Radical-chain oxidative addition mechanism for the reaction of an [Re(CO)5]- anion with α-bromostilbene

Article abstract of DOI:10.1039/c2dt32552g

E-α-Bromostilbene spontaneously reacts with Na[Re(CO)₅] at 22 °C in THF to give Na[ReBr(CO)₄{Z-C(Ph)CHPh}] and Na[Re ₂(CO)₉{Z-C(Ph)CHPh}] as the main products. Z-α-Bromostilbene is less reactive, but gives the same products. The reaction is stimulated by visible light or a source of solvated electrons (NaK_2.8) and can be inhibited by a quinomethide radical trap. With an excess of Na[Re(CO)₅] one can observe the initial formation of Na[ReBr(CO)₄{Z-C(Ph)CHPh}] and its complete transformation into Na[Re₂(CO)₉{Z-C(Ph)CHPh}]. Treatment of Na[ReBr(CO) ₄{Z-C(Ph)CHPh}] with CO almost quantitatively converts it to [Re(CO)₅{Z-C(Ph)CHPh}], the structure of which is established by a single-crystal X-ray diffraction study. A radical-chain mechanism is proposed for the reaction comprising the following steps: (a) coupling of a Vin radical with Na[Re(CO)₅], (b) CO-dissociation from the formed 19-electron radical-anion and (c) bromine atom abstraction by [Re(CO)₄{Z-C(Ph) CHPh}]^- from α-bromostilbene. The mechanism is confirmed by the formation of the same Na[ReBr(CO)₄{Z-C(Ph)CHPh}] product in the presence of NaI. When the radical-chain process is inhibited, a slow halogenophilic reaction is observed, mainly giving the Z and E-isomers of the acylrhenate Na[Re₂(CO)₉{C(O)C(Ph)CHPh}].

Full text of DOI:10.1039/c2dt32552g

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Radicalꢀchain oxidative addition mechanism for the reaction of
[Re(CO)5]ꢀ anion with αꢀbromostilbene
Petr K. Sazonova*, Dmitry S. Ptushkina, Victor N. Khrustalevb, Natal'ya G. Kolotyrkinac, and Irina P.
Beletskayaa*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
EꢀαꢀBromostilbene spontaneously reacts with Na[Re(CO)5] at 22°C in THF to give
Na[ReBr(CO)4{ZꢀC(Ph)=CHPh}] and Na[Re2(CO)9{ZꢀC(Ph)=CHPh}] as the main products.
ZꢀαꢀBromostilbene is less reactive, but gives the same products. The reaction is stimulated by visible
10 light or a source of solvated electrons (NaK2.8) and can be inhibited by a quinomethide radical trap. With
an excess of Na[Re(CO)5] one can observe the initial formation of Na[ReBr(CO)4{ZꢀC(Ph)=CHPh}] and
its complete transformation into Na[Re2(CO)9{ZꢀC(Ph)=CHPh}]. Treatment of
Na[ReBr(CO)4{ZꢀC(Ph)=CHPh}] with CO almost quantitatively converts it to
[Re(CO)5{ZꢀC(Ph)=CHPh}], the structure of which is established by singleꢀcrystal Xꢀray diffraction
15 study. A radicalꢀchain mechanism is proposed for the reaction comprising the following steps: (a)
coupling of Vinꢁ radical with Na[Re(CO)5], (b) COꢀdissociation from the formed 19ꢀelecron radicalꢀanion
and (c) bromine atom abstraction by [Re(CO)4{ZꢀC(Ph)=CHPh}]ꢁꢀ from αꢀbromostilbene. The
mechanism is confirmed by the formation of the same Na[ReBr(CO)4{ZꢀC(Ph)=CHPh}] product in the
presence of NaI. When the radicalꢀchain process is inhibited, a slow halogenophilic reaction is observed,
20 mainly giving the Z and Eꢀisomers of the acylrhenate Na[Re2(CO)9{C(O)C(Ph)=CHPh}].
Introduction
M(CO)nL
Hal-
Metal carbonyl anions (carbonylmetallates) are an important class
of metalꢀcentred nucleophiles, and since 1960s have been widely
-
25 used for the introduction of transition metalꢀcarbon bond in
various organic electrophiles.1ꢀ4 From a mechanistic viewpoint,
carbonylmetallates often behave differently from ordinary
nucleophiles, such as alcoholates, thiolates and carbanions. They
are characterized by supernucleophilicity ([Fe(CO)2Cp]ꢀ and
30 [Fe(CO)4]2ꢀ),3, 5ꢀ7 “supersoftness” (e.g. higher than usual kI/kOTos
or kI/kBr ratios)5 and high reducing power. Evidence of an outerꢀ
[M(CO)nL] -
+ L(CO)nMHal
Hal
C
Re(CO)4Hal
M(CO)nL = Re(CO)5
O
sphere single electron transfer (SET) mechanism was obtained for
Scheme 1. The halogenophilic mechanism of nucleophilic vinylic
6
the reactions of [Fe(CO)2Cp]ꢀ anion1,
and other iron
substitution with carbonylmetallates
carbonylmetallates8 with certain alkyl halides.
As a matter of fact, various [Re(CO)5X] complexes are highly
50 susceptible towards nucleophilic addition. Studies of the anionic
35
In the reactions with aryl and vinyl halides, an area of our
close interest, carbonylmetallates tend to attack not the carbon,
but the halogen atom, which results in a “halogenophilic”
mechanism of nucleophilic substitution (Scheme 1).9ꢀ11 A specific
behaviour of pentacarbonylrhenate anion, Na[Re(CO)5], played a
(acyl)rhenates
resulting
from
[Re(CO)5(αꢀfluorovinyl)]
complexes have led us to another interesting finding, that of
strong CF…M+ interaction (Fig. 1).12 The CF…M+ interaction in
these systems is facilitated by the chelate effect and by the back
55 donation from rhenium, which enhances the donor ability of
fluorine.
40 key role in proving this mechanism. The intermediate carbanion
attacks [Re(CO)5Hal] at the coordinated CO and instead of
neutral σꢀvinylꢀcomplexes, indistinguishable from the “direct”
nucleophilic substitution products, the halo(acyl)rhenates are
formed (Scheme 1).10, 11
45
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(CO)4
Re
with halo(acyl)rhenates? We think it quite likely and in the
20 present paper provide a detailed study of the mechanism of this
reaction.
F
Nu
R
O
F
M'
M=Li, Na, K
Results and Discussion
Fig. 1 CꢀF…M+ interaction in anionic αꢀfluorovinyl
Identification of the reaction products
tetracarbonyl(acyl)rhenates
Mixing of the THF solution of Na[Re(CO)5] with Eꢀαꢀ
In this context we were not surprised to observe anionic
tetracarbonylrhenate as the main product in the reaction of αꢀ
bromostilbene with Na[Re(CO)5]. But a closer examination of the
13C NMR spectrum showed no acyl carbon signal (δ >240 ppm)
25 bromostilbene results in immediate evolution of gas, which
5
1
ceases after several min, and after 20 min the H NMR spectrum
shows the formation of one main product (Table 1, en. 1). Its
yield does not increase further with time, and according to 13C
NMR spectrum recorded after 3 hr no Na[Re(CO)5] is left. The
30 product is easily identified as a cisꢀNa[Re(CO)4XY] complex, the
COꢀsignals patterns in IR and 13C NMR spectra closely
resembling that of halo(acyl)rhenates studied previously,10, 11 but
with no bridging acyl carbon signal (δ >240 ppm). This, together
with the data of HR ESI MS, allowed us to identify the product as
35 Na[ReBr(CO)4{C(Ph)=CHPh}]. The gas evolving in the reaction
must then be carbon monoxide. It is worth noting that ESI MS
does not show a quasimolecular ion peak, but only fragmentary
ion peaks [Mꢀ2CO]ꢀ, [Mꢀ3CO]ꢀ and [Mꢀ4CO]ꢀ. However, such
behavior was also observed for the previously studied
40 tetracarbonylrhenate anions.11 The stilbene ligand in the product
has Zꢀgeometry, as evidenced by a large value of 3J(C,H) (13 Hz)
between the vinylic hydrogen and a vicinal Cipso carbon,
and
the
product
was
consequently
identified
as
bromo(vinyl)rhenate:
Ph
H
Ph
Re(CO)4
Br
Na
10
The product is formally a result of the oxidative addition of Cꢀ
Br bond to [Re(CO)5]ꢀ with a concomitant release of CO ligand, a
result quite unusual for an 18ꢀelectron metal carbonyl anion. The
fact of spontaneous reaction of nonactivated αꢀbromostilbene
15 with medium nucleophilic rhenium carbonylmetallate is also
remarkable, because the classical additionꢀelimination
mechanism in this case is unlikely. Can a specific structure of the
product give a clue to the reaction mechanism, as was the case
Ph
Ph
Ph
H
Ph
Ph
H
Ph
H
Ph
H
Ph
Br
Na[Re(CO)5]
+
+
THF, rt, hν
- CO
H
Re(CO)4
Br
Re(CO)4
Re(CO)5
Na
Na
45 Table 1. Reactions of αꢀbromostilbene with Na[Re(CO)5], THF, 22°С a
entry
E/Zꢀratio of
Reaction conditions
Na[Re(CO)5]
concentration,
molꢁlꢀ1
Time
Product yields, %
(Vin= ꢀC(Ph)=CHPh)
PhCH=CBrPh
E/Z=80:20
Na[ReBr(CO)4Vin]
Na[Re2(CO)9Vin]
VinH
6 (E/Z=2:1)
3 (E/Z>5)
4 (E/Z>5)
1 (E/Z<3)
2 (E/Z<3)
6 (E/Z=2:3)
hν b
1
2
2a
3
3a
3b
0.26
20 min
1 h
48 h
60
44
<1
25
25
68
12
27 e
38
10
31
<1
8
<2
41
76
2
6
9
<4
<4
4
<1
17
<1
8
15
<2
30
<2
13
2
hν,b excess of
Na[Re(CO)5] f
E/Z=92:8
0.27
10 min
1 h
dark
E/Z=92:8
0.27
30 min d
40 min
4 h
hν с
<4
hν b
hν b, tꢀBuOH (4 eq.)
hν c
4
E/Z=92:8
E/Z=80:20
E/Z=2:98
0.015
0.07
0.27
18 (E/Z=1:1)
7 (E/Z=1:2)
2 (E/Z>10)
7 (E/Z>20)
1 (E/Z=1:1)
7 (E/Z=1:2)
9 (E/Z=1:1)
6 (E/Z=1:1)
8 (E/Z=1:1)
7 (E/Z=3:1)
19 (E/Z=2:1)
4 (E/Z=4:1)
8 (E/Z=4:1)
5
6
6a
7
7а
7б
1 h
8 min
90 min
7 min
100 min
8 h
30 min
2 h
10 min
1 h
10 min
40 min
hν,c
E/Z=92:8
0.27
inhibitor j
5
28
50
38
75
43
72
hν c
8
E/Z=92:8
E/Z=92:8
E/Z=92:8
0.19
0.19
0.27
9
9а
10
10a
hν c, NaK2.8
dark, NaK2.8
9
a Reactions were performed in sealed vessels with roughly equimolar ratio of Na[Re(CO)5] and αꢀbromostilbene. Product yields were determined by 1H
NMR spectroscopy using durene as internal standard. The yield of Na[Re2(CO)9Vin] is calculated relative to the amount of Na[Re(CO)5]. b Under ambient
laboratory lighting. c Irradiation with a halogen lamp (300 W) placed at 80 cm from the reaction vessel. d 20 min in the dark + 30 min under irradiation. e
[Re(CO)5{ZꢀC(Ph)=CHPh}] is also formed in 15% yield. f Ratio of Na[Re(CO)5] : αꢀbromostilbene =2.5:1 . j In the presence of 4ꢀ(diphenylmethyliden)ꢀ
50 2,6ꢀdiꢀtertꢀbutylcyclohexaꢀ2,5ꢀdienon (20 ꢀ 30 mol%).
2
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corresponding to their transꢀposition.13
Products of this type, M′[MHal(CO)nR], are quite unusual for
the reactions of metal carbonyl anions with organic halides.
Similar products, [CoHal(CO)3{C(O)R}]ꢀ, were observed only in
the reactions of [Co(CO)4]ꢀ anion (and its rhodium and iridium
analogs) with reactive alkyl halides.4, 14 Their formation can be
explained by the direct SN2 substitution followed by migratory
insertion, with Halꢀ anion occupying the coordination vacancy,
the mechanism not applicable in our case. The COꢀligand
5
10 dissociation (and exchange for halide) in group 9 metal carbonyls
is also much easier than in rhenium carbonyl complexes.
The attempted isolation of the product by precipitating it from
THF solution using petroleum ether resulted in its complete
decomposition. It was noted, however, that upon standing
15 overnight under the reaction atmosphere (containing CO, evolved
in the reaction) a new product emerged in the reaction mixture,
tentatively identified as [Re(CO)5{C(Ph)=CHPh}]. When the
reaction mixture was stirred under CO (1 bar) for 3.5 hr (eqn (1))
Na[ReBr(CO)4{C(Ph)=CHPh}] (formed in 75% yield) was
20 completely converted to [Re(CO)5{C(Ph)=CHPh}] (65% yield
3
based on Na[Re(CO)5]). A large vicinal J(Cipso,Hvinyl) constant
Fig.2. Molecular structure of [Re(CO)5{C(Ph)=CHPh}]. Anisotropic
displacement ellipsoids are drawn at the 50% probability level. Selected
bond distances (Å) and angles (°): Re1ꢀC1 2.238(4), Re1ꢀC15 1.979(5),
Re1ꢀC16 1.995(5), Re1ꢀC17 2.014(5), Re1ꢀC18 2.026(5), Re1ꢀC19
1.999(5), C1ꢀC2 1.355(6), C1ꢀRe1ꢀC15 177.0(2), C16ꢀRe1ꢀC18 176.9(2),
C17ꢀRe1ꢀC19 170.6(2), Re1ꢀC1ꢀC2 121.4(3), C2ꢀC1ꢀC3 121.6(4), C1ꢀ
(12 Hz) confirms that the double bond in this product also has
cisꢀgeometry.
50
Ph
H
Ph
Ph
H
Ph
CO, 1 bar
THF, rt, 3.5 h
- NaBr
C2ꢀC9 132.5(4)
.
Re(CO)4
Br
Re(CO)5
Na
In the crystal, the molecules of [Re(CO)5{C(Ph)=CHPh}] are
(1)
55 bound to each other by the attractive intermolecular carbonylꢀ
carbonyl interactions [the O…O distances are in the range of
2.981(5)ꢀ3.216(5) Å]16 into 3ꢀdimensional framework.
25
The
structure
of
[Re(CO)5{C(Ph)=CHPh}]
was
unambiguously established by singleꢀcrystal Xꢀray diffraction
study and is shown in Figure 2 along with the atomic numbering
scheme and selected bond lengths and angles.
NMR monitoring of the reactions of Na[Re(CO)5] with Eꢀαꢀ
bromostilbene showed the formation of another byproduct with
60 1H and 13C chemical shifts of the stilbene moiety very close to
Na[ReBr(CO)4{C(Ph)=CHPh}] (Table 1). Its 13C NMR signal
pattern in the metal carbonyl region corresponded to equatorial
[Re2(CO)9X]ꢀ complex.17 Identification of the complex as
Na[Re2(CO)9{ZꢀC(Ph)=CHPh}] was completed by the HR ESI
65 MS spectrum showing [Mꢀ3CO]ꢀ и [Mꢀ4CO]ꢀ ion peaks.
The Na[Re2(CO)9{C(Ph)=CHPh}] product is absent in the first
minutes of the reaction, while the signals belonging to
Na[ReBr(CO)4{C(Ph)=CHPh}] are already present in the 1H
NMR spectrum. When the reaction is carried out with an excess
Octahedral Re complex contains five terminal carbonyl ligands
30 and a σꢀbonded 1,2ꢀdiꢀphenyl substituted vinyl ligand (Fig. 2).
The C1=C2 double bond has cis-configuration. The Re1ꢀC1
distance (2.238(4) Å) is significantly longer than the ReꢀCCO
distances (1.979(5)ꢀ2.026(5) Å) indicative of the stronger πꢀ
acceptor nature of carbonyl ligands compared to the vinyl one.
35 Two carbonyl groups, which are practically orthogonal to the
Re1ꢀC1=C2 plane, are slightly pushed towards the C1=C2 double
bond (the “umbrella” effect 15). Due to steric reasons, the phenyl
ring at the C1 carbon atom is orthogonal to the Re1ꢀC1=C2 plane,
whereas the phenyl ring at the C2 carbon atom is coplanar to this
40 plane. Such hindered conformation of the stilbene moiety is
characterized by the substantially increased C1ꢀC2ꢀC9 bond
angle (132.5(4)°). However, this conformation is stabilized by the
70 of
Na[Re(CO)5]
the
initial
formation
of
Na[ReBr(CO)4{C(Ph)=CHPh}] (Table 1, en. 2) and its
subsequent complete transformation into the binuclear product
are observed (Table 1, en. 2a). Thus, the binuclear complex
Na[Re2(CO)9{C(Ph)=CHPh}] is formed by the nucleophilic
75 substitution of bromide in Na[ReBr(CO)4{C(Ph)=CHPh}] with
Na[Re(CO)5] (eqn (2)). Unlike the initial reaction with
bromostilbene this reaction proceeds also in the dark, though
more slowly than under ambient lighting.
intramolecular
C10ꢀH10…π(C4ꢀC3ꢀC8)
[H10…C3 2.47, H10…C4 2.79 and H10…C8 2.75 Å].
hydrogen
bond
45
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50 the experiment in the presence of tꢀBuOH as an “anion trap”
confirms that it is not a halogenophilic process. The yield of
stilbene is not increased in the presence of tꢀBuOH (en. 5, Table
1), which shows that stilbene results from radical, and not
H
Re(CO)4Br
H
Re2(CO)9
Na[Re(CO)5]
Na
Ph
THF, rt
- NaBr
Na
Ph
Ph
Ph
(2)
carbanion
intermediates.
A
lower
yield
of
It should be noted that previously we observed similar
Na[Re2(CO)9Vin] complexes in the reactions of Na[Re(CO)5]
with several other vinyl halides, such as α,β,βꢀtrifluorostyrene.17
The present observation of binuclear complex being formed from
Na[ReBr(CO)4{C(Ph)=CHPh}], sheds new light on how such
complexes are formed in the reactions of Na[Re(CO)5] with other
vinyl halides.
55 Na[ReBr(CO)4{C(Ph)=CHPh}] product in this experiment can be
explained by the lower starting concentration of Na[Re(CO)5]
(vide supra).
5
50
40
30
20
10
Mechanistic considerations
10 The reaction of Na[Re(CO)5] with Eꢀαꢀbromostilbene proceeds
105ꢀ107 times faster than predicted for an additionꢀelimination
substitution mechanism (Scheme 2). The scheme for predicting
the reactivity of vinyl halides towards carbonylmetallates is based
upon the correlation of a large body of experimental rate data
15 with certain quantum chemical parameters.11
Z'
Nu
Y
Z'
Z
Nu
Y
Z'
X
Z
light on
light off
100
[X]-
[Nu]
X
Z
Y
0
Z or(and) Z' are EWG
0
200
300
400
Scheme 2. The additionꢀelimination mechanism
t, min
On the other hand, the halogenophilic mechanism is equally
not feasible, as it would have resulted in a bromo(acyl)rhenate
20 (Scheme 1), instead of a bromo(vinyl)rhenate that is actually
formed in the reaction. It is worth mentioning that the reaction of
Na[Re(CO)5] with Zꢀαꢀiodostilbene gives the iodo(acyl)rhenate,
Na[ReI(CO)4{C(O)C(Ph)=CHPh}], and hence proceeds through
a halogenophilic mechanism.11 This iodo(acyl)rhenate is quite
25 stable at room temperature, as well as other halo(acyl)rhenates
studied previously, which makes it unlikely that
bromo(acyl)rhenate is formed as an intermediate in the reaction
with bromostilbene.
60
Fig. 3. The effect of light in the reaction of Na[Re(CO)5] with Eꢀαꢀ
bromostilbene
The Zꢀisomer of αꢀbromostilbene also reacts with
Na[Re(CO)5] under the same conditions (visible light irradiation)
and gives the same set of products as the Eꢀisomer. The reaction
65 is slower and the overall yield of Na[ReBr(CO)4{C(Ph)=CHPh}]
and Na[Re2(CO)9{C(Ph)=CHPh}] is lower than from Eꢀisomer,
reaching 48% after 1.5 hr (en. 6a, Table 1) without any further
increase. Formation of only one isomer of the product from both
Zꢀ and Eꢀisomers of the starting vinyl halides means complete
70 loss of the initial stereochemistry (stereoconvergence) and is
suggestive of vinyl radical or radicalꢀanion intermediate.19
All the mechanistic evidence obtained so far points to a radical
mechanism involving both vinyl and Reꢀcentered radical
intermediates. However, it does not allow for a choice between a
75 chain and a nonꢀchain radical mechanism. For that purpose an
initiation/inhibition test must be performed.
The substitution of CO ligand for bromide anion in the course
30 of the reaction was the first clue that suggested a radical
intermediate, or more precisely, an oddꢀelectron rhenium
complex as an intermediate. It is well known that ligand
18
substitution is facilitated in oddꢀelectron complexes.1,
The
second clue came when it was found that the reaction is
35 stimulated with visible light. When the experiment was carried
out in the dark the yield of Na[ReBr(CO)4{C(Ph)=CHPh}]
reached 25% after 10 min, and did not increase any further (en.3
and 3а, Table 1). The reaction started again when the light was
turned on and was completed within 1 h giving the product in
40 68% yield (en. 3b, Table 1). Thus the reaction begins as a
thermally initiated process but after several min (if judged by the
gas evolution) is inhibited and needs photostimulation for the
completion. The final yield of Na[ReBr(CO)4{C(Ph)=CHPh}]
varies slightly between the experiments, and at lower starting
45 concentration of Na[Re(CO)5] the final yield of the product is
significantly lower (en. 4, Table 1), though it is easier to observe
the effect of light (Fig. 3).
Ph
O
Ph
Fig. 4 Quinomethide used as an inhibitor
An inhibitor commonly used in the studies of SRN1 reactions,19,
20
80
pꢀdinitrobenzene, cannot be used in our case, since
Na[Re(CO)5] is oxidized by pꢀdinitrobenzene. We used 4ꢀ
(diphenylmethyliden)ꢀ2,6ꢀdiꢀtertꢀbutylcyclohexaꢀ2,5ꢀdienon (Fig.
4), which can act as both a “radical trap” and an “electron sink”,
terminating the chain propagation sequence. We have already
A small quantity of stilbene observed in the reaction (Table 1)
is an expected side product for a radical reaction mechanism and
85 used this compound an effective trap to prove the radicalꢀchain
4
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Ph
H
Ph
Ph
H
Ph
Br
Ph
H
Ph
H
Na[Re(CO)5]
+
+
Re2(CO)10
Re(CO)4
Re(CO)5
inhibitor
dark reaction
THF, rt, 48 h
O
Na
Z:E=8:92
Conditions:
Yields:
7% (Z:E=1:1)
20%
77%
without a proton donor
60% (Z:E=3:4)
<1%
with t-BuOH (5 eq.)
49% (Z:E=4:1)
Scheme 3. Dark reaction in the presence of the inhibitor.
mechanism of the reaction of Na[Re(CO)5] with α,β,βꢀ
Na[Re2(CO)9{C(Ph)=CHPh}]) reached 89% (en. 9a, Table 1).
The reaction proceeded somewhat faster than with
trifluorostyrene.17
5
There was no immediate reaction between Na[Re(CO)5] and 45 photoinitiation only, as can be seen from the comparison of en. 8
Eꢀαꢀbromostilbene in the presence of the trap with no visible gas
evolution and no product signals were observed in the H NMR
and en. 9 in Table 1. The reaction can be also carried out in the
dark with NaK2.8 initiation, resulting in almost the same rate and
product yields as with photoinitiation (en. 10, 10a, Table 1).
We also attempted to initiate the reaction of Eꢀαꢀ
1
spectrum after 7 min (en. 7, Table 1). After several hours of
1
irradiation the signals of the products do appear in H NMR (en.
10 7a and 7b, Table 1), yet the reaction is retarded by a factor of 50 bromostilbene with
a
less reactive carbonylmetallate,
~20, reaching only 30% conversion of Na[Re(CO)5] after 3 hr.
When the reaction in the presence of the inhibitor is carried out in
the dark the maximum yield of Na[Re2(CO)9{C(Ph)=CHPh}] is
as low as 5%, but a slow thermal reaction (completed in two
15 days) is observed giving two other [Re2(CO)9X]ꢀ type products
(Scheme 3).
K[Mn(CO)5] using NaK2.8/light, but only the products of
bromostilbene reaction with NaK2.8, Eꢀ and Zꢀstilbene and tolane,
were observed in the 1H NMR spectrum. The starting
K[Mn(CO)5] remained unreacted after 24 hr.
55 Possible mechanism of the reaction
At least two different schemes of a radicalꢀchain mechanism may
be proposed for the reaction of Na[Re(CO)5] with αꢀ
bromostilbene. In both mechanisms (Schemes 4 and 5) the
reaction is initiated by electron transfer to αꢀbromostilbene
60 generating the corresponding radicalꢀanion,20 either photoinduced
The products are identified as Zꢀ and Eꢀisomers of the
corresponding acylrhenates, Na[Re2(CO)9{C(O)C(Ph)=CHPh}],
as they show the bridging acyl carbon signals at ~250 ꢀ275 ppm
20 besides the characteristic pattern of Re2(CO)9 moiety in 13C
NMR. The formation of these acylrhenates is completely
suppressed in the presence of an “anion trap”, tꢀBuOH, with a
simultaneous increase in the yield of stilbene, that becomes the
main product (Scheme 3). The effect of tꢀBuOH together with the
25 acylrhenate structure of the products leaves no doubt as to the
nature of the slow thermal process: it is the halogenophilic
reaction. Its initial product must be the bromo(acyl)rhenate,
which then transforms into binuclear acylrhenate by a process
analogous to that leading to Na[Re2(CO)9{C(Ph)=CHPh}] (eqn
30 (2)).
electron transfer from Na[Re(CO)5]21 or capturing of the solvated
22
electron from NaK2.8
.
The radicalꢀanion eliminates bromide
anion to give vinyl radical, which begins the chain propagation
sequence by coupling with Na[Re(CO)5] nucleophile.
65
The first one (Scheme 4) is essentially the same as the
generally accepted SRN1 mechanism19, 20 with an added step of
COꢀligand substitution for a bromide anion. The CO exchange
must occur in one of the intermediates, since the starting
Na[Re(CO)5] and the [Re(CO)5{C(Ph)=CHPh}] do not exchange
70 CO for halide anions under the reaction conditions and
furthermore NaBr is almost insoluble in THF. It may be the
radicalꢀanion of the product, since ligand substitution is
facilitated in oddꢀelectron complexes.1, 18
Carbonylmetallates can react by SRN1ꢀpathway as was
75 previously shown by two examples: the electrochemically
It is well known that SRN1 type reactions can be initiated by a
source of solvated electrons.20 The reaction of Na[Re(CO)5] with
Eꢀαꢀbromostilbene proceeds with photostimulation, but we
surmised that additional initiation with NaK2.8 may increase the
35 yield of the product. Since final yields of the products vary from
one experiment to another and are dependent on Na[Re(CO)5]
concentration and possibly on other unknown factors, an
additional blank experiment was performed under strictly
identical conditions (en. 8, Table 1). The maximum yield of
40 Na[ReBr(CO)4{C(Ph)=CHPh}] (75%) in the presence of NaK2.8
was indeed higher than in any other experiment, and the
combined yield of two substitution products (together with
induced
aromatic
nucleophilic
substitution
by
carbonylmetallates23 and photoinduced carbonylation of aryl and
vinyl halides catalyzed by [Co(CO)4]ꢀ.24
80
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ARTICLE TYPE
H
Re(CO)4Br
Ph
H
H
Br
- Br-
Ph
Ph
Ph
Ph
Ph
hν
H
Br
initiation
[Re(CO)5]-
H
Br
Ph
Ph
[Re(CO)5]-
+
H
Re(CO)4Br
Ph
Ph
Ph
Ph
H
Re(CO)5
Ph
H
Re(CO)4
Ph
Br -
- CO
Ph
Ph
Scheme 4. The SRN1ꢀtype mechanism for the reaction of Na[Re(CO)5] with αꢀbromostilbene
The second scheme (Scheme 5) represents an adopted radicalꢀ
added halide nucleophile, namely an excess of NaI. The result
25
chain oxidative addition mechanism,1,
well documented for 20 was negative, since the initial product observed in the presence of
5
unsaturated 16ꢀelectron transition metal complexes (such as
Vaska’s complex)26, but unusual for metal carbonyl anions. The
19ꢀelectron radicalꢀanion [Re(CO)5{C(Ph)=CHPh}]ꢁꢀ can expel
the COꢀligand to give the 17ꢀelectron [Re(CO)4{C(Ph)=CHPh}]ꢁꢀ
NaI was Na[ReBr(CO)4{C(Ph)=CHPh}] (40 min, 70% yield),
which slowly exchanged bromide for iodide to give
Na[ReI(CO)4{C(Ph)=CHPh}] (Scheme 6).
A similar result was obtained in the presence of PEt3, with
intermediate.27, 28, 29 The difference from Scheme 4 is in the next 25 Na[ReBr(CO)4{C(Ph)=CHPh}] still being the initial product.
10 and last propagation step, in which, instead of transferring an
electron to αꢀbromostilbene, the [Re(CO)4{C(Ph)=CHPh}]ꢁꢀ
radicalꢀanion abstracts the bromine atom. Halogen atom
abstraction is a wellꢀknown pathway for the reactions of 17ꢀ
Thus the second mechanism (Scheme 5) at present offers a better
explanation of the observed chemistry.
Fast inhibition of the initial thermal reaction may be explained
by the reversibility of CO dissociation step and the shift of the
electron metal complexes with organic halides,1, 8, 30 and in case 30 equilibrium towards the 19ꢀelectron [VinRe(CO)5]ꢁꢀ radicalꢀanion
15 of Reꢀcentred radicals it can be a very fast process.31, 32
by the CO accumulating in the reaction mixture.
To probe the feasibility of Scheme 4 we attempted to intercept
the intermediate in which the COꢀligand is supposedly exchanged
for bromide. For that purpose we performed the reaction with the
H
Re(CO)4Br
Ph
H
Ph
Ph
Ph
- Br-
H
Br
H
Br
Ph
Ph
[Re(CO)5]-
+
hν
H
Re(CO)4
Ph
Ph
initiation
H
Br
Ph
Ph
17-electron
Ph
Ph
[Re(CO)5]-
H
Re(CO)5
Ph
Ph
CO
19-electron
35
Scheme 5. The radicalꢀchain oxidative addition mechanism for the reaction Na[Re(CO)5] with αꢀbromostilbene
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ARTICLE TYPE
H
Br
H
H
Re(CO)4Br
Re(CO)4I
H
Re2(CO)9
Na[Re(CO)5]
+
+
THF, rt, hν
NaI (1 eq)
Na
Ph
Na
Ph
Na
Ph
Ph
Ph
Ph
Ph
Ph
Yields:
Time:
40 min
4 h
8%
<1%
11%
70%
11%
70%
Scheme 6. The reaction of Na[Re(CO)5] with αꢀbromostilbene in the presence of NaI
NMR spectroscopy (1H and 13C) was used as a primary tool for
the analysis of the reaction mixtures usually combined with the
45 IR spectra in the metal carbonyl stretching region (1550ꢀ2200 cmꢀ
Conclusions
5
It was COꢀligand substitution for bromide resulting in a formal
oxidative addition product that first suggested the idea of a
radical mechanism for the reaction of [Re(CO)5]ꢀ anion with
αꢀbromostilbene. Several additional probes, such as
stereoconvergence, lightꢀstimulation and formation of silbene
10 byproduct, confirmed the reaction pathway through radical
intermediates. The radicalꢀchain nature of the process became
evident when the inhibition of the reaction was demonstrated.
The mechanism proposed for the reaction (Scheme 5) includes a
bromineꢀatom transfer step and is well documented in oxidative
15 addition reactions to unsaturated (16ꢀelectron) transition metal
complexes, but to the best of our knowledge is new for 18ꢀ
electron carbonylmetallates.
The establishment of this radical pathway is important for a
wider class of carbonylmetallate reactions with vinyl and aryl
20 halides, described as halogenophilic (Scheme 1). It allows for a
clearer distinction between radical and polar halogenophilic
mechanisms. The latter is also observed in the studied reaction
when the radicalꢀchain pathway is inhibited. It seems unrealistic
that the same radical intermediates should be involved in two
25 different and competing processes, radicalꢀchain oxidative
addition and halogenophilic reaction. Thus there are even fewer
reasons to suppose some hidden radical intermediates in the
halogenophilic reactions of [Re(CO)5]ꢀ carbonylmetallate with
vinyl and aryl halides.
1
1). Product yields were determined by the integration of the H
NMR spectra of the reaction solutions containing an internal
standard (durene). The signals were referenced to the individual
products by comparison to the spectra of the isolated compounds
50 or literature data.
1H NMR (400.13 MHz) and 13C NMR (100.61 MHz) spectra
were obtained on a Bruker Avance spectrometer at 22°C and
referenced to the signals of the solvent. IR spectra were recorded
on Thermo Nicolet IRꢀ200 spectrometer in THF in
a
55 0.2 mm CaF2 cell. High resolution mass spectra (HR MS) were
measured on the Bruker micrOTOF II instrument using
electrospray ionization (ESI). The measurements were done in a
negative ion mode (3200 V) for anionic complexes and in
positive ion mode for [Re(CO)5{C(Ph)=CHPh}]; external or
60 internal calibration was done with Electrospray Calibrant
Solution (Fluka).
Reaction of Na[Re(CO)5] with αꢀbromostilbene
Typically 0.3ꢀ0.7 ml of Na[Re(CO)5] solution in THF (c ~0.2 M)
was mixed at r.t. with additive (if appropriate), αꢀbromostilbene
65 (equimolar to 20% excess), internal standard (durene). Reactions
were performed in closed vessels fitted with a vacuum valve with
the free volume (~50 cm3) sufficient to keep the partial pressure
of carbon monoxide released in the reaction below 0.1 bar. For
photostimulation the reactions were irradiated by a 300W halogen
70 lamp placed at 80 cm from the reactor flask, in other cases the
reactions were performed at ambient laboratory lighting. After
stirring for a certain period of time samples of the reaction
mixture were transferred under vacuum into thinꢀwalled glass
tubes (~3.5 mm diameter) which were flameꢀsealed and placed
75 into standard NMR tubes containing acetoneꢀd6 for the lock
signal. Composition of the reaction solution was then determined
by 1H and 13C NMR spectroscopy. The details of each experiment
are given in Table 1.
30 Experimental
General
Preparation of metal carbonyl anion salts and their reactions with
vinyl halides were carried out in “allꢀfused” glassware using
vacuumꢀline techniques. Breakꢀseal ampoules were used for the
35 transfer of substances. Na[Re(CO)5] was prepared by the
reduction of [Re2(CO)10] with 0.5% NaHg (30ꢀ50% excess) and
purified by the low temperature crystallization from THF. Eꢀαꢀ
Bromostilbene was prepared by consecutive bromination and
dehydrobromination of stilbene33, and Zꢀαꢀbromostilbene was
40 prepared by thermal isomerisation (200°, 1h) of the Eꢀisomer and
repeated crystallization of the obtained isomer mixture from
EtOH.
Photostimulated reaction (en. 3, Table 1), 0.39 ml of 0.27 M
80 solution of Na[Re(CO)5] (0.105 mmol) in THF, 26 mg αꢀ
bromostilbene (0.100 mmol, E/Z=92:8). After 30 min stirring
1
under halogen lamp the H and 13C NMR spectra showed that
almost no starting reagents were left and one main product was
formed, Na[ReBr(CO)4{ZꢀC(Ph)=CHPh}]. 1Н NMR (THF,
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400.13 MHz),
δ
: 7.24 (s, 1H), 7.09 (t, 2H), 6.93 (t, 2H), 6.77ꢀ
intensity, %): m/z = 548.9371 [Mꢀ2CO]ꢀ (100%), 520.9419 [Mꢀ
60 3CO]ꢀ (70%). Calcd. for C16H11IO2Re 548.9361. Calcd. for
C15H11IORe 520.9412.
Reaction of Na[ReBr(CO)4{ZꢀC(Ph)=CHPh}] with CO. The
reaction mixture obtained from 0.37 ml of 0.19 M solution of
Na[Re(CO)5] in THF (0.070 mmol), 21.5 mg αꢀbromostilbene
6.85 (m, 3H), 6.69ꢀ6.74 (m, 3H). 13C NMR (THF, 100.6 MHz),
δ
: 193.19, 191.75, 191.24 (4CO), 164.62, 160.91, 142.97 (Cquat),
138.30, 128.91, 127.98, 127.47, 125.17, 123.66, 122.43 (CH). IR
(THF), ν/cmꢀ1: 2075 m, 1973 vs, 1951 s, 1898 s. HR ESI MS
5
(relative intensity, %): m/z
=
500.9494 [Mꢀ2CO]ꢀ (70%),
472.9534 [Mꢀ3CO]ꢀ (100%). Calcd. for C16H11BrO2Re 500.9500. 65 (0.083 mmol, E/Z=92:8), ~2 mg NaK2.8 (0.05 mmol) and
Calcd. for C15H11BrORe 472.9551.
Reaction with an excess of Na[Re(CO)5] (en.2, Table 1), 0.37
10 ml of 0.27 M solution of Na[Re(CO)5] in THF (0.100 mmol),
10.3 mg αꢀbromostilbene (0.040 mmol, E/Z=92:8). After two
containing 0.0525 mmol of Na[ReBr(CO)4{ZꢀC(Ph)=CHPh}]
1
(en. 9a, Table 1) was stirred under CO (1 bar) for 3.5 h. The H,
13C NMR and IR spectra showed that no Na[ReBr(CO)4{Zꢀ
C(Ph)=CHPh}] was left and [Re(CO)5{ZꢀC(Ph)=CHPh}] was
days under ambient laboratory lighting the predominating product 70 formed in 65% yield (based on Na[Re(CO)5]). The reaction
1
mixtures from several other experiments were reacted with CO
pNrMesRen(tTiHn Fth, e40s0o.l1u3tioMnHwza)s, δNa:[7R.0e0(C(sO, 1)H{)Z,ꢀ7C.0(P1h(t),=2CHH),P6h.}9]3. (dН,
following the same procedure and were all combined for the
isolation of the product by column chromatography on silica gel
using petroleum etherꢀCH2Cl2, 5:1 as eluent. [Re(CO)5{Zꢀ
2
9
15 2H), 6.69ꢀ6.76 (m, 3H), 6.57ꢀ6.64 (m, 3H). 13C NMR (THF,
100.6 MHz),
δ
: 203.47 (2CO), 201.06 br. (4CO), 195.91 (1CO),
1
138.31, 128.94, 127.59, 127.32, 126.09, 123.14, 122.44 (CH). IR 75 δ
195.84 (1CO), 190.3 br (1CO), 161.12, 159.52, 143.49 (Cquat),
(THF), ν/cmꢀ1: 2083 m, 2016 s, 1972 vs, 1924 s 1882 s. HR ESI
20 MS (relative intensity, %): m/z = 718.9662 [Mꢀ3CO]ꢀ (100%).
Calcd. for C20H11O6Re2 718.9643.
C(Ph)=CHPh}], mp 105ꢀ106°C. Н NMR (THF, 400.13 MHz),
1H:),76.2.735((t,d,22HH),).71.30C5N(sM, 1RH()a,c6et.o9n0eꢀ6ꢀd.869, 1(0m0,.65MH)H, z6).,8δ4ꢀ6:.18885(.m11,
(4CO), 183.13 (1CO), 159.06, 146.56, 141.64 (Cquat), 129.47,
129.24, 128.46, 125.94, 125.06, 124.90 (Co, Cм, Cп), 143.27
Dark reaction in the presence of the inhibitor, 0.43 ml of 0.27 80 (CH=). IR (THF), ν/cmꢀ1: 2131 m, 2016 vs, 1984 s. EI MS (Irel.
,
M solution of Na[Re(CO)5] in THF (0.116 mmol), 34 mg αꢀ
bromostilbene (0.131 mmol, E/Z=92:8), 13 mg 4ꢀ
25 (diphenylmethyliden)ꢀ2,6ꢀdiꢀtertꢀbutylcyclohexaꢀ2,5ꢀdienon
(0.035 mmol). After two days no Na[Re(CO)5] was left according
%): m/z = 506 (10) M+, 450 (15) [M ꢀ 2CО]+, 422 (10) [M ꢀ
3CО]+, 394 (5) [M ꢀ 4CО]+, 366 (40) [M ꢀ 5CО]+, 178 (100)
[C14Н10]+. HR ESI MS (relative intensity, %): m/z = 449.0252
[Mꢀ2CO, ꢀH]+. Calcd. for C17H10O3Re 449.0188. Crystals suitable
to IR and 13C NMR spectra and a mixture of Zꢀ and Eꢀisomers of 85 for Xꢀray analysis were grown from petroleum ether.
Xꢀray crystal structure determination.
acylrhenates Na[Re2(CO)9{C(O)C(Ph)=CHPh}] were formed as
the main products. 18ꢀCrownꢀ6 (~30 mg) was added to the
30 reaction solution and the acylrhenate products were precipitated
as an orangeꢀbrown oil by the addition of 1 ml of petroleum
The crystal of [Re(CO)5{ZꢀC(Ph)=CHPh}] (C19H11O5Re, M =
505.48) is monoclinic, space group P21/c, at T = 100 K: a =
7.4699(3) Å, b = 34.0964(14) Å, c = 6.7664(3) Å,
β = 92.041(1)°,
1
ether, and washed with benzene and petroleum ether. Н NMR
90 V = 1722.29(12) Å3, Z = 4, dcalc = 1.949 g/cm3, F(000) = 960, ꢁ =
7.081 mm−1. 22105 reflections (4999 independent reflections, Rint
= 0.026) were collected on a Bruker SMART APEXꢀII CCD
(THF+18ꢀcrownꢀ6, 400.13 MHz),
isomer), 6.71 (s, 1H, =CHPh, Eꢀisomer), 6.95ꢀ7.23 (m, Haromatic
δ
: 5.73 (s, 1H, =CH, Zꢀ
,
35 Z+Eꢀisomers), 7.47 (d, 2Hortho,
Zꢀ
ortho,
diffractometer (λ(MoKα)ꢀradiation, graphite monochromator, ω
isomer). 13C NMR (THF+18ꢀcrownꢀ6, 100.6 MHz),
Zꢀisomer), 7.51 (dδ, ,2ZHꢀisomer:
and φ scan mode, θmax = 30.0o) and corrected for absorption using
95 the SADABS program (Tmin = 0.318; Tmax = 0.460).34 The
structure was solved by direct methods and refined by fullꢀmatrix
least squares technique on F2 with anisotropic displacement
parameters for nonꢀhydrogen atoms. The hydrogen atoms were
placed in calculated positions and refined within the riding model
258.67 (=CC(O)), 205.14 (2CO), 200.19 br. (4CO), 196.35 (1
CO), 193.02 (1CO), 190.0 br (1CO), 163.89 (=CC(O)), 138.75,
138.19 (Cipso), 130.18, 128.60, 128.36, 128.31 (CHmeta+ortho),
40 127.21, 126.25 (CHpara), 115.08 (=CHPh). Eꢀisomer: 254.44
(=CC(O)), 204.58 (2CO), 199.70 br. (4CO), 196.75 (1 CO),
194.94 (1CO), 189.4 br (1CO), 162.10 (=CC(O)), 138.87, 138.71
(Cipso), 130.65, 130.38, 128.27, 128.03 (CHmeta+ortho), 126.80,
126.49 (CHpara), 129.18 (=CHPh). IR (THF+18ꢀcrownꢀ6), ν/cmꢀ1:
45 2085 m, 2015 s, 1971 vs, 1930 s 1888 s. HR ESI MS (relative
intensity, %): m/z = 718.9647 [Mꢀ4CO]ꢀ (100%). Calcd. for
C20H11O6Re2 718.9643.
100 with fixed isotropic displacement parameters (Uiso(H)
=
1.2Ueq(C)). The final divergence factors were R1 = 0.044 for 4850
independent reflections with I > 2σ(I) and wR2 = 0.091 for all
4999 independent reflections, S = 1.001. All calculations were
carried out using the SHELXTL program.35 Crystallographic data
105 for [Re(CO)5{ZꢀC(Ph)=CHPh}] have been deposited with the
Cambridge Crystallographic Data Center. CCDC 903491
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax:
Reaction in the presence of NaI, 0.59 ml of 0.27 M solution of
Na[Re(CO)5] in THF (0.160 mmol), 31.3 mg αꢀbromostilbene
50 (0.121 mmol, E/Z=92:8), 23,2 mg NaI (0.154 mmol). After 4h the
1H and 13C NMR signals of the initially formed
Na[ReBr(CO)4{ZꢀC(Ph)=CHPh}] were replaced by the signals of
110 +44 1223 336033;
www.ccdc.cam.ac.uk).
eꢀmail:
deposit@ccdc.cam.ac.uk
or
1
Na[ReI(CO)4{ZꢀC(Ph)=CHPh}]. Н NMR (THF, 400.13 MHz),
δ
: 7.30 (s, 1H), 7.07 (t, 2H), 6.94 (t, 2H), 6.75ꢀ6.82 (m, 3H),
55 6.65ꢀ6.71 (m, 3H). 13C NMR (THF, 100.6 MHz),
δ
: 191.73,
Acknowledgements
191.01, 189.77 (4CO), 161.47, 161.40, 143.27 (Cquat), 140.77,
128.87, 127.95, 127.41, 124.99, 123.59, 122.37 (CH). IR (THF),
ν/cmꢀ1: 2072 m, 1971 vs, 1951 s, 1899 s. HR ESI MS (relative
Financial support of this work by the Program of the President of
the Russian Federation for the State Support of Leading Research
8
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22 Though light irradiation is required only for the initiation step, the
reaction is not expected to proceed when the light is turned off. It
involves highly reactive radical intermediates,20 which means very
fast termination step(s), with all radicals disappearing in a matter of
seconds. For instance, the rate constant for hydrogen abstraction
from CH3CN solvent by Ph2C=CPh• is 1.2•105 Mꢀ1•sꢀ1.19.
23 T. V. Magdesieva, Kukhareva, II, G. A. Artamkina, K. P. Butin, and I.
P. Beletskaya, J. Organomet. Chem., 1994, 468, 213ꢀ221; T. V.
Magdesieva, I. I. Kukhareva, E. N. Shaposhnikova, G. A.
Artamkina, I. P. Beletskaya, and K. P. Butin, J. Organomet. Chem.,
1996, 526, 51ꢀ58.
Schools (Grant № НШꢀ6965.2012.3) and the Russian Academy
of Sciences Program 1ꢀОХ is gratefully acknowledged.
70
75
80
Notes and references
a Chemistry Department of the M.V. Lomonosov Moscow State University,
119992, Moscow, Russia. Fax: +7 495 939 3618; Eꢀmail:
5
beletska@org.chem.msu.ru, petr@elorg.chem.msu.ru
b A.N. Nesmeyanov Institute of Organoelement Compounds (INEOS ),
Russian Academy of Sciences, Vavilov Street 28, 119991, Moscow, Russia
c ND Zelinsky Institute of Organic Chemistry, Department of Structural
24 J. J. Brunet, C. Sidot, and P. Caubere, J. Org. Chem., 1983, 48, 1166ꢀ
1171.
10 Studies (IOC), Russian Academy of Sciences, Moscow 119991, Russia
25 R. H. Hill and R. J. Puddephatt, J. Am. Chem. Soc., 1985, 107, 1218ꢀ
1225.
26 J. A. Labinger, J. A. Osborn, and N. J. Coville, Inorg. Chem., 1980,
19, 3236ꢀ3243.
1J. P. Collman, L. S. Hegedus, R. G. Finke, and J. R. Norton, Principles
and Applications of Organotransition Metal Chemistry University
Science Books, Mill Valley, California, 1987.
15 2 R. B. King, J. Organomet. Chem., 1975, 100, 111ꢀ125; M. J.
Schweiger, T. Ederer, K. Sunkel, and W. Beck, J. Organomet.
Chem., 1997, 545, 17ꢀ25; W. Beck, B. Niemer, and M. Wieser,
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21 Most plausible mechanisms of photoinitiation are photoexcitation of a
charge transfer complex formed between the nucleophile and the
20
65
substrate, and electron transfer from an exited nucleophile (the
UVꢀabsorption band of Na[Re(CO)5], λmax=334 nm, extends into
visible region, giving yellow colour to its solutions).
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Graphical abstract
An oxidative addition reaction of α-bromostilbene to Na[Re(CO)5] carbonylmetallate is shown to
proceed by a radical-chain mechanism involving a 17-electron [Re(CO)4{C(Ph)=CHPh}]-· radical-
anion intermediate.
Ph
H
Ph
Br
Ph
H
Ph
Ph
H
Ph
Na[Re(CO)5]
+
THF, rt, hν
-CO
Re(CO)4
Br
Re(CO)4
Re(CO)5
Na
Na