Synthesis and structure of tricarbonyl(η6-arene)chromium complexes of phenyl and benzyl D-glycopyranosides

Article abstract of DOI:10.3762/bjoc.8.118

A series of 15 glycoside-derived tricarbonyl(η⁶-arene) chromium complexes were prepared in 19-87% yield by heating fully acetylated or methylated aryl O-, S-, N- and C-glycosides of D-glucopyranose and D-mannopyranose with hexacarbonylchromium.

Full text of DOI:10.3762/bjoc.8.118

Synthesis and structure of
tricarbonyl(η6-arene)chromium complexes
of phenyl and benzyl D-glycopyranosides
Thomas Ziegler* and Ulrich Heber
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1059–1070.
Institute of Organic Chemistry, University of Tuebingen, Auf der
Morgenstelle 18, 72076 Tuebingen, Germany
doi:10.3762/bjoc.8.118
Received: 14 March 2012
Accepted: 19 June 2012
Published: 11 July 2012
Email:
Thomas Ziegler* - thomas.ziegler@uni-tuebingen.de
* Corresponding author
This article is part of the Thematic Series "Synthesis in the
glycosciences II".
Keywords:
aryl glycosides; carbohydrates; transition-metal complex;
tricarbonyl(arene)chromium
Guest Editor: T. K. Lindhorst
© 2012 Ziegler and Heber; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A series of 15 glycoside-derived tricarbonyl(η6-arene)chromium complexes were prepared in 19–87% yield by heating fully
acetylated or methylated aryl O-, S-, N- and C-glycosides of D-glucopyranose and D-mannopyranose with hexacarbonylchromium.
All tricarbonylchromium complexes were fully characterized. The structures of nine crystalline complexes were determined by
X-ray diffraction, revealing unusual intra- and intermolecular nonclassical hydrogen bonds.
Introduction
In 1957, Fischer and Öfele published the preparation of tricar- increasing the acidity of the aromatic protons and the elec-
bonyl(η6-benzene)chromium, which was the first arene tricar- trophilicity of the aromatic ring and, thus, making the arene
bonylchromium complex [1]. Since then, a plethora of tran- more susceptible towards SNAr reactions. Likewise, the
sition-metal complexes of arenes have been prepared, character- benzylic and homo-benzylic positions in tricarbonyl(η6-
ized and described in the literature. Among the multitude of arene)chromium complexes are more acidic and more prone to
transition-metal complexes of aromatic compounds, however, solvolysis, nucleophilic substitution and deprotonation than in
only tricarbonyl(η6-arene)chromium compounds are widely the parent arenes due to the fact that the tricarbonylchromium
used for organic syntheses [2-4]. This is due to the fact that ligand stabilizes both benzylic and homo-benzylic carbenium
tricarbonyl(η6-arene)chromium complexes are relatively stable ions and carbanions [5,6]. Asymmetric ortho- or meta-substi-
compounds, which can be easily prepared and also easily recon- tuted tricarbonyl(η6-arene)chromium compounds display planar
verted into the parent arenes. Furthermore, the tricar- chirality, which, in turn, makes these chiral complexes attrac-
bonylchromium group is an electron-withdrawing substituent tive catalysts for enantioselective reactions [6-8].
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Beilstein J. Org. Chem. 2012, 8, 1059–1070.
Despite the broad applications that tricarbonyl(η6- chromium. For method (b) high-boiling-point solvents, such as
arene)chromium complexes have found in organic synthesis di-n-butylether, decalin or dioxane, can be used. The addition of
since their discovery in 1957, only a very few tricar- THF to these solvents was shown to prevent the excessive subli-
bonylchromium complexes of sugar derivatives are known mation of Cr(CO)6 during the formation of the arene–chromium
today. Figure 1 shows the types of such carbohydrate-derived complexes [17]. Therefore, we used method (b) for the prepar-
chromium complexes that have been described in the literature ation of tricarbonyl(η6-arene)chromium complexes of glyco-
so far. Complexes of type A and B were obtained from the sides as follows.
corresponding glycopyranosides and were studied as substrates
for chiral-auxiliary-directed asymmetric ortholithiation and as One equivalent of phenyl or benzyl glycoside 1 and one equiva-
catalysts for enantioselective Diels–Alder reactions [9-12]. lent of Cr(CO)6 were dissolved in di-n-butylether containing
Tricarbonylchromium complexes of type C and D were 10% THF, and the solution was stirred at 140 °C under argon
obtained via benzannulation of glucal-derived pentacar- and under the exclusion of light (brown glassware). After the
bonylchromium carbenes or by reaction of alkynyl C-glyco- reaction of 1 with Cr(CO)6 was complete (16–96 h), the solvent
sides with pentacarbonylchromium carbenes [13].
was evaporated and the crude complexes 2 were purified by
chromatography under argon on silica gel with n-hexane/ethyl
acetate mixtures as eluent. Crystalline chromium complexes 2
were recrystallized from ethanol, and for suitable crystals X-ray
structures were determined. Table 1 summarizes the results for
the preparation of the complexes 2 from simple acetylated and
methylated phenyl, benzyl and 1-O-benzoyl glycosides 1. All
glycosides 1a–k were prepared according to literature pro-
cedures (for details see the Supporting Information File 1).
All reactions of glycosides 1a–k with Cr(CO)6 proceeded
smoothly and gave the corresponding tricarbonyl(η6-
arene)chromium complexes 2a–k in medium yield. The some-
what lower yields in some cases are due to oxidative decompo-
sition of the products during purification by column chromatog-
raphy on silica gel.
Figure 1: Known types of η6-tricarbonylchromium complexes of sugar
derivatives [9-13].
Further syntheses and characterizations of more examples of
carbohydrate-derived tricarbonyl(η6-arene)chromium The reaction times also varied significantly between 16 and
complexes are highly desirable in order to allow studies of the 96 h. This was due to the purity of the hexacarbonyl chromium
rich chemistry of such complexes in greater detail. In this paper charges we purchased from several companies. In general,
we describe the synthesis and structural elucidation of a series however, electron-donating protecting groups in the sugar
of tricarbonyl-(η6-arene)chromium complexes of some simple moiety accelerated the formation of the chromium complexes at
phenyl and benzyl O-, N-, S- and C-glycosides.
the aglycon (Table 1, entries 3 and 4). Treatment of the
mannose orthoester 1f (Table 1, entry 6) under the standard
conditions applied here resulted in the exclusive formation of
tricarbonyl[(2,3,4,6-tetra-O-acetyl-α-D-mannopyranosyloxy)-
η6-benzene]chromium (2f) in 47% yield. In the glucose series it
is well known that orthoesters similar to 1f rearrange to the
corresponding glycosides upon heating [18]. Therefore, it is
very likely that 1f isomerized to the corresponding phenyl
2,3,4,6-tetra-O-acetyl-α-D-mannopyranoside, which was then
converted into complex 2f. The direct complexation of
orthoester 1f is unlikely because no such chromium complex
was detected. The anomeric α-configuration of 2f was proven
by decomplexation of an analytical sample with iodine in
CHCl3 followed by measurement of the NMR spectrum of the
formed glycoside. The latter showed a CH-coupling constant at
the anomeric center of 173.9 Hz, which is indicative of an
Results and Discussion
Preparation of η6-tricarbonylchromium
complexes of glycosides
In general, tricarbonyl(η6-arene)chromium compounds can be
prepared in a wide variety of methods [4]. However, the
following two methods are the most commonly employed ones:
(a) ligand-exchange reaction between an arene and, most
conveniently, either naphthalene–Cr(CO)3 complex or
(MeCN)3Cr(CO)3 in which the chromium ligand is only weakly
bound [14]; (b) simply heating the arene with hexacarbonyl-
chromium in an inert solvent (Mahaffy–Pauson method)
[15,16]. Method (a) has the disadvantage that the applied
chromium complexes for the ligand exchange reaction are
extremely sensitive toward oxidation, due to the weakly bound
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Beilstein J. Org. Chem. 2012, 8, 1059–1070.
Table 1: Synthesis of η6-tricarbonylchromium complexes 2a–k from glycosides 1a–k and Cr(CO)6 in di-n-butylether/THF 9:1 at 140 °C under Ar and
exclusion of light.
Entry
1
Glycoside 1
Time
96 h
Complex 2
Yield
29%
1a
2a
2b
2c
2d
2
3
4
90 h
80 h
16 h
87%
29%
19%
1b
1c
1d
1e
5
6
70 h
53%
47%
2e
42 h
1f
2f
2g
2h
7
8
42 h
67 h
35%
30%
1g
1h
1i
9a
24 h
46%
2i
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Beilstein J. Org. Chem. 2012, 8, 1059–1070.
Table 1: Synthesis of η6-tricarbonylchromium complexes 2a–k from glycosides 1a–k and Cr(CO)6 in di-n-butylether/THF 9:1 at 140 °C under Ar and
exclusion of light. (continued)
10
24 h
80 h
49%
81%
1j
2j
11
1k
2k
aAnomeric mixture α:β = 1:2.
α-anomer [19]. As additional proof for the anomeric configur- Next, we also prepared some sugar-derived tricar-
ation of 2f, we also prepared β-anomer 2g (Table 1, entry 7). bonylchromium complexes of glycosides having a prochiral
For the preparation of chromium complex 2i we used a 1:2 aglycon, i.e., an ortho-substituted phenyl or benzyl aglycon.
anomeric mixture of aminoglucoside 1i, which did not change Glycosides 1m [21] and 1q [22] were prepared according to the
during the complexation (Table 1, entry 9).
respective literature procedures. Glycosides 1n–p were not
known in the literature and were prepared as follows: o-Tolyl
For the anticipated synthesis of a glucose-derived 2,3,4-6-tetra-O-acetyl-β-D-glucopyranoside (1m) was first
tricarbonyl(η6-pyridine)chromium complex we prepared glyco- deacetylated (cat. NaOMe in MeOH) followed by treatment
side 1l by reacting acetobromoglucose with 6-tert-butyl-2- with pivaloyl chloride in pyridine to give 1n in 86% yield.
hydroxypyridine [20] under Helferich conditions (Hg(CN)2). Treatment of pentaacetylglucose and 2-tert-butylphenol with
Only the corresponding O-glycoside 1l was obtained, and no BF3-etherate in dichloromethane afforded 1o in 31% yield.
N-glycoside was formed in this Helferich glycosylation Glycoside 1p was prepared by a Helferich glycosylation of
(Scheme 1). Sterically hindered 6-tert-butyl-2-hydroxypyridine o-methylbenzyl alcohol with acetobromoglucose. For further
was chosen as the aglycon in order to avoid complexation of details see Supporting Information File 1. Table 2 summarizes
hexacarbonylchromium with the basic nitrogen atom. However, the results of the complexation of glucosides 1m–q affording
all attempts to convert 1l into the corresponding tricarbonyl(η6- the diastereomeric tricarbonyl(η6-arene)chromium complexes
pyridine)chromium complex failed. Attempts to prepare a 2m–q.
chromium complex of 1l by a ligand exchange reaction with
naphthalene–Cr(CO)3 or (MeCN)3Cr(CO)3 were also unsuc- Treatment of o-tolyl glucoside 1m with hexacarbonyl-
cessful (no further experimental details shown).
chromium under the standard conditions for 70 h afforded a 7:3
mixture of the diastereomeric tricarbonylchromium complexes
2m in 76% overall yield (Table 2, entry 1). Upon slow crystal-
lization of the diastereomeric mixture from ethanol, isomer
pR-2m could be obtained in pure form. From the mother liquor
a small amount of pure isomer pS-2m could be isolated upon
repeated recrystallization as well. The absolute configuration of
the planar chiral tricarbonyl(η6-o-tolyl)chromium aglycon in
both diastereomers 2m could be unambiguously assigned by
X-ray crystallography. Since the two diastereomers 2m were
not formed in equal amounts during complexation of glucoside
1m, we contemplated that an even higher diastereoselectivity
should be obtained when sterically more demanding aglycons or
sugar moieties are present during reaction with Cr(CO)6. There-
fore, we also reacted glucosides 1n and 1o bearing either bulky
pivaloyl groups or a tert-butyl group in the aglycon in the sugar
moiety with Cr(CO)6. However, no complexation could be
Scheme 1: Synthesis of glucoside 1l.
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Beilstein J. Org. Chem. 2012, 8, 1059–1070.
Table 2: Synthesis of tricarbonylchromium complexes 2m–q from glycosides 1m–q containing a prochiral aglycon and Cr(CO)6 in di-n-butylether/THF
9:1 at 140 °C under Ar in the dark.
Entry
Glycoside 1
Time
Complex 2
Yield
Ratioa
pR-2m:pS-2m
1
70 h
76%
pR-2m
= 7:3b
1m
pS-2m
2
no product
–
–
1n R1 = Piv, R2 = Me
1o R1 = Ac, R2 = C(CH3)3
pR-2p:pS-2p
3
4
15 h
16 h
42%
42%
= 1:1c
1p
2p
2q
pR-2q:pS-2q
= 1:1c
1q
aDetermined by 1H NMR; bdiastereomers can be separated by crystallization; cno separation possible.
detected even under prolonged reaction time (Table 2, entry 2).
Placing the o-tolyl group in a position more distant from the
sugar part, as in glucoside 1p (Table 2, entry 3), or using o-tolyl
C-glucoside 1q (Table 2, entry 4) gave 1:1 mixtures of the
corresponding diastereomers 2p and 2q, respectively. A sep-
aration of the diastereomers by crystallization was not possible
for complexes 2p and 2q.
The O-acetylated carbohydrate-derived tricarbonyl(η6-
arene)chromium complexes prepared here can be deprotected
without affecting the chromium complex, as exemplified in
Scheme 2. Zemplén deacetylation of 2c afforded tricarbonyl(β-
Scheme 2: Deprotection of 2c and enzymatic cleavage of 3.
D-glucopyranosyloxy-η6-benzene)chromium (3) in quantitative
yield. Glucoside 3 is a good substrate for β-glucosidases, such
as almond glucosidase E.C. 3.2.1.21, and is converted into does not significantly affect the conformation of the sugar
D-glucose and tricarbonyl(η6-phenol)chromium in 98% yield. moieties. This is evident from the 1H NMR spectra of com-
pounds 2, which show no significant change of chemical shifts
Structures of tricarbonyl(η6-arene)chromium
and coupling constants of the carbohydrate protons compared to
complexes of glycosides
those of the educts 1. As an example, Table 3 shows the chem-
In general, complexation of glycosides 1 with Cr(CO)6 to give ical shifts of the protons of compounds 1a and 2a. All NMR
the corresponding tricarbonyl(η6-arene)chromium compounds 2 signals of the sugar protons remain practically unchanged,
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Beilstein J. Org. Chem. 2012, 8, 1059–1070.
Table 3: Chemical shifts (δ in ppm) of the protons of compounds 1a and 2a derived from the 1H NMR spectra of the compounds measured in
acetone-d6.
H1
H2
H3
H4
H5
H6a
H6b
OCH2
HAr
1a
2a
4.83 d
4.98 d
4.98 t
4.98 t
5.25 t
5.28 t
5.05 t
5.06 t
3.97 m
4.01 m
4.29 dd
4.27 dd
4.15 dd
4.16 dd
4.87 d 4.67 d
4.68 d 4.45 d
7.28–7.34 m
5.56–5.72 m
whereas the protons of the benzylic methylene group and the Figure 2 shows the asymmetric unit of compound 2a, which
benzene moiety appear, as was expected, at higher fields in 2a. contains two slightly differently distorted molecules. Table 4
Therefore, it can be concluded that the sugar rings remain in the summarizes some selected atomic distances and angles of the
4C1 conformation. The high-field shifts of the benzylic meth- two molecules in the asymmetric unit (2aa refers to the right
ylene group and the aromatic moiety are in accordance with molecule and 2ab to the left one in Figure 2). Most signifi-
other tricarbonyl(η6-arene)chromium complexes. This is further cantly, the biggest differences are found around the benzylic
confirmed by the X-ray structures obtained from crystalline methylene groups (C7 and C37, respectively). Another signifi-
chromium complexes.
cant feature of 2a that was not observed in any other complex 2
is the parallel face-to-face orientation of the two benzene rings.
For the tricarbonylchromium complexes 2a–e,j,k,m which gave However, the distance of the two ring planes of 3.627 Å and the
suitable crystals, structures were determined by X-ray diffrac- angle of 16° between the ring planes indicate no π-interaction of
tion [23]. Crystals were grown for all compounds by slow crys- the two benzene rings.
tallization of the compounds from ethanol. The conformation of
the Cr(CO)3 group in relation to the aromatic ring shows some
deviations compared to other tricarbonyl(η6-arene)chromium
complexes. In complexes where the benzene ring carries an
electron-donating substituent, the Cr(CO)3 group is preferably
in an eclipsed orientation, whereas in cases where the benzene
carries an electron-withdrawing substituent, a staggered con-
formation is commonly found [24]. Here, eclipsed conforma-
tions of the Cr(CO)3 group were found in compounds 2c and
2e, which both carry an electron-donating glycosyloxy
substituent. However, compound 2k, which carries an electron-
withdrawing substituent also shows an eclipsed conformation of
the Cr(CO)3 group. Likewise, all compounds 2a, 2b, 2d, 2j and
2m show a staggered conformation of the Cr(CO)3 group,
although they all carry an electron-donating substituent at the
benzene ring. All complexes show unusual intra- and intermole-
cular nonclassical hydrogen bonds [25], which will be discussed
in more details for each individual X-ray structure in the
Figure 2: ORTEP-plot of the asymmetric unit containing two mole-
cules of compound 2a showing 30% probability ellipsoids; the arrows
show the nonclassical H-bonds between O1 and H34 and O21 and H4.
following. For technical details of the X-ray structures see
Supporting Information File 1.
Table 4: Selected corresponding distances and angles of 2aa and 2ab.
2aa
2ab
C1’–O10
1.375 Å
1.427 Å
1.491 Å
112.8°
108.4°
147.8°
C41–O30
1.378 Å
1.407 Å
1.477 Å
113.2°
112.6°
157.9°
O10–C7
O30–C37
C7–C1
C37–C31
C1’–O10–C7
C1–C7–O10
C1’–O10–C7–C1
C41–O30–C37
C31–C37–O30
C41–O30–C37–C31
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Beilstein J. Org. Chem. 2012, 8, 1059–1070.
The most significant features in the crystal structure of 2a are Figure 4 shows the structure of compound 2c. Once again the
the two intermolecular nonclassical hydrogen bonds between most significant feature is a nonclassical intermolecular
O1 and H34 and O21 and H4 (Figure 2). That these hydrogen hydrogen bond between the oxygen of a carbonyl group (O12)
bonds are indeed true bonds is evident from the distances of and the hydrogen (H6’A) at C6 of the neighboring molecule.
2.659 Å for O1/H34 and 2.680 Å for O21/H4 and the angles Distance (2.628 Å) and C≡O···H and C–H···O angles (123 and
101.9° for C1’–O1–H34, 123.7° for C5’–O1–H34, 122.8° for 101.5°) are indicative of the H-bond. Compound 2c also shows
C41–O21–H4 and 105.6° for C41–O21–H4. These distances that complexation of an aromatic aglycon with tricar-
and angles allow protons H4 and H34 to interact with the lone bonylchromium does not have any significant influence on the
pairs of the oxygen atoms O1 and O21 of the sugar rings [26]. sugar ring. In Table 5, the distances and angles at the anomeric
Most likely, the two nonclassical hydrogen bonds in 2a are the center of 2c are compared to those for phenyl β-D-gluco-
reasons why two molecules crystallize as slightly different pyranoside (Ph-Glc) [27] showing that only the anomeric bond
pairs; a feature not observed in the other complexes.
is slightly shortened upon complexation.
Figure 3 shows the structure of compound 2b, which is the
anomer of 2a. Once again a nonclassical hydrogen bond is re-
sponsible for the interaction of the molecules. Contrary to 2a, in
which a nonclassical H-bond was found between the oxygen of
a sugar ring and the hydrogen of a benzene ring, the H-bond
forms in 2b between the oxygen of a carbonyl group (O13) and
the hydrogen of an acetyl group (H26B). However, the length of
this H-bond (2.212 Å) and the angles (161.6° for C≡O···H and
159.8° for C–H···O) prove the presence of the H-bond.
Figure 4: ORTEP-plot of the asymmetrical unit showing two mole-
cules of compound 2c and 30% probability ellipsoids; the arrows show
the nonclassical H-bonds between O12 and H6’A.
Figure 5 shows the structure of compound 2d, which contains a
hydrogen bond between carbonyl oxygen O13 of the left mole-
cule and hydrogen H5 of the benzene ring of the right molecule.
The length of this hydrogen bond is 2.498 Å and the angles
C≡O···H and C–H···O are 166.2 and 158.3°, respectively. All
atomic distances and angles of the tricarbonyl(η6-
benzene)chromium aglycon are almost identical to those of the
acetylated counterpart 2c. However, in 2d, the carbonyl groups
are in a staggered conformation (like in compound 2a) whereas
in 2c the carbonyl ligands adopt an eclipsed conformation.
Figure 3: ORTEP-plot of the asymmetric unit showing two molecules
of compound 2b and 30% probability ellipsoids; the arrows show the
nonclassical H-bonds between O13 and H26B.
Table 5: Comparison of selected distances and angles in 2c with phenyl β-D-glucopyranoside.
Bond
2c
phenyl β-D-glucopyranoside
C1’–O1
1.417 Å
1.418 Å
1.360 Å
119.1°
1.394 Å
1.434 Å
1.388 Å
118.0°
O1’–C1’
O1–C1
C1’–O1–C1
O1’–C1’–O1
106.7°
107.4°
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Beilstein J. Org. Chem. 2012, 8, 1059–1070.
Table 6: Bond lengths and angles for the nonclassical H-bonds in 2e.
H-bond
Length
Angle
Angle
C≡O···H
C–H···O
C≡O···H
O11–H4
O12–H5
O13–H3
2.594 Å
2.668 Å
2.656 Å
154.4°
142.5°
133.4°
94.5°
118.9°
125.6°
found between O13 of a carbonyl group and H22a of the acetyl
group at O6 of the sugar moiety. The bond lengths and angles
of this H-bond are in the expected range (2.709 Å for O···H,
138.2° for angle C–H···O and 93.5° for angle C≡O···H).
Figure 5: ORTEP-plot of the asymmetric unit showing two molecules
of compound 2d and 30% probability ellipsoids; the arrows show the
nonclassical H-bonds between O13 and H5.
All bond distances and angles for both the carbohydrate moiety
and the tricarbonyl(η6-benzene)chromium aglycon in com-
pound 2e (Figure 6) are within the values found for all of the
other complexes. The carbonyl groups adopt an eclipsed con-
formation like in 2c. In the crystal, two complexed benzene
rings face each other and are aligned parallel, with the tricar-
bonylchromium groups facing in opposite directions. One car-
bonyl group of each molecule is placed in between the benzene
rings and, thus, allows for the formation of a “network” of three
nonclassical H-bonds in the crystal. The lengths and angles of
these three hydrogen bonds in 2e are within the typical range
for nonclassical H-bonds [28-31] (Table 6).
Figure 7: ORTEP-plot of the asymmetric unit showing three mole-
cules of compound 2j and 30% probability ellipsoids; the arrows show
the nonclassical H-bonds between O13 and H22a.
The structure of the C-glycoside 2k (Figure 8) also shows a
“network” or nonclassical hydrogen bond in the crystal. The
molecules group similarly to compound 2e; however, the
benzene rings between two molecules (Figure 8, molecules on
the right side) are not oriented in parallel. Four nonclassical
hydrogen bonds form between the oxygen atoms of three car-
bonyl groups (O12 and O13) and the hydrogen atoms of two
methyl groups (H2’ and H22a), one methylene group (H1’) and
one aromatic hydrogen (H6). Table 7 summarizes the bond
lengths and angles of these nonclassical H-bonds.
Figure 6: ORTEP-plot of the asymmetrical unit showing two mole-
cules of compound 2e and 30% probability ellipsoids; the arrows show
the nonclassical H-bonds between O13 and H3, O11 and H4 and O12
and H5.
Table 7: Bond lengths and angles for the nonclassical H-bonds in 2k.
H-bond
Length
C≡O···H
Angle
C–H···O
Angle
C≡O···H
Figure 7 shows the crystal structure of the S-glycoside 2j in
which the molecules are stacked with all benzene rings in an
almost symmetric parallel orientation on one side. However, an
interaction of the tricarbonyl(η6-benzene)chromium rings can
be excluded because the distance between the benzene rings is,
with >3.37 Å, too long for such interactions, which only occur
at shorter distances [32-34]. A nonclassical hydrogen bond is
O11–H6
2.714 Å
2.719 Å
2.720 Å
2.632 Å
129.5°
127.1°
150.5°
164.2°
126.3°
118.9°
118.4°
130.3°
O13–H22a
O13–H1’
O12–H2’
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Beilstein J. Org. Chem. 2012, 8, 1059–1070.
Figure 8: ORTEP-plot of the asymmetric unit showing three molecules of compound 2k and 30% probability ellipsoids; the arrows show the nonclas-
sical H-bonds between O12 and H2’, O13 and H1’, O11 and H6 and O13 and H22A.
Figure 9 and Figure 10 show the crystal structures of the two
diastereomers 2m. The isomer with (pR)-configuration of the
tricarbonyl(η6-benzene)chromium group (Figure 9) shows one
intramolecular nonclassical hydrogen bond between carbonyl
oxygen O11 and H22B of the 2-O-acetyl group and one inter-
molecular H-bond between carbonyl oxygen O13 of the right
molecule and H24A of the 3-O-acetyl group of the left mole-
cule. All bond lengths and angles of these hydrogen bonds are
in the expected range for nonclassical H-bonds.
The structure of the diastereomer 2m with (pS)-configuration of
the tricarbonyl(η6-benzene)chromium group (Figure 10) resem-
bles compound 2j (Figure 7) in that the benzene rings are also
in a symmetric parallel orientation. However, the distance of the
benzene rings is, with 3.178 Å, shorter than in 2j but still too
long for an interaction of the aromatic rings. One nonclassical
hydrogen bond is found in pS-2m, between the oxygen O13 of a
carbonyl group and the anomeric hydrogen H1’ of the sugar
moiety. The bond lengths and angles of this H-bond are also in
Figure 10: ORTEP-plot of the asymmetric unit showing three mole-
cules of compound pS-2m and 30% probability ellipsoids; the arrows
show the nonclassical H-bonds between O13 and H1’.
Figure 9: ORTEP-plot of the asymmetric unit showing two molecules of compound pR-2m and 30% probability ellipsoids; the arrows show the
nonclassical intramolecular H-bonds between O11 and H22B and the intermolecular ones between O13 and H24A.
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Beilstein J. Org. Chem. 2012, 8, 1059–1070.
the expected range for nonclassical hydrogen bonds (2.699 Å nyl-2,3,4,6-tetra-O-acetyl-D-glucopyranosylamine (1i) [42],
for O···H, 128.7 degree for angle C–H···O and 167.3 degree for phenyl 2,3,4,6-tetra-O-acetyl-1-thio-β-D-glucopyranoside (1j)
angle C≡O···H).
[43], 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosylbenzene (1k)
[44], 2-methylphenyl 2,3,4,6-tetra-O-acetyl-β-D-gluco-
pyranoside (1m) [21], 2-(2,3,4,6-tetra-O-acetyl-β-D-glucopyra-
Conclusion
We have prepared and characterized a series of tricarbonyl(η6- nosyl)methylbenzene (1q) [22].
benzene)chromium complexes from phenyl and benzyl O-, N-,
S- and C-glycosides, and which were hitherto unknown. The Chromium complexes: general procedure. A solution of
X-ray diffraction of some of these glycoside-derived tricar- glycoside 1 (1 mol equiv) and Cr(CO)6 (1 mol equiv) in di-n-
bonylchromium complexes revealed crystal structures that butylether/THF 9:1 was heated in the dark under Ar at 140 °C
contain numerous nonclassical hydrogen bonds.
until TLC indicated complete consumption of 1 and was then
concentrated. Chromatography of the residue under Ar with
n-hexane/ethyl acetate 2:1 and immediate concentration
Experimental
General details. All solvents were dried and distilled prior to of the fractions containing the chromium complex gave 2. Crys-
their use. Reactions were performed under Ar and monitored by talline complexes 2 were slowly recrystallized from ethanol.
TLC on Polygram Sil G/UV silica gel plates from Machery & Suitable crystals were submitted to X-ray crystallographic
Nagel. Detection was effected by charring with H2SO4 (5% in analysis.
EtOH) or by inspection of the TLC plates under UV light. Reac-
Tricarbonyl[(2,3,4,6-tetra-O-acetyl-β-D-
tions involving Cr(CO)6 or chromium complexes were
glucopyranosyloxymethyl)-η6-
benzene]chromium (2a)
performed in brown glassware or in the dark. NMR spectra
were recorded on a Bruker ARX 250 spectrometer at 250 MHz
for proton spectra and 62.5 MHz for carbon spectra, on a
Bruker Avance 400 spectrometer at 400 MHz for proton spectra
and 100 MHz for carbon spectra, and on a Bruker AMX 600
spectrometer at 600 MHz for proton spectra and 150 MHz for
carbon spectra. Tetramethylsilane was used at the internal stan-
dard. Chemical shifts δ are given in parts per million (ppm) and
coupling constants in hertz (Hz). All NMR spectra were treated
as first-order spectra. HRMS was performed on a Bruker
Daltonics APEX 2 FT–ICR spectrometer. FAB–MS was
performed on a Finnigan MAT TSQ 70 spectrometer and
ionization with Xe. IR spectra were recorded with a Bruker
Tensor 27 IR spectrometer. UV spectra were recorded with a
Shimadzu UV 2102 PC spectrometer. Elemental analyses were
performed on a Hekatech Euro 3000 CHN analyzer. Optical
rotations were measured with a Perkin-Elmer Polarimeter 341.
Melting points were determined with a Büchi B-540 apparatus
and are uncorrected. Preparative chromatography was
performed on silica gel (0.032–0.063 mm) from Machery &
Nagel with different mixtures of solvents as eluent.
Treatment of 1a (3.0 g, 6.8 mmol) and Cr(CO)6 (1.50 g,
6.8 mmol) in di-n-butylether/THF (100 mL) for 96 h according
to the general procedure afforded 2a (1.14 g, 29%) as yellow
triclinic crystals: Mp 140–141 °C (EtOH); [α]D −11.0 (c 1.0,
toluene); IR (KBr): 1952, 1895 cm−1; FAB–MS (m/z): 597 [M
+ Na]+, 574 [M]+, 490 [M − 3CO]+; 1H NMR (acetone-d6) δ
5.74–5.56 (m, 5H, H-aryl), 5.28 (t, 1H, 3-H), 5.06 (t, 1H, 4-H),
4.98 (t, 1H, 2-H), 4.98 (dd, J1,2 = 7.3 Hz, 1H, 1-H), 4.68, 4.45
(dd, 2H, OCH2Ph), 4.27 (dd, 1H, 6a-H), 4.16 (dd, 1H, 6b-H),
4.01 (m, 1H, 5-H), 2.06–1.94 (m, 12H, OCH3); 13C NMR
(acetone-d6) δ 234.1 (Cr-CO), 170.7, 170.3, 170.0, 169.7
(O=CO), 109.3 (C1-aryl), 100.9 (C1), 95.3, 95.2, 94.0, 93.8,
93.8 (C-aryl), 72.8 (C3), 72.6 (C5), 71.9 (C2), 70.0 (OCH2),
69.3 (C4), 62.7 (C6), 20.6 (3C, OCH3), 20.5 (OCH3); Anal.
calcd for C24H26CrO13 (574.5): C, 50.18; H, 4.56; found: C,
50.10; H, 4.40.
Supporting Information
Supporting Information File 1
Experimental data.
Starting materials. The following glycosides 1 were prepared
according to literature procedures: Benzyl 2,3,4,6-tetra-O-
acetyl-β-D-glucopyranoside (1a) [35], benzyl 2,3,4,6-tetra-O-
acetyl-α-D-glucopyranoside (1b) [36], phenyl 2,3,4,6-tetra-O-
acetyl-β-D-glucopyranoside (1c) [37], phenyl 2,3,4,6-tetra-O-
methyl-β-D-glucopyranoside (1d) [38], phenyl 2,3,4,6-tetra-O-
acetyl-α-D-glucopyranoside (1e) [39], 3,4,6-tri-O-acetyl-1,2-(1-
phenoxy-1-ethylidene)-β-D-mannopyranose (1f) [37], phenyl
2,3,4,6-tetra-O-acetyl-β-D-mannopyranoside (1g) [40], benzoyl
2,3,4,6-tetra-O-acetyl-β-D-glucopyranoside (1h) [41], N-phe-
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-118-S1.pdf]
Acknowledgements
This work was financially supported by the Deutsche
Forschungsgemeinschaft. We thank the groups of Professor K.
Albert and Professor K. Zeller for measuring the NMR and MS
spectra.
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Beilstein J. Org. Chem. 2012, 8, 1059–1070.
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