Fe-catalyzed multicomponent reactions: The regioselective alkoxy allylation of activated olefins and its application in sequential Fe catalysis

Article abstract of DOI:10.1002/chem.201103009

We present herein a versatile and broadly applicable Fe-catalyzed regioselective alkoxy allylation of activated double bonds. Substituted allylic carbonates are converted into the corresponding σ-enyl Fe complexes by reaction with Bu₄N[Fe(CO)

Full text of DOI:10.1002/chem.201103009

FULL PAPER
DOI: 10.1002/chem.201103009
Fe-Catalyzed Multicomponent Reactions: The Regioselective Alkoxy
ACHTUNGTRENNUNG llylation of Activated Olefins and its Application in Sequential Fe Catalysis
Andrꢀ P. Dieskau, Michael S. Holzwarth, and Bernd Plietker*[a]
Abstract: We present herein a versatile
with the generation of a C-nucleophile,
which is trapped by the s-enyl Fe com-
plex in a regioselective manner. Alter-
natively, the alkoxide acts as a base in
deprotonating an external pronucleo-
phile, which undergoes Michael addi-
tion. The method is characterized by a
broad functional group tolerance, mild
reaction conditions, low catalyst load-
ings, and high regioselectivities in favor
of the ipso-substitution product.
and broadly applicable Fe-catalyzed re-
gioselective alkoxy allylation of activat-
ed double bonds. Substituted allylic
carbonates are converted into the cor-
responding s-enyl Fe complexes by re-
Keywords: allylation
· catalysis ·
action
with
Bu4N[Fe(CO)3(NO)]
iron · Michael addition · multicom-
ponent reactions
(TBAFe) at 308C. The liberated alkox-
ide adds to an activated double bond
Introduction
terest in allylic substitutions catalyzed by the electron-rich
ferrate Bu4N[Fe(CO)3(NO)] (TBAFe),[7a–f,10] we were won-
dering whether this low-valent complex might be able to
catalyze the regioselective alkoxy allylation of activated ole-
fins (Scheme 1). This transformation was originally reported
by Yamamoto[11] and has been developed into an interesting
reaction manifold that allows the formation of two different
bonds in a one-pot fashion.[12] Very recently, Stoltzꢀs group
developed an enantioselective intercepted allylic substitu-
Despite enormous progress in the field of organic synthesis,
the generation of molecular building blocks with defined
and dense structural characteristics still remains one of the
most important and noble challenges in organic chemistry.
In particular, the preparation of two adjacent quaternary
carbon centers is problematic due to unfavorable steric in-
teractions not only within the product itself but also within
the transition state.[1] Allylic substitution, in which a carban-
ion reacts with the sp2-hybridized and planar g-carbon atom
of an activated allylating agent (e.g., allyl halide, acetate,
allyl–metal complex) in an SN2’-fashion, circumvents the
problem of interfering substituents at the two reacting
carbon atoms. In particular, metal-catalyzed allylic substitu-
tion has evolved as a versatile tool for the construction of
new bonds in a chemo- and stereoselective way.[2] Despite
the progress made, control over the regioselectivity remains
a major issue in this powerful field of metal catalysis. Over
the past years, different metals have proven to offer solu-
tions for this long-standing problem. Whereas Mo,[3] Ru,[4]
and Ir complexes[5] favor the formation of branched prod-
ucts, irrespective of the structure of the starting material,
Rh[6] and Fe complexes[7] were shown to favor ipso-substitu-
tion along with conservation of structural information.
In view of the tremendous resurgence in the use of Fe cat-
alysis[8,9] in organic synthesis and as part of our ongoing in-
[a] Dipl.-Chem. Dipl.-Ing. A. P. Dieskau, Dipl.-Chem. M. S. Holzwarth,
Prof. Dr. B. Plietker
Institut fꢁr Organische Chemie
Universitꢂt Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Fax : (+49)711-685 64285
E-mail: bernd.plietker@oc.uni-stuttgart.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103009.
Scheme 1. Proposed reaction pathway for the regioselective Fe-catalyzed
alkoxy allylation.
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2423

tion by combining their allyl vinyl carbonate palladium cat-
alysis with highly activated olefins.[13] However, these reac-
tions do not allow for strict control of the regioselective
course within the allylation step.
With regard to the high levels of regiocontrol that we ob-
served in Fe-catalyzed allylic substitutions, this transforma-
tion appeared particularly suitable for confirming the cata-
lytic potential of TBAFe, since 1) two new bonds are
formed in a one-pot fashion starting from an allylic carbon-
ate and a Michael acceptor, leading to CO2 as the sole by-
product, 2) sterically hindered allylic carbonates would pro-
vide access to densely substituted small molecules, and
3) the basic leaving group should not act as a base to gener-
ate a nucleophile by deprotonation, but rather has to gener-
ate the free nucleophile upon Michael addition.
In particular, the latter two aspects constitute a challenge,
since 1) the generation of two adjacent quaternary carbon
centers by transition-metal-catalyzed allylic substitution is
difficult to achieve if the s–p–s equilibrium is faster than
the nucleophilic addition, and 2) free alkoxide bases are
known to promote decarbonylation of TBAFe through a
Hieber base reaction[14] (Scheme 1).
From a mechanistic point of view, the reaction represents
an interesting extension of the allylic substitution mecha-
nism. By adding a reactive Michael acceptor such as vii, the
liberated alkoxide vi can act in two different ways. Either it
reacts as a nucleophile with the allyl–Fe complex iv or
regio-iv, or it adds to the olefin vii with concomitant genera-
tion of a carbanionic species viii, which in turn reacts as a
nucleophile with the allyl–Fe complex. Hence, from this
transformation, various important indications can be de-
duced. Apart from the relative reactivities of an oxo-anion
versus a carbanion as a nucleophile, the timescale of the Mi-
chael addition relative to the s–p–s equilibrium of the two
allyl–Fe complexes iv and regio-iv could provide interesting
insights into the relative rates of the individual steps in-
volved in the Fe-catalyzed allylic substitution.
Scheme 2. Alkoxy allylation of different Michael acceptors. (The reac-
tions were performed on a 0.5 mmol scale using benzylidene malodini-
trile (1 equiv), carbonate 2 (1.2 equiv), TBAFe (5 mol%), and ligand
(7.5 mol%) in THF at 808C. The yields were determined by H NMR in-
tegration of the crude reaction mixture using mesitylene as an internal
standard).
1
(Scheme 2); however, the best regioselectivity and conver-
sion were obtained using benzimidazole-derived ligand L10.
A subsequent temperature screening revealed that the
catalytic system operated with the same efficiency at tem-
peratures down to 308C. Moreover, lowering the reaction
temperature proved beneficial for the regioselectivity. Final-
ly, the Fe-catalyzed alkoxy allylation of Michael acceptor 1
with carbonate 2 was conducted under neutral conditions to
give the corresponding allylation products 3A and 3B in
high yield and with good regioselectivity in favor of the
ipso-substitution product (Scheme 3).
Results and Discussion
Both the basicity and nucleophilicity of a reagent are func-
tions of the solvent polarity. Since we wanted the free alkox-
ide to act as a nucleophile rather than as a base, a detailed
solvent screening served as a starting point for the develop-
ment of the Fe-catalyzed alkoxy allylation. We initially
chose conditions that were recently established in our labo-
ratories as optimal for allylic substitutions. We were pleased
to observe some conversion in different solvents, giving the
desired product as a mixture of two regioisomers. THF
proved to be the solvent of choice.[15]
Substrate scope: Having established the optimal conditions,
we proceeded to study the scope of this transformation
These initial investigations on solvent effects were fol-
lowed by an extensive ligand screening (Scheme 2). Various
N-heterocyclic carbene ligands were tested and were found
to facilitate successful alkoxy allylation in varying yields
with moderate to good regioselectivities. Amongst the li-
gands tested, L1, L7, L9, and L10 gave good conversions
Scheme 3. The regioselective Fe-catalyzed alkoxy allylation of activated
olefins; r.s.=regioselectivity.
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Fe-Catalyzed Alkoxy Allylation of Activated Olefins
FULL PAPER
using substituted benzylidene malodinitriles (Scheme 4).
Different para-substituted acceptors reacted to give the cor-
responding products in good to excellent yields with high re-
gioselectivities. A variety of functional groups, such as ester,
conditions, sensitive functional groups, such as the THP-
ether in 23, were tolerated (Table 1, entry 9). Subjecting the
bis-methoxycarbonyl-protected diol 17 to the reaction condi-
tions afforded exclusively the mono-alkoxy allylated product
26A in good yield (Table 1, entry 3). Apparently, the second
alkoxy allylation is significantly slower due to the decreased
reactivity of the methyl carbonate functionality in 26A.
Remarkably, the non-conjugated chiral enyne 16 was con-
verted into the corresponding 1:1 diastereomeric mixture of
enyne 25A without an intermediate shift of the double bond
into conjugation (Table 1, entry 2). This result reflects the
characteristics of the Fe-catalyzed alkoxy allylation; the ste-
reoselective course of the Michael addition is not influenced
by the stereochemical information provided in the allyl–Fe
complex and the stereochemical integrity of the allyl–Fe
complex remains unchanged. Employing aryl-substituted al-
lylic carbonates, such as 19, however, leads to a significant
shift of the regioisomeric ratio, favoring the formation of
the thermodynamically preferred cross-conjugated product
28B (Table 1, entry 5).
The reaction of a chiral secondary carbonate such as 16
and a Michael acceptor with two prochiral carbon atoms, as
found in a-cyanobenzopyrone 32, potentially generates
three contiguous stereocenters in a single step. If a dynamic
p-allyl mechanism is operative, the diastereoselective out-
come of the reaction might be influenced by the stereocen-
ter formed in the Michael addition. Subjecting carbonate 16
to this transformation resulted in the regioselective forma-
tion of the ipso-substitution product 33A, albeit as a 1:1
mixture of diastereoisomers (Scheme 5). Apparently, the
Scheme 4. Alkoxy allylation of functionalized Michael acceptors. The re-
actions were performed on a 0.5 mmol scale using arylidene malodinitrile
(1 equiv), carbonate
2
(1.2 equiv), TBAFe (5 mol%), and L10
(7.5 mol%) in THF (1m) at 308C. [a] Reaction run at 608C. [b] Allyl
methyl carbonate was used instead of 2.
Scheme 5. Diastereoselective course of the alkoxy allylation of prochiral
benzopyrone 32 with carbonate 16.
halide, nitro, and ether substituents, were tolerated and the
regioselective course of the reaction seemed to be independ-
ent of the electronic properties of the substituents
(Scheme 4, 4 vs. 8). However, a drop in regioselectivity to
75:25 was observed for an ortho-substituted acceptor, giving
rise to 5 in 94% yield. Notably, even an indolydene malodi-
nitrile could be transformed into 13 in 59% yield. The com-
bination of a cyanobenzopyrone and allyl methyl carbonate
yielded the alkoxy allylation product 14A in 79% yield as a
single diastereoisomer. This stereochemical outcome has al-
ready been observed by Yamamoto and co-workers.[12f]
Subsequently, we set out to investigate the scope of differ-
ent allyl carbonates amenable to the alkoxy allylation under
our optimized reaction conditions (Table 1). The alkoxy ally-
lation proved to be broadly applicable to a variety of differ-
ent carbonates, yielding the desired products in high yields
and with good regioselectivities. Due to the mild reaction
stereocenters that were introduced by the Michael addition
of the alkoxide do not influence the stereoselective course
of the nucleophilic substitution at the allyl–Fe complex.
The selective mono-functionalization of biscarbonate 17
sets the stage for sequential Fe-catalyzed allylic substitu-
tions. Based upon our recent studies on allylic sulfenyla-
tions,[16] in which we were able to show that TBAFe, in the
presence of an excess of a thiol, is converted into electron-
rich binuclear iron–sulfur clusters of the type
[Bu4N]2[(NO)2Fe(SR)]2, we were wondering whether the re-
spective processes could be combined (Scheme 6).
Indeed, the addition of benzyl mercaptan to a reaction
mixture containing 26A resulted in clean formation of the
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B. Plietker et al.
Table 1. Variation of the allyl donor.
desired product 34 as a single regioisomer in good
overall yield.
Three-component coupling: In the transformations
presented so far, both residues of the starting car-
bonate, the allyl moiety and the alkoxy moiety, are
incorporated into the final product, generating
CO2 as the only byproduct. However, this protocol
is limited to the use of substituted allylic carbo-
nates with a simple alkoxy substituent. Transfer of
a more complex alkoxy group necessitates a tedi-
ous synthesis of the respective mixed allylic car-
bonate using phosgene. Hence, we were wondering
whether we could extend the existing protocol into
a three-component coupling reaction by perform-
ing the reaction in the presence of an external al-
cohol R3OH as a second nucleophilic species
(Scheme 7).
At first sight, this variation looks like a simple
extension; however, with regard to the mechanistic
manifold, the situation becomes significantly more
complex. As pointed out above, the generation of
a second nucleophile viii by a Michael addition
competes with the direct addition of the alkoxide
vi. Adding an external alcohol R3OH as a pronu-
cleophile results in a situation in which the primar-
ily formed alkoxide vi can act either as a nucleo-
phile, adding to the allyl–Fe complex xii or to the
olefin vii, or it can act as a base, being protonated
by R3OH. Similarly to vi, the resulting new alkox-
ide xiii can operate in different mechanistic mani-
folds. From the experiments described above, cata-
lyst deactivation and the direct allylation of alkox-
ide vi or xiii appeared to be unfavorable. To slow
down the Michael addition of alkoxide vi, Boc-pro-
tected allylic alcohols were chosen as starting ma-
terials. We anticipated that the higher basicity of
tert-butoxide, coupled with its slower Michael addi-
tion due to steric hindrance, would resolve some of
the chemoselectivity issues discussed above.
Entry[a]
Allyl donor
Product
A/B
Yield
[%][b]
1[c]
2[c]
3
15
16
17
24A
25A
26A
81:19
86
71
69
83:17
>99:<1
4
18
19
20
21
22
27A
28A
28B
29A
30A
–
63
84
72
76
53
5[c]
6[d]
7
45:55
<1:>99
>99:<1
–
8
To our delight, the combination of a tert-butyl
allyl carbonate, a Michael acceptor, and two equiv-
alents of an external alcohol led to clean formation
of the desired alkoxy allylated products in good to
excellent yields with good to very good regioselec-
tivities (Scheme 8).
In particular, p-activated alcohols, such as
benzyl, allyl, dienyl, propargyl, and even homopro-
pargyl alcohol, were successfully added (36–41).
Side reactions with other allylic activated function-
9
23
31A
–
50
[a] The reactions were performed on a 0.5 mmol scale using benzylidene malodinitrile
(1 equiv), carbonate (1.2 equiv), TBAFe (5 mol%), and L10 (7.5 mol%) in THF at
308C. [b] Isolated yield. [c] The diastereomeric ratio of the branched product was
ꢀ1:1. [d] Reaction run at 608C.
al groups proved to be slow on the timescale of the reaction.
Hence, using N-allyloxycarbonylethanolamine led to clean
formation of 43 in 47% yield without corruption of the allyl-
carbamate moiety. Notably, even the sterically crowded sec-
ondary alcohol (À)-menthol was converted into the ipso
product 42A in excellent yield. The diastereoselective
course was clearly influenced by the chiral nucleophile. This
Scheme 6. Sequential iron catalysis.
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Chem. Eur. J. 2012, 18, 2423 – 2429

Fe-Catalyzed Alkoxy Allylation of Activated Olefins
FULL PAPER
stereoselectivity paves the way for the future development
of diastereo- and enantioselective Fe-catalyzed alkoxy ally-
lations.
Conclusion
We have reported a regioselective Fe-catalyzed alkoxy ally-
lation of activated double bonds, which delivers sterically
congested small molecules in high yields. This transforma-
tion, in which two different bonds are formed in a decarbox-
ylative two-component reaction with release of CO2 as the
sole byproduct, is characterized by broad functional group
tolerance and mild, neutral reaction conditions. Further de-
velopment led to a three-component reaction, of which CO2
and tert-butanol are the sole byproducts. Besides the broad
substrate scope, the first example of a sequential Fe catalysis
consisting of regioselective alkoxy allylation and allylic sul-
fenylation has been reported. This transformation particular-
ly underlines the catalytic potential of TBAFe in the synthe-
sis of complex molecules. Future work will be directed to-
wards the development of novel Fe-catalyzed tandem reac-
tions and a stereoselective version of this useful transforma-
tion.
Scheme 7. Fe-catalyzed three-component coupling: mechanism and cata-
lytic challenges.
Experimental Section
General remarks: All reactions and manipulations sensitive to air or
moisture were performed under dry nitrogen using standard Schlenk
techniques. All solvents were purified prior to use. All chemicals were
purchased from Acros Organics, Sigma-Aldrich, or Alfa Aesar. Allylic
carbonates[7a] and N-heterocyclic carbene ligands[17] were synthesized ac-
cording to established literature procedures. The catalyst TBAFe was
prepared from commercial iron pentacarbonyl according to a previously
published procedure.[7b] NMR spectra were recorded on a Bruker AV 300
spectrometer at 300 MHz (1H NMR), 75 MHz (13C NMR), a Bruker AV
500 spectrometer at 500 MHz (1H NMR), 125.6 MHz (13C NMR), or a
Bruker AV 250 spectrometer at 250 MHz (1H NMR), 62.5 MHz
(13C NMR). Chemical shifts are reported in ppm downfield from tetrame-
thylsilane, which was used as an internal standard. IR spectra were mea-
sured on a Bruker Vector 22 FTIR spectrometer in attenuated total re-
flectance (ATR) mode. Mass spectra were measured in electrospray ioni-
zation mode on a Bruker Micro-TOF-Q.
Representative procedure for the TBAFe-catalyzed alkoxy allylation of
1: A 10 mL Schlenk tube was charged with ligand L10 (0.0375 mmol,
13.4 mg) and THF (500 mL). Potassium tert-pentoxide (1.7m in toluene,
0.0413 mmol, 24.3 mL) was added and the reaction mixture was stirred
for 15 min at room temperature. Thereafter, Bu4N[Fe(CO)3(NO)]
(0.025 mmol, 10.3 mg) was added to the pale-yellow solution and allowed
to coordinate for 1 h at 808C. After cooling to room temperature, the
corresponding carbonate
2 (0.6 mmol) and the Michael acceptor 1
(0.5 mmol) were added. The reaction mixture was stirred at 308C until
TLC showed full conversion. After purification by column chromatogra-
phy on silica gel (petroleum ether/ethyl acetate, 10:1), the product was
obtained as a mixture of regioisomers 3A/3B (91:9) in 94% (119 mg)
combined yield. The regioisomers were separated by means of semi-prep-
arative HPLC. 3A: Colorless oil; Rf =0.53 (petroleum ether/ethyl ace-
tate, 8:1); 1H NMR (300 MHz, CDCl3): d=7.52–7.47 (m, 2H), 7.46–7.41
(m, 3H), 6.06 (dd, J=17.2, 10.8 Hz, 1H), 5.34–5.26 (m, 2H), 4.39 (s, 1H),
3.20 (s, 3H), 1.46 (s, 3H), 1.39 ppm (s, 3H); 13C NMR (126 MHz,
CDCl3): d=140.6, 135.9, 133.3, 130.1, 128.9, 128.1, 114.5, 114.4, 114.0,
83.4, 57.8, 45.1, 33.3, 26.0, 18.3 ppm; IR (film): n˜ =1456 (w), 1213 (w),
Scheme 8. Regioselective Fe-catalyzed three-component coupling with an
external alcohol source. The reactions were performed on a 0.5 mmol
scale using Michael acceptor (1 equiv), carbonate (1.2 equiv), alcohol
(2 equiv), TBAFe (5 mol%), and L10 (7.5 mol%) in THF at 308C.
[a] Cinnamyl Boc-carbonate was used instead of 35.
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B. Plietker et al.
1156 (m), 1107 (vs), 933 (m), 729 (s), 699 (s), 628 cmÀ1 (w); GC/MS
(ESI): m/z (%)=277 (22) [M+Na+], 209 (44), 149 (42), 121 (100);
HRMS (ESI): m/z: calcd for C16H18N2O+Na+: 277.1311; found:
277.1314.
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Representative procedure for the three-component coupling (41A): A
10 mL Schlenk tube was charged with ligand L10 (0.0375 mmol, 13.4 mg)
and THF (500 mL). Potassium tert-pentoxide (1.7m in toluene,
0.0413 mmol, 24.3 mL) was added and the reaction mixture was stirred
for 15 min at room temperature. Thereafter, Bu4N[Fe(CO)3(NO)]
(0.025 mmol, 10.3 mg) was added to the pale-yellow solution and allowed
to coordinate for 1 h at 808C. After cooling to room temperature, the
Boc-carbonate (0.6 mmol), 2-benzylidene-malononitrile
1 (0.5 mmol,
77.1 mg), and the corresponding external alcohol (1.0 mmol) were added.
The reaction mixture was stirred at 308C until TLC showed full conver-
sion. After purification by column chromatography on silica gel (petrole-
um ether/ethyl acetate, 20:1), the product was obtained as a mixture of
regioisomers 41A/41B (90:10) in 95% (168 mg) combined yield. The re-
gioisomers were separated by means of semi-preparative HPLC. 41A:
Colorless oil; Rf =0.19 (petroleum ether/ethyl acetate, 20:1); 1H NMR
(300 MHz, CDCl3): d=7.58–7.53 (m, 2H), 7.48–7.32 (m, 8H), 6.12 (dd,
J=17.3, 10.7 Hz, 1H), 5.35–5.28 (m, 2H), 5.04 (s, 1H), 4.50 (d, J=
16.2 Hz, 1H), 4.03 (d, J=16.2 Hz, 1H), 1.54 (s, 3H), 1.46 ppm (s, 3H);
13C NMR (75 MHz, CDCl3): d=139.8, 134.4, 131.7, 130.3, 128.9, 128.9,
128.5, 128.4, 122.0, 117.3, 113.5, 113.0, 88.2, 83.0, 78.2, 56.4, 54.0, 44.6,
25.5, 23.6 ppm; IR (film): n˜ =2978 (w), 1491 (m), 1456 (w), 1443 (w),
1351 (w), 1278 (w), 1202 (w), 1157 (m), 1087 (s), 1070 (vs), 1028 (m), 999
(m), 934 cmÀ1 (m); MS (ESI): m/z (%): 377 (100) [M+Na+], 309 (2), 221
(1), 193 (4), 115 (4); HRMS (ESI): m/z: calcd for C24H22N2O+Na+:
377.1624; found: 377.1617.
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TBAFe-catalyzed alkoxy allylation and allylic sulfenylation of biscarbon-
ate 17:
A 10 mL Schlenk tube was charged with ligand L10
(0.0235 mmol, 8.4 mg) and THF (400 mL). Potassium tert-pentoxide (1.7m
in toluene, 0.0255 mmol, 15 mL) was added and the reaction mixture was
stirred for 15 min at room temperature. Thereafter, Bu4N[Fe(CO)3(NO)]
(0.015 mmol, 6.2 mg) was added to the pale-yellow solution and allowed
to coordinate for 1 h at 808C. After cooling to room temperature, (E)-
but-2-ene-1,4-diyl dimethyl dicarbonate (0.4 mmol, 81.8 mg) and 2-ben-
zylidene-malononitrile 1 (0.3 mmol, 46.3 mg) were added. The reaction
mixture was stirred at 308C for 16 h (whereupon TLC showed full con-
version). Benzyl mercaptan (0.75 mmol, 88 mL) was subsequently added
and the reaction mixture was stirred at 808C for 5 h, whereupon TLC
showed full conversion. The product 34 was obtained as a yellow oil after
column chromatography on silica (petroleum ether/ethyl acetate, 10:1) in
71% (77 mg) yield. Rf =0.47 (petroleum ether/ethyl acetate, 5:1);
1H NMR (300 MHz, CDCl3): d=7.47–7.41 (m, 5H), 7.34–7.26 (m, 5H),
5.88 (dd, J=9.3, 1.4 Hz, 1H), 5.66–5.62 (m, 1H), 4.33 (s, 1H), 3.69 (s,
2H), 3.31 (s, 3H), 3.06 (dd, J=7.8, 1.0 Hz, 2H), 2.55 ppm (d, J=7.6 Hz,
2H); 13C NMR (75 MHz, CDCl3): d=137.8, 133.2, 133.0, 130.3, 128.9,
128.9, 128.6, 128.0, 127.1, 122.3, 113.9, 113.6, 83.5, 57.8, 44.7, 35.7, 32.1,
27.6 ppm; IR (film): n˜ =2937 (w), 1493 (w), 1454 (w), 1209 (w), 1105 (m),
1072 (w), 971 (w), 729 (m), 699 cmÀ1 (s); MS (EI, 70 eV): m/z (%): 362
(0.1) [M+], 329 (0.1), 271 (0.1), 239 (0.2), 121 (100), 105 (4), 91 (54), 77
(17), 65 (7); HRMS (ESI): m/z: calcd for C22H22N2OS+H+: 363.1526;
found: 363.1523.
Acknowledgements
Financial support by the Studienstiftung des Deutschen Volkes (Ph.D.
grant for A.P.D.) and the Deutsche Forschungsgemeinschaft is gratefully
acknowledged.
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Received: September 26, 2011
Published online: January 16, 2012
Chem. Eur. J. 2012, 18, 2423 – 2429
ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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