Palladium pincer complex-catalyzed condensation of sulfonimines and isocyanoacetate to imidazoline derivatives. Dependence of the stereoselectivity on the ligand effects

Article abstract of DOI:10.1002/adsc.200700242

Palladium-catalyzed condensation reactions of sulfonimines with isocyanoacetate were performed using various PCP, SCS, SeCSe and NCN pincer complexes as catalysts. The reactions proceeded rapidly (2 h) at room temperature using only 1 mol % pincer complex

Full text of DOI:10.1002/adsc.200700242

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DOI: 10.1002/adsc.200700242
Palladium Pincer Complex-Catalyzed Condensation of
Sulfonimines and Isocyanoacetate to Imidazoline Derivatives.
Dependence of the Stereoselectivity on the Ligand Effects
Juhanes Aydin,a K. Senthil Kumar,a Lars Eriksson,b and Kµlmµn J. Szabóa,*
a
Department of Organic Chemistry, Stockholm University, Arrhenius Laboratory; SE-106 91 Stockholm, Sweden
Fax : (+46)-8-15-4908; e-mail: kalman@organ.su.se
Department of Structural Chemistry, Stockholm University, Arrhenius Laboratory; SE-106 91 Stockholm, Sweden
b
Received: May 14, 2007; Revised: August 7, 2007; Published online: November 21, 2007
This paper is dedicated to Professor Jan-E. Bäckvall on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Palladium-catalyzed condensation reac- PCP complex-catalyzed process does not depend sig-
tions of sulfonimines with isocyanoacetate were per- nificantly on the steric bulk of the sulfonimine com-
formed using various PCP, SCS, SeCSe and NCN ponent. Mechanistic studies revealed that the PCP
pincer complexes as catalysts. The reactions proceed- complex-catalyzed reaction proceeds via an h1-coor-
ed rapidly (2 h) at room temperature using only 1 dinated palladium isocyanoacetate pincer intermedi-
mol% pincer complex catalyst without any additives. ate. This intermediate could also be isolated and its
The electron-deficient and relatively bulky PCP com- structure was determined by X-ray diffraction. The
plex provides imidazoline derivatives with a very X-ray structure of this reaction intermediate indi-
high syn diastereoselectivity. The applied PCP cata- cates a surprisingly strong carbon-metal bond be-
lyst proved to be very robust under the applied reac- tween the palladium atom and the coordinated iso-
tion conditions, as it could be recovered without any cyanoacetate molecule. Our mechanistic studies
decomposition after the completed catalytic process. show that the pincer complex catalyst does not un-
The stereoselectivity of the condensation reactions is dergo redox reactions and, thus it retains a +2 oxi-
reversed by employing the electron-rich SeCSe type dation state under the catalytic process.
of complexes. Simple palladium salts, such as palladi-
um acetate, Pd
A
poor stereoselectivity. The stereoselectivity of the palladium; pincer complexes; stereoselectivity
Introduction
alysts because of a tight terdentate coordination of
the ligand to the metal center. One of the main appli-
Professor Jan-E. Bäckavall has exceptionally forward- cation areas of palladium pincer complex catalysis is
ed Swedish and world-wide research in organic the aldol reaction of aldehydes with isocyanides af-
chemistry and catalysis inspiring generations of young fording oxazoline derivatives,[20,22,23,35,38–42] which can
colleagues. Jan is a great friend and colleague and be hydrolyzed to amino acids. As far as we know, ana-
with this paper, we would like to congratulate him to logue pincer complex-catalyzed reactions employing
the 60th birthday as well as show our appreciation of imine substrates in place of aldehydes have never
his great achievements in metal and bio-catalysis, in been reported. Notwithstanding, this reaction is par-
particular in palladium chemistry.[1–12]
ticularly interesting, since condensation of imines with
Catalytic applications of palladium pincer com- isocyanides leads to 2-imidazolines, which can easily
plexes[13–19] (e.g., 1a–g) have attracted considerable at- be converted to a,b-diamino acids, which represent[43]
tention in the last couple of years,[20–39] since employ- a class of biologically important compounds. Al-
ment of pincer complex catalysts in place of the com- though, this important condensation reaction could be
monly used palladium salts often opens new and performed using various sulfonimines (2) and isocya-
highly selective synthetic routes. Furthermore, palladi- noacetate (3) in the presence of gold,[44–46] rutheni-
um pincer complexes are highly stable and robust cat- um[47] and copper[48] catalysts, reports on palladium-
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catalyzed reactions are very scarce.[44,47] This can be at room temperature in the presence of catalytic
explained by the fact that the palladium-catalyzed amounts (1 mol%) of pincer complexes 1a–g afford-
coupling of imines with isocyanides was characterized ing 2-imidazoline derivatives 4a–g with nearly quanti-
as a slow and unselective process.[44,47]
tive yields [Eq. (1)]. In contrast to the palladium
We have now found that palladium pincer com- pincer complex-catalyzed condensation of aldehydes
plexes show a relatively high catalytic activity in the with isocyanoacetate 3, the reactions with imine sub-
coupling of sulfonimines with isocyanoacetate; and strates 2a–g could be achieved without base catalysis.
that the stereoselectivity of the process is highly de- The condensation reactions of 2a with 3 in the pres-
pendent on the electronic properties of the applied ence of catalytic amounts of 1a and b are completed
pincer complex catalysts (Scheme 1). Therefore, we within two hours indicating that the catalytic activity
have studied the synthetic scope and selectivity of this of 1a and b in this process is much higher than in the
reaction employing various sulfonimines (2a–g) and corresponding allylation reaction of sulfonimines
isocyanoacetate 3 in the presence of catalytic amounts (such as 2a), which requires at least 16 h to be com-
of pincer complexes 1a–g and a commonly employed pleted.[33,34]
palladium salt Pd
A
The 2-imidazoline products [Eq. (1), Table 1] are
products 4a–g [Eq. (1)]. In addition to the synthetic relatively stable in the absence of water, however
studies we have also studied the mechanistic aspects they displayed some tendency for decomposition
of the condensation reaction.
under silica gel or neutral alumina chromatography.
In fact the corresponding diamino acid derivatives 5a
and 5b could be easily obtained by stirring 2-imidazo-
line derivatives 4a and 4b in the presence of water
and neutral aluminium oxide [Eq. (2)]. As silica and
alumina chromatography leads to a partial ring open-
ing [Eq. (2)], the final products were purified with
quick flash chromatography on celite. We did not ob-
serve a condensation reaction between sulfonimines
and 3 in the absence of a palladium catalyst.
Results and Discussion
Synthetic Scope and Selectivity
Probably, the most interesting feature of the cou-
pling reaction is the stereoselectivity for the forma-
The condensation reactions between various imines tion of the imidazoline derivatives. The condensation
2a–g and isocyanoacetate 3 could easily be performed reaction of 2a and 3 proceeds with a high syn selectiv-
Scheme 1. Palladium pincer complexes 1a–g employed in this study.
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Condensation of Sulfonimines and Isocyanoacetate to Imidazoline Derivatives
Table 1. Condensation of sulfonimines with isocyanoacetate
in the presence of various palladium pincer complex cata-
lysts.[a]
ity (syn/anti ratio 10:1) in the presence of the trifluor-
oacetate salt of PCP complex 1a (entry 1). Change of
the counterion on palladium to chloride (1b) leads to
a slight decrease of the stereoselectivity (syn/anti ratio
7:1), however the syn selectivity is still maintained
(entry 2). We have studied the effects of the electron-
withdrawing methoxycarbonyl group on the stereose-
lectivity. However, using para-methoxycarbonyl-sub-
stituted complex 1c in place of 1a did not change sig-
nificantly the stereoselectivity (entry 3). Surprisingly,
employment of the SeCSe complex 1d[37] instead of
PCP complex 1a led to the reversal of the stereoselec-
tivity affording the anti stereoisomer of 4a (syn/anti
ratio 1:3) as the major product (entry 4). A further in-
crease of the electron density on the complex can be
achieved by applying a para-methoxy substituent[36] in
the catalyst (1e). Compared to 1d application of the
methoxy-substituted SeCSe complex 1e slightly in-
creases (syn/anti ratio 1:4) the amount of the anti
product in the condensation reaction (Entry 5). Fur-
thermore, SCS (1f) and NCN (1g) complexes react
with a low selectivity (syn/anti ratio 2:3), however the
major product is still the anti form of 4a (entries 6
and 7). Finally, we employed Pd(OAc)2 (1h) as cata-
A
lyst (entry 8), which afforded the imidazoline product
(4a) with somewhat lower yield than the pincer com-
plex-catalyzed reactions (entries 1–7). The Pd(OAc)2-
U
catalyzed process also provides the anti form as major
diastereomer, however the selectivity is relatively low
(syn/anti ratio 1:2). A similar reactivity and selectivity
was reported for Pd(II) and Pd(0) salts applied as cat-
alysts in the analogous condensation reactions.[44,47]
As mentioned above, the syn/anti selectivity of the
condensation reaction of 2a and 3 changes from 10:1
to 1:4 as one goes from catalyst 1a to 1e. Monitoring
1
the progress of the catalytic reactions using H NMR
spectroscopy revealed that the catalytic activity of
these complexes is also different (Figure 1). The con-
[a]
All reactions were performed using 1 mol% catalyst 1 at
208C in THF.
Isolated yield.
[b]
[c]
Ratio of the syn and anti products determined by
1H NMR spectroscopy.
Figure 1. Formation of 4a from 2a and 3 catalyzed by 1
&
mol% of 1a ( ) or 1e (+) in THF-d8.
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densation reaction catalyzed by PCP complex 1a is Stoichiometric Studies
faster than the corresponding process with SeCSe cat-
alyst 1e. It was found that the reaction with 1a is com- In order to get mechanistic insights to the above [Eq.
pleted in one hour, while the full conversion of 2a (1)] catalytic condensation reactions, we have studied
and 3 to imidazoline product 4a with the electron-rich the stoichiometric reaction of PCP complex 1a with
SeCSe complex 1e as catalyst required about twice as isocyanoacetate 3. Addition of a three-fold excess of
much time as with 1a. These studies have also shown 3 to the CDCl3 solution of 1a led to clearly observable
1
that the syn/anti ratio of 4a was constant with both changes in the H NMR spectrum of these reagents
catalysts under the condensation reaction conditions.
Subsequently, we have studied the electronic effects
(Scheme 2).
The proton signal of 3 appearing at 4.23 ppm as a
of the aromatic substituents in the sulfonimine com- singlet (lb=3.0 Hz) is shifted to 4.86 ppm and consid-
ponent. The fluoro substituted 2b was reacted with 3 erably broadened (lb=202 Hz). At the same time, the
in the presence of 1a (entry 9) with the same reactivi- doublet (6.72 ppm) and triplet shifts (7.10) of the aro-
ty and about the same selectivity (syn/anti ratio 9:1), matic ring of complex 1a have also increased to
as the parent sulfonimine 1a. Similar to the above 6.85 ppm and 7.22 ppm respectively (Scheme 2). A
condensation of 2a and 3, the stereoselectivity of the further significant change occurred in the 31P NMR
reaction was reversed (entry 10) when SeCSe complex spectrum. The 31P NMR shift of the phosphorus atom
1e was employed instead of PCP complex 1a (syn/anti in the side arms in 1a (146.2 ppm) was increased by
ratio 1:3). Application of 2c with an electron-supply- eight ppm (154.1 ppm) on addition of 3. In this pro-
ing para-methoxy group on the phenyl group cess only a single phosphorus shift was observed indi-
(entry 11) gave somewhat higher selectivity (syn/anti cating that the symmetrical tridentate pincer architec-
ratio 11:1) than 2a.
ture of the resulted complex (6a) remained intact.
The steric effects of the aromatic rings of 2 are sur- Considering the above systematic changes we rea-
prisingly small on the stereoselectivity of the reaction. soned that 3 was coordinated to the palladium atom
Thus, the high syn selectivity (syn/anti ratio 7:1) of of the pincer complex affording complex 6a. Forma-
the condensation reaction was maintained even in the tion of similar complexes was postulated in condensa-
presence of the bulky naphthyl substituent (2d), in tion reactions of 3 with aldehydes in pincer complex-
spite of the fact that the syn diastereomer of 4d is catalyzed processes.[49,50]
thermodynamically probably less stable than the cor-
After 1a was completely converted to 6a the sol-
responding anti form (entry 12). The diastereoselec- vent and the excess of 3 were evaporated. Complex
tivity is also insensitive to the increase of the bulki- 6a proved to be surprisingly stable, as the NMR shift
ness of the sulfonyl substituent. For example, replace- values of 6a after evaporation and dissolution in
ment of the tolyl group with a naphthyl one (2e) CDCl3 were identical to the corresponding data ob-
leads to a highly syn selective process (entry 13). The served under the stoichiometric reaction. Subsequent-
selectivity and reactivity of the palladium-catalyzed ly, sulfonimine 2a was added to the solution of 6a and
1
reaction is unchanged in the presence of sulfur-con- the reaction was monitored with H NMR spectrosco-
taining heterocycles, such as 2f (entry 14). Further- py. It was found that the reaction of 2a and 6a leads
more, the catalytic reaction proceeds smoothly with to a rapid formation of the condensed product 4a
high selectivity even with non-aromatic sulfonimines, [Eq. (3)]. Thus, the stoichiometric reaction of 6a and
such as 2g (entry 15).
2a gives the same product as the catalytic transforma-
tion of 3 with 2a in the presence of 1 mol% of 1a
(entry 1). Although several peaks appeared in the
31P NMR spectrum of the reaction mixture indicating
Scheme 2. Change of the NMR shifts (in ppm) under the stoichiometric reaction of 1a and 3. Unless otherwise stated the
1H NMR shift values are given.
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Condensation of Sulfonimines and Isocyanoacetate to Imidazoline Derivatives
that different ligands may have coordinated to the re- and 2.00 Š) in 6a are very close to the corresponding
sulted pincer complex, addition of organic chloride bond lengths in the parent 1a reported by Bedford
salt PPh4Cl led to appearance of only two 31P NMR and co-workers.[30] As a typical feature of PCP pincer
+
shifts. One of these shifts was assigned to the PPh4
complexes, the PCP angle (1598) deviates from the
counterion, while the other one was assigned to linear alignment (1808), which would have been re-
chloro complex 1b, which could also be isolated by quired for an ideal MOoverlap in tetragonal planar
chromatography and identified.
complexes. Most interestingly, the palladium-carbon
bond to the coordinated isocyanoacetate molecule
(Pd C2) is relatively short, 2.025 Š. This indicates
that the palladium-carbon bonding to the aryl ring
and to the carbon atom of the coordinated isocyanoa-
ꢀ
X-ray Structure of6a
We were also able to isolate and crystallize catalytic cetate are about equally strong, Thus, the interaction
intermediate 6a. The obtained crystals were suitable between the isocyanide carbon and palladium can be
for X-ray diffraction analysis of this species. The geo- classified as a strong covalent bond, instead of a
metric parameters determined for the palladium- donor-acceptor interaction between a Lewis acid (the
ligand bonding and for the coordinated isocyanoace- pincer complex) and a Lewis base (3). This is proba-
tate molecule reveal some interesting features. The X- bly an important structural feature, as in the aldol re-
ray diffraction structure of 6a clearly shows a pincer action of isocyanoacetates with aldehydes, the pincer
complex architecture with two palladacycles complexes are often referred to as Lewis acid cata-
(Figure 2). The Pd P and Pd C1 bond lengths (2.27 lysts.[20] Other interesting structural features of the co-
ordinated isocyanoacetate moiety are the bond
ꢀ
ꢀ
ꢀ
lengths of the C3 C4 bond (1.532 Š) and the carbon-
yl carbon oxygen bond (C4=O1, 1.198 Š) bonds,
which clearly indicate the presence of a typical
carbon-carbon single bond and a carbon-oxygen
double bond. Accordingly, coordination of isocyanoa-
cetate 3 to palladium in 6a does not induce spontane-
ous enolization, which is required to increase the nu-
cleophilicity of the coordinated isocyanoacetate.[16,49]
The Catalytic Cycle ofthe Reaction
The above stoichiometric studies clearly show two im-
portant mechanistic features of the catalytic reaction.
The first step is formation of complex 6a from catalyst
1a and 3 followed by a reaction of 6 and sulfonimine
2a. Under this process the tridentate coordination of
the pincer complex catalyst was retained. A complete
recovery of complex 1 clearly shows that the catalytic
Figure 2. X-ray structure of 6a. Selected bond lengths (Š)
and angles: Pd P1 2.2784(11); Pd P2 2.2688(10); Pd C1
reaction does not involve palladium(0) species, since
ꢀ
ꢀ
ꢀ
reduction of the palladium atom in 1 would have in-
volved decomposition of the complex.[51–53] Further-
more, the X-ray structure of 6a (Figure 2) clearly
shows that the isocyanoacetate molecule is firmly co-
ordinated to palladium without enolization of the sub-
strate.
ꢀ
ꢀ
ꢀ
2.004(5); Pd C2 2.025(5); C2 N 1.123(6); N C3 1.424(6);
ꢀ
ꢀ
ꢀ
ꢀ
C3 C4 1.532(6); C4=O1 1.198(7); P2 Pd P1 159.00(6); N
ꢀ
ꢀ
ꢀ
C2 Pd 173.3(4); N C3 C4 108.8(4). For sake of clarity the
trifluoroacetate counter ion and the included CHCl3 mole-
cule is not shown in the Figure. The displacement ellipsoids
are drawn at the 30% probability level.
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Considering the above studies a plausible catalytic syn isomers of 4a–g according to the above mecha-
cycle was constructed (Scheme 3). Accordingly, the nism (Scheme 3), while changing of the heteroatoms
catalytic reaction starts with deprotonation of 6a. As in side arms to selenium (1d and e), sulfur (1f) or ni-
we have pointed out, under catalytic conditions this trogen (1g) may change the reaction mechanism shift-
proton dissociation takes place without added base. ing the stereoselectivity toward formation of the anti
The deprotonated complex can be described by two diastereomer. The highly different stereoselectivities
resonance structures 6b and the enolate form 6b’. The obtained with palladium acetate (1h) and PCP com-
next step is a nucleophilic attack of the electron-rich plex 1a also suggest a completely different mechanism
enolate moiety by the sulfonimine substrate to give for condensation of sulfonimines with isocyanoacetate
6c. The condensation reaction is accomplished by a 3.
nucleophilic attack on the carbon atom of the isocya-
nate group, which is probably still coordinated to pal-
ladium affording complex 6d. Protonation of the C2 Conclusions
carbon of the imidazoline ring leads decomplexation
of the product (4a) and regeneration of the catalyst.
We have shown that condensation of sulfonimines
The stereoselectivity of the process is determined with isocyanoacetate can be accomplished by using
in the addition step of 2a in the 6b!6c process. Em- palladium pincer complexes in low (1 mol%) catalyst
ploying PCP complex 1a as catalyst the syn selectivity loadings. The reaction proceeds rapidly at 208C with-
is very high. Our studies indicate that using electron- out addition of base or other additives. The stereose-
supplying substituents, such as sulfur, nitrogen and se- lectivity of the condensation reaction is strongly de-
lenium instead of phosphorus leads to an increase of pendent on the applied pincer ligand. Using the elec-
the amount of the anti product (Table 1). Previous tron-deficient and relatively bulky PCP complex 1a
studies indicate that PCP complexes and pincer com- the major product is the syn form, however the diaste-
plexes[50,54] with sulfur or nitrogen atoms in the side reoselectivity is reversed on applying SeCSe-based
arms (such as 1f and 1g) probably react by different catalyst 1e. The condensation reaction proceeds with
mechanisms in aldol reactions. In addition, the differ- a poor regioselectivity using common palladium
ence in the geometric flexibility[55] of complexes 1a–g salts[44,47] (such as 1h), providing mainly the anti prod-
may also influence the stereochemical outcome of the uct. The synthetic scope of the condensation reaction
cyclization reactions.
using 1a is relatively broad, as the reactivity and the
It appears from the presented studies that applica- selectivity of the reaction are not significantly depen-
tion of PCP complexes 1a and b leads to formation of dent on the steric bulk of the sulfonimine component.
Scheme 3. Plausible catalytic cycle based on the mechanistic studies.
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Condensation of Sulfonimines and Isocyanoacetate to Imidazoline Derivatives
data were determined from the purified mixture of the dia-
stereomers. Spectral data for the major diastereomer –
1H NMR (400 MHz, CDCl3): d=7.91 (dd, J=2.0 and 7.8 Hz,
4H), 7.37–7.27 (m, 6H), 6.49 (s, 2H), 4.47 (d, J=13.7 Hz,
2H), 4.19 (d, J=13.9 Hz, 2H), 3.67 (bs, 3H), 1.69 (bs, 3H);
Our mechanistic studies show that the active inter-
mediate of the reaction is an isocyanoacetate coordi-
nated pincer complex (6a); and that the reaction pro-
ceeds without involvement of palladium(0) species.
The pincer complex catalyst is very stable under the
entire process and it can be recovered unchanged
after the condensation reaction.
1
spectral data for minor diastereomer – H NMR (400 MHz,
CDCl3): d=7.85 (d, J=7.2 Hz, 4H), 7.37–7.27 (m, 6H), 6.45
(s, 2H), 4.47 (d, J=13.7 Hz, 2H), 4.09 (d, J=14.5 Hz, 2H),
3.65 (bs, 3H), 1.80 (bs, 3H); 13C NMR data for the mixture
of two diastereomers – 13C NMR (100.5 MHz, CDCl3): d=
176.9, 176.7, 156.8, 156.6, 151.2, 150.6, 142.1, 141.6, 133.0,
132.95, 130.0, 129.7, 129.4, 129.35, 129.2, 109.7, 109.6, 55.1,
55.0, 42.7, 42.2, 23.5, 23.3.
Experimental Section
All experiments were conducted under argon atmosphere
employing standard manifold techniques. 1,3-Bis[(methyl-
thio)methyl]benzene was prepared according to the proce-
dure described by Furukawa and co-workers[56]. The palladi-
um pincer complexes 1a,[30] 1b,[57] 1d,[58] and 1g[59] were pre-
pared using published procedures. All solvents used in the
reactions were freshly distilled prior to use. NMR spectra
were recorded in CDCl3 on Varian or Bruker spectrometers
(1H at 400 MHz, 13C at 100.5 MHz, 31P at 161.9 MHz, and
Synthesis ofComplex 1f
A suspension of PdCl2 (0.1 g, 0.56 mmol) in anhydrous ace-
tonitrile (30 mL) was refluxed under argon atmosphere until
PdCl2 was completely dissolved. Thereafter, AgBF4 (0.22 g,
1.2 mmol) was added to the reaction mixture. The resulting
mixture was refluxed for 2 h and subsequently cooled to
room temperature. The precipitate was filtered off and the
filtrate was added to 1,3-bis[(methylthio)methyl]benzene
(0.11 g, 0.56 mmol), and then this mixture was refluxed for
4 h under argon atmosphere. After filtration the solvent was
removed and the crude product was purified by chromatog-
raphy (CH2Cl2:acetonitrile, 9:1) affording 0.268 g (82%) of
19F at 376.3 MHz) using CHCl3 (d[1H]=7.26, d[13C]=77.0),
T
H3PO4 and a,a,a-trifluorotoluene as standards. Mass data
(ESI) were obtained with a Bruker MicrOTOF spectrome-
ter. For column chromatography, Merck silica gel 60 (230–
400 mesh) was used.
Synthesis ofComplex 1c
1
1f. H NMR (400 MHz, CDCl3): d=7.04–6.94 (m, 3H), 4.44
(bs, 2H), 4.06 (bs, 2H), 2.78 (s, 6H), 2.40 (s, 3H); 13C NMR
To
methyl
4-bromo-3,5-dihydroxybenzoate
(0.2 g,
(100.5 MHz, CDCl3): d=149.5, 126, 123.4, 47.1, 22.7, 2.9.
0.81 mmol) in THF (5 mL), Ph2PCl (0.36 g, 1.62 mmol) was
added, followed by dropwise addition of NEt3 (0.18 g,
1.78 mmol) at room temperature. This reaction mixture was
stirred for 5 h followed by filtration through a thin pad of General Procedure for the Palladium Pincer
Celite, and evaporation to give the crude pro-ligand. The
pro-ligand was dissolved in toluene (1 mL) and added to
Complex-Catalyzed Aldol Reaction with N-
Sulfonylimines 2a–g
Pd2(dba)3 (0.37 g, 0.4 mmol) followed by stirring for 3 h at
A
A mixture of N-sulfonylimine 2a–g (0.2 mmol), methyl iso-
cyanoacetate 3 (0.02 g, 0.2 mmol) and the corresponding cat-
alyst 1a–h (0.002 mmol, 1 mol%) was stirred in THF
(1.0 mL) at 208C for 2 h. After filtration through a thin pad
of Celite, the solvent was removed to yield a pure cis/trans
mixture of 2-imidazoline derivative 4a–g. The cis/trans ratio
758C. Purification using silica gel chromatography (CH2Cl2
as eluent) afforded 0.29 g (50%) of the bromide of complex
1c. To this bromide complex (0.29 g, 0.4 mmol) in CH2Cl2
(20 mL) AgTFA (0.26 g, 1.2 mmol) was added and then the
resulting mixture was stirred for 3 h at room temperature. A
subsequent filtration through a pad of silica yielded 1c as a
1
1
was determined from crude H NMR spectra.
white solid in quantitative yield (0.3 g). H NMR (400 MHz,
Methyl
1-[(4-methylphenyl)sulfonyl]-5-phenyl-4,5-dihy-
CDCl3): d=7.85 (m, 8H), 7.52 (m, 12H), 7.38 (s, 2H), 3.88
(s, 3H); 13C NMR (100.5 MHz, CDCl3): d=166.3, 164.5 (t,
8.0 Hz), 133.7, 133.0, 132.7, 132.5, 132.3, 132.2, 131.8 (t, J=
8.7 Hz), 131.6, 131.5, 128.9 (t, J=6.0 Hz), 108.5 (t, J=
8.0 Hz), 52.2; 19F NMR (376.3 MHz, CDCl3): d=ꢀ74.03;
31P NMR (161.9 MHz, CDCl3): d=145.5.
dro-1H-4-imidazolecarboxylate (4a): The NMR data ob-
tained for 4a are identical with the literature values.[44,47]
Spectral data for the syn isomer (entry 1) – 1H NMR
(400 MHz, CDCl3): d=7.76 (d, J=2.0 Hz, 1H), 7.40 (d, J=
8.0 Hz, 2H), 7.21–7.08 (m, 5H), 6.99 (d, J=8.0 Hz, 2H),
5.21 (dd, J=11.5 and 2.0 Hz, 1H), 5.14 (d, J=11.5 Hz, 1H),
3.13 (s, 3H), 2.37 (s, 3H); 13C NMR (100.5 MHz, CDCl3):
d=168.3, 151, 144.7, 134.7, 133.9, 129.7, 128.5, 128, 127.5,
127.2, 76.2, 63.5, 51.7, 21.5; HR-MS: m/z=381.0867, calcd.
for [M + Na]+, C18H18 N2NaO4S: 381.0879. Spectral data for
Synthesis ofComplex 1e
This complex was synthesized following a procedure by Yao
and co-workers[37] described for similar species. A round
bottomed flask was charged with (5-methoxy-1,3-phenyl-
ene)bis(methylene)bis(phenylselane) (0.3 g, 0.68 mmol) and
1
the anti isomer (entry 5) – H NMR (400 MHz, CDCl3): d=
7.63 (d, J=2.1 Hz, 1H), 7.45 (d, J=8.3 Hz, 2H), 7.25–7.11
(m, 7H), 5.06 (d, J=7.5 Hz, 1H), 4.64 (dd, J=7.5 and
2.1 Hz, 1H), 3.68 (s, 3H), 2.37 (s, 3H); 13C NMR
(100.5 MHz, CDCl3): d=169.7, 150, 144.7, 138, 134.3, 129.7,
128.7, 128.4, 127.2, 126.8, 79.8, 63.7, 52.8, 21.5; HR-MS:
glacial acetic acid (0.7 mL). To this mixture Pd(OAc)2 and a
G
further portion of glacial acetic acid (0.7 mL) was added.
This mixture was refluxed for 3 h. After evaporation the res-
idue was purified by silica gel chromatography (CH2Cl2/
MeOH, 9:1) affording 1e (0.37 g, 89%). Complex 1e is
m/z=381.0871, calcd. for [M
+
Na]+; C18H18N2NaO4S:
1
formed as a mixture of two diastereomers.[37,58] The H NMR
381.0879.
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Methyl 5-(4-fluorophenyl)-1-[(4-methylphenyl)sulfonyl]-
4,5-dihydro-1H-4-imidazole-carboxylate (4b): The NMR
data obtained for 4b are identical with the literature
values.[44] Spectral data for the syn isomer (entry 9) –
1H NMR (400 MHz, CDCl3): d=7.75 (d, J=2.0 Hz, 1H),
7.41 (d, J=8.3 Hz, 2H), 7.16 (d, J=8.3 Hz, 2H), 7.0–6.95
(m, 2H), 6.79 (t, J=8.6 Hz, 2H), 5.19 (dd, J=11.4 and
2.0 Hz, 1H), 5.13 (d, J=11.4 Hz, 1H), 3.18 (s, 3H), 2.38 (s,
3H). 13C NMR (100.5 MHz, CDCl3): d=168.2, 163.8, 161.4,
150.9, 145, 134.6, 129.7, 129.4 (d, J=8.4 Hz), 127.2, 115 (d,
J=21.4 Hz), 76, 62.7, 51.8, 21.5; HR-MS: m/z=399.0784,
calcd. for [M + Na]+, C18H17FN2NaO4S: 399.0785.
Methyl 5-(4-methoxyphenyl)-1-[(4-methylphenyl)sulfon-
yl]-4,5-dihydro-1H-4-imidazole-carboxylate (4c): The NMR
data obtained for 4c are identical with the literature
values.[44] Spectral data for the syn isomer (entry 11) –
1H NMR (400 MHz, CDCl3): d=7.37 (d, J=2.0 Hz, 1H),
7.38 (d, J=8.2 Hz, 2H), 7.11 (d, J=8.2 Hz, 2H), 6.88 (d, J=
8.7 Hz, 2H), 6.59 (d, J=8.7 Hz, 2H) 5.15 (dd, J=11.3 and
2.0 Hz, 1H), 5.10 (d, J=11.3 Hz, 1H), 3.71 (s, 3H), 3.18 (s,
3H), 2.36 (s, 3H); 13C NMR (100.5 MHz, CDCl3): d=168.4,
159.6, 150.8, 144.5, 134.8, 129.6, 128.8, 127.1, 125.7, 113.3,
75.9, 63.1, 55.1, 51.7, 21.4; HR-MS: m/z=411.0972, calcd.
for [M + Na]+, C19H20N2NaO5S: 411.0985.
5.54 (dd, J=15.8 and 9.5 Hz, 1H), 5.03 (dd, J=10.6 and
2.0 Hz, 1H), 4.81 (dd, J=11.0 and 9.5 Hz, 1H), 3.58 (s, 3H),
2.32 (s, 3H); 13C NMR (100.5 MHz, CDCl3): d=168.6, 150.5,
144.9, 135.9, 135.5, 135.2, 129.8, 128.51, 128.48, 127.7, 126.6,
121.2, 74.6, 62.4, 52.2, 21.5; HR-MS: m/z=407.1036, calcd.
for [M + Na]+, C20H20 N2NaO4S: 407.1036.
General Procedure for the Preparation of Diamino
Esters 5a and b from the Hydrolysis of 2-Imidazolines
4a and b
To the 2-imidazoline 4 (0.195 mmol) in CHCl3 (2 mL) was
added water (0.007 g, 0.39 mmol) followed by neutral alumi-
num oxide (0.20 g, 1.96 mmol). The mixture was stirred at
room temperature for 16 h and thereafter, filtered and puri-
fied by chromatography using pentane/EtOAc (1:1) as
eluent.
Methyl 2-formylamino-3-[(4-methylphenyl)sulfonyl]ami-
no-3-phenylpropanoate (5a): 1H NMR (400 MHz, CDCl3):
d=8.17 (bs, 1H), 7.60 (d, J=8.3 Hz, 2H), 7.10–7.20 (m,
5H), 7.01–6.96 (m, 2H), 6.64 (d, J=8.6 Hz, 1H), 6.42 (d, J=
6.6 Hz, 1H), 5.04 (dd, J=8.6 and 3.3 Hz, 1H), 4.89 (dd, J=
7.6 and 3.3 Hz, 1H), 3.70 (s, 3H), 2.34 (s, 3H); 13C NMR
(100.5 MHz, CDCl3): d=169.6, 161.8, 143.2, 137.2, 135.3,
129.2, 128.3, 127.9, 127, 126.5, 59.2, 55.9, 52.7, 21.3; HR-MS:
m/z=399.0983, calcd. for [M + Na]+, C18H18 N2NaO5S:
399.0985.
Methyl 1-[(4-methylphenyl)sulfonyl]-5-(2-naphthyl)-4,5-
dihydro-1H-4-imidazolecarboxylate (4d): Spectral data for
1
the syn isomer (entry 12) – H NMR (400 MHz, CDCl3): d=
Methyl
3-(4-fluorophenyl)-2-formylamino-3-[(4-methyl-
7.83 (d, J=2.0 Hz, 1H), 7.73–7.37 (m, 6H), 7.29 (d, J=
8.3 Hz, 2H), 7.04 (dd, J=8.3 and 2.0 Hz, 1H), 6.84 (d, J=
8.3 Hz, 2H) 5.37 (d, J=11.3 Hz, 1H), 5.31 (dd, J=11.3 and
2.3 Hz, 1H), 3.03 (s, 3H), 2.15 (s, 3H); 13C NMR
(100.5 MHz, CDCl3): d=168.3, 150.8, 144.6, 134.8, 133,
132.5, 130.9, 129.4, 127.9, 127.8, 127.4, 127.3, 127, 126.4,
126.1, 124.7, 76.1, 63.7, 51.7, 21.2; HR-MS: m/z=431.1043,
calcd. [M + Na]+; C22H20 N2NaO4S: 431.1036.
Methyl 1-[(6-methyl-2-naphthyl)sulfonyl]-5-phenyl-4,5-di-
hydro-1H-4-imidazolecarboxylate (4e): Spectral data for the
syn isomer (entry 13) – 1H NMR (400 MHz, CDCl3): d=
7.90 (d, J=2.0 Hz, 1H), 7.87–7.36 (m, 6H), 7.14–7.00 (m,
1H), 6.99–6.92 (m, 4H), 5.22 (bs, 2H) 3.10 (s, 3H), 2.53 (s,
3H); 13C NMR (100.5 MHz, CDCl3): d=168.3, 150.9, 139.8,
135.2, 133.5, 133.4, 129.9, 129.1, 129.0, 128.8, 128.6, 128.5,
127.9, 127.4, 126.7, 121.5, 76.1, 63.6, 51.6, 21.9; HR-MS:
m/z=431.1027, calcd. for [M + Na]+, C22H20 N2NaO4S:
431.1036.
Methyl 1-[(4-methylphenyl)sulfonyl]-5-(2-thienyl)-4,5-di-
hydro-1H-4-imidazolecarboxylate (4f): Spectral data for the
syn isomer (entry 14) – 1H NMR (400 MHz, CDCl3): d=
7.69 (d, J=2.2 Hz, 1H), 7.43 (d, J=8.1 Hz, 2H), 7.14 (d, J=
8.1 Hz, 2H), 7.09 (dd, J=4.6 and 1.5 Hz, 1H), 6.75–6.70 (m,
2H) 5.49 (d, J=10.9 Hz, 1H), 5.17 (dd, J=10.9 and 2.2 Hz,
1H), 3.32 (s, 3H), 2.36 (s, 3H); 13C NMR (100.5 MHz,
CDCl3): d=168, 150.2, 144.7, 136.2, 134.8, 129.7, 127.6, 127,
126.5, 126.1, 75.9, 58.9, 52, 21.5; HR-MS: m/z=387.0435,
calcd. for [M + Na]+, C16H16 N2NaO4S2: 387.0444.
phenyl)sulfonyl]amino-propanoate
(5b):
1H NMR
(400 MHz, CDCl3): d=8.21 (s, 1H), 7.58 (d, J=8.2 Hz, 2H),
6.74–6.15 (m, 6H), 6.76 (d, J=7.8 Hz, 1H), 6.46 (d, J=
8.8 Hz, 1H), 5.0 (dd, J=8.8 and 3.5 Hz, 1H), 4.95 (dd, J=
7.8 and 3.5 Hz, 1H), 3.68 (s, 3H), 2.33 (s, 3H); 13C NMR
(100.5 MHz, CDCl3): d=169.4, 161.8, 143.5, 137.3, 131.2,
129.4, 128.4, 128.3, 127.0, 115.2, 115.4, 58.8, 56.2, 53.0, 21.4;
HR-MS: m/z=417.0890, calcd. for [M
+
Na]+,
C18H19FN2NaO5S: 417.0891.
Stoichiometric Reaction with Pincer Complex 1a and
Methyl Isocyanoacetate 3
Complex 1a (0.008 g, 0.012 mmol) and methyl isocyanoace-
tate
3 (0.003 g, 0.036 mmol) were dissolved in CDCl3
1
(0.5 mL). The reaction was monitored by H and 31P NMR
spectroscopy at 258C. After consumption of 1a the reaction
mixture was evaporated. After re-dissolving the residue in
CDCl3 the NMR shift values for 6a were found to be identi-
cal to the corresponding data recorded under the catalytic
reaction. After addition of sulfonimine 2a (0.003 g,
0.012 mmol) to the solution of 6a, the reaction was moni-
tored by 1H and 31P NMR spectroscopy. Change of the
NMR shifts clearly showed formation of 4a, which was also
the product of the condensation of 2a and 3 catalyzed by 1a
(entry 1). After the stoichiometric reaction of 6a and 2a was
completed, PPh4Cl (0.005 g, 0.012 mmol) was added and for-
1
mation of catalyst 1b could be detected by H and 31P NMR
Methyl
1-[(4-methylphenyl)sulfonyl]-5-[(E)-2-phenyl-1-
spectroscopy. The identity of complex 1b could be con-
firmed after purification of the reaction mixture by silica gel
chromatography using CH2Cl2/pentane (2:1) as eluent.
ethenyl]-4,5-dihydro-1H-4-imidazolecarboxylate (4g): The
NMR data obtained for 4g are identical with the literature
values.[44] Spectral data for the syn isomer (entry 15) –
1H NMR (400 MHz, CDCl3): d=7.66 (J=d, 2.0 Hz, 1H),
7.64 (d, J=8.2 Hz, 2H), 7.30–7.23 (m, 2H), 7.15 (d, J=
8.2 Hz, 2H), 7.12–7.06 (m, 3H), 6.52 (d, J=15.8 Hz, 1H),
2592
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FULL PAPERS
Condensation of Sulfonimines and Isocyanoacetate to Imidazoline Derivatives
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Monitoring the Pincer-Complex Catalyzed
Condensation of3 with 2a
In an NMR tube 2a (0.2 mmol) and the corresponding cata-
lyst 1a or 1e (0.002 mmol, 1 mol%) were dissolved in THF-
d8 (0.5 mL) at 258C. Thereafter, methyl isocyanoacetate 3
(0.02 g, 0.2 mmol) in THF-d8 (0.5 mL) was added and this
reaction mixture was monitored by 1H NMR spectroscopy
(Figure 1).
[9] C. Jonasson, M. Kritikos, J.-E. Bäckvall, K. J. Szabó,
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Complex 1a (0.05 g, 0.07 mmol) and methyl isocyanoacetate
3 (0.015 g, 0.146 mmol) were dissolved in CDCl3 (1.5 mL).
1
Formation of 6a could be observed in 10 min by H, 31P and
13C NMR spectroscopy. Then, the CDCl3 solution of 6a was
transferred to a vial followed by addition of diethyl ether
(0.5 mL), and then this mixture was stored at 58C affording
crystalline form of 6a suitable for X-ray structure determi-
1
nation. H NMR (400 MHz, CDCl3): d=7.82–7.74 (m, 8H),
7.62–7.55 (m, 12H), 7.22 (t, J=8.2 Hz, 1H), 6.85 (d, J=
8.2 Hz, 2H), 4.86 (bs, lb=202 Hz, 2H), 3.77 (s, 3H);
13C NMR (100.5 MHz, CDCl3): d=165 (t, J=7 Hz), 164.4,
160.75 (apparent q, J=33 Hz), 133.3, 131.6 (t, J=8.4 Hz),
130.9 (t, J=28.6 Hz), 129.6 (t, J=6.4 Hz), 107.7 (t, J=
8.4 Hz), 53.4, 47.2 (bs, lb=34 Hz); 19F NMR (376.3 MHz,
CDCl3): d=ꢀ74.6; 31P NMR (161.9 MHz, CDCl3): d=154.1.
[18] K. J. Szabó, Synlett 2006, 811.
[19] The Chemistry of Pincer Compounds, (Eds.: D. Mo-
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2007.
[20] M. A. Stark, C. J. Richards, Tetrahedron Lett. 1997, 38,
X-ray Analysis ofComplex 6a
5881.
The diffraction data were measured at 100 K with an
Oxford Diffraction Xcalibur-2 kappa-diffractometer with a
Sapphire-III CCD-detector.[60] The structure was solved with
direct methods using SHELXS-97[61] and refined with con-
ventional full matrix least square methods using SHELXL-
97.[61] File CCDC 645985 contains the supplementary crystal-
lographic data for complex 6a in this paper. These data can
be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data request/
cif.
[21] N. C. Mehendale, C. Bezemer, C. A. v. Walree,
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[22] M. Q. Slagt, G. Rodríguez, M. M. P. Grutters,
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H. J. v. Ramesdonk, J. W. Verhoeven, D. D. Ellis, A. L.
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Acknowledgements
This work was mainly supported by the Swedish Natural Sci-
ence Research Council (VR). A post-doctoral fellowship for
K.S.K. by the Wennergren Foundations is greatly acknowl-
edged. The authors are indebted to the Alice and Knut Wal-
lenberg Foundation for funding a UPLC instrument.
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Orpen, L. T. Pilarski, P. G. Pringle, R. L. Wingad,
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