Activation of aliphatic carbon-carbon bonds of esters and amides by rhodium(II) porphyrin

Article abstract of DOI:10.1016/j.jorganchem.2007.01.012

Aliphatic carbon-carbon bonds of esters and amides were activated successfully with rhodium(II) porphyrin radical to give rhodium(III) porphyrin alkyls in moderate yields.

Full text of DOI:10.1016/j.jorganchem.2007.01.012

Journal of Organometallic Chemistry 692 (2007) 2021–2027
www.elsevier.com/locate/jorganchem
Activation of aliphatic carbon–carbon bonds of esters and amides
by rhodium(II) porphyrin
*
Lirong Zhang, Kin Shing Chan
Department of Chemistry, The Chinese University of Hong Kong, Shatin, Hong Kong, China
Received 21 August 2006; received in revised form 3 January 2007; accepted 11 January 2007
Available online 18 January 2007
Abstract
Aliphatic carbon–carbon bonds of esters and amides were activated successfully with rhodium(II) porphyrin radical to give rho-
dium(III) porphyrin alkyls in moderate yields.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Rhodium(II) porphyrin; Esters; Amides; Carbon–carbon bond activation
1. Introduction
the activation of unstrained aliphatic carbon–carbon bonds
[17] lie in the activation of kinetically more favorable, ste-
rically unhindered carbon–hydrogen over carbon–carbon
bonds.
The activations of inert chemical bonds are of fundamen-
tal importance in chemical research and industrial applica-
tions [1–3]. The activation of an aliphatic carbon–carbon
bond (CCA) is one of the most challenging reactions. The
development of efficient methods for selective cleavage of
carbon–carbon bonds in homogeneous media by transition
metal complexes is an important topic of organometallic
chemistry [4]. Carbon–carbon bond activation could be
applied in developing an environmentally benign process
by the depolymerization of synthetic polymer waste into
lower and biodegradable hydrocarbon fragments [5].
Examples of carbon–carbon bonds activation by transi-
tion metal complexes mainly involved ring strain relief
[6–8], proximal carbonyl group assistance [9–11], pre-
anchored phosphine ligands [12], and nucleophilic open-
ings of three-membered rings [13]. Also photochemically
induced C–CN bond cleavage mediated by iron complexes
was documented by Nakazawa et al. [14,15]. Silylation of
nitriles involving C–CN bond cleavage in catalytic manner
was reported by Chatani recently [16]. The difficulties of
We have reported the first examples of activation of ali-
phatic carbon–carbon bonds of nitroxide [18] and ketones
[19] by 5,10,15,20-tetra(2,4,6-trimethylphenyl)porphyrinate
rhodium(II) [Rh(tmp)]. We have explored the activation of
carbon–carbon bond of esters and amides to examine the
importance of heteroatom assisted pre-coordination [20–
22] in CCA. The CCA results of ketones have demonstrated
that non-enolizable ketones showed higher selectivity than
that of enolizable ketones towards to carbon–carbon bond
activation due to elimination of competitive carbon hydro-
gen bond activation [19]. The CCA reactions were also pro-
moted by Ph3P. Hence, we now report the successful
activation of aliphatic carbon–carbon bonds in non-enoliz-
able esters and amides by Rh(tmp) radical in the presence
of triphenylphosphine as a promoter ligand [23–25].
2. Results and discussion
The metal centered radical RhII(tmp) (2) was generated
in about 80% yield by the photolytic cleavage of
Rh(tmp)CH3 (1) in benzene for ꢀ8 h (Eq. (1)) [19] (see
Fig. 1).
*
Corresponding author. Tel.: +86 852 26096376; fax: +86 852
26035057.
E-mail address: ksc@cuhk.edu.hk (K.S. Chan).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.01.012

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L. Zhang, K.S. Chan / Journal of Organometallic Chemistry 692 (2007) 2021–2027
Table 1
Results of CCA at the alkoxy group (type A)
Entry
1
Esters
Reaction time (h)
24
Rh(tmp)CH3 (1)/%a
N
29
O
N
Rh
N
Ph
O
O
O
O
N
3a
2
3
4
24
30
38
27
O
Rh(tmp)
Ph
Fig. 1. Structure of Rh(tmp).
3b
C6H6; hm; 6–10ꢁ
C
RhðtmpÞCH3
!
RhðtmpÞ
ð1Þ
ð2Þ
8 h; N2
1
2
6
O
Ph
Ph
2.1. CCA results of Rh(tmp) and esters
3c
PPh3 ð1 equiv:Þ
C6H6; N2; 130 ꢁ
RhðtmpÞ þ ester
!
RhðtmpÞCH3
24
O
C
Ph
When Rh(tmp) reacted with ethyl benzoate (3a) in the pres-
ence of Ph3P at 130 ꢁC in 2 days, Rh(tmp)CH2CH3 (4) was
observed in about 7% yield. No reaction occurred at 100 ꢁC
for 2 days.
3e
Two possible pathways exist for this formation of
Rh(tmp)CH2CH3. First, the ethyl group cleavage via car-
bon–oxygen bond activation of PhCO2Et (3a) occurred.
Second, CCA of –OCH2–CH3 occurred to give
Rh(tmp)CH3, Rh(tmp)CH3 then reacted with the unre-
acted Rh(tmp) to give Rh(tmp)H and Rh(tmp)CH2, which
further underwent a bimolecular substitution [26,27] to
give Rh(tmp)CH2CH3 and Rh(tmp) (Scheme 1). Therefore,
the reaction conditions were optimized to be 130 ꢁC for a
shorter time of 1 day to prevent the formation of
Rh(tmp)Et and other side reactions.
To our delight, when Rh(tmp) reacted with esters at
130 ꢁC in the presence of 1 equiv. of triphenylphosphine
in 1 day or less, carbon–carbon bond activation occurred
with Rh(tmp)Me formed (Eq. (2)). Table 1 lists results of
the CCA.
5
24
23
O
O
O
3f
a
Yield (%) was based on 80% of Rh(tmp) generated through photolysis.
1-Benzoyloxy-1-phenylethane (3c) (entry 3) was the
most reactive among the series of esters examined, and
reacted with Rh(tmp) at 130 ꢁC for 6 h to produce
Rh(tmp)CH3 in 38% yield. The electron withdrawing influ-
ence of phenyl ring weakened the methyl–carbon bond
energy ðBDEPhCH–CH ¼ 76:4 kcal molꢂ1Þ [28], and facili-
3
tated the CCA. As a radical mechanism has been proposed
of the CCA of ketones with Rh(tmp) [26,27], the resultant
radical A would also be further resonance-stabilized (Eq.
(3)) [29]. In this case, the steric hindrance of the phenyl
group did not slow down the reactivity (Table 1, entry 3
vs. 1 and 2).
O
Rh(tmp) +
O
Ph
Ph
Ph
2
3e
ð3Þ
Ethyl benzoate (3a) ðBDEOCH –CH ¼ 90:2 kcal molꢂ1
Þ
O
2
3
+
Rh(tmp)CH
3
[28], isopropyl benzoate (3b) ðBDEOCH–CH ¼ 88:2
3
O
Ph
kcal molꢂ1Þ [28], and tert-butyl benzoate (3e) gave similar
product yields, further demonstrating the steric hindrance
1
A
Rh(tmp)
+
Rh(tmp)CH3
Rh(tmp)H +
Rh(tmp)CH2
Rh(tmp)
Rh(tmp)CH2CH3
+
Rh(tmp)CH3
Rh(tmp)CH2
+
Scheme 1. Proposed mechanism for forming Rh(tmp)CH2CH3.

L. Zhang, K.S. Chan / Journal of Organometallic Chemistry 692 (2007) 2021–2027
2023
Table 2
Results of CCA a- to the carbonyl group (type B)
Rh
O
CH3
Entry Esters
1
Reaction time (h) Rh(tmp)CH3 (1)/%a
Ph
O
24 33
O
B
O
Fig. 2. Six-membered ring transition state of CCA (type A).
Ph
3g
Rh
O
2
3
4
24
25
O
O
CH
3
Ph
Ph
O
CH
3
C
3h
Fig. 3. Five-membered ring transition state of CCA (type B).
24
34
O
NC
and type B (Fig. 3), six-membered and five-membered rings
were formed, respectively. The more facile formation of
five-membered ring transition state [30] may account the
higher reactivity of type B substrates.
O
CH3
3i
48
No reaction
2.2. CCA of Rh(tmp) and esters under solvent-free
conditions
O
CH
3
Ph
O
Ph
RhðtmpÞ þ ester solvent-free; PPh
!
3 product
ð4Þ
3j
N2; temp:; time
a
Yield (%) was based on 80% of Rh(tmp) generated through photolysis.
Solvent-free conditions were carried out for the CCA to in-
crease the rate through high concentration of substrate.
However, 3a did not react at 70 ꢁC for 5 days. At 100 ꢁC,
only about 10% Rh(tmp)CH3 was isolated after 3 days.
did not play a significant role. Comparing with isopropyl
benzoate (3b), 1-benzoyloxy-1-phenylethane (3c) gave
higher ꢀ8% yield of CCA product. It also supported that
electron withdrawing group closer to the activation posi-
tion increased the reactivity of substrates. For the case of
diethyl carbonate (3f), about 23% yield of Rh(tmp)CH3
was produced.
Table 3
CCA results of esters under solvent-free conditions
Entry Esters
Temperature Time
Product (yield/%a)
(ꢁC)
(days)
The CCA at the a,b position to the carbonyl group of
esters was also successful. Phenyl-2,2-dimethyl-propiono-
ate (3g) (Table 2, entry 1) was slightly more reactive than
tert-butyl benzoate (3e) (Table 1, entry 4), which might
be due to the carbonyl group closer to the activation posi-
1
2
70
100
5
3
No reaction
Rh(tmp)CH3 (1)
(10)
O
Ph
O
O
O
3a
tion in 3g ðBDE
¼ 84:5 kcal molꢂ1Þ [28].
ðCH3Þ3–CCOOCH3
3
100
130
3
1
Rh(tmp)CH2CH3
(4) (trace)
Substituent effect was also examined. Compound 3h was
less reactive (Table 2, entry 2) than 3g. Presumably, it is
due to the sterically more hindered phenyl group. Com-
pound 3i was more reactive as the nitrile group is more
electron-withdrawing and less sterically hindered to result
in a higher yield of CCA product. However, in the more
bulky case of 2,3-diphenylmethyl propionate (3j) (Table
2, entry 4), no CCA or COA reaction occurred.
O
Ph
3d
4
Rh(tmp)CH3 (1)
(18)
O
Ph
Type B substrates were more reactive than type A sub-
strates. For both types A and B substrates, the pre-coordi-
nation of rhodium with carbonyl likely occurred. We
rationalized that in the transition states of type A (Fig. 2)
3e
a
Yield (%) was based on 80% of Rh(tmp) generated through photolysis.

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L. Zhang, K.S. Chan / Journal of Organometallic Chemistry 692 (2007) 2021–2027
Table 4
Table 4 (continued)
CCA results of amides
Entry
Amides
Temperature Time Rh(tmp)CH3
Entry Amides
Temperature
(ꢁC)
Time
(h)
Rh(tmp)CH3
(ꢁC)
(h)
(1)/%a
(1)/%a
13
100
48
No reaction
O
1
2
130
130
100
24
6
6
33
41
37
O
N
3
Ph
N
N
Ph
O
5a
5h
5i
14
15
16
100
100
100
100
6
6
6
6
28
22
25
32
4
100
6
29
O
N
Ph
Ph
O
5b
5
6
7
130
130
100
24
6
6
40
37
39
O
N
O
N
O
5j
5c
5d
5e
5f
O
N
8
100
6
33
O
5k
N
O
17
O
N
9
100
6
39
5l
O
a
Yield (%) was based on 80% of Rh(tmp) generated through photolysis.
N
Ph
O
Propyl benzoate (3d) reacted at 100 ꢁC for 3 days to give
trace amount of Rh(tmp)CH2CH3 via CCA at –OCH2–
CH2CH3 moiety. Ester 3e reacted at 130 ꢁC for 1 day to
produce about 18% yield of Rh(tmp)CH3. From Table 3,
it suggested that increasing concentration of substrates
did not result in the higher activity of CCA, likely due to
change of solvent property [31].
10
11
130
100
6
6
43
40
O
N
O
2.3. Sealed tube experiments to confirm CCA of Rh(tmp)
and esters
The reaction mixture of Rh(tmp), Ph3P and 3a was
heated in anaerobic conditions at 130 ꢁC for 30 h in an
NMR tube. The extent of the reaction was monitored by
1H NMR spectroscopy. The increasing intensity with time
12
100
24
No reaction
O
N
Si
2
of the characteristic peaks (doublet, JRh–H = 3.0 Hz, d:
O
ꢂ5.25 ppm) of Rh–CH3 confirmed the occurrence of car-
5g
bon–carbon bond activation in the substrate.

L. Zhang, K.S. Chan / Journal of Organometallic Chemistry 692 (2007) 2021–2027
2025
O
2.4. CCA results of Rh(tmp) and amides
Rh(tmp)
+
N
PPh3
RhðtmpÞ þ amide
!
RhðtmpÞCH3
ð5Þ
O
C6H6; N2; temp:; time
2
5c
O
N
ð6Þ
Table 4 shows the results of CCA of the more coordinating
substrates, amides, with Rh(tmp). Carbon–carbon bond
activation occurred when Rh(tmp) reacted with amides at
100–130 ꢁC within 1 day to give Rh(tmp)CH3.
PPh
o
3
Rh(tmp)CH
+
3
100 C, 6 h
(tmp)Rh
O
The effect of reaction time was examined. When 5a was
heated at 130 ꢁC and 100 ꢁC for 6 h, similar yields of
Rh(tmp)CH3 were obtained in 41% and 37%, respectively.
The same results were observed for 5c and 5f. Tempera-
tures from 100 to 130 ꢁC did not affect the yields of
Rh(tmp)CH3 (Table 4, entries 2, 3; 6, 7; 10, 11) and extent
of reaction time giving similar results (Table 4, entries 1, 2;
5, 6). Even longer time of 24 h did not result in higher
yields. Therefore, the CCA of Rh(tmp) and amides were
carried out at 100 ꢁC for 6 h.
6 ?
1 39 %
NaBH , EtOH
4
O
N
o
N , 50-60 C-r.t., 1 h
2
Rh(tmp)I
N-(bromomethyl)phthalimide
o
0 C-r.t., 1 h
(tmp)Rh
O
6 82 %
ð7Þ
For N-phenyl benzanilide (5b), the CCA yield was lower
than that of 5a. It might be due to the steric hindrance of
phenyl ring. Comparing the cases of 5c, 5d, 5e, and 5f, sim-
ilar reactivity was observed and Rh(tmp)CH3 was formed
in moderate yields. However, the more hindered trimethyl-
silyl substituted amide 5g and benzyl substituted amide 5h
did not react with Rh(tmp). The amides with one carbonyl
group such as 5i and 5j, gave lower CCA yields. Substrates
5k and 5l gave about 25% and 32% yield of CCA product,
respectively.
2.6. Sealed tube experiment to confirm CCA of Rh(tmp)
and amides
The mixture of a solution of Rh(tmp), 1 equiv. of Ph3P
and 10 equiv. of substrate 5f in C6D6 was placed in the
NMR tube, degassed and sealed under vacuo and heated
at 100 ꢁC for 10 h. The increasing intensity with time of
2
the characteristic peaks (doublet, JRh–H = 3.0 Hz, d:
ꢂ5.25 ppm) of Rh–CH3 confirmed the formation of
Rh(tmp)Me as the CCA product.
2.5. Identification of co-product
2.7. Comparison of CCA of esters and amides
Since only Rh(tmp)CH3 was isolated in low to moderate
yields, other coproducts formed was therefore searched.
For 5c, compound 6 may also form (Eq. (6)). An authentic
sample of compound 6 was obtained by reductive alkyl-
ation of Rh(tmp)I (Eq. (7)). However, analysis of the crude
reaction mixture showed the absence of 6. As6 was ther-
mally stable in C6D6 with 1 equiv. Ph3P at 100 ꢁC for 3
days, its decomposition was not possible. We have earlier
proposed a bimolecular substitution mechanism for the
CCA reaction, so it is likely that radical D, once formed,
underwent more efficient hydrogen abstraction rather than
coupled [32] with Rh(tmp) (see Fig. 4).
The pre-coordination of carbonyl groups with rhodium–
metal center was proposed in the esters and amides CCA.
The higher reactivities of amides than those of esters are
likely due to the more electron donating ability of nitrogen
in amides than oxygen in esters [33].
3. Summary
In conclusion, esters and amides underwent carbon–
carbon bond activation by Rh(tmp) successfully to give
Rh(tmp) alkyls in moderate yields.
4. Experimental
All materials were obtained from commercial suppliers
and used without further purification unless otherwise
specified. Benzene was distilled from sodium. Benzene-d6
was vacuum distilled from sodium, degassed thrice by
freeze–thaw–pump cycle and stored in a Teflon scrawhead
stoppered flask. Pyridine was distilled over KOH under
N2. Triphenylphosphine was recrystallized from EtOH.
Thin layer chromatography was performed on Merck
pre-coated silica gel 60 F254 plates. Silica gel (Merck,
70–230 and 230–400 mesh) or neutral aluminum oxide
O
N CH2
O
D
Fig. 4. Proposed intermediate for 5c.

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L. Zhang, K.S. Chan / Journal of Organometallic Chemistry 692 (2007) 2021–2027
(Merck, activity I, 70–230 mesh) was used for column
chromatography.
4.4. Sealed tube experiments
1H NMR spectra were recorded on a Bruker DPX 300
Triphenylphosphine solution (0.01 mL, 0.001 mmol,
0.1 M in C6D6, 1 equiv.) was added to the solution of
[Rh(tmp)] (2) in the NMR tube at r.t. Degassed ester 3a
solution (10 equiv.) in C6D6 was added to the adduct solu-
tion, then the NMR tube was sealed under vacuum. The
mixture was heated at 130 ꢁC for 30 h under N2 in the
absence of light. The reaction was monitored with 1H
NMR spectroscopy.
¨
(300 MHz) spectrometer. Spectra were referenced inter-
nally to the residual proton resonance in CDCl3 (d
7.26 ppm), tetramethylsilane (TMS, d 0.00 ppm) or with
C6D6 (d 7.15 ppm) as the internal standard. Chemical shifts
(d) were reported as part per million (ppm) in d scale down-
field from TMS or (Me3Si)4Si.
4.1. Preparation of 5,10,15,20-
tetramesitylporphyrinatorhodium(II) [Rh(tmp)] (2) [34]
4.5. Reaction of [Rh(tmp)] (2) and amides 5a–5l with PPh3
added
To a Teflon screwheaded stoppered flask, Rh(tmp)CH3
(1) (10.0 mg, 0.011 mmol) was charged and dissolved in
C6H6 (4.0 mL) to obtain a clear orange solution. The reac-
tion mixture was then degassed by the freeze–pump–thaw
method (three cycles) and refilled with N2. The reaction mix-
ture was irradiated under a 400 W Hg-lamp at 6–10 ꢁC until
all the starting material was consumed as indicated by TLC
analysis (ꢀ8 h) to give Rh(tmp) (2) in 80% yield.
Triphenylphosphine solution (0.1 mL, 0.01 mmol, 0.1 M
in C6H6, 1 equiv.) was added to the solution of [Rh(tmp)]
(2) at r.t. Degassed amide solution (5 equiv.) in benzene
was added to the adduct solution, and the mixture was
heated under N2 in the absence of light. The crude product
was purified by chromatography on silica gel to give the
product.
4.2. Reaction of [Rh(tmp)] (2) and esters 3a–3j with PPh3
added
4.6. Preparation of (5,10,15,20-
tetramesitylporphyrinato)(N-phthalimido)methyl-
rhodium(III) [Rh(tmp)CH2N-Pht] (6) [34]
Triphenylphosphine solution (0.1 mL, 0.01 mmol, 0.1 M
in C6H6, 1 equiv.) was added to the solution of [Rh(tmp)] (2)
at r.t. Degassed ester solution (5 equiv.) in benzene was
added to the adduct solution, and the mixture was heated
under N2 in the absence of light. The crude product was
purified by chromatography on silica gel to give the product.
To a solution of Rh(tmp)I (50 mg, 0.049 mmol) in EtOH
(30 mL), a solution of NaBH4 (9.5 mg, 0.25 mmol) in aqu-
ous NaOH (0.1 M, 2 mL) was added under N2. The solu-
tion mixture was heated at 50–60 ꢁC under N2 for 1 h
and then cooled to r.t. N-(bromomethyl)phthalimide
(157 mg, 0.49 mmol) was added under N2. The mixture
was stirred at r.t. for 15 min. A reddish orange suspension
was formed. The reaction was worked up by addition of
CH2Cl2 and H2O. the crude product was extracted with
CH2Cl2 (200 mL), washed with H2O (25 mL · 3), dried
over anhydrous MgSO4, filtered and rotary evaporated
off to dryness. After purification by column chromatogra-
phy on silica gel using hexane:CH2Cl2 (5:1) to hex-
ane:CH2Cl2 (2:1) as the gradient eluent, a red solid of 6
was obtained (42.0 mg, 0.040 mmol, 82%). Rf = 0.51 (hex-
4.3. Preparation of (5,10,15,20-
tetramesitylporphyrinato)rhodium(III) ethyl
[Rh(tmp)CH2CH3] (4) [34]
Red suspension of Rh(tmp)I (200 mg, 0.15 mmol) in
EtOH (100 mL) and the solution of NaBH4 (28 mg,
0.74 mmol) in 0.5 M NaOH (8 mL) wre purged with N2
separately for about 15 min. The NaBH4 solution was
added to the suspension of Rh(tmp)I via cannular. The
reaction mixture was heated at 55 ꢁC for 1 h under N2.
After cooled to r.t., EtI was added via syringe. A bright
red suspension formed immediately and it was stirred over-
night at room temperature. The reaction was worked up by
addition of CH2Cl2 and H2O. The crude product was
extracted with CH2Cl2 (200 mL), washed with H2O
(25 mL · 3), dried over anhydrous MgSO4, filtered and
rotary evaporated off to dryness. After purification by col-
umn chromatography on silica gel using hexane:CH2Cl2
(10:1) to hexane:CH2Cl2 (5:1) as the gradient eluent, bright
red solid (4) (113 mg, 0.12 mmol, 83%) was obtained which
was further purified by recrystallization from CH2Cl2/hex-
ane. Rf = 0.67 (hexane:CH2Cl2 = 5:1); 1H NMR
1
ane:CH2Cl2 = 1:1). H NMR (300 MHz, C6D6) d ꢂ2.19
(d, 2H, JRh–CH ¼ 3:6 Hz), 1.88 (s, 12H), 2.25 (s, 12H),
2
2.45 (s, 12H), 6.42 (d, 2H, J = 8.4 Hz), 6.55 (dd, 2H,
J = 3.0 Hz, J = 5.4 Hz), 6.90 (s, 2H), 7.22 (s, 4H), 7.42 (s,
2H), 8.80 (s, 8H). 13C NMR (CDCl3, 100 MHz) 21.98,
22.12, 22.66, 120.61, 122.54, 128.41, 131.42, 131.56,
133.37, 138.08, 139.15, 139.48, 140.19, 143.64, 163.43.
Anal. Calc. for C65H58N5O2Rh: C, 74.77; H, 5.60; N,
6.70. Found: C, 74.40; H, 5.99; N, 6.43%. HRMS (FAB):
Calc. for (C65H58N5O2Rh)+: m/z 1043.3640. Found: m/z
1043.36097.
4.7. Sealed tube experiments
(300 MHz, CDCl3)
d
ꢂ4.74 (dq, 2H, J = 7.5 Hz,
JRh–CH ¼ 3:6 Hz), ꢂ4.22 (t, 3H, J = 5.7 Hz), 1.93 (s,
Triphenylphosphine solution (0.01 mL, 0.001 mmol,
0.1 M in C6D6, 1 equiv.) was added to the solution of
2
12H), 1.90 (s, 12H), 2.61 (s, 12H), 7.25 (s, 4H), 8.45 (s, 8H).

L. Zhang, K.S. Chan / Journal of Organometallic Chemistry 692 (2007) 2021–2027
2027
[12] (a) C.-H. Jun, H. Lee, S.G. Lim, J. Am. Chem. Soc. 123 (2001)
751;
[Rh(tmp)] (2) in the NMR tube at r.t. Degassed amide 5f
solution (10 equiv.) in C6D6 was added to the adduct solu-
tion, then the NMR tube was sealed under vacuum. The
mixture was heated at 100 ꢁC for 10 h under N2 in the
absence of light. The reaction was monitored with 1H
NMR spectroscopy.
(b) C.-H. Jun, K.Y. Chung, J.B. Hong, Org. Lett. 3 (2001) 785;
(c) C.-H. Jun, D.H. Lee, Y.H. Kim, H. Lee, Organometallics 20
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[14] H. Nakazawa, T. Kawasaki, K. Miyoshi, C.H. Suresh, N. Koga,
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Acknowledgments
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We thank the Research Grants Council of Hong Kong
of the SAR of China for financial support (No. 400104).
[18] M.K. Tse, K.S. Chan, J. Chem. Soc., Dalton Trans. (2001) 510.
[19] L. Zhang, K.S. Chan, J. Organomet. Chem. 691 (2006) 3782.
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