Generation of α,β-unsaturated iminium ions by laser flash photolysis

Article abstract of DOI:10.1002/anie.201103683

Two at a time: α,β-Unsaturated iminium ions can be generated by laser flash photolysis of enaminophosphonium ions (see scheme). The rate constants of their reactions with nucleophiles provide the first direct comparison of the electrophilicities of iminiu

Full text of DOI:10.1002/anie.201103683

DOI: 10.1002/anie.201103683
Organocatalysis
Generation of a,b-Unsaturated Iminium Ions by Laser Flash
Photolysis**
Sami Lakhdar,* Johannes Ammer, and Herbert Mayr*
Dedicated to Professor Gerhard Bringmann on the occasion of his 60th birthday
Iminium activation has become one of the most important
methods in enantioselective synthesis.[1] For the optimization
and the rational design of organocatalytic cycles, knowledge
of the mechanism of these reactions is crucial.[2] In previous
work, we have shown that the rate constants for the reactions
of unsaturated iminium ions with ketene acetals,[2d] sulfur
ylides,[3] and pyrroles[4] can be determined by UV/Vis
spectroscopy employing conventional spectrometers or
stopped-flow equipment. Both methods require the mixing
of the reactants, and therefore are not applicable to reactions
that proceed on the sub-millisecond time scale.
We now report on the in situ laser-flash-photolytic
generation of iminium ions derived from cinnamaldehyde
Scheme 1.
and imidazolidinones, which allowed us to measure rate
constants for the reactions of iminium ions with strong
nucleophiles. This method along with previously reported
kinetic procedures have been employed to directly compare
the electrophilic reactivities of iminium ions derived from
different imidazolidinones.
Treatment of the imidazolidinonium salts 1a–c with
cinnamaldehyde (2) in methanol or ethanol following liter-
ature procedures[5,6] gave precipitates of the iminium salts 3a–
c (Scheme 1), which were previously analyzed by X-ray
crystallography.[5a,6c] When these crystals were dissolved in
acetonitrile, only the E isomers of 3a–c were observed by
NMR spectroscopy.[7]
Combination of the iminium salts 3a–c with one equiv-
alent of tributylphosphine gave the (E)-enaminophospho-
nium salts 4a–c as mixtures of two diastereoisomers (2:1 for
4a and 4c and 1:1 for 4b; Scheme 1). Selective formation of
the (E)-enamines 5a–c (1:1 ratio of two diastereoisomers)
was observed when solutions of 3a–c in acetonitrile were
treated with excess piperidine (Scheme 1).[8]
As organocatalytic processes involving 3a–c are often
highly enantioselective,[9] we have to conclude that the low
stereoselectivities of the stoichiometric reactions with PBu3
and piperidine in Scheme 1 are due to reversible reactions
under the conditions employed.
Tri-n-butylphosphine has previously been reported to be
an effective photo-leaving group for the laser-flash-photolytic
generation of stabilized carbocations.[10] Irradiation of aceto-
nitrile solutions of the phosphonium salts 4a–c with 7 ns laser
pulses from the fourth harmonic of a Nd/YAG laser (266 nm,
30–60 mJpulseꢀ1) yielded the iminium ions 3a–c which
showed the same UV/Vis absorption maxima lmax as solutions
of the isolated iminium salts in acetonitrile (Figure 1a).
When salts 3a–c were generated in the presence of a large
excess of the nucleophiles 6j or 6l–o, we observed mono-
exponentional decays of their absorbances, from which the
rate constants kobs (sꢀ1) were obtained (Figure 1b). Plots of
kobs versus the nucleophile concentrations were linear (Fig-
ure 1c) and provided the second-order rate constants k2
(mꢀ1 sꢀ1) which are listed in Table 1.
In order to provide a broader experimental basis for the
comparison of the electrophilicities of iminium ions derived
from different imidazolidinones we have also determined rate
constants of the reactions of 3a, 3b, and 3c with weaker
nucleophiles using convential UV spectrometers and
stopped-flow techniques. The rate of the reaction of 3a with
DBU (6l) has been determined in two ways, with laser-flash-
photolytically generated iminium ions as well as with
solutions of isolated iminium salts, and the values differed
by less than 6%. This agreement is remarkable in view of
Seebachꢀs hypothesis that (E)-iminium ions are more reactive
than their Z isomers.[6e] As we do not know the configuration
of the photolytically generated iminium ions, the monoexpo-
[*] Dr. S. Lakhdar, Dipl.-Ing. J. Ammer, Prof. Dr. H. Mayr
Department Chemie, Ludwig-Maximilians-Universitꢀt Mꢁnchen
Butenandtstrasse 5–13 (Haus F), 81377 Mꢁnchen (Germany)
E-mail: sami.lakhdar@cup.uni-muenchen.de
herbert.mayr@cup.uni-muenchen.de
Homepage: http://www.cup.uni-muenchen.de/oc/mayr
[**] We thank the Alexander von Humboldt Foundation (research
fellowship for S.L.) and the Deutsche Forschungsgemeinschaft (Ma
673/21-3) for support of this work, Dr. P. Mayer for the X-ray
structure determination, Prof. S. Kobayashi for installing the laser
flash photolysis working station, and Dr. A. R. Ofial and Prof. D.
Seebach for helpful comments.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103683.
Angew. Chem. Int. Ed. 2011, 50, 9953 –9956
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9953

Communications
Figure 1. a) UV/Vis spectrum of 3b immediately after the laser pulse in CH3CN. b) Decay of the absorbance of 3b obtained after irradiation of a
1.51ꢂ10ꢀ4 m solution of 4b in CH3CN in the presence of piperidine (6n; 1.86ꢂ10ꢀ3 m). c) Plot of the pseudo-first-order rate constants kobs (sꢀ1
)
versus the concentration of piperidine.
Table 1: Second-order rate constants (k2) for the reactions of the iminium ions 3a–c with the nucleophiles 6a–o (208C, MeCN).
N[a]
sN
k2 (3a)
[mꢀ1 sꢀ1
k2 (3b)
[mꢀ1 sꢀ1
k2 (3c)
[a]
Nucleophile
]
]
[mꢀ1 sꢀ1
]
pyrrole
N-methylpyrrole
6a
6b
6c
6d
6e
6 f
6g
6h
6i
6j
6k
6l
6m
6n
6o
4.63
5.85
6.57
8.01
8.69
10.61
10.67
11.63
12.56
14.11
14.33
15.29
15.49
17.35
18.80
1.00
1.03
0.93
0.96
1.07
0.86
0.91
0.95
0.70
0.71
0.65
0.70
0.69
0.68
0.70
6.8ꢂ10ꢀ4[b]
7.2ꢂ10ꢀ3[b]
–
–
–
–
–
1-(trimethylsiloxy)-pentene
2,5-dimethylpyrrole
1,2,5-trimethylpyrrole
2-(trimethylsiloxy)-5,6-dihydro-4H-pyran
2,4-dimethylpyrrole
kryptopyrrole
2-(trimethylsiloxy)-4,5-dihydrofuran
H2NCH2CH2OH
5.18ꢂ10ꢀ1
1.34ꢂ103[c]
–
4.28ꢂ10ꢀ2
3.6[b]
–
–
–
–
5.3[b]
5.23ꢂ102[d]
3.5ꢂ103[b]
1.3ꢂ104[b]
1.14ꢂ104[b,e]
7.56ꢂ105
2.40ꢂ105
6.81ꢂ105
3.69ꢂ105
1.86ꢂ107
4.95ꢂ108
–
6.87ꢂ104[c]
1.33ꢂ105[c]
1.12ꢂ105
5.27ꢂ107
9.91ꢂ105
7.54ꢂ107
1.96ꢂ107
4.02ꢂ107
5.88ꢂ108
5.20ꢂ103[c]
–
–
P(Ph)3
1.53ꢂ104
–
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
P(nBu)3
piperidine
2.86ꢂ105
1.51ꢂ107
4.86ꢂ108
1,4-diazabicyclo[2.2.2]octane (DABCO)
[a] See reference [11] for the origin of the nucleophilicity parameters N and sN determined in MeCN or CH2Cl2. [b] From reference [4]. [c] These rate
constants were derived in the presence of potassium trifluoroacetate (as base) from plots of 1/kobs versus 1/[base] as described in reference [4]
ꢀ
because the initial C C bond-forming step is reversible. [d] Second-order rate constant k2 for the reaction of 3a-OTf with 6 f in CH2Cl2, from
reference [2d]. [e] In CH2Cl2.
nential decays of the photolytically generated iminium ions
and the identical reactivities of the iminium ions generated in
different ways either imply that only the E isomers are
formed by the photolytic process or that the E and Z isomers
have the same reactivities.
In previous work, we have shown that the reactions of
carbocations and Michael acceptors with s, n, and p nucleo-
philes follow Equation (1), in which electrophiles are de-
scribed by E (electrophilicity parameter) and nucleophiles are
described by N (nucleophilicity parameter) and sN (nucleo-
phile-specific sensitivity parameter).[12]
Figure 2, in which (lgk2)/sN is plotted versus the nucleo-
philicity parameter N, demonstrates not only that the rate
constants obtained with different kinetic methods are con-
sistent, but also that the N and sN parameters of nucleo-
philes,[11,12] which were derived from their reactions with
benzhydrylium ions such as 7, are suitable for predicting the
rates of the reactions of these nucleophiles with the iminium
ions 3. Therefore, the electrophilicity parameters E of 3a–c
were determined by a least-squares fit, that is, by minimiza-
tion of D2 = S[lgk2ꢀsN(N+E)]2, using k2, N, and sN from
Table 1.
Apart from the rate constants for DABCO (6o) which are
close to the diffusion limit, only the second-order rate
constants for the reactions of ethanolamine (6j) were
excluded from these correlations. For unknown reasons, the
observed rate constants for the reactions of 6j with 3a and 3b
are 11 and 44 times larger, respectively, than the values
calculated by Equation (1). As these deviations are still within
the confidence interval of Equation (1), we will not speculate
about their origin.
lg k2 ð20 ꢁCÞ ¼ sNðE þ NÞ
ð1Þ
In this way, we were able to set up comprehensive
electrophilicity and nucleophilicity scales, covering more than
30 orders of magnitude.[13] These scales have found wide
application for the design of polar organic reactions, in
particular in organocatalysis.[14]
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 9953 –9956

reactions. We have also determined the first quantitative data
on the extraordinarily high electrophilicities of iminium ions
derived from MacMillanꢀs second-generation catalyst 1b and
finally demonstrated the applicability of the benzhydrylium-
derived nucleophilicity parameters N and sN for analyzing
scope and limitations of iminium-activated reactions.
Received: May 30, 2011
Published online: September 5, 2011
Keywords: electrophilicity · kinetics ·
.
linear free energy relationships · organocatalysis ·
reactive intermediates
[1] a) P. I. Dalko, L. Moisan, Angew. Chem. 2004, 116, 5248 – 5286;
Angew. Chem. Int. Ed. 2004, 43, 5138 – 5175; b) A. Berkessel, H.
Grçger, Asymmetric Organocatalysis, Wiley-VCH, Weinheim,
2004; c) A. M. Walji, D. W. C. MacMillan, Synlett 2007, 1477 –
1489; d) D. W. C. MacMillan, Nature 2008, 455, 304 – 308; e) A.
Dondoni, A. Massi, Angew. Chem. 2008, 120, 4716 – 4739;
Angew. Chem. Int. Ed. 2008, 47, 4638 – 4660; f) P. Melchiorre,
M. Marigo, A. Carlone, G. Bartoli, Angew. Chem. 2008, 120,
6232 – 6265; Angew. Chem. Int. Ed. 2008, 47, 6138 – 6171; g) S.
Bertelsen, K. A. Jørgensen, Chem. Soc. Rev. 2009, 38, 2178 –
2189; h) M. Nielsen, D. Worgull, T. Zweifel, B. Gschwend, S.
Bertelsen, K. A. Jørgensen, Chem. Commun. 2011, 47, 632 – 649.
[2] a) R. Gordillo, K. N. Houk, J. Am. Chem. Soc. 2006, 128, 3543 –
3553; b) P. Dinꢁr, M. Nielsen, M. Marigo, K. A. Jørgensen,
Angew. Chem. 2007, 119, 2029 – 2033; Angew. Chem. Int. Ed.
2007, 46, 1983 – 1987; c) G. Evans, T. J. K. Gibbs, R. L. Jenkins,
S. J. Coles, M. B. Hursthouse, J. A. Platts, N. C. O. Tomkinson,
Angew. Chem. 2008, 120, 2862 – 2865; Angew. Chem. Int. Ed.
2008, 47, 2820 – 2823; d) S. Lakhdar, T. Tokuyasu, H. Mayr,
Angew. Chem. 2008, 120, 8851 – 8854; Angew. Chem. Int. Ed.
2008, 47, 8723 – 8726; e) S. Lakhdar, A. R. Ofial, H. Mayr, J.
Phys. Org. Chem. 2010, 23, 886 – 892.
Figure 2. Correlation of (lgk2)/sN against the nucleophilicity parameters
N of the nucleophiles 6a–n for their reactions with the iminium ions
3a and 3b and the benzhydrylium ion 7 (correlation for 3c is omitted
for the sake of clarity; it is shown on page S25 of the Supporting
Information).
Table 2: Electrophilicity parameters E of 3a–c.
Electrophile
3a
3b
3c
E[a]
ꢀ7.37
ꢀ5.52
ꢀ7.67
[a] Determined from data in Table 1 by minimization of the term
D2 =ꢀ[lgk2ꢀsN(N+E)]2.
The electrophilicity parameters of the a,b-unsaturated
iminium ions 3a–c in Table 2 show that 3b is about 102 times
more reactive than 3a and 3c, which have quite similar
electrophilicities. This finding is in line with Larsenꢀs obser-
[3] S. Lakhdar, R. Appel, H. Mayr, Angew. Chem. 2009, 121, 5134 –
5137; Angew. Chem. Int. Ed. 2009, 48, 5034 – 5037.
ꢀ
vation that 1b-CF3CO2 is a more active catalyst in Diels–
ꢀ
Alder reactions of cinnamaldehyde than 1a-CF3CO2
,
[4] S. Lakhdar, H. Mayr, Chem. Commun. 2011, 47, 1866 – 1868.
[5] a) J. B. Brazier, G. Evans, T. J. K. Gibbs, S. J. Coles, M. B.
Hursthouse, J. A. Platts, N. C. O. Tomkinson, Org. Lett. 2009,
11, 133 – 136; b) J. B. Brazier, K. M. Jones, J. A. Platts, N. C. O.
Tomkinson, Angew. Chem. 2011, 123, 1651 – 1654; Angew. Chem.
Int. Ed. 2011, 50, 1613 – 1616.
despite the fact that the equilibrium concentration of the
ꢀ
iminium salt 3b-CF3CO2 is only half of that of 3a-
ꢀ
[15,16]
CF3CO2
.
The greater scope of reactions accessible
with MacMillanꢀs second-generation catalyst 1b[17] can now
be unambiguously attributed to the significantly higher
electrophilicity of the iminium ion 3b.
[6] For comprehensive studies on the isolation and X-ray structures
of iminium salts derived from imidazolidinones see: a) D.
What is the origin of the high electrophilicity of 3b?
Seebachꢀs structural studies of iminium ions by X-ray analysis,
NMR spectroscopy, and DFT calculations have shown that
the benzylic phenyl group of 3a resides preferentially over the
heterocyclic ring, while in the case of 3b benzyl is sitting
above the iminium p system and blocking the approach of
nucleophiles from the Re face. An X-ray crystal structure of
3c (page S5 in the Supporting Information) shows that its
conformation resembles that of 3a. While the preferred Si
approach to 3a and 3c is slowed down by the steric shielding
of a methyl group and the cyclopentane ring, respectively, the
reactive Si face of 3b is free from any steric hindrance and,
therefore, exhibits higher electrophilicity.
ˇ
Seebach, U. Groselj, D. M. Badine, W. B. Schweizer, A. K.
ˇ
Beck, Helv. Chim. Acta 2008, 91, 1999 – 2034; b) U. Groselj, D.
Seebach, D. M. Badine, W. B. Schweizer, A. K. Beck, I. Krossing,
P. Klose, Y. Hayashi, T. Uchimaru, Helv. Chim. Acta 2009, 92,
1225 – 1259; c) U. Groselj, W. B. Schweizer, M.-O. Ebert, D.
Seebach, Helv. Chim. Acta 2009, 92, 1 – 13; d) D. Seebach, U.
Groselj, W. B. Schweizer, S. Grimme, C. Mꢂck-Lichtenfeld,
Helv. Chim. Acta 2010, 93, 1 – 16; e) D. Seebach, R. Gilmour, U.
Groselj, G. Deniau, C. Sparr, M.-O. Ebert, A. K. Beck, L. B.
McCusker, D. Sisak, T. Uchimaru, Helv. Chim. Acta 2010, 93,
603 – 634; f) C. Sparr, R. Gilmour, Angew. Chem. 2010, 122,
6670 – 6673; Angew. Chem. Int. Ed. 2010, 49, 6520 – 6523; g) for
the X-ray structure of 3c see: page S5 of the Supporting
Information. CCDC 827498 (3c) contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
ˇ
ˇ
ˇ
ˇ
ˇ
In conclusion, we have shown that the laser-flash-photo-
lytic generation of iminium ions has allowed us to extend our
kinetic investigations over the whole conceivable reactivity
range, from the slowest to diffusion-controlled bimolecular
[7] While the presence of Z isomers could not be detected in the
solutions of 3b,c, a small, nonintegrable peak at d = 8.5 ppm in
Angew. Chem. Int. Ed. 2011, 50, 9953 –9956
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www.angewandte.org
9955

Communications
1
the H NMR spectrum of 3a might be assigned to (Z)-3a. E/Z
[11] Nucleophilicity parameters N and sN a) for 6d, 6e, 6g, and 6h:
T. A. Nigst, M. Westermaier, A. R. Ofial, H. Mayr, Eur. J. Org.
Chem. 2008, 2369 – 2374; b) for 6a – 6c, 6 f, and 6i: H. Mayr, T.
Bug, M. F. Gotta, N. Hering, B. Irrgang, B. Janker, B. Kempf, R.
Loos, A. R. Ofial, G. Remennikov, H. Schimmel, J. Am. Chem.
Soc. 2001, 123, 9500 – 9512; c) for 6j and 6n: T. Kanzian, T. A.
Nigst, A. Maier, S. Pichl, H. Mayr, Eur. J. Org. Chem. 2009,
6379 – 6385; d) for 6k and 6m: B. Kempf, H. Mayr, Chem. Eur. J.
2005, 11, 917 – 927; e) for 6l: M. Baidya, H. Mayr, Chem.
Commun. 2008, 1792 – 1794; f) for 6o: M. Baidya, S. Kobayashi,
F. Brotzel, U. Schmidhammer, E. Riedle, H. Mayr, Angew.
Chem. 2007, 119, 6288 – 6292; Angew. Chem. Int. Ed. 2007, 46,
6176 – 6179.
[12] a) H. Mayr, M. Patz, Angew. Chem. 1994, 106, 990 – 1010;
Angew. Chem. Int. Ed. Engl. 1994, 33, 938 – 957; b) H. Mayr, B.
Kempf, A. R. Ofial, Acc. Chem. Res. 2003, 36, 66 – 77; c) H.
Mayr, A. R. Ofial, Pure Appl. Chem. 2005, 77, 1807 – 1821; d) H.
Mayr, A. R. Ofial, J. Phys. Org. Chem. 2008, 21, 584 – 595.
[13] For a comprehensive database of nucleophilicity and electro-
philicity parameters, see: http://www.cup.lmu.de/oc/mayr/DBin-
tro.html.
[14] For selected examples on the use of Equation (1) in organo-
catalysis see: a) P. G. Cozzi, F. Benfatti and L. Zoli, Angew.
Chem. 2009, 121, 1339 – 1342; Angew. Chem. Int. Ed. 2009, 48,
1313 – 1316; b) F. Benfatti, E. Benedetto, P. G. Cozzi, Chem.
Asian J. 2010, 5, 2047 – 2052; c) M. G. Capdevila, F. Benfatti, L.
Zoli, M. Stenta, P. G. Cozzi, Chem. Eur. J. 2010, 16, 11237 –
11241; d) L. Zhang, L. Cui, X. Li, J. Li, S. Luo, J.-P. Cheng,
Chem. Eur. J. 2010, 16, 2045 – 2049; e) A. R. Brown, W.-H. Kuo,
E. N. Jacobsen, J. Am. Chem. Soc. 2010, 132, 9286 – 9288; f) G.
Bergonzini, S. Vera, P. Melchiorre, Angew. Chem. 2010, 122,
9879 – 9882; Angew. Chem. Int. Ed. 2010, 49, 9685 – 9688.
[15] C. H.-M. Larsen, PhD Thesis, California Institute of Technology,
2005, pp. 41 – 42. We thank Professor D. W. C. MacMillan for
bringing this work to our attention.
isomerization of 3b as observed by Seebach et al. (Ref. [6e])
during the formation of 3b from 1b and 2 was not detected under
our conditions.
[8] For NMR characterization of chiral enamines see: a) T. J.
Peelen, Y. Chi, S. H. Gellman, J. Am. Chem. Soc. 2005, 127,
11598 – 11599; b) M. B. Schmid, K. Zeitler, R. M. Gschwind,
Angew. Chem. 2010, 122, 5117 – 5123; Angew. Chem. Int. Ed.
2010, 49, 4997 – 5003; c) M. B. Schmid, K. Zeitler, R. M.
Gschwind, J. Org. Chem. 2011, 76, 3005 – 3015; d) M. B.
Schmid, K. Zeitler, R. M. Gschwind, J. Am. Chem. Soc. 2011,
133, 7065 – 7074; e) K. Patora-Komisarska, M. Benohoud, H.
Ishikawa, D. Seebach, Y. Hayashi, Helv. Chim. Acta 2011, 94,
719 – 745.
[9] a) K. A. Ahrendt, C. J. Borths, D. W. C. MacMillan, J. Am.
Chem. Soc. 2000, 122, 4243 – 4244; b) N. A. Paras, D. W. C.
MacMillan, J. Am. Chem. Soc. 2001, 123, 4370 – 4371; c) A. B.
Northrup, D. W. C. MacMillan, J. Am. Chem. Soc. 2002, 124,
2458 – 2460; d) J. F. Austin, D. W. C. MacMillan, J. Am. Chem.
Soc. 2002, 124, 1172 – 1173; e) N. A. Paras, D. W. C. MacMillan,
J. Am. Chem. Soc. 2002, 124, 7894 – 7895; f) S. P. Brown, N. C.
Goodwin, D. W. C. MacMillan, J. Am. Chem. Soc. 2003, 125,
1192 – 1194; g) S. G. Ouellet, J. B. Tuttle, D. W. C. MacMillan, J.
Am. Chem. Soc. 2005, 127, 32 – 33; h) Y. K. Chen, M. Yoshida,
D. W. C. MacMillan, J. Am. Chem. Soc. 2006, 128, 9328 – 9329;
i) J. B. Tuttle, S. G. Ouellet, D. W. C. MacMillan, J. Am. Chem.
Soc. 2006, 128, 12662 – 12663; j) G. Lelais, D. W. C. MacMillan,
Aldrichimica Acta 2006, 39, 79 – 87.
[10] For the photoheterolysis of quaternary phosphonium salts see:
a) E. O. Alonso, L. Johnston, J. C. Scaiano, V. G. Toscano, J. Am.
Chem. Soc. 1990, 112, 1270 – 1271; b) E. O. Alonso, L. J.
Johnston, J. C. Scaiano, V. G. Toscano, Can. J. Chem. 1992, 70,
1784 – 1794; c) C. Imrie, T. A. Modro, E. R. Rohwer, C. C. P.
Wagener, J. Org. Chem. 1993, 58, 5643 – 5649; d) C. Imrie, T. A.
Modro, C. C. P. Wagener, J. Chem. Soc. Perkin Trans. 2 1994,
1379 – 1382; e) L. Shi, M. Horn, S. Kobayashi, H. Mayr, Chem.
Eur. J. 2009, 15, 8533 – 8541; f) J. Ammer, H. Mayr, Macro-
molecules 2010, 43, 1719 – 1723; For the generation of benzhy-
dryl cations from tri-n-butylphosphonium tetrafluoroborates
see: g) T. Kanzian, S. Lakhdar, H. Mayr, Angew. Chem. 2010,
122, 9717 – 9720; Angew. Chem. Int. Ed. 2010, 49, 9526 – 9529;
h) N. Streidl, R. Branzan, H. Mayr, Eur. J. Org. Chem. 2010,
4205 – 4210; i) M. Baidya, S. Kobayashi, H. Mayr, J. Am. Chem.
Soc. 2010, 132, 4796 – 4805.
[16] A quantitative comparison of the overall rates of imidazolidi-
none and diarylprolinol ether catalyzed reactions has recently
been published: J. B. Brazier, G. P. Hopkins, M. Jirari, S. Mutter,
R. Pommereuil, L. Samulis, J. A. Platts, N. C. O. Tomkinson,
Tetrahedron Lett. 2011, 52, 2783 – 2785.
[17] (2S,5S)-5-Benzyl-2-tert-butyl-3-methyl-imidazolidin-4-one (1b)
was first been described in: R. Naef, D. Seebach, Helv. Chim.
Acta 1985, 68, 135 – 143.
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Angew. Chem. Int. Ed. 2011, 50, 9953 –9956