Biphasic manganese carbonyl reactions: A new approach to making carbon-carbon bonds

Article abstract of DOI:10.1016/S0040-4039(02)00323-4

A new, mild and practical biphasic method for conducting manganese carbonyl reactions, which lead to the formation of carbon-carbon bonds, has been developed: the mechanisms of these reactions can involve radical and/or ionic pathways.

Full text of DOI:10.1016/S0040-4039(02)00323-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2535–2538
Biphasic manganese carbonyl reactions: a new approach to
making carbon–carbon bonds
Nathalie Huther,a P. Terry McGrailb and Andrew F. Parsonsa,*
aDepartment of Chemistry, University of York, Heslington, York YO10 5DD, UK
bCytec Fiberite Ltd, N131 Wilton Centre, Wilton, Redcar TS10 4RF, UK
Received 2 January 2002; revised 5 February 2002; accepted 15 February 2002
Abstract—A new, mild and practical biphasic method for conducting manganese carbonyl reactions, which lead to the formation
of carbon–carbon bonds, has been developed: the mechanisms of these reactions can involve radical and/or ionic pathways.
© 2002 Elsevier Science Ltd. All rights reserved.
Free-radical reactions, particularly cyclisations, have
been extensively studied over the past 20 years and by
far the most common method of initiation involves
reaction of organohalide (or related) precursors with
tributyltin hydride.1 However, this method is far from
ideal and the toxicity and difficulty of removing tin-
containing by-products has led to the development of
alternative reagents for radical generation.2 Recent
examples include silylated cyclohexadienes,3 hypophos-
phorous acid (and its salts)4 and phosphites,5 although
the use of tributyltin hydride still dominates. As part of
a programme to develop alternative and more versatile
free-radical initiators we have recently investigated the
use of dimanganese decacarbonyl [Mn2(CO)10] in
organic synthesis. This work has shown that photolysis
ence of an aqueous solution of sodium hydroxide and
the phase-transfer catalyst benzyltriethylammonium
chloride (BTAC). Under these conditions, it was antici-
pated that the manganese halide byproducts would
react with hydroxide to form water-soluble manganese
complexes [e.g. Et3(Bn)N+Mn2(CO)9Br−, Et3(Bn)N+
Mn(CO)−5].8
Our first experiments centred on the preparation of
bibenzyl 2 from benzyl bromide 1 (Scheme 1). Hence
photolysis of
1 with Mn2(CO)10 (0.5 equiv.) in
dichloromethane in the presence of aqueous sodium
hydroxide and BTAC gave the desired dimer 2 in 36%
yield.9,10 This reaction also produced benzylmanganese
pentacarbonyl 3 in 43% yield, which was unexpected as
this compound was not formed when the same reaction
was carried out solely in dichloromethane.6 Similar
biphasic reactions using different amounts of
Mn2(CO)10 also gave both 2 and 3 (e.g. using 1 equiva-
lent of Mn2(CO)10 gave 2 and 3 in 81 and 19% yield,
respectively) although the yield of 2 generally increased
(and 3 decreased) with reaction times of >2 h. This
represents an exceptionally mild method of forming 3,
which is traditionally prepared from reaction of
Mn2(CO)10 with Na/Hg and benzyl bromide (or chlo-
ride) in anhydrous diethyl ether.11 Indeed, in the
absence of photolysis, 3 is the major compound [e.g.
’
of Mn2(CO)10 generates Mn(CO)5, which reacts with
organohalides in CH2Cl2 to efficiently initiate a variety
of radical coupling6 and cyclisation reactions under
mild conditions.7
For these reactions the manganese halide byproducts
[of type XMn(CO)5] can be removed by treating the
crude reaction mixture with excess DBU; this forms a
polar manganese complex, which can be separated from
organic products by column chromatography. For
polar organic compounds removal of excess DBU and/
or the manganese–DBU complex can be problematic
and so we sought to develop an alternative method,
which involves carrying out the reactions in the pres-
Keywords: manganese and compounds; radicals and radical reactions;
cyclisation; coupling reactions.
* Corresponding author. Tel.: +44-1904-432608; fax: +44-1904-
432516; e-mail: afp2@york.ac.uk
Scheme 1.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00323-4

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N. Huther et al. / Tetrahedron Letters 43 (2002) 2535–2538
The pronounced effect of employing biphasic condi-
tions was also apparent for reactions involving sec-
ondary bromide 4 (Scheme 2). Whereas only unreacted
starting material was isolated on photolysis of 4 with
Mn2(CO)10 in CH2Cl2, the same reaction in the pres-
ence of aqueous hydroxide and BTAC produced dimer
5 (as a 1:1 mixture of diastereomers) in 75% yield.
When BrMn(CO)5 was used in place of Mn2(CO)10 in
the biphasic reaction, dimer 5 was isolated in the same
yield (75%).16 It is therefore likely that 5 is formed via
reaction of bromide 4 with −Mn(CO)5 rather than
after 2 h, 3 was isolated in 30% yield (or 96% based on
recovered 1)] and only trace amounts of 2 are formed.
The most likely route to compound 3 involves nucle-
ophilic substitution of bromide 1 by the manganese
−
pentacarbonyl anion Mn(CO)5. Benzylmanganese pen-
tacarbonyl 3 could then react with bromide 1 to form 2,
in a second nucleophilic substitution reaction, or alter-
’
natively, the manganese radical Mn(CO)5 could
abstract a bromine atom from 1 to form the benzyl
radical which dimerises: the formation of the benzyl
radical was confirmed by carrying out photolysis exper-
iments in the presence of TEMPO, which led to the
formation of 1-benzyloxy-2,2,6,6-tetramethylpiperidine
(in 30–37% yield). It should also be noted that the
benzyl radical could be formed on homolysis of the
weak Mn–C bond in 3 (ꢀ105 kJ mol−1)11,12, although
attempts to convert 3 to 2 (using the same photolysis
conditions as shown in Scheme 1) were unsuccessful.
’
Mn(CO)5.
Radical cyclisation reactions can also be carried out
using biphasic conditions. For example, reaction of
bromotrichloromethane with diene 6 and Mn2(CO)10
(0.1 equiv. w.r.t. BrCCl3 or 0.3 equiv. w.r.t. diene 6) or
BrMn(CO)5 (0.2 equiv. w.r.t. BrCCl3) afforded tetra-
hydrofuran 7 in essentially quantitative yield (Scheme
3). The use of BrMn(CO)5 is of particular interest
because this is the first time that this reagent has been
reported to initiate radical cyclisation reactions. A sim-
ilar result was obtained using dienes 8 and 9, and in all
cases the crude products were not contaminated with
any BrMn(CO)5 as shown by the infrared spectra.
−
The Mn(CO)5 could arise from reaction of Mn2(CO)10
and/or BrMn(CO)5 with hydroxide.8,13 To clarify the
mechanism, bromide 1 was reacted under the same
conditions with BrMn(CO)5 (2 equiv.) in place of
Mn2(CO)10.14 This produced bibenzyl 2 and benzylman-
ganese pentacarbonyl 3 in 60 and 34% yield, respec-
A more complex result was obtained on reaction of
BrMn(CO)5 [or Mn2(CO)10] with trichloroacetamide 9
in the absence of BrCCl3 (Scheme 4). This gave rise to
four products in a combined 75% yield. Each of the
products 10–13 could be formed by radical reactions.
For example, dichloroamide 10 could be formed by
−
tively, which points to the formation of the Mn(CO)5
anion from BrMn(CO)5. However, the formation of
’
Mn(CO)5 cannot be completely discounted from this
reaction as photolysis of BrMn(CO)5 is known to gen-
erate Mn2(CO)10 and this was confirmed by both TLC
and IR analysis of the crude reaction mixture.15
’
abstraction of a chlorine atom from 9 [by Mn(CO)5]
followed by abstraction of a hydrogen atom (from, for
example, CH2Cl2) by the intermediate dichloroamide
radical. However, 10–13 could also be formed from
intermediate organomanganese adducts of the type R–
Mn(CO)5. Thus, dichloroamide 10 could be prepared
on hydrolysis of an intermediate organomanganese
Mn2(CO)10 or BrMn(CO)5
Ph
Ph
Ph
Br
CH2Cl2, hv, NaOH,
H2O, BTAC
75%
4
5
compound generated from reaction of
9
with
Scheme 2.
Scheme 3.

N. Huther et al. / Tetrahedron Letters 43 (2002) 2535–2538
2537
Cl
Cl
Cl
Cl
O
N
O
N
BrMn(CO)5
(1 equiv.)
CCl3
N
10 (18%)
11 (16%)
+
O
CH2Cl2, hv,
NaOH, H2O,
BTAC
H
Cl
Cl
9
O
N
O
N
12 (21%)
13 (20%)
Scheme 4.
−Mn(CO)5. In order to distinguish these pathways, the
same reaction [using BrMn(CO)5] was carried out in
D2O rather than H2O. Any incorporation of deuterium
in the products would identify compounds derived from
intermediate organomanganese adducts. Interestingly,
monochlorinated products 12 and 13 showed complete
deuteration of the CH2Cl and CHCl groups adjacent to
the amide carbonyl (by 1H NMR spectroscopy). In
comparison, dichloroamide 10 showed 50% incorpora-
tion of deuterium at the CHCl2 position while cyclic
dichloride 11 did not contain any deuterium. This
suggests that initial organomanganese adducts are
Acknowledgements
We thank Cytec Fiberite Ltd for support and the
University of York for funding.
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N. Huther et al. / Tetrahedron Letters 43 (2002) 2535–2538
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