Carbon-nitrogen bond activation of amines by rhodium(III) porphyrin complexes

Article abstract of DOI:10.1016/j.jorganchem.2010.02.007

Carbon-nitrogen bond activation of amines by rhodium porphyrin chloride has been achieved to give rhodium porphyrin alkyl complexes. Rhodium porphyrin hydride and rhodium porphyrin dimer were proposed as the intermediates in cleaving the C-N bond.

Full text of DOI:10.1016/j.jorganchem.2010.02.007

Journal of Organometallic Chemistry 695 (2010) 1370–1374
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Journal of Organometallic Chemistry
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Carbon–nitrogen bond activation of amines by rhodium(III) porphyrin complexes
*
Ching Chi Au, Tsz Ho Lai, Kin Shing Chan
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon–nitrogen bond activation of amines by rhodium porphyrin chloride has been achieved to give
rhodium porphyrin alkyl complexes. Rhodium porphyrin hydride and rhodium porphyrin dimer were
proposed as the intermediates in cleaving the C–N bond.
Received 23 November 2009
Received in revised form 3 February 2010
Accepted 9 February 2010
Ó 2010 Elsevier B.V. All rights reserved.
Available online 12 February 2010
Keyword:
Carbon–nitrogen bond activation
1. Introduction
of toluenes [19] and alkanes [20]. In expanding the substrate scope
in bond activation chemistry, we have examined the reactions of
Transition metal catalyzed cross-coupling reaction is one of the
most powerful means leading to bond forming in organic synthe-
sis. It is usually achieved through the selective cleavage of unreac-
tive bonds, such as carbon–hydrogen bond [1], carbon–carbon
bond [2] as well as carbon–oxygen bond [3]. However, cross-cou-
pling reactions related to carbon–nitrogen bond activation (CNA)
are relative fewer. Recently, it was discovered that cleavage of car-
bon–nitrogen bonds in aryltrimethylammonium salt [4] and aryl-
triazenes [5] can lead to the formation of carbon–carbon bonds
to give a series of biaryls through Suzuki coupling. In addition,
cleavage of unactivated aromatic carbon–nitrogen bonds in aniline
derivatives [6] can also lead to the carbon–carbon bond formation
to give ketones. This brings about our interest in carbon–nitrogen
bond activation.
Furthermore, reported examples of CNA mainly made use of
low valent transition metal complexes, examples include CNA of
amines by palladium black [7]; CNA of arylamines [8], bridged lac-
tams [9] and diaza [2.2.1] bicyclic alkenes [10] by platinum(II)
complexes; chelation-assisted CNA with pincer-type ligand using
monovalent rhodium complexes [11]; CNA of aziridines by early
transition metal complexes [12] as well as CNA of cyanamides by
a silyl–iron complex [13]. However, CNA by high valent transition
metal complexes are rarely reported, examples include CNA of pyr-
idine by thorium(IV) complexes [14] and CNA of amines by a rho-
dium(III) corrole complex [15] (corrole = tetradehydrocorrin).
Previously we have reported a series of bond activation chemistry
with Group 9 metalloporphyrin complexes, including the carbon–
carbon bond activation (CCA) of nitroxides [16], nitriles [17], esters
and amide [18], base-promoted carbon–hydrogen bond activation
amines with rhodium porphyrin complexes and discovered the
CNA of amines to give rhodium(III) porphyrin alkyls.
2. Results and discussion
Initially, Rh(ttp)Cl (ttp = 5,10,15,20-tetratolylporphyrin) reacted
with tri-n-pentylamine (20 equiv.) in benzene at 120 °C for 16 days
to give 28% yield of Rh(ttp)nPent. When THF was used as the sol-
vent, the rate and yield were enhanced with 63% Rh(ttp)nPent ob-
tained after 9 days. With the reaction performed in solvent free
conditions, the rate of CNA was greatly enhanced and the reaction
was completed in 1 day with 59% yield of Rh(ttp)nPent obtained
(Table 1, Eq. (1), entry 9). The enhanced rate of reaction is probably
due to the increased concentration of substrate. Therefore, CNA of
amine was studied with neat amines.
ð1Þ
The scope of amine substrates was then investigated (Table 1
and Eq. (1)). Both secondary and tertiary amines reacted with
Rh(ttp)Cl to give the corresponding Rh(ttp)R.
The CNA yields for secondary amines were not satisfactory with
only 6% yield of Rh(ttp)Et and trace amount of Rh(ttp)nBu obtained
in 1 day and 3 days when diethylamine and dibutylamine were
used, respectively (Table 1, entries 1 and 2). When tertiary amines
were used, CNA yield was greatly enhanced (Table 1, entries 3–5
and 9). A 64% yield of Rh(ttp)nBu was obtained when Rh(ttp)Cl re-
acted with tri-n-butylamine for 1 day (Table 1, entry 5). Also, it was
found that CNA yield gradually increased as the carbon chain
length of the amine molecule increased (Table 1, entries 3–5),
* Corresponding author. Tel.: +852 26096376; fax: +852 26095057.
E-mail address: ksc@cuhk.edu.hk (K.S. Chan).
0022-328X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.02.007

C.C. Au et al. / Journal of Organometallic Chemistry 695 (2010) 1370–1374
1371
Table 1
CNA of amines.
crude reaction mixture probably due to their low concentration
and overlapping of 1H NMR signals with the substrate. The pro-
posed step is supported by the fact that Rh(ttp)H reacted directly
with nBu3N to give Rh(ttp)nBu in 62% yield (Eq. (2)). In addition,
Rh(ttp)Cl reacted with iBu3N to give (iBu2NH)Rh(ttp)Cl (Table 1, en-
Entry
Amine
Rh(ttp)R
Yield (%)
1
Et2NH 2a
Rh(ttp)Et 3a
6
2a
3
nBu2NH 2b
Et3N 2c
Rh(ttp)nBu 3b
Rh(ttp)Et 3a
Trace
17
47
64
41
25
43
59
57
18
i
try 7). This suggests that Bu2NH is generated in the reaction sys-
4
5
nPr3N 2d
Rh(ttp)nPr 3c
Rh(ttp)nBu 3b
Rh(ttp)nBu 3b
Rh(ttp)nBu 3b
Rh(ttp)nBu 3b
Rh(ttp)nPent 3d
(iBu2NH)Rh(ttp)Cl 3e
Rh(ttp)Me 3f
tem. The carbon–nitrogen bond cleavage likely occurs through
Rh(ttp)H directly without the generation of Rh(ttp)ꢀ and Bu3NH+
as the reaction between the acidic Rh(ttp)H and the weak base
Bu3N is endoergically unfavorable [24].
nBu3N 2e
6b
7c
8d
9
nBu3N 2e
nBu3N 2e
nBu3N 2e
nPent3N 2f
10
11
iBu3N 2g
N-methylpiperidine 2h
a
b
c
Reaction was carried out for 3 days.
Rh(ttp)I was used instead of Rh(ttp)Cl.
10 equiv. of KOH added.
ð2Þ
d
10 equiv. of K2CO3 added.
Under thermal conditions, it was reported that Rh(ttp)H would
be converted to [Rh(ttp)]2 and dihydrogen which was removed
during the workup process [25]. The possible intermediacy of
[Rh(ttp)]2 to give CNA product was also examined. From the inde-
pendent experiment between [Rh(ttp)]2 and nBu3N, we found that
[Rh(ttp)]2 cleaved the C–N bond of nBu3N, probably through car-
bon–hydrogen bond activation to give Rh(ttp)H and 4 which then
likely gives also Rh(ttp)H a b-hydride elimination for subsequently
C–N bond activation. Indeed [Rh(ttp)]2 reacted with nBu3N to give
Rh(ttp)nBu with 59% yield (Eq. (3)). However, no Rh(ttp)NBu2 was
not detected since late transition metal–amido complex with
nitrogen center bearing group is relatively unstable [26]. Even if
it had formed, it would have decomposed quickly. More probably,
this direct radical substitution does not occur since it is an energet-
ically very uphill reaction to yield a unstable Bu2N nitrogen-cen-
tered radical intermediate or transition state. Eq. (3) supports the
possible occurrence of pathway ii (Scheme 1). This also explains
the lower yield for CNA reactions of secondary amines. Since sec-
ondary amines, such as Et2NH, are less hindered ligand, they coor-
dinate better and induce disproportionation of [RhII(ttp)]2 species
to give [(ttp)RhL2]+ and [Rh(ttp)ꢀ] [27] to result in a lower CNA
yield. Alternatively, [RhII(ttp)]2 can be rationalized to undergo ra-
pid CHA to give 4 and Rh(ttp)H for the CNA.
reaction yield increased from 17% for triethylamine to 64% for tri-
n-butylamine. This gradual change in CNA yield is due to the en-
hanced solubility of Rh(ttp)Cl in the substrate. However, for
i
branched chain trialkylamine of Bu3N (Table 1, entry 10), the ex-
pected CNA product (Rh(ttp)iBu) was not observed. Instead
(iBu2NH)Rh(ttp)Cl was obtained in high yield likely from the coor-
dination of the organic coproduct after the CN bond cleavage and
its formation will be discussed in the mechanism section. Reaction
with a cyclic amine was also preformed (Table 1, entry 11) with
18% yield of Rh(ttp)Me obtained from N-methylpiperidine.
The base-promoted alkane CHA by Rh(ttp)Cl [20] and other
base-promoted bond activation by transition metal complexes
[19,21], have prompted us to examine the promoting effect of base
on the CNA of amines. However, it was found that the addition of a
base was not beneficial to product yield with only 25% and 43%
yields of Rh(ttp)nBu obtained in the presence of KOH and K2CO3,
respectively for the reactions between Rh(ttp)Cl and nBu3N (Ta-
ble 1, entries 7 and 8).
Counter anion effect was also investigated. Rh(ttp)I gave a low-
er yield of 41% of Rh(ttp)nBu in 1 day (Table 1, entry 6). Therefore,
the use of weakly coordinating anion on rhodium porphyrin com-
plex was not beneficial.
To gain an idea of the reactive intermediate and the reaction
mechanism of CNA of amine, several reactive rhodium porphyrin
species such as [Rh(ttp)]ꢀ, [Rh(ttp)]2 and Rh(ttp)H were treated
to react with tri-n-butylamine under the same reaction conditions
[20].
ð3Þ
Scheme 1 illustrates the proposed mechanism of the CNA of
amines. Initially, Rh(ttp)Cl reacts with the weakest C(a)–H bond
of amine to give 4 [22,23]. Subsequent b-hydride elimination of 4
produces Rh(ttp)H and an enamine. Finally, Rh(ttp)H reacts
with another amine molecule probably through sigma-bond
metathesis to give Rh(ttp)R as the CNA product. The co-products,
Bu2N(CH@CHCH2CH3) and nBu2NH, could not be detected in the
As shown previously, addition of an inorganic base lowered the
CNA yield (Table 1, entries 7 and 8). In the presence of base,
Rh(ttp)H will likely be converted to the inactive [Rh(ttp)]ꢀ [28]. In-
deed, no Rh(ttp)nBu was obtained in the independent experiment
between [Rh(ttp)]ꢀ and nBu3N.
Scheme 1. Proposed mechanism for CNA of amine.

1372
C.C. Au et al. / Journal of Organometallic Chemistry 695 (2010) 1370–1374
solvent mixture of hexane: CH2Cl2 (1:1).
A red product
3. Conclusion
Rh(ttp)nPent [20] (6.6 mg, 0.008 mmol, 63%) was collected.
In summary, we have discovered the carbon–nitrogen bond
activation of amines by rhodium(III) porphyrin complexes. The
CNA yield increased as the substrate changed from a secondary
to a tertiary amine. Rh(ttp)H and [Rh(ttp)]2 are shown to be the
probable intermediates for the C–N bond cleavage.
4.4. Reaction between Rh(ttp)Cl and diethylamine
Rh(ttp)Cl (11.5 mg, 0.014 mmol) and diethylamine (2.0 mL)
were degassed for three freeze–pump–thaw cycles. The mixture
was then heated at 120 °C under N2 for 1 day. Excess amine was
then removed under vacuum and the red crude mixture was puri-
fied by silica gel column chromatography eluting with a solvent
mixture of hexane: CH2Cl2 (1:1). A red product Rh(ttp)Et [16]
(0.6 mg, 0.001 mmol, 6%), with Rf = 0.72 (hexane/CH2Cl2 = 1:1)
4. Experimental
4.1. General
was collected. 1H NMR (CDCl3, 300 MHz), d ꢀ4.87 (dq, 2H, 2JRh–H
=
3.0 Hz, J = 7.5 Hz), ꢀ4.47 (dt, 3H, 3JRh–H = 1.5 Hz, J = 7.5 Hz), 2.69 (s,
12H), 7.53 (t, 8H, J = 6.3 Hz), 8.00 (d, 4H, J = 7.5 Hz), 8.08 (d, 4H,
J = 7.2 Hz), 8.71 (s, 8H).
All materials were purchased from commercial suppliers and
used without further purification unless otherwise specified. Rho-
dium trichloride (RhCl3ꢁxH2O) was obtained from Johnson Mat-
they. n-Hexane for chromatography was distilled from calcium
hydride. Tetrahydrofuran (THF) was freshly distilled from sodium
benzophenone ketyl under N2. Benzene was distilled from sodium
under N2. Rh(ttp)Cl [29], Rh(ttp)I [30], Rh(ttp)H [31], [Rh(ttp)]2
[31] and Rh(ttp)Na [32] were prepared according to the literature
methods. All CNA reactions were conducted under N2 and shielded
from light with aluminium foil wrapping. Thin-layer chromatogra-
phy was performed on precoated silica gel 60 F254 plates. Column
chromatography was performed on silica gel (70–230 and 230–
400 mesh).
4.5. Reaction between Rh(ttp)Cl and dibutylamine
Rh(ttp)Cl (11.5 mg, 0.014 mmol) and dibutylamine (2.0 mL)
were degassed for three freeze–pump–thaw cycles. The mixture
was then heated at 120 °C under N2 for 1 day. Excess amine was
then removed under vacuum and the red crude mixture was puri-
fied by silica gel column chromatography eluting with a solvent
mixture of hexane: CH2Cl2 (1:1). A red product Rh(ttp)Bu, with
Rf = 0.72 (hexane/CH2Cl2 = 1:1) was collected. 1H NMR (CDCl3,
1H NMR spectra were recorded on either a Bruker DPX-300
(300 MHz) or Bruker AV 400 (400 MHz) spectrometer. Chemical
shifts were referenced with the residual solvent protons in CDCl3
(d 7.26 ppm) as the internal standard. Chemical shifts (d) were re-
ported as part per million (ppm) in scale downfield from TMS (d
0.00 ppm). 13C NMR spectra were recorded on Bruker AV 400
(100 MHz) spectrometer. Chemical shifts were referenced to the
solvent peak of CDCl3 (d 77.0 ppm). Coupling constants (J) were re-
ported in Hertz (Hz).
2
300 MHz) d ꢀ4.95 (dt, 2H, JRh–H = 3.0 Hz, J = 8.1 Hz), ꢀ4.50 (quin,
2H, J = 7.9 Hz), ꢀ1.57 (sext, 2H, J = 7.4 Hz), ꢀ0.83 (t, 3H, J =
7.2 Hz), 2.70 (s, 12H), 7.53 (t, 8H, J = 6 Hz), 7.99 (dd, 4H, J =
2.1 Hz, J = 7.8 Hz), 8.08 (dd, 4H, J = 2.1 Hz, 7.8 Hz), 8.71 (s, 8H).
1
13C NMR (CDCl3, 100 MHz) 12.2, 15.1(d, JRh–C = 27 Hz), 19.3, 21.5,
29.5, 122.4, 127.3, 127.4, 131.3, 133.6, 134.0, 137.1, 139.3, 143.2;
HRMS (FABMS): calcd for (C52H46N4Rh)+ m/z 829.2772, found m/z
829.27688.
High resolution mass spectra (HRMS) were performed on a
ThermoFinnigan MAT 95 XL mass spectrometer in fast atom bom-
bardment (FAB) mode using 3-nitrobenzyl alcohol (NBA) matrix.
4.6. Reaction between Rh(ttp)Cl and triethylamine
Rh(ttp)Cl (11.5 mg, 0.014 mmol) and triethylamine (2.0 mL)
were degassed for three freeze–pump–thaw cycles and filled with
N2. The mixture was then heated at 120 °C for 1 day. Excess amine
was then removed under vacuum and the red crude mixture was
purified by silica gel column chromatography eluting with a sol-
vent mixture of hexane: CH2Cl2 (1:1). A red product Rh(ttp)Et
[16] (2.0 mg, 0.002 mmol, 17%) was collected.
4.2. Reaction between Rh(ttp)Cl and tripentylamine in benzene
Rh(ttp)Cl (10.1 mg, 0.012 mmol) and tripentylamine (0.07 mL,
20 equiv.) were added to benzene (2.0 mL) and the mixture was
degassed for three freeze–pump–thaw cycles and filled with N2.
The mixture was then heated at 120 °C for 16 days. The solvent
was then removed under vacuum and the red crude mixture
was purified by silica gel column chromatography eluting with
4.7. Reaction between Rh(ttp)Cl and tripropylamine
Rh(ttp)Cl (10.1 mg, 0.012 mmol) and tripropylamine (2.0 mL)
were degassed for three freeze–pump–thaw cycles and filled with
N2. The mixture was then heated at 120 °C for 1 day. Excess amine
was then removed under vacuum and the red crude mixture was
purified by silica gel column chromatography eluting with a solvent
mixture of hexane: CH2Cl2 (1:1). A red product Rh(ttp)nPr [16]
a
solvent mixture of hexane: CH2Cl2 (1:1). A red product
Rh(ttp)nPent [20] (0.3 mg, 0.004 mmol, 28%), with Rf = 0.78
(hexane/CH2Cl2 = 1:1) was collected. 1H NMR (CDCl3, 300 MHz)
2
d ꢀ4.96 (dt, 2H, JRh–H = 2.8 Hz, J = 8.1 Hz), ꢀ4.49 (quin, 2H, J =
7.8 Hz), ꢀ1.62 (quin, 2H, J = 7.4 Hz), ꢀ0.52 (sext, 2H, J = 7.3 Hz),
ꢀ0.27 (t, 3H, J = 7.4 Hz), 2.69 (s, 12H), 7.52 (t, 8H, J = 5.9 Hz),
7.98 (dd, 4H, J = 2.1 Hz, 8.6 Hz), 8.08 (dd, 4H, J = 2.7 Hz, 8.6 Hz),
8.70 (s, 8H).
(5.3 mg, 0.007 mmol, 47%), with Rf = 0.72 (hexane/CH2Cl2 = 1:1)
was collected. 1H NMR (CDCl3, 300 MHz) d ꢀ4.98 (dt, 2H, JRh–H
=
2
3.3 Hz, J = 8.3 Hz), ꢀ4.46 (sext, 2H, J = 8.7 Hz), ꢀ1.75 (t, 3H,
J = 7.2 Hz), 2.69 (s, 12H), 7.53 (t, 8H, J = 5.7 Hz), 8.00 (dd, 4H,
J = 2.1 Hz, 7.7 Hz), 8.07 (dd, 4H, J = 2.1 Hz, 8.0 Hz), 8.71 (s, 8H).
4.3. Reaction between Rh(ttp)Cl and tripentylamine in tetrahydrofuran
(THF)
4.8. Reaction between Rh(ttp)Cl and tributylamine
Rh(ttp)Cl (10.1 mg, 0.012 mmol) and tripentylamine (0.07 mL,
20 equiv.) were added to THF (2.0 mL) and the mixture was de-
gassed for three freeze–pump–thaw cycles and filled with N2.
The mixture was then heated at 120 °C for 9 days. The solvent
was then removed under vacuum and the red crude mixture was
purified by silica gel column chromatography eluting with a
Rh(ttp)Cl (11.3 mg, 0.014 mmol) and tributylamine (2.0 mL)
were degassed for three freeze–pump–thaw cycles and filled with
N2. The mixture was then heated at 120 °C for 1 day. Excess amine
was then removed under vacuum and the red crude mixture was
purified by silica gel column chromatography eluting with a

C.C. Au et al. / Journal of Organometallic Chemistry 695 (2010) 1370–1374
1373
solvent mixture of hexane: CH2Cl2 (1:1). A red product Rh(ttp)nBu
4.14. Reaction between Rh(ttp)Cl and N-methylpiperidine
(7.4 mg, 0.009 mmol, 64%) was collected.
Rh(ttp)Cl (10.3 mg, 0.013 mmol) and N-methylpiperidine
(2.0 mL) were degassed for three freeze–pump–thaw cycles. The
mixture was then heated at 120 °C under N2 for 1 day. Excess amine
was then removed under vacuum and the red crude mixture was
purified by silica gel column chromatography eluting with a solvent
mixture of hexane: CH2Cl2 (1:1). A red product Rh(ttp)Me [16]
4.9. Reaction between Rh(ttp)I and tributylamine
Rh(ttp)I (11.3 mg, 0.012 mmol) and tributylamine (2.0 mL)
were degassed for three freeze–pump–thaw cycles and filled with
N2. The mixture was then heated at 120 °C for 1 day. Excess amine
was then removed under vacuum and the red crude mixture was
purified by silica gel column chromatography eluting with a sol-
vent mixture of hexane: CH2Cl2 (1:1). A red product Rh(ttp)nBu
(4.3 mg, 0.005 mmol, 41%) was collected.
(1.8 mg, 0.002 mmol, 18%), with Rf = 0.72 (hexane/CH2Cl2 = 1:1)
2
was collected. 1H NMR (CDCl3, 300 MHz) d ꢀ5.82 (d, 3H, JRh–H
=
3 Hz), 2.69 (s, 12H), 7.53 (d, 8H, J = 7.5 Hz), 8.01 (dd, 4H, J = 2.4,
8.4 Hz), 8.07 (dd, 4H, J = 2.4, 8.4 Hz), 8.73 (s, 8H).
4.15. Reaction between Rh(ttp)H and tributylamine
4.10. Reaction between Rh(ttp)Cl and tributylamine in the presence of
KOH
Rh(ttp)H (10.0 mg, 0.013 mmol) and tributylamine (2.0 mL)
were degassed for three freeze–pump–thaw cycles. The mixture
was then heated at 120 °C under N2 for 1 day. Excess amine was
then removed under vacuum and the red crude mixture was puri-
fied by silica gel column chromatography eluting with a solvent
Rh(ttp)Cl (11.3 mg, 0.014 mmol), tributylamine (2.0 mL) and
KOH (7.8 mg, 0.14 mmol) were degassed for three freeze–pump–
thaw cycles. The mixture was then heated at 120 °C under N2 for
1 day. Excess amine was then removed under vacuum and the
red crude mixture was purified by silica gel column chromatogra-
phy eluting with a solvent mixture of hexane: CH2Cl2 (1:1). A red
product Rh(ttp)nBu (3.0 mg, 0.003 mmol, 25%) was collected.
mixture of hexane: CH2Cl2 (1:1).
(6.5 mg, 0.008 mmol, 62%) was collected.
A
red product Rh(ttp)nBu
4.16. Reaction between [Rh(ttp)]2 and tributylamine
4.11. Reaction between Rh(ttp)Cl and tributylamine in the presence of
K2CO3
[Rh(ttp)]2 (10.0 mg, 0.0065 mmol) and tributylamine (2.0 mL)
were degassed for three freeze–pump–thaw cycles. The mixture
was then heated at 120 °C under N2 for 1 day. Excess amine was
then removed under vacuum and the red crude mixture was puri-
fied by silica gel column chromatography eluting with a solvent
mixture of hexane: CH2Cl2 (1:1).
(3.0 mg, 0.003 mmol, 59%) with Rf = 0.72 (hexane/CH2Cl2 = 1:1)
was collected.
Rh(ttp)Cl (11.3 mg, 0.014 mmol), tributylamine (2.0 mL) and
K2CO3 (19.3 mg, 0.14 mmol) were degassed for three freeze–
pump–thaw cycles. The mixture was then heated at 120 °C under
N2 for 1 day. Excess amine was then removed under vacuum and
the red crude mixture was purified by silica gel column chroma-
tography eluting with a solvent mixture of hexane: CH2Cl2 (1:1).
A red product Rh(ttp)nBu (5.0 mg, 0.006 mmol, 43%) was collected.
A
red product Rh(ttp)nBu
4.17. Reaction between Rh(ttp)Na and tributylamine
Rh(ttp)Na (0.012 mmol) and tributylamine (2.0 mL) were de-
gassed for three freeze–pump–thaw cycles. The mixture was then
heated at 120 °C under N2 for 1 day. Excess amine was then re-
moved under vacuum and a green crude mixture was obtained.
4.12. Reaction between Rh(ttp)Cl and tripentylamine
Rh(ttp)Cl (10.1 mg, 0.013 mmol) and tripentylamine (2.0 mL)
were degassed for three freeze–pump–thaw cycles and filled with
N2. The mixture was then heated at 120 °C for 1 day. Excess amine
was then removed under vacuum and the red crude mixture was
purified by silica gel column chromatography eluting with a sol-
vent mixture of hexane: CH2Cl2 (1:1). A red product Rh(ttp)nPent
[20] (6.2 mg, 0.007 mmol, 59%) was collected.
Acknowledgement
We thank the Research Grants Council of Hong Kong of the SAR
of China for financial support [CUHK 400206].
References
4.13. Reaction between Rh(ttp)Cl and triisobutylamine
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followed by ethyl acetate: CH2Cl2 (1:2).
A
red product
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2
ꢀ5.58 (br, 1H), d ꢀ4.69 (dt, 2H, JRh–H = 3.0 Hz, J = 8.1 Hz), ꢀ2.76
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(c) E. Wenkert, E.L. Michelotti, C.S. Swindell, J. Am. Chem. Soc. 108 (1979)
2246–2247.
(quin, 2H, J = 7.9 Hz), ꢀ1.88 (hept, 2H, J = 6.8 Hz), ꢀ1.26 (d, 6H,
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