New Reagent for Reductive Coupling of Carbonyl and Imine Compounds: Highly Reactive Manganese-Mediated Pinacol Coupling of Aryl Aldehydes, Aryl Ketones, and Aldimines

Full text of DOI:10.1021/jo971942y

J . Org. Chem. 1998, 63, 5235-5239
5235
Generally, the reaction is effected by treatment of
carbonyl compounds with an appropriate metal reagent
and/or metal complex to give rise to the corresponding
coupled product. A number of different types of metal
reagents have been used to carry out the pinacol reaction.
For instance, the reaction with various low-valent metal
complexes of Al,5 Ce,6 Fe,7 Mg,8 Nb,9 Sm,10 Si,11 Ti,12 V,13
Yb,14 Zr,15 and main group organometallic hydrides (Bu3-
SnH, Ph3SnH, Bu3GeH, and (TMS)3SiH)16 afforded inter-
or intramolecular coupling products of carbonyls.
New Rea gen t for Red u ctive Cou p lin g of
Ca r bon yl a n d Im in e Com p ou n d s: High ly
Rea ctive Ma n ga n ese-Med ia ted P in a col
Cou p lin g of Ar yl Ald eh yd es, Ar yl Keton es,
a n d Ald im in es
Reuben D. Rieke* and Seung-Hoi Kim
Department of Chemistry, University of NebraskasLincoln,
Lincoln, Nebraska 68588-0304
The coupling products can have two newly formed
stereocenters. As a consequence, efficient reaction condi-
tions have been required to control the stereochemistry
of the 1,2-diols. Recent efforts have focused on the
development of new reagents and reaction systems to
improve the reactivity of the reagents and diasteroselec-
tivity of the products. In addition, functional group
tolerance has been a challenge for reductive coupling of
carbonyl compounds using the reaction systems men-
tioned above, and many successful results have been
reported.
In 1984, Lukehart17 reported a formal reductive cou-
pling of mangana-â-diketonato complexes to give carbon-
carbon bond formation. Manganese complexes also have
been used as a good single-electron source in oxidative
free-radical cyclizations.18 On the basis of these findings,
we postulated that our active manganese might be a good
single-electron donor and, therefore, be utilized for pi-
nacol coupling.
Received October 21, 1997
In tr od u ction
One of the most powerful methods for constructing
carbon-carbon bond is the reductive coupling of carbonyl
compounds giving olefins and/or 1,2-diols.1 Of these
methods, the pinacol coupling,2 which was described in
1859, is still a useful tool for the synthesis of vicinal diols.
The corresponding products of this reaction can be used
as intermediates for the preparation of ketones and
alkenes.3 More importantly, this methodology has been
applied to the synthesis of biologically active natural
compounds.4
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1994, 35, 6671. (c) Corey, E. J .; Danheiser, R. L.; Chandrasekaran, S.;
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During the course of our investigation on active metals,
we have found that treatment of aryl aldehydes and aryl
ketones with active manganese (Mn*) prepared via the
Rieke method gave the corresponding pinacolic coupling
product (Tables 1 and 2). As shown in Scheme 1, the
reaction of aldimines with this active manganese also
afforded the corresponding coupling products, vicinal
diamines. Due to the potential utility of vicinal diamines
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S0022-3263(97)01942-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/07/1998

5236 J . Org. Chem., Vol. 63, No. 15, 1998
Notes
Ta ble 2. Cou p lin g Rea ction of Ald eh yd es w ith Active
Ta ble 1. Stu d y of Rea ctivity Dep en d in g on Ha lid es
Ma n ga n ese
MnX2
condns (T (°C)/time)
yielda (%)
dl/mesob
MnCl2
MnBr2
MnI2
rt/30 min
rt/overnight
rt/24 h
86
89
80
63:37
68:32
54:46
a
b
Isolated yields (based on aldehyde). Purified from recrystal-
lization (2% ethyl ether/hexanes).
in organic synthesis, especially in natural products and
medicinal reagents,19 synthetic methodologies of vicinal
diamines have attracted considerable attention. To date,
several methods have been described to perform the
reductive dimerization of imines into vicinal diamines.20
The most widely used method is metal-mediated pinacolic
coupling of imines. Several metals have been utilized,
including samarium,21 sodium,20c,22 ziroconium,23 alumi-
num, bismuth,24 zinc,25 titanium,26 indium,27 niobium,28
and ytterbium.29
To date, no report has appeared using manganese
metal. In this paper, we wish to report a mild method
for reductive coupling reactions of aryl aldehydes, aryl
ketones, and aldimines.
Resu lts a n d Discu ssion
Reduction of manganese halides (MnI2, MnBr2, and
MnCl2) by Li using naphthalene as an electron carrier
in THF affords a slurry of Mn* at room temperature. The
resulting Mn*, however, appears to be partially soluble
in THF. Due to this, no washing was performed to
remove naphthalene and other salts, and the Mn* was
used as a slurry. The preparation of the active metals
and the subsequent reaction of the organometallic re-
agents are conducted under an argon atmosphere.
Red u ctive Cou p lin g Rea ction s of Ar yl Ald eh yd es.
As described above, three different types of manganese
halides can be used to prepare active manganese. There-
fore, reactivity depending on metal halide was studied
first. The results are summarized in Table 1, and it was
found that there are no significant differences in both
(19) (a) Pasini, A.; Zunino, F. Angew. Chem., Int. Ed. Engl. 1987,
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H.; Schonenerger, H. J . Med. Chem. 1982, 25, 832.
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(b) Natsugari, H.; Whittle, R. R.; Weinreb, S. M. J . Am. Chem. Soc.
1984, 106, 7987. (c) Smith, J . G.; Ho, I. J . Org. Chem. 1972, 37, 653.
(d) Smith, J . G.; Veach, C. D. Can. J . Chem. 1966, 44, 2497.
(21) Enholm, E. J .; Forbes, D. C.; Holub, D. P. Synth. Commun.
1990, 20, 981.
(22) Eisch, J . J .; Kaska, D. D.; Peterson, C. J . J . Org. Chem. 1966,
31, 453.
(23) Buchwald, S. J .; Watson, B. T.; Wannamaker, M. W.; Dewan,
J . C. J . Am. Chem. Soc. 1989, 14, 4486.
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36, 6747.
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Q. Tetrahedron Lett. 1992, 33, 5559. (b) Khan, N. H.; Zubari, R. H.;
Siddiqui, A. A. Synth. Commun. 1980, 363. (c) Smith, J . G.; Boettger,
T. J . Synth. Commun. 1981, 61.
a
All products have been fully characterized by 1H NMR, 13C
b
NMR, and HRFAB-MS. Isolated yields (based on aldehydes).
c Obtained from recrystallization (1-2% ethyl ether/hexanes)
unless mentioned. Determined by 1H NMR (300 MHz) analysis
d
of the isolated product. e Reaction was carried out at
0 °C.
f Obtained from flash chromatography. g According to TLC analy-
sis, starting material (aldehyde) was recovered after hydrolysis
of the reaction mixture.
(26) Mangeney, P.; Tejero, T.; Alexakis, A.; Grosjean, F.; Normant,
J . Synthesis 1988, 255.
(27) Kalyanam, N.; Rao, G. V. Tetrahedron Lett. 1993, 34, 1647.
(28) Roskamp, E. J .; Pedersen, S. F. J . Am. Chem. Soc. 1987, 109,
6551.
(29) Takaki, K.; Tsubaki, Y.; Tanaka, S.; Beppu, F.; Fujiwara, Y.
Chem. Lett. 1990, 203.
yield and stereochemistry of the product (2a ). Good
isolated yields (80-89%) and an almost 1:1 mixture of
dl/meso isomers were obtained from three different
attempts described in Table 1. Longer reaction times
were required to complete the coupling reaction in the

Notes
Sch em e 1. Red u ctive Cou p lin g of Ald im in esa
J . Org. Chem., Vol. 63, No. 15, 1998 5237
Ta ble 3. Cou p lin g Rea ction of Keton es w ith Active
Ma n ga n ese
a
Key: (a) Determined by 13C NMR (75 MNH) analysis.
case of manganese iodide. While we have not conducted
a detailed study of the reaction pathway, we believe that
this reaction proceeds via one of the mechanisms pre-
sented in ref 2b.
Various aromatic aldehydes were reacted with the
active manganese prepared from using manganese chlo-
ride. Table 2 represents that simple treatment of aryl
aldehydes with active manganese gives the corresponding
1,2-diols in moderate to good isolated yields under very
mild conditions, room temperature in THF. The results
are shown in Table 2.
It is proposed that the reaction proceeds via a single-
electron transfer (SET).2b With the active manganese
supplying the electrons, the ratio of manganese to alde-
hyde was found to be very important with respect to
overall yield. Entry 1 in Table 2 shows that a higher
yield is obtained using 2 equiv of Mn* and 1 equiv of
aldehyde. This appears to be general for all aldehydes
and ketones. All of the reactions with different mole
ratios were carried out at room temperature in 30 min.
Reaction temperature had little effect on the yield.
Reactions at room temperature and 0 °C in 30 min gave
64% and 62% isolated yields, respectively (entry 1 in
Table 2).
a
All products have been fully characterized by 1H NMR, 13C
NMR, and HRFAB-MS. Isolated yields (based on ketones). c Pu-
b
d
rified from chromatography unless mentioned. Determined by
1H NMR (300 MHz) or 13C NMR (75 MHz) analysis of the isolated
product. e Obtained from recrystallization (1-2% ethyl ether/
hexanes).
It should be mentioned that this reaction exhibits
tolerance of several functional groups on the aromatic
ring, including bromine, chlorine, cyano, and methoxy
(entries 2-5 in Table 2). According to the high-resolution
FAB-MS of 2b-g in Table 2 and 6 in Table 3, the
functional groups are still retained in the final products.
Unfortunately, according to TLC analysis of the reaction
mixture after hydrolysis, the starting material was solely
recovered in the coupling reaction of vanillin, which bears
a hydroxy group on the aromatic ring (entry 9 in Table
2). Even when 5 equiv of Mn* was used the same result
was observed. Probably, the acidic hydrogen of phenolic
OH reacts with the Mn* quenching the reaction.
To investigate possible steric effects of substitutents
on the coupling reaction, o-, m-, and p-substituted alde-
hydes were examined. o-, m-, and p-methoxybenzalde-
hydes were reacted with Mn* under the same conditions
(entries 5-7 in Table 2). For these molecules, overnight
stirring at room temperature was required to complete
the reaction. The results shown in Table 2 indicate that
no steric hindrance is observed in terms of yield (83-
90%). Table 2 also shows that high diasteroselectivity
is not observed in this reaction system except in the case
of 3-bromobenzaldehyde. In the reaction of 3-bromoben-
zaldehyde, the dl-isomer was the major product (dl/meso
93:7), but the reason for this is not clear. To investigate
the change of dl/meso ratios depending upon the purifica-
tion protocols, two different methods, column chroma-
tography and recrystallization, were used to purify the
crude reaction mixture obtained from the reaction of 1
equiv of Mn* and 0.5 equiv of benzaldehyde at room
temperature. In both cases, the same dl/meso (76:24) was
observed. This ratio was essentially the same as that of
1
the crude mixture. The H NMR spectrum of the crude
mixture indicated a dl/meso ratio of 74:26. A slightly
different dl/meso ratio (79:21) was obtained from the
recrystallization (20% ethyl ether/hexanes) of prechro-
matographically purified product. Interestingly, a sig-
nificant change in the dl/meso ratio was observed in the
coupling product of 4-chlorobenzaldehyde. Recrystalli-
zation of the prechromatographically purified product (dl/
meso, 63:37) was carried out by using 20% ethyl ether/
hexanes. The 1H NMR spectrum of the recrystallized
product showed a higher dl/meso ratio (95:5). This is
clearly a result of partial separation upon purification.
In addition to the coupling of aryl aldehydes, it should
be mentioned that a multicompound mixture was ob-
tained from the reactions using aliphatic aldehydes with
Mn*. According to Yanada’s work, the radical intermedi-
ates from the aliphatic aldehydes are unstable and do
not have enough time to react with each other to give
diols.
Red u ctive Cou p lin g Rea ction of Ar yl Keton es.
Table 3 represents the reductive coupling reactions of

5238 J . Org. Chem., Vol. 63, No. 15, 1998
Notes
prepurified-grade argon was further purified by passage over a
BASF R3-11 catalyst column at 150 °C, a phosphorus pentoxide
column, and a column of granular potassium hydroxide. Lithium,
naphthalene, and metal halides were weighed out and charged
into reaction flasks under in a Vacuum Atmospheres Co. drybox.
Tetrahydrofuran was distilled immediately before use from Na/K
alloy under an atmosphere of argon.
Analytical thin-layer chromatography was performed using
Merck 5735 indicating plates precoated with silica gel 60 F254
(layer thickness 0.2 mm). The product spots were visualized
with either UV light (254 nm) or a solution of vanillin. Prepara-
tive thin-layer chromatographic separations were obtained using
Analtech silica gel GF (layer thickness 2 mm) preparative plates.
Liquid chromatographic purifications were performed by flash
column chromatography using glass columns packed with Merck
silica gel 60 (230-400 mesh).
P r ep a r a t ion of H igh ly R ea ct ive Ma n ga n ese (Mn *).
Highly reactive manganese was prepared by the reduction of
anhydrous manganese halides (chloride, bromide, and iodide)
with lithium using naphthalene as an electron carrier. In a
typical preparation, lithium (9.68 mmol), naphthalene (1.48
mmol), and anhydrous manganese chloride (4.71 mmol) were
stirred in freshly distilled THF (15 mL) for 1-3 h at room
temperature. A black slurry was obtained and ready for use.
(Note: The number of millimoles of Mn* cited in this paper
refers to the theoretical amount possible, based on the original
amount of anhydrous manganese halide).
A Typ ica l P r oced u r e for th e P r ep a r a tion of 1,2-Diols
(2a -2h ) fr om th e Rea ction s of Ar yl Ald eh yd es w ith Mn *.
To a slurry of Mn* was added aryl aldehyde (0.5-0.8 equiv,
based on Mn*) via syringe (or cannula) at room temperature.
The resulting mixture was allowed to stir for 30 min to
overnight. The reaction was monitored by TLC. After being
stirred, the reaction was quenched with an aqueous solution of
3 M HCl (20 mL) and then extracted with ethyl acetate (3 × 20
mL). The combined organic layers were washed with saturated
NaCl solution (2 × 30 mL) and dried over anhydrous MgSO4.
Evaporation of solvents and flash chromatography (hexanes/
ethyl acetate) and/or recrystallization afforded the desired diols
in the indicated yield and diastereoselectivity (Table 2).
Diol 2a (Mixture of dl/Meso-Isomers).30 Recrystallization
(hexanes/ethyl ether) of the crude mixture afforded 2a in 86%
yield: 1H NMR (CDCl3) δ 7.35-7.10 (m, 10H), 4.82, 4.69 (ss,
2H), 2.71 (br, 2H); 13C NMR (CDCl3) δ 139.83, 139.71, 128.17,
128.08, 128.04, 127.86, 127.06, 126.92, 79.05, 78.01; HRFAB-
MS calcd for C14H14O2 217.0994, found (M + Na)+ 237.0898.
Typ ica l P r oced u r e for th e P r ep a r a tion of 1,2-Diols fr om
th e Rea ction of Ar yl Keton es w ith Mn *. The standard
procedure used for aryl aldehyde was followed with aryl ketone.
Appropriate workup afforded 1,2-diols (4, 6, 8, 10, and 12) in
71-93% isolated yields.
aryl ketones using active manganese. These reactions
were also conducted under almost the same conditions
used in aryl aldehyde couplings, and the results are
summarized in Table 3. Isolated yields were good to
excellent (71-93%), and much higher stereoselectivity
was observed. As observed in the aldehyde couplings,
the mole ratio of carbonyl to metal reagent greatly effects
the yield of the final product as is shown in entry 1 in
Table 3. For instance, the reaction of 0.5 equiv of
acetophenone (3) with 1 equiv of active manganese
afforded the 1,2-diol derivative 4 in higher yield (71%)
than that (54%) using 0.8 equiv of ketone with 1.0 equiv
of Mn*. Interestingly, much higher diasteroselectivity
is observed. The reason for this may be steric. The
corresponding coupling products (6, 8, and 10) of alkyl-
aryl-substituted ketones (5, 7, and 9), were obtained in
good yields (88-93%) with moderate to good diastereo-
selectivities under the conditions presented in Table 3.
Even for a bulky biaryl ketone, the expected coupling
product was obtained in high yield. For example, treat-
ment of benzophenone (11) with active manganese gave
1,1,2,2-tetraphenylethanediol (12) in 91% isolated yield.
Once again, attempts to reductively couple alkyl ketones
with Mn* gave a multicompound mixture, which was not
identified.
Red u ctive Cou p lin g of Ald im in es. It was also
found that the active manganese could be used for the
coupling reaction of aldimines with Mn*. As shown in
Scheme 1, mild reaction conditions, room temperature
in THF, were used to carry out the reaction. The imines
(13 and 15) and active manganese were allowed to stir
overnight at room temperature. After appropriate work-
up, the corresponding vicinal diamines (14 and 16) were
afforded in 62% and 56% isolated yields, respectively,
with poor diastereoselectivity. 1H NMR and 13C NMR
data are consistent with the literature values. Mecha-
nism and optimization studies have not been conducted.
However, we believe that this coupling also proceeds via
a pinacol-type, single-electron-transfer (SET) mechanism.2b
We were not able to detect any of the monoamines
resulting from the reduction of the starting aldimines.
Con clu sion
In conclusion, we have demonstrated that highly
reactive manganese can be used for the reductive dimer-
ization of aryl aldehydes, aryl ketones, and aldimines.
With this highly reactive manganese, the corresponding
coupling products, 1,2-diols and vicinal diamines, were
obtained in good isolated yields. The diasteroselectivity
is poor except for aryl ketones. It is well-known that
pinacol coupling proceeds via a single-electron-transfer
mechanism. Accordingly, this work demonstrates that
the highly reactive manganese can be used as a good
single-electron donor in organic synthesis. Other reac-
tions are under investigation.
Diol
4 (mixture of dl/meso-isomers) was obtained from
recrystallization (hexanes/ethyl ether) of the crude mixture in
71% yield: 1H NMR (CDCl3) δ 7.27-7.18 (m, 10H), 2.58 (br, 2H),
1.60, 1.51 (ss, 6H); 13C NMR (CDCl3) δ 143.41, 127.34, 127.12,
127.03, 78.83, 24.92; HRFAB-MS calcd for C16H18O2 242.1307,
found (M + Na)+ 265.1197.
Red u ctive Cou p lin g Rea ction of Ald im in es (13 a n d 15)
in to Vicin a l Dia m in es (14 a n d 16). The standard procedure
used for aryl aldehyde was followed using aldimine and Mn*
prepared from MnI2. During the workup step, the combined
organic layers were washed with saturated Na2S2O3 solution to
remove iodine species from the reaction mixture. Flash chro-
matography (hexanes/ethyl acetate) afforded the corresponding
vicinal diamines in 62% and 56% isolated yields, respectively.
Dia m in e 14 (Mixture of dl/Meso-Isomers). Flash chroma-
tography (hexanes/ethyl acetate) of the crude mixture afforded
14 in 62% yield: 1H NMR (CDCl3) δ 7.25-6.52 (m, 20H), 4.95,
4.57 (ss, 4H); 13C NMR (CDCl3) δ 146.49, 139.90, 138.21, 129.20,
129.08, 128.40, 128.26, 127.56, 127.51, 127.35, 118.10, 117.82,
114.09, 113.74, 63.97, 61.96; HRFAB-MS calcd for C26H24N2
364.1939, found (M + H)+ 365.2023.
Exp er im en ta l Section
Gen er a l Meth od s. 1H NMR (300 MHz) spectra were re-
corded in CDCl3 or CDCl3/DMSO-d6 solution. All chemical shifts
are reported in parts per million (δ) downfield from internal
tetramethylsilane. Fully decoupled 13C NMR (75 MHz) spectra
were recorded in CDCl3 or CDCl3/DMSO-d6 solution. The center
peak of CDCl3 (77.0 ppm) was used as the internal reference.
Mass spectra were performed by the Nebraska Center for Mass
Spectrometry at the University of NebraskasLincoln.
(30) The R-proton to the hydroxyl group in the dl-isomer appears
ca. 0.1-0.2 ppm higher field than that of the meso-isomer: see ref
12k.
All manipulations were carried out under an atmosphere of
argon on a dual manifold vacuum/argon system. The Linde

Notes
Dia m in e 16 (Mixture of dl/Meso-Isomers). Flash chroma-
tography (hexanes/ethyl acetate) of the crude mixture afforded
16 in 56% yield: 1H NMR (CDCl3) δ 7.33-7.19 (m, 16H), 6.99-
6.98 (m, 4H), 3.77, 3.76 (ss, 2H), 3.44 (center of AB system, J )
17.1 Hz, 4H), 1.72 (s, 2H); 13C NMR (CDCl3) δ 140.80, 140.31,
128.58, 128.34, 128.17, 127.86, 127.63, 126.65, 67.17, 50.91.
J . Org. Chem., Vol. 63, No. 15, 1998 5239
Su p p or tin g In for m a tion Ava ila ble: Characterization
data of 2b-h , 6, 8, 10, and 12 and copies of the 1H and 13C
NMR spectra for 2a -h , 4, 6, 8, 10, 14, and 16 (33 pages). This
material is contained in libraries on microfiche, immediately
follows this article in the microfilm version of the journal, and
can be ordered from the ACS; see any current masthead page
for ordering information.
Ack n ow led gm en t. The financial support provided
by the National Science Foundation is gratefully ac-
knowledged.
J O971942Y