.
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16039–16041. (b) A related N,N0-diamidocarbene was recently reported, see:
(3) (a) Igau, A.; Baceiredo, A.; Trinquier, B. G. Angew. Chem., Int. Ed.
1989, 28, 621–622. (b) Boyer, J.; Beverung, W. J. Chem. Soc. D 1969, 23,
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Chem. Soc. 2002, 124, 6806–6807. (d) Green, J.; Singer, L. Tetrahedron Lett.
1969, 10, 5093–5095. (e) Cheng, L.-Q.; Cheng, Y. Tetrahedron 2007, 63,
9359–9364. (f) Canac, Y.; Conejero, S.; Donnadieu, B.; Schoeller, W. W.;
Bertrand, G. J. Am. Chem. Soc. 2005, 127, 7312–7313. (g) Busetto, L.;
Marchetti, F.; Zacchini, S.; Zanotti, B. Organometallics 2008, 27, 5058–5066.
(h) Despagnet-Ayoub, E.; Gornitzka, H.; Bourissou, D.; Bertrand, G. Eur. J.
Org. Chem. 2003, 2039–2042. (i) Ciganek, E. J. Org. Chem. 1970, 35, 862–
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Cesar, V.; Lugan, N.; Lavigne, G. Eur. J. Inorg. Chem. 2010, 361–365.
(6) Similar reactivites have been exhibited by cyclic alkyl amino carbenes
although coupling reactions with isocyanides have not been reported, see:
(a) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G.
Science 2007, 316, 439–441. (b) Zeng, X.; Frey, G. D.; Kinjo, R.; Donnadieu,
B.; Bertrand, G. J. Am. Chem. Soc. 2009, 131, 8690–8696. (c) Lavallo, V.;
Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. Angew. Chem.,
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DOI: 10.1021/jo100427g
r
Published on Web 03/18/2010
J. Org. Chem. 2010, 75, 2763–2766 2763
2010 American Chemical Society

Hudnall et al.
JOCNote
TABLE 1. Selected Spectroscopic and Crystallographic Data for 2
C1C2Na (ppm)
C1C2Na (ppm)
νCCNb (cm-1
)
d(C1-C2)c (A)
d(C2-N2)c (A)
—
d (deg)
C1-C2-N2
2a
2b
2c
2d
2e
2f
198.1
201.4
195.4
203.6
205.7
191.0
108.9
109.8
108.0
109.7
111.5
108.5
2019
2014
2026
2010
2030
2024
1.307(5)
1.318(2)
1.297(4)
1.319(2)
1.316(3)
1.328(4)
1.240(6)
1.235(2)
1.248(4)
1.224(2)
1.230(3)
1.231(4)
167.0(6)
176.7(2)
173.9(4)
174.8(2)
173.9(2)
173.6(3)
aThe 13C NMR chemical shifts (δ) of the emphasized atoms noted were acquired in C6D6. bIR spectra were obtained as KBr pellets. The frequency
noted was attributed to the C1dC2dN group in the respective product. cDistance between the noted atoms in the respective product. dAngle between the
noted atoms in the respective product.
SCHEME 1. Syntheses of Ketenimines 2a
aCy = cyclohexyl. Dipp = 2,6-(iPr)2Ph. Isolated yields are indicated
in parentheses.
the aforementioned isocyanides did not appear to be reversible.
On the contrary, the products formed from these coupling
reactions were thermally robust and exhibited melting points
of 193-242 ꢀC. The products were also found to be stable to air
and moisture, which facilitated their isolation as pale- to dark-
yellow solids.
FIGURE 2. ORTEP view of the crystal structure of 2f (50%
thermal ellipsoids; hydrogen atoms have been omitted for clarity).
Selected bond lengths (A) and angles (deg): C1-C2 1.328(4),
C2-N2 1.231(4), N2-C19 1.450(3), N1-C1 1.402(2), N1-C3
1.376(2), C3-O1 1.210(2), C1-C2-N2 173.6(3), C2-N2-C19
122.1(2), N1-C1-N1A 116.1(2), N1-C3-O1 120.76(17).
The IR spectra of 2 revealed sharp bands at 2010-
2030 cm-1 (KBr) which were consistent with a CdCdN
stretching mode typical of ketenimine functional groups
(1962-2040 cm-1); see Table 1.1-3 Likewise, the 13C NMR
spectra of 2 exhibited resonances at δ 190-204 and 108-
110 ppm (C6D6) which were attributed to the nitrile CCN
and carbenoid CCN carbon nuclei, respectively. Although
the former chemical shifts were consistent with previously
reported ketenimines (δ 160-200 ppm),3 the latter were
relatively downfield (the 13C CCN resonances of known
ketenimines typically fall in the range of 45-65 ppm).3 The
discrepancies are presumably due to the two electronegative
heteroatoms bound to the carbon nucleus under interroga-
tion. Bertrand’s phosphino-amino ketenimine, which was
reported to display a 13C resonance at δ 94.0 ppm, provided
additional support for this assignment.3c
agreement with analogous angles exhibited by other keteni-
mines in the solid state.3g,7,8
To gain insight into the unusual reactivity exhibited by 1, a
series of DFT calculations were performed at the PBE0/6-
311þG(d,p) level of theory9 on a range of NHCs and their
CNtBu coupled products.10 Emphasis was placed on compar-
ing 1, as well as a truncated version that features N-methyl
substituents (3), with two prototypical NHCs: 1,3-dimethyli-
midazolinylidene (4) and 1,3-dimethylimidazolylidene (5).11
As summarized in Table 2, the change in enthalpy of the
coupling reactions involving 1 and 3 with CNtBu was found
to be significantly more favored than analogous reactions
involving 4 and 5.12 Supporting this assessment, the DFT
X-ray diffraction analyses performed on single crystals of
2 confirmed the structures of the desired ketenimine pro-
ducts (a representative example is shown in Figure 2; see the
SI for additional structures) and key data are summarized in
Table 1. In all cases, short C1-C2 and C2-N2 distances
were observed (1.297(4)-1.328(4) and 1.224(2)-1.248(4) A,
respectively), consistent with substantial multiple bond char-
acter for each of these linkages. Moreover, the C1-C2-N2
angles were nearly linear (167.0(6)-176.7(2)ꢀ) and in good
(9) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158–6170.
(10) For additional computational analyses and detailed discussions of
the electronic structures of NHCs, see: (a) Heinemann, C.; Thiel, W. Chem.
Phys. Lett. 1994, 217, 11–16. (b) Arduengo, A. J. III; Dias, H. V. R.; Dixon,
D. A.; Harlow, R. L.; Klooster, W. T.; Koetzle, T. F. J. Am. Chem. Soc. 1994,
116, 6812–6822. (c) Arduengo, A. J. III; Bock, H.; Chen, H.; Denk, M.;
Dixon, D. A.; Green, J. C.; Herrmann, W. A.; Jones, N. L.; Wagner, M.;
West, R. J. Am. Chem. Soc. 1994, 116, 6641–6649. (d) Boehme, C.; Frenking,
€
G. J. Am. Chem. Soc. 1996, 118, 2039–2046. (e) Heinemann, C.; Muller, T.;
Apeloig, Y.; Schwarz, H. J. Am. Chem. Soc. 1996, 118, 2023–2038. (f) Tukov,
A. A.; Normand, A. T.; Nechaev, M. S. Dalton Trans. 2009, 7015–7028.
(11) See SI for additional information and further discussion.
(12) In agreement with the computational analyses, 1,3-dimesitylimida-
zol-2-ylidene exhibited no reactivity toward an assortment of isocyanides
under various conditions (solvents, temperatures, etc.) whereas analogous
reactions involving 1,3-dimesitylimidazolin-2-ylidene formed complex mix-
tures of unidentifiable products.
(7) Jochims, J. C.; Lambrecht, J.; Burkert, U.; Zsolnai, L.; Huttner, G.
Tetrahedron 1984, 40, 893–903.
(8) Ruiz, J.; Riera, V.; Vivanco, M.; Lanfranchi, M.; Tiripicchio, A.
Organometallics 1998, 17, 3835–3837.
2764 J. Org. Chem. Vol. 75, No. 8, 2010

Hudnall et al.
JOCNote
TABLE 2. Selected Calculated Dataa
-ΔHb (kcal/mol)
d(C1-C2)c (A)
d(C2-N2)c (A)
—
d (deg)
ΔHS-Te (kcal/mol)
C1-C2-N2
1
3
4
5
38.0
34.0
16.2
1.324
1.328
1.325
1.396
1.218
1.222
1.239
1.262
176.1
166.5
156.4
132.0
41.8
45.2
68.6
81.1
7.40
aThe DFT calculations were performed at the PBE0/6-311þG(d,p) level of theory. bChange in enthalpy associated with the reaction: NHC þ
CNtBu f NHCdCNtBu, thermal corrections obtained from frequency calculations performed on optimized geometries were applied. cDistance
between the noted atoms in the product NHCdCNtBu. dAngle between the noted atoms in the product NHCdCNtBu. eChange in enthalpy between the
singlet and triplet states of the respective NHC.
optimized structure of 2d was in excellent agreement with that
determined experimentally. The optimized geometries of the
calculated ketenimines formed from 4 and 5 feature relatively
acute C1-C2-N bond angles and were accompanied with
significant charge buildup on the C2 atom (see the SI). As
such, they can be expected to adopt charge separated states
(see eq 1) which may contribute to their relatively high
energies. These effects may be exacerbated in the N,N0-
diamidoketenimine derived from 5 and CNtBu, due to a
preferenceof the NHC-derivedmoiety toretain aromaticity.10
Considering compounds 3-5 were calculated to exhibit
relatively large singlet-triplet gaps (ΔHS-T), minimizing the
possibility of radical-based coupling mechanisms with iso-
cyanides, subsequent efforts turned toward examining the
molecular orbitals of these systems to gain additional insight
into the unique reactivity of 1. Figure 3 depicts the frontier
FIGURE 3. Selected molecular orbitals (MOs) of 3 (left), 4
molecular orbitals of carbenes 3-5. The relevant HOMOs of
(middle), and 5 (right) optimized at the HF/STO-3G level of
these systems were calculated to be of similar energies,
theory. The values listed are the energies of the respective MO in
suggesting that they each can be expected to display similar
hartrees.
nucleophilicities. Indeed, various N,N0-diamidocarbenes
were previously reported to couple to electrophilic sub-
mines. DFT analyses revealed that a unique low-energy
strates, including transition metals and CS2, in a manner
LUMOinherent to1 enhancesitselectrophilicity andprovides
similar to typical NHCs.5 However, the LUMO of 3, which
an explanation for the unusual reactivity observed. On a
features an empty p-orbital on the carbene nucleus, was
broader level, the coupling chemistry described herein effec-
calculated to lie at a significantly lower energy than the
tivelyexpands the limited number ofnonmetalcatalyzed CdC
corresponding LUMOs of 4 and 5. We believe this unique
bond forming reactions4e and is expected to facilitate access to
MO enhances the electrophilicity exhibited by 1 and explains
a spectrum of structurally and functionally diverse organic
its reactivity toward isocyanides, carbon monoxide, and
molecules of broad interest. Efforts directed toward exploring
potentially other nucleophiles.13
the chemistry of the ketenimines described above are under-
way and will be reported in due course.
In conclusion, we report the first coupling reaction be-
tween an NHC and an isocyanide. The reaction, which was
enabled by 1, was found to proceed smoothly with a range of
isocyanides and facilitated access to a novel class of keteni-
Experimental Section
General Procedure for the Synthesis of 2. To a freshly prepared
solution of 1 in 5 mL of toluene (generated in situ via deproto-
(13) It is tempting to compare the coupling reaction reported herein with
nation of [1H][OTf]5a with 1.0 mol equiv of NaHMDS) was
NHC dimerization; see: (a) Wanzlick, H. W.; Schikora, E. Angew. Chem.
1960, 72, 494. (b) Alder, R. W.; Blake, M. E.; Chaker, L.; Harvey, J. N.;
€
added 1.0 mol equiv of the corresponding isocyanide. An
instantaneous color change from pale-yellow to a deep yellow-
orange was observed. After the resulting solution was stirred at
ambient temperature in a drybox for 12 h, the solvent was
removed under reduced pressure to afford a yellow-orange
powder. The crude material was purified by recrystallization
Paolini, F.; Schutz, J. Angew. Chem., Int. Ed. 2004, 43, 5896–5911.
(c) Kamplain, J. W.; Bielawski, C. W. Chem. Commun. 2006, 1727–1729.
(d) Poater, A.; Ragone, F.; Giudice, S.; Costabile, C.; Dorta, R.; Nolan, S. P.;
Cavallo, L. Organometallics 2008, 27, 2679–2681. (e) Graham, D. C.; Cavell,
K. J.; Yates, B. F. J. Phys. Org. Chem. 2005, 18, 298–309. (f) Luan, X.; Mariz,
R.; Gatti, M.; Costabile, C.; Poater, A.; Cavallo, L.; Linden, A.; Dorta, R. J.
Am. Chem. Soc. 2008, 130, 6848–6858.
J. Org. Chem. Vol. 75, No. 8, 2010 2765

Hudnall et al.
JOCNote
from a hydrocarbon solvent to afford analytically pure 2 as a
yellow solid. The above conditions were successfully performed
on multiple scales, ranging from 20 to 250 mg of [1H][OTf].
(CdCdN), 1678 cm-1 (CdO); UV-vis (CH2Cl2) λmax = 272 nm,
ε = 1.48 ꢀ 104 M-1 cm-1; HRMS [M þ H]þ calcd for
C37H46N3O2 564.3590, found 564.3581. Anal. Calcd for
C37H45N3O2: C, 78.83; H, 8.05; N, 7.45. Found: C, 78.94; H,
8.06; N, 7.41.
1
2a: mp 193-194 ꢀC, recrystallized from hexanes; H NMR
(C6D6, 300.47 MHz) δ 0.49-0.63 (m, 7H), 1.30 (d, 3J = 6.6 Hz,
24H), 1.87 (s, 6H), 2.59 (m, 2H), 3.28 (sept, 3J = 6.9 Hz),
7.00-7.03 (m, 4H), 7.11-7.13 (m, 2H); 13C (C6D6, 150.83 MHz)
δ 13.7, 19.9, 23.9, 24.7, 29.3, 30.2, 31.6, 47.3, 57.6, 108.9, 124.2,
129.9, 133.0, 168.6, 198.1; IR (KBr) ν 2018 (CdCdN),
1675 cm-1 (CdO); UV-vis (CH2Cl2) λmax = 272 nm, ε =
1.50 ꢀ 104 M-1 cm-1; HRMS [M þ H]þ calcd for C35H50N3O2
544.3903, found 544.3896. Anal. Calcd for C35H49N3O2: C,
77.31; H, 9.08; N, 7.73. Found: C, 76.98; H, 8.82; N, 7.83.
1
2f: mp 241-242 ꢀC; recrystallized from toluene; H NMR
(C6D6, 300.47 MHz) δ 1.18 (d, 3J = 6.9 Hz, 12H), 1.19 (s, 6H),
1.27 (d, 3J = 6.6 Hz, 12H), 1.84 (s, 6H), 3.31 (sept, 3J = 6.9 Hz,
4H), 6.54-6.64 (m, 3H), 6.96-7.02 (m, 4H), 7.06-7.12 (m, 2H);
13C (C6D6, 150.84 MHz) δ 17.5, 23.2, 24.6, 29.3, 47.2, 109.7,
124.7, 125.9, 127.9, 128.1, 128.3, 129.0, 130.3, 133.1, 141.4,
147.3, 168.8, 203.6; IR (KBr) ν 2010 (CdCdN), 1684 cm-1
(CdO); UV-vis (CH2Cl2) λmax = 283 nm, ε = 2.06 ꢀ 104 M-1
cm-1; HRMS [M þ H]þ calcd for C39H50N3O2 592.3903, found
592.3895. Anal. Calcd for C39H49N3O2: C, 79.15; H, 8.35; N,
7.10. Found: C, 79.31; H, 8.25; N, 7.02.
1
2b: mp 204-206 ꢀC, recrystallized from hexanes; H NMR
(C6D6, 300.47 MHz) δ 1.30 (d, 3J = 6.6 Hz, 24H), 1.87 (s, 6H),
3.29 (br s, 4H), 3.84 (br s, 2H), 6.14 (d, 3J = 7.8 2H), 6.86-6.94
(m, 4H), 6.98-7.08 (m, 5H); 13C (C6D6, 75.47 MHz) δ 24.6, 29.3,
47.3, 61.8, 109.8, 127.3, 127.5, 127.9, 128.1, 128.3, 128.3, 130.1,
132.8, 136.3, 168.6, 201.4; IR (KBr) ν 2014 (CdCdN),
1671 cm-1 (CdO); UV-vis (CH2Cl2) λmax = 272 nm, ε =
1.53 ꢀ 104 M-1 cm-1; HRMS [M þ H]þ calcd for C38H48N3O2
578.3747, found 578.3752. Anal. Calcd for C38H47N3O2: C,
78.99; H, 8.20; N, 7.27. Found: C, 79.04; H, 8.19; N, 7.09.
Computational Details. The calculations were carried out with
Gaussian 09 (Revision A.02).14 All geometries were optimized at
the PBE09/6-31G(d) level and were verified to have all real
harmonic frequencies by frequency calculations, which also
provided thermal corrections to enthalpy and APT15 charges.
The geometries were subsequently reoptimized at the PBE0/6-
311þG(d,p) level. The enthalpies reported in Table 2 were
obtained by combining the electronic energies from the PBE0/
6-311þG(d,p) calculations with the thermal corrections from the
PBE0/6-31G(d) frequency calculations. The molecular orbitals
shown in Figure 3 were obtained by reoptimizing the carbenes at
the HF/STO-3G level. Full symmetry (when present) was used in
every calculation reported herein. Some of the values listed in
Table S2 (SI) were taken from a recent report.16
1
2c: mp 209-210 ꢀC, recrystallized from n-pentane; H NMR
(C6D6, 300.47 MHz) δ 0.50-0.77 (m, 10H), 1.32 (d, 3J = 6.9 Hz,
3
24H), 1.88 (s, 6H), 2.55 (m, 1H), 3.31 (sept, J = 6.9 Hz, 4H),
7.01-7.04 (m, 4H), 7.11-7.14 (m, 2H); 13C (C6D6, 150.82 MHz)
δ 23.4, 23.9, 24.8, 25.4, 29.4, 32.4, 47.5, 67.1, 108.0, 124.2, 127.9,
128.1, 128.3, 129.9, 133.5, 168.8, 195.4; IR (KBr) ν2026 (CdCdN),
1675 cm-1 (CdO); UV-vis (CH2Cl2) λmax = 272 nm, ε = 1.90 ꢀ
104 M-1 cm-1; HRMS [M þ H]þ calcd for C37H52N3O2
570.4060, found 570.4057. Anal. Calcd for C37H51N3O2: C,
77.99; H, 9.02; N, 7.37. Found: C, 77.73; H, 8.71; N, 7.03.
Acknowledgment. This work was made possible by the
generous financial support of the National Science Foundation
(grant No. CHE-0645563), the Robert A. Welch Foundation
(grant No. F-1621), and the facilities of the Shared Hierarchical
Academic Research Computing Network. We thank Dr. V. M.
Lynch for assistance with the crystal structure analyses.
1
2d: mp 207-208 ꢀC, recrystallized from hexanes; H NMR
(C6D6, 300.47 MHz) δ 0.34 (s, 9H), 1.27 (br s, 24H), 1.86 (s, 6H),
3.32 (br s, 4H), 7.00-7.03 (m, 4H), 7.10-7.13 (m, 2H); 13C
(C6D6, 150.81 MHz) δ 25.2, 28.5, 29.5, 31.9, 48.1, 60.6, 108.5,
124.1, 127.9, 128.1, 128.3, 129.8, 134.3, 169.2, 191.0; IR (KBr) ν
2024 (CdCdN), 1669 cm-1 (CdO); UV-vis (CH2Cl2) λmax
=
Supporting Information Available: Additional experimental
details, characterization data, and computational analyses. This
material is available free of charge via the Internet at http://
pubs.acs.org.
272 nm, ε = 1.47 ꢀ 104 M-1 cm-1; HRMS [M þ H]þ calcd for
C35H50N3O2 544.3903, found 544.3900. Anal. Calcd for
C35H49N3O2: C, 77.31; H, 9.08; N, 7.73. Found: C, 77.37; H,
8.87; N, 7.55.
2e: mp 202-203 ꢀC, recrystallized from n-pentane; 1H NMR
(C6D6, 400.27 MHz) δ 1.22 (d, 3J = 6.4 Hz, 12H), 1.29 (d, 3J =
6.8 Hz, 12H), 1.89 (s, 6H), 3.30 (sept, 3J = 6.8 Hz, 4H), 6.05 (d,
3J = 7.6 Hz, 2H), 6.66 (m, 1H), 6.73 (t, 3J = 7.6 Hz, 2H), 7.01 (d,
3J = 7.6 Hz, 4H), 7.10 (t, 3J = 7.6 Hz, 4H); 13C (C6D6, 150.82
MHz) δ 23.1, 24.7, 24.8, 29.4, 47.6, 111.5, 120.7, 124.4, 126.8,
128.3, 128.7, 130.2, 145.1, 146.9, 168.8, 205.7; IR (KBr) ν 2030
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