Alkylation of Rhodium Porphyrin Complexes with Primary Alcohols under Basic Conditions

Article abstract of DOI:10.1021/acs.organomet.9b00454

Primary alcohols were successfully utilized as the alkylating reagents to conveniently access rhodium porphyrin alkyl complexes in up to 91% yields under basic conditions. Mechanistic investigations suggest two possible pathways for the C-O bond cleavage: (1) nucleophilic substitution with rhodium(I) porphyrin anion and (2) a borrowing hydrogen pathway via rhodium(III) porphyrin hydride.

Full text of DOI:10.1021/acs.organomet.9b00454

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Alkylation of Rhodium Porphyrin Complexes with Primary Alcohols
under Basic Conditions
Yongjun Bian,† Chun Meng Tam, Ching Tat To, Xingyu Qu, and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin 00, New Territories, Hong Kong, People’s Republic of
China
ABSTRACT: Primary alcohols were successfully utilized as
the alkylating reagents to conveniently access rhodium
porphyrin alkyl complexes in up to 91% yields under basic
conditions. Mechanistic investigations suggest two possible
pathways for the C−O bond cleavage: (1) nucleophilic substitution with rhodium(I) porphyrin anion and (2) a borrowing
hydrogen pathway via rhodium(III) porphyrin hydride.
group (pKa of RhIII(ttp)H = 11 in DMSO, and pKa of PhOH =
INTRODUCTION
■
9.95 in H2O).9 Alternatively, the C−O bond is relatively strong
Rhodium porphyrin alkyl complexes, RhIII(por)R (por =
with BDE values of 92.0 kcal/mol in Me−OH and 79.9 kcal/
porphyrinato dianion ligand), are convenient and air-stable
precursors to rhodium(II) porphyrin metalloradicals, which
display distinct reactivity on C−H and C−C bond activation of
mol in PhCH2−OH.10 Our group has reported the C−O bond
cleavage of MeOH with RhIII(ttp)Cl under basic conditions to
give RhIII(ttp)Me.11 RhIII(ttp)H is proposed as the active
various organic substrates under mild reaction conditions.1,2
intermediate and cleaves the C−O bond in MeOH via σ-bond
metathesis to yield RhIII(ttp)Me and H2O. Other C−O
cleavage strategies involve the Lewis acid promoted SN1-type
nucleophilic reaction of alcohol,12a conversion of the hydroxy
group into a better leaving group followed by substitution,12b
and a borrowing hydrogen strategy.12c In this work, we have
successfully expanded the scope of the alcohol from MeOH.
Herein, we report the reaction of RhIII(ttp)Cl with primary
alkyl alcohols under basic conditions to access a series of
RhIII(ttp)R species in benzonitrile solvent as a result of C−O
bond cleavage (Scheme 1d). Mechanistic investigations
suggest that both RhIII(ttp)H and RhI(ttp)− anion are viable
intermediates for the C−O cleavage.
RhIII(ttp)R (ttp = 5,10,15,20-tetratolylporphyrinato dianion)
has been used as a precatalyst in the catalytic transfer
hydrogenation of [2.2]paracyclophane,2d,e catalytic anaerobic
oxidation of ketones,3 and hydrodebromination of allylic and
benzylic bromides with water by hydrolysis or photolysis of the
Rh−R bond.4 Rhodium porphyrin alkyl complexes allow the
access of air-sensitive intermediates from benchtop-stable
materials for the activation of inert chemical bonds. The first
systematic synthesis of rhodium porphyrin alkyl complexes was
reported by Ogoshi in 1975.5 RhIII(oep)Cl (oep =
2,3,7,8,12,13,17,18-octaethylporphyrinato dianion) was first
treated with NaBH4 and aqueous NaOH in EtOH at 60 °C
to first generate the RhI(oep)− anion. The RhI(oep)− anion
then reacted with alkyl bromides or iodides, R−X (R = 1°, 2°
alkyl, X = Br, I), to give the corresponding RhIII(oep)R via
nucleophilic substitution (Scheme 1a). In 2014, Dong utilized
tetraalkylammonium and alkylquinolinium salts as the
alkylating agents for RhIII(tpp)Cl (tpp = tetraphenylporphyr-
inato dianion).6 This air-tolerant transformation also takes
advantage of the nucleophilicity of the RhI(tpp)− anion
generated in situ under basic conditions. Our group has
communicated the alkylation of RhIII(ttp)Cl with alkyl halides
under basic conditions.7 Both the RhII(ttp) metalloradical and
RhI(ttp)− anion generated in situ from RhIII(ttp)Cl are
responsible for cleaving the alkyl R−X bond (X = Cl, Br, I)
by halogen atom abstraction and nucleophilic substitution,
respectively (Scheme 1c). This alkylation protocol gives access
to RhIII(ttp)R with 1°, 2°, and even 3° alkyl ligands in air.
Alkyl alcohols are poor alkylating agents because direct
nucleophilic substitution leads to hydroxide as a poor leaving
group under neutral or basic conditions. We have previously
disclosed the cleavage of the alkyl PhO−Me bond in anisole
with RhI(ttp)− anion via a nucleophilic substitution process to
give RhIII(ttp)Me.8 In this scenario phenoxide is the leaving
RESULTS AND DISCUSSION
■
Initial Discovery. Initially, we employed the previously
reported conditions11 and examined the C−O cleavage of
benzyl alcohol (2a) with RhIII(ttp)Cl (1). To our delight,
RhIII(ttp)Cl (1) reacted with PhCH2OH (2a) in the presence
of 10 equiv of K2CO3 under solventless conditions at 150 °C
to afford the C−O cleavage product RhIII(ttp)CH2Ph (3a) in
50% yield in 1 day (eq 1). We then further pursued the
optimization of reaction conditions.
III
Rh (ttp)Cl + PhCH2OH
1
2
K2CO3 (10 equiv)
III
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→⎯ Rh (ttp)CH2Ph
N2, dark, 150 °C, 24 h
3a, 50%
(1)
Temperature Effects. The reaction temperature was first
screened (Table 1). At lower reaction temperatures of 80 and
Received: July 3, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00454
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Organometallics
Article
Scheme 1. Alkylation of RhIII(por)Cl To Give RhIII(por)R
Table 2. Solvent Effects on the C−O Cleavage of Benzyl
Alcohol
a
entry
solvent
iPrOH
iPrOH
time/h
yield/%
1
15
24
24
24
2
13
4
24
24
24
4
31
b
2
57
b c
,
3
4
5
6
7
8
9
10
MeOH
EtOH
PhCN
PhCN
CH3CN
HOCH2CH2OH
DMF
pyridine
CH3NO2
<5
<5
b
88
d
81
65
b
36
30
b
<5
<5
b
11
a
b
c
Isolated yield, 1H NMR yield. A 28% yield of RhIII(ttp)Me was
d
isolated. 80 °C.
negligible yields of RhIII(ttp)Bn (3a) when the reaction was
conducted in methanol or ethanol solvent (Table 2, entries 3
and 4). The optimal result was obtained by employing
benzonitrile as the solvent, and RhIII(ttp)CH2Ph (3a) was
formed in 88% yield after 2 h (Table 2, entry 5). The reaction
also proceeded smoothly at 80 °C in PhCN to give
RhIII(ttp)CH2Ph (3a) in 81% yield in 13 h (Table 2, entry
6). Acetonitrile was a suitable solvent and afforded a 65% yield
of 3a in 4 h (Table 2, entry 7). Other polar solvents such as
ethylene glycol, DMF, pyridine and nitromthane gave poor to
negligible yields of RhIII(ttp)CH2Ph 3a (Table 2, entries 8−
11). At this stage we could not observe the clear relationships
between product yields and solvent properties except that
nitrile solvents were promising. Therefore, benzonitrile was
chosen to be the optimal solvent for the screening of alcohol
substrates.
Table 1. Temperature Effects on the C−O Cleavage of
Benzyl Alcohol
Substrate Scope. RhIII(ttp)Cl (1) reacted smoothly with
electron-rich and electron-poor benzyl alcohols 2a−g under
the optimized reaction conditions to afford the corresponding
C−O cleavage product RhIII(ttp)CH2C6H4(FG) (3) in good
yields in 2−4 h (Table 3). 4-Fluoro- and 4-chloro-substituted
a
entry
temp/°C
yield/%
b
1
2
3
4
80
100
120
150
19
b
38
85
50
Table 3. Scope of Substituted Benzyl Alcohols
a
b
Isolated yield. 1H NMR yield with DCE (1,2-dichloroethane) as the
internal standard.
100 °C, RhIII(ttp)CH2Ph (3a) was produced in poor yields of
19% and 38%, respectively, after 1 day. Increasing the
temperature to 120 °C afforded RhIII(ttp)CH2Ph (3a) in
85% yield. The reduction in product yield at 150 °C was
probably due to the partial decomposition of RhIII(ttp)CH2Ph
(3a) at elevated temperature. Therefore, 120 °C was chosen to
be the optimal reaction temperature.
Solvent Effect. We then examined the use of various
solvents to reduce the loading of benzyl alcohol (2a) from
solventless to 100 equiv with respect to RhIII(ttp)Cl (1)
(Table 2). When isopropyl alcohol was used as the solvent,
RhIII(ttp)Bn (3a) was formed in 31% yield after 15 h (Table 2,
entry 1). The yield was increased to 57% when the reaction
time was prolonged to 24 h (Table 2, entry 2). There were
a
entry
FG
time/h
yield/%
1
2
3
5
4
5
6
7
8
H (2a)
4-F (2b)
2
2
2
3
2
2
4
2
88 (3a)
81 (3b)
63 (3c)
65 (3d)
91 (3e)
78 (3f)
73 (3g)
62 (3h)
19 (3i)
4-Cl (2c)
4-CF3 (2d)
4-tBu (2e)
4-Me (2f)
2-Me (2g)
4-OMe (2h)
4-NO2 (2i)
0.5
a
Isolated yield.
B
DOI: 10.1021/acs.organomet.9b00454
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benzyl alcohols gave RhIII(ttp)CH2C6H4(4-F) (3b( and
RhIII(ttp)CH2C6H4(4-Cl) (3c) in 81% and 63% yields,
respectively (Table 3, entries 2 and 3). No C−F or C−Cl
cleavage products were observed.13 Alkylation of RhIII(ttp)Cl
(1) with 4-trifluoromethylbenzyl alcohol (2d) successfully
afforded RhIII(ttp)CH2C6H4(4-CF3) (3d) in 65% yield in 3 h.
The reactions with benzyl alcohols bearing 4-tBu and 4-Me
substituents were completed in 2 h to give RhIII(ttp)-
CH2C6H4(4-tBu) (3e) and RhIII(ttp)CH2C6H4(4-Me) (3f)
in 81% and 63% yields, respectively (Table 3, entries 4 and 5).
2-Methylbenzyl alcohol (2g) required a longer reaction time of
4 h to yield 73% of RhIII(ttp)CH2C6H4(2-Me) (3g) due to the
steric hindrance. No benzylic C−H activation products were
observed.1d Electron-rich p-methoxybenzyl alcohol (2h)
underwent regioselective C−O cleavage to produce RhIII(ttp)-
CH2C6H4(4-OMe) (3g) in 62% yield in 2 h (Table 3, entry 6).
p-Nitrobenzyl alcohol was a poor candidate and only led to a
19% yield of RhIII(ttp)CH2C6H4(4-NO2) (3i) presumably due
to the reduction of the nitro group (Table 3, entry 7).14
To expand the synthetic utility of this alkylating protocol, we
then explored the C−O cleavage reaction with other primary
alkyl alcohols (Table 4). Under the optimized reaction
CH2CH2Ph (3o) to give RhIII(ttp)H and styrene followed by
the reinsertion.15
To further examine whether this C−O cleavage protocol is
applicable to secondary alcohols, the reaction of 1-phenyl-
ethanol (2p) with RhIII(ttp)Cl (1) was carried out. However,
this reaction proceeded sluggishly to give the products
RhIII(ttp)CH2CH2Ph (3o) and RhIII(ttp)CH(CH3)Ph (3p)
in 8% total yield with a 3o:3p ratio of 1.0:1.2 (eq 2).
Therefore, we did not pursue other secondary alcohols.
Mechanistic Studies. It has been reported that RhIII(ttp)
Cl readily reacts with K2CO3 to give RhIII(ttp)CO3K via ligand
substitution in MeOH,11 and we proposed that it is the same
case in benzonitrile solvent, given its polar nature. RhIII(ttp)-
CO3K might then react with PhCH2OH to give RhIII(ttp)-
OCH2Ph. Alternatively, deprotonation of coordinated benzyl
alcohol gives RhIII(ttp)OCH2Ph as well (pKa of PhCH2OH =
15.4). RhIII(ttp)OCH2Ph then converts to RhIII(ttp)H and
benzaldehyde via β-H elimination,16 in a pathway analogous to
that proposed in the C−O activation of MeOH. To probe this
possibility, we attempted to detect any organic coproducts
formed in the C−O cleavage of benzyl alcohol. The reaction of
RhIII(ttp)Cl (1) with 100 equiv of PhCH2OH (2a) and 10
equiv of K2CO3 was conducted in CD3CN at 120 °C. The
reaction gave RhIII(ttp)CH2Ph (3a) in 67% yield and
benzaldehyde (4) in 61% yield with respect to RhIII(ttp)Cl
(1) (eq 3). This suggests that the benzyl alcohol plays a dual
Table 4. Scope of Primary Alkyl Alcohols
a
entry
R
time/h
yield/%
1
2
3
4
5
6
Me (2j)
Et (2k)
nPr (2l)
24
24
24
24
1
69 (3j)
37 (3k)
12 (3l)
9 (3m)
63(3n)
nBu (2m)
CNCH2CH2 (2n)
2-phenylethyl (2o)
b
24
29 (3o + 3p)
a
b
Isolated yield. RhIII(ttp)CH2CH2Ph (3o) and RhIII(ttp)CH(CH3)-
Ph (3p) in a 2.2:1.0 ratio.
role in the reaction: as a reductant for RhIII(ttp)Cl (1) to
RhIII(ttp)H via an analogous RhIII(ttp)OCH2Ph intermediate
and as an alkylating agent. Therefore, the reaction
stoichiometry for benzyl alcohol (2a) and RhIII(ttp)Cl (1) is
2:1.
conditions, methanol (2j) without β-hydrogens reacted with
RhIII(ttp)Cl (1) to afford the desired product RhIII(ttp)Me
(3j) in 69% yield after 24 h (Table 4, entry 1). This yield is
similar to that with the previously reported solventless
conditions.11 To our delight, the C−O cleavage of ethanol
(2k) took place to give a 37% yield of RhIII(ttp)Et (3k) in 24 h
(Table 4, entry 2). Both n-propanol (2l) and n-butanol (2m)
were poor alkylating candidates to afford RhIII(ttp)nPr (3l) and
RhIII(ttp)nBu (3m) in 12% and 9% yields, respectively (Table
4, entries 3 and 4). The low yields were presumably due to the
product decomposition by the facile β-hydride elimination to
give RhIII(ttp)H and gaseous olefins. To our delight, the cyano
group in 2-cyanoethanol (2n) effectively suppressed the β-
hydride elimination and afforded a 63% yield of RhIII(ttp)-
CH2CH2CN (3n) in 1 h (Table 4, entry 5). When 2-
phenylethanol (2o) was employed as the substrate, a mixture
of RhIII(ttp)CH2CH2Ph (3o) and RhIII(ttp)CH(CH3)Ph (3p)
was obtained in 29% total yield in a 2.2:1.0 ratio (Table 3,
entry 6). The formation of RhIII(ttp)CH2CH2Ph (3o) can be
accounted for by the C−O cleavage of 2-phenylethanol (2o),
while the formation of branched RhIII(ttp)CH(CH3)Ph (3p) is
likely derived from the β-hydride elimination of RhIII(ttp)-
In principle, RhIII(ttp)H generated from the reduction of
RhIII(ttp)Cl could be deprotonated by K2CO3 to give an
equilibrium mixture of RhI(ttp)− anion and RhIII(ttp)H (pKa
of RhIII(ttp)H = 11 in DMSO and pKa of KHCO3 = 10.25 in
H2O).17 To identify which species is the active intermediate
for C−O cleavage, independent reactions between benzyl
alcohol and RhIII(ttp)H with and without K2CO3 were
performed.
In the absence of K2CO3, RhIII(ttp)H (5) reacted with 100
equiv of benzyl alcohol (2a) in CD3CN at 120 °C for 4 h to
give only a trace amount of RhIII(ttp)CH2Ph (3a) (Scheme
2a). Therefore, RhIII(ttp)H (5) is unlikely the active
intermediate for the C−O bond cleavage. The lack of reaction
is likely due to the formation of coordinatively saturated
(CD3CN)RhIII(ttp)H. At the same time, RhIII(ttp)H under-
went dehydrogenation to give [RhII(ttp)]2,18 which is in
thermal equilibrium with the RhII(ttp) metalloradical (BDE of
Rh−Rh = 12 kcal/mol).19 The estimated equilibrium constant
at 120 °C is 1.53 M. Therefore, the direct cleavage of the C−O
bond in benzyl alcohol by [RhII(ttp)]2 or RhII(ttp) metal-
C
DOI: 10.1021/acs.organomet.9b00454
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RhIII(ttp)Cl were carried out (Table 5). The Hammett plot
from these experiments shows that both electron-donating and
Scheme 2. Reactivity of RhIII(ttp)H toward C−O Bond
Cleavage
Table 5. Competition Experiment of C−O Activation
a
yield/%
entry
FG
σp
3a
3
log(kFG/kH)
1
2
3
4
5
6
OMe
Me
H
F
Cl
−0.27
−0.17
0
0.06
0.23
0.54
23
40
81
27
15
20
66
53
0.45
0.12
63
44
70
0.38
0.46
0.54
loradical is unlikely as well, in agreement with the estimated
endothermicity of about 8.8 kcal/mol.20
CF3
a
NMR yield.
When 10 equiv of K2CO3 was added into the reaction
mixture containing RhIII(ttp)H (5) and benzyl alcohol (2a) in
CD3CN to generate RhI(ttp)− in situ, RhIII(ttp)CH2Ph (3a)
was formed in 44% yield after heating at 120 °C for 4 h
(Scheme 2b). These results point to two possible mechanistic
possibilities: (1) RhI(ttp)− anion is responsible for the direct
C−O cleavage of benzyl alcohol, likely via nucleophilic
substitution and (2) K2CO3 is essential and plays a promoting
role in the C−O cleavage reaction with RhIII(ttp)H. We favor
the latter mechanistic proposal over the former because the
direct nucleophilic substitution of alcohol with a nucleophile
weaker than hydroxide is unlikely to be feasible under basic
conditions.
-withdrawing substituents promoted the rate of C−O cleavage
(Figure 1). Due to the different sensitivities of each reaction
We reason that, in addition to the nucleophilic substitution
with RhI(ttp)− anion, other K2CO3-dependent C−O cleavage
pathways were concurrently operating. We examined the C−O
cleavage of benzyl alcohol in the presence of aniline to probe
whether the borrowing hydrogen pathway was operating.
When RhIII(ttp)Cl (1) was treated with 100 equiv of
PhCH2OH (2a) and 100 equiv of PhNH2 at 80 °C for 13 h,
RhIII(ttp)CH2Ph (3a) was isolated in 40% yield (Scheme 3).
Figure 1. Hammett plot of C−O cleavage of benzyl alcohols using the
Hammett constant σp.
Scheme 3. Probing for the Borrowing Hydrogen Mechanism
step to the electronic effects and the possibilities of two C−O
bond activation pathways, this concave upward Hammett plot
could not be interpreted easily.
On the basis of the previous studies on C−O cleavage of
MeOH11 and current experimental findings, Scheme 4
illustrates the proposed mechanism of C−O bond cleavage
in primary alcohols using benzyl alcohol as an example.
Initially, K2CO3 deprotonates benzyl alcohol to generate a
small amount of PhCH2O− anion, which reacts with RhIII(ttp)
Cl via ligand substitution to give the RhIII(ttp)OCH2Ph
intermediate. Alternatively, RhIII(ttp)OCH2Ph can be pro-
duced from the ligand substitution of RhIII(ttp)Cl with K2CO3
followed by the reaction with benzyl alcohol. Then RhIII(ttp)-
OCH2Ph undergoes β-H elimination to yield RhIII(ttp)H and
benzaldehyde.16 In PhCN solvent, (PhCN)RhIII(ttp)− anion is
rapidly produced from the deprotonation of RhIII(ttp)H by
K2CO3 followed by the solvent ligation (pKa of RhIII(ttp)H =
11 in DMSO and pKa of KHCO3 = 10.25 in H2O). The
(PhCN)RhIII(ttp)− anion is more nucleophilic than the
RhI(ttp)− anion.21 Then, (PhCN)RhI(ttp)− attacks the
carbonyl carbon of benzaldehyde, which leads to the α-
hydroxyalkyl complex RhIII(ttp)CH(OH)Ph upon protona-
tion.17,22 RhIII(ttp)CH(OH)Ph is then reduced by RhIII(ttp)H
to afford the final product RhIII(ttp)CH2Ph (3a) and
Analysis of the reaction aliquot with GC-MS revealed the
formation of benzaldehyde, N-benzylideneaniline, and N-
benzylaniline. The formation of N-benzylideneaniline is
accounted for by the condensation of benzaldehyde with
aniline with the elimination of H2O. Subsequently, hydro-
genation of N-benzylideneaniline can yield N-benzylaniline.
The observation of all these organic coproducts suggests that
the borrowing hydrogen pathway is likely operating for the C−
O cleavage reaction.
Finally, to gain some mechanistic insights from the
electronic effects of C−O bond cleavage in para-substituted
benzyl alcohols, competition experiments using an equimolar
ratio of benzyl alcohol and para-substituted benzyl alcohol with
D
DOI: 10.1021/acs.organomet.9b00454
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alcohol (2a) (3.0 mL) was degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 150 °C for 24 h. After the excess
benzyl alcohol was removed by vacuum distillation, the mixture was
worked up by extraction with CH2Cl2 and H2O. The organic extract
was dried by anhydrous Na2SO4, filtered, and rotary evaporated. The
red residue was purified by column chromatography over silica gel
with a solvent mixture of hexane/CH2Cl2 (1/1) as eluent to give
Scheme 4. Proposed Reaction Mechanism
1d
1
RhIII(ttp)Bn (3a) (10.7 mg, 0.013 mmol, 50%). H NMR (CDCl3,
400 MHz): δ −3.77 (d, 2 H, 3JRh−H = 3.7 Hz), 2.72 (s, 12 H), 2.98 (d,
2 H, J = 7.5 Hz), 5.89 (t, 2 H, J = 7.6 Hz), 6.43 (t, 1 H, J = 7.3 Hz),
7.56 (t, 8 H, J = 6.4 Hz), 8.02 (d, 4 H, J = 7.0 Hz), 8.09 (d, 4 H, J =
6.8 Hz), 8.69 (s, 8 H).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 at
120 °C. In a Teflon screw-capped tube, a mixture of RhIII(ttp)Cl (1)
(20.2 mg, 0.025 mmol), K2CO3 (34.5 mg, 0.25 mmol) and benzyl
alcohol (2a) (3.0 mL) was degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 120 °C for 24 h. After the excess
benzyl alcohol was removed by vacuum distillation, the mixture was
worked up by extraction with CH2Cl2 and H2O. The organic extract
was dried by anhydrous Na2SO4, filtered, and rotary evaporated. The
residue was washed twice with hexane to give RhIII(ttp)Bn (3a) (18.4
mg, 0.021 mmol, 85%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 at
100 °C. In a Teflon screw-capped tube, a mixture of RhIII(ttp)Cl (1)
(20.2 mg, 0.025 mmol), K2CO3 (34.5 mg, 0.25 mmol) and benzyl
alcohol (2a) (3.0 mL) was degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 100 °C for 24 h. After the excess
benzyl alcohol was removed by vacuum distillation, the mixture was
worked up by extraction with CH2Cl2 and H2O. The organic extract
was dried by anhydrous Na2SO4, filtered, and rotary evaporated. Then
CDCl3 and a smaller amount of 1,2-dichloroethane (DCE) were
added to measure the NMR yield of RhIII(ttp)Bn (3a) (38%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 at 80
°C. In a Teflon screw-capped tube, a mixture of RhIII(ttp)Cl (1) (20.2
mg, 0.025 mmol), K2CO3 (34.5 mg, 0.25 mmol), and benzyl alcohol
(2a) (3.0 mL) was degassed for three freeze−pump−thaw cycles,
refilled with N2, and heated at 100 °C for 24 h. After the excess benzyl
alcohol was removed by vacuum distillation, the mixture was worked
up by extraction with CH2Cl2 and H2O. The organic extract was dried
by anhydrous Na2SO4, filtered, and rotary evaporated. Then CDCl3
and a smaller amount of 1,2-dichloroethane (DCE) were added to
measure the NMR yield of RhIII(ttp)Bn (3a) (19%).
RhIII(ttp)OH, likely via σ-bond metathesis. RhIII(ttp)OH then
undergoes rapid reductive dimerization to produce [RhII(ttp)]2
and H2O2 at 120 °C.23 Under basic conditions and polar
medium, K2CO3 promotes the conversion of [RhII(ttp)]2 to
RhI(ttp)−K+ anion and RhIII(ttp)CO3K,24 which can partic-
ipate in the C−O cleavage reaction. At the same time, H2O2 is
decomposed rapidly to H2O and O2.25 We do not consider the
direct nucleophilic substitution pathway because hydroxide is a
very poor leaving group.
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
Isopropyl Alcohol for 15 h. RhIII(ttp)Cl (1) (10.1 mg, 0.0125
mmol), K2CO3 (17.3 mg, 0.125 mmol), and benzyl alcohol (2a)
(135.2 mg, 1.25 mmol) were added to isopropyl alcohol (1.5 mL) in a
Teflon screw-capped tube and degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 120 °C for 15 h. After the
isopropyl alcohol was removed by vacuum distillation, the mixture
was worked up by extraction with CH2Cl2 and H2O. The organic
extract was dried by anhydrous Na2SO4, filtered, and rotary
evaporated. The residue was purified by column chromatography
over silica gel with a solvent mixture of hexane/CH2Cl2 (1/1) as
eluent to give RhIII(ttp)Bn (3a) (3.3 mg, 0.0038 mmol, 31%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
Isopropyl Alcohol for 24 h. RhIII(ttp)Cl (1) (10.1 mg, 0.0125
mmol), K2CO3 (17.3 mg, 0.125 mmol), and benzyl alcohol (2a; 135.2
mg, 1.25 mmol) were added to isopropyl alcohol (1.5 mL) in a Teflon
screw-capped tube and degassed for three freeze−pump−thaw cycles,
refilled with N2, and heated at 120 °C for 24 h. After the isopropyl
alcohol was removed by vacuum distillation, the mixture was worked
up by extraction with CH2Cl2 and H2O. The organic extract was dried
by anhydrous Na2SO4, filtered, and rotary evaporated. The residue
was purified by column chromatography over silica gel with a solvent
mixture of hexane/CH2Cl2 (1/1) as eluent to give RhIII(ttp)Bn (3a)
(6.1 mg, 0.0071 mmol, 57%).
CONCLUSION
■
In summary, RhIII(ttp)Cl reacted with primary alcohols at 80−
120 °C in the presence of base to give moderate to good yields
of RhIII(ttp)(alkyl) complexes in PhCN. Mechanistic studies
suggest that the base-promoted borrowing hydrogen pathway
with RhIII(ttp)H as the key intermediate is operating. Further
studies to look for more evidence for the nucleophilic
substitution pathway are ongoing.
EXPERIMENTAL SECTION
■
Unless otherwise noted, all reagents were purchased from commercial
suppliers and directly used without further purification. Hexane was
distilled from anhydrous calcium chloride. Thin-layer chromatography
(TLC) was performed on precoated silica gel 60 F254 plates. Silica gel
(Merck, 70−230 mesh) was used for column chromatography. All
reactions were protected from light by wrapping with aluminum foil.
The reaction mixtures in Teflon screw-capped pressure tubes were
heated in heating blocks on heaters and monitored by TLC. RhIII(ttp)
Cl (1)26 and RhIII(ttp)H (5)17 have been characterized and were
prepared according to the literature process.
1H NMR spectra were recorded on a Bruker AV400 instrument at
400 MHz. Chemical shifts were referenced with the residual solvent
protons in CDCl3 (δ 7.26 ppm) and CD3CN (δ 1.94 ppm). Unless
otherwise specified, the coupling constant J refers to H−H coupling.
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 at
150 °C. In a Teflon screw-capped tube, a mixture of RhIII(ttp)Cl (1)
(20.2 mg, 0.025 mmol), K2CO3 (34.5 mg, 0.25 mmol) and benzyl
Reaction of RhIII(ttp)Cl and 50 equiv of Benzyl Alcohol with
K2CO3 in Isopropyl Alcohol. RhIII(ttp)Cl (1) (10.1 mg, 0.0125
mmol), K2CO3 (17.3 mg, 0.125 mmol), and benzyl alcohol (2a) (67.5
mg, 0.625 mmol) were added to isopropyl alcohol (1.5 mL) in a
E
DOI: 10.1021/acs.organomet.9b00454
Organometallics XXXX, XXX, XXX−XXX

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Article
Teflon screw-capped tube and degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 120 °C for 24 h. After the
isopropyl alcohol was removed by vacuum distillation, the mixture
was worked up by extraction with CH2Cl2 and H2O. The organic
extract was dried by anhydrous Na2SO4, filtered, and rotary
evaporated. Then CDCl3 and a smaller amount of DCE were added
to measure the NMR yield of RhIII(ttp)Bn (3a) (38%).
up by extraction with CH2Cl2 and H2O. The organic extract was dried
by anhydrous Na2SO4, filtered, and rotary evaporated. The residue
was purified by column chromatography over silica gel with a solvent
mixture of hexane/CH2Cl2 (1/1) as eluent to give RhIII(ttp)Bn (3a)
(7.0 mg, 0.008 mmol, 65%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
Ethylene Glycol. RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3
(17.3 mg, 0.125 mmol), and benzyl alcohol (2a) (135.2 mg, 1.25
mmol) were added to ethylene glycol (1.5 mL) in a Teflon screw-
capped tube and degassed for three freeze−pump−thaw cycles,
refilled with N2, and heated at 120 °C for 24 h. After the ethylene
glycol and excess benzyl alcohol were removed by vacuum distillation,
the mixture was worked up by extraction with CH2Cl2 and H2O. The
organic extract was dried by anhydrous Na2SO4, filtered, and rotary
evaporated. Then CDCl3 and a smaller amount of DCE were added to
measure the NMR yield of RhIII(ttp)Bn (3a) (36%).
Reaction of RhIII(ttp)Cl and 10 equiv of Benzyl Alcohol with
K2CO3 in Isopropyl Alcohol. RhIII(ttp)Cl (1) (10.1 mg, 0.0125
mmol), K2CO3 (17.3 mg, 0.125 mmol), and benzyl alcohol (2a) (13.5
mg, 0.125 mmol) were added to isopropyl alcohol (1.5 mL) in a
Teflon screw-capped tube and degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 120 °C for 24 h. After the
isopropyl alcohol was removed by vacuum distillation, the mixture
was worked up by extraction with CH2Cl2 and H2O. The organic
extract was dried by anhydrous Na2SO4, filtered, and rotary
evaporated. Then CDCl3 and a smaller amount of DCE were added
to measure the NMR yield of RhIII(ttp)Bn (3a) (9%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
Pyridine. RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3
mg, 0.125 mmol), and benzyl alcohol (2a) (135.2 mg, 1.25 mmol)
were added to pyridine (1.5 mL) in a Teflon screw-capped tube and
degassed for three freeze−pump−thaw cycles, refilled with N2, and
heated at 120 °C for 24 h. After the pyridine and excess benzyl
alcohol were removed by vacuum distillation, the mixture was worked
up by extraction with CH2Cl2 and H2O. The organic extract was dried
by anhydrous Na2SO4, filtered, and rotary evaporated. Then CDCl3
and a smaller amount of DCE were added to measure the NMR yield
of RhIII(ttp)Bn (3a) (<5%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
Methanol. RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3
mg, 0.125 mmol), and benzyl alcohol (2a) (135.2 mg, 1.25 mmol)
were added to methanol (1.5 mL) in a Teflon screw-capped tube and
degassed for three freeze−pump−thaw cycles, refilled with N2, and
heated at 120 °C for 24 h. After the methanol was removed by
vacuum distillation, the mixture was worked up by extraction with
CH2Cl2 and H2O. The organic extract was dried by anhydrous
Na2SO4, filtered, and rotary evaporated. Then CDCl3 and a smaller
amount of DCE were added to measure the NMR yield of
RhIII(ttp)Bn (3a) (<5%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
N,N-Dimethylformamide. RhIII(ttp)Cl (1) (10.1 mg, 0.0125
mmol), K2CO3 (17.3 mg, 0.125 mmol), and benzyl alcohol (2a)
(135.2 mg, 1.25 mmol) were added to DMF (1.5 mL) in a Teflon
screw-capped tube and degassed for three freeze−pump−thaw cycles,
refilled with N2, and heated at 120 °C for 24 h. After DMF and excess
benzyl alcohol were removed by vacuum distillation, the mixture was
worked up by extraction with CH2Cl2 and H2O. The organic extract
was dried by anhydrous Na2SO4, filtered, and rotary evaporated. The
residue was purified by column chromatography over silica gel with a
solvent mixture of hexane/CH2Cl2 (1/1) as eluent to give
RhIII(ttp)Bn (3a) (3.2 mg, 0.0037 mmol, 30%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
Ethanol. RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg,
0.125 mmol), and benzyl alcohol (2a) (135.2 mg, 1.25 mmol) were
added to ethanol (1.5 mL) in a Teflon screw-capped tube and
degassed for three freeze−pump−thaw cycles, refilled with N2, and
heated at 120 °C for 24 h. After the ethanol was removed by vacuum
distillation, the mixture was worked up by extraction with CH2Cl2 and
H2O. The organic extract was dried by anhydrous Na2SO4, filtered,
and rotary evaporated. Then CDCl3 and a smaller amount of DCE
were added to measure the NMR yield of RhIII(ttp)Bn (3a) (<5%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
Benzonitrile. RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3
(17.3 mg, 0.125 mmol), and benzyl alcohol (2a) (135.2 mg, 1.25
mmol) were added to benzonitrile (1.5 mL) in a Teflon screw-capped
tube and degassed for three freeze−pump−thaw cycles, refilled with
N2, and heated at 120 °C for 2 h. After the benzonitrile was removed
by vacuum distillation, the mixture was worked up by extraction with
CH2Cl2 and H2O. The organic extract was dried by anhydrous
Na2SO4, filtered, and rotary evaporated. The residue was purified by
column chromatography over silica gel with a solvent mixture of
hexane/CH2Cl2 (1/1) as eluent to give RhIII(ttp)Bn (3a) (9.5 mg,
0.011 mmol, 88%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
Nitroethane. RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3
(17.3 mg, 0.125 mmol), and benzyl alcohol (2a) (135.2 mg, 1.25
mmol) were added to nitroethane (1.5 mL) in a Teflon screw-capped
tube and degassed for three freeze−pump−thaw cycles, refilled with
N2, and heated at 120 °C for 4 h. After nitroethane and excess benzyl
alcohol were removed by vacuum distillation, the mixture as worked
up by extraction with CH2Cl2 and H2O. The organic extract was dried
by anhydrous Na2SO4, filtered, and rotary evaporated. Then CDCl3
and a smaller amount of DCE were added to measure the NMR yield
of RhIII(ttp)Bn (3a) (<5%).
Reaction of RhIII(ttp)Cl and 4-Fluorobenzyl Alcohol.
RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125
mmol), and 4-fluorobenzyl alcohol (2b) (157.7 mg, 1.25 mmol) were
added to benzonitrile (1.5 mL) in a Teflon screw-capped tube and
degassed for three freeze−pump−thaw cycles, refilled with N2, and
heated at 120 °C for 2 h. After benzonitrile and excess 4-fluorobenzyl
alcohol were removed by vacuum distillation, the residue was quickly
purified by column chromatography over silica gel with a solvent
mixture of hexane/CH2Cl2 (1/1) as eluent to give RhIII(ttp)Bn(4-F)
(3b)1d (8.9 mg, 0.0101 mmol, 81%). Rf = 0.60 (hexane/CH2Cl2 = 1/
1). 1H NMR (CDCl3, 400 MHz): δ −3.81 (d, 2 H, 3JRh−H = 3.7 Hz),
2.73 (s, 12 H), 2.93−2.89 (m, 2 H), 5.57 (t, 2 H, J = 8.7 Hz), 7.58 (t,
8 H, J = 7.9 Hz), 8.00 (dd, 4 H, J = 7.5, 1.6 Hz), 8.09 (dd, 4 H, J =
7.5, 1.6 Hz), 8.71 (s, 8 H).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
Benzonitrile at 80 °C. RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol),
K2CO3 (17.3 mg, 0.125 mmol), and benzyl alcohol (2a) (135.2 mg,
1.25 mmol) were added to benzonitrile (1.5 mL) in a Teflon screw-
capped tube and degassed for three freeze−pump−thaw cycles,
refilled with N2, and heated at 80 °C for 13 h. After the benzonitrile
was removed by vacuum distillation, the mixture was worked up by
extraction with CH2Cl2 and H2O. The organic extract was dried by
anhydrous Na2SO4, filtered, and rotary evaporated. The residue was
purified by column chromatography over silica gel with a solvent
mixture of hexane/CH2Cl2 (1/1) as eluent to give RhIII(ttp)Bn (3a)
(8.7 mg, 0.010 mmol, 81%).
Reaction of RhIII(ttp)Cl and Benzyl Alcohol with K2CO3 in
Acetonitrile. RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3
mg, 0.125 mmol), and benzyl alcohol (2a) (135.2 mg, 1.25 mmol)
were added to acetonitrile (1.5 mL) in a Teflon screw-capped tube
and degassed for three freeze−pump−thaw cycles, refilled with N2,
and heated at 120 °C for 4 h. After the acetonitrile and excess benzyl
alcohol were removed by vacuum distillation, the mixture was worked
Reaction of RhIII(ttp)Cl and 4-Chlorobenzyl Alcohol.
RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125
mmol), and 4-chlorobenzyl alcohol (2c) (178.2 mg, 1.25 mmol) were
added to benzonitrile (1.5 mL) in a Teflon screw-capped tube and
degassed for three freeze−pump−thaw cycles, refilled with N2, and
heated at 120 °C for 2 h. After benzonitrile was removed by vacuum
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DOI: 10.1021/acs.organomet.9b00454
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
distillation, the residue was quickly purified by column chromatog-
raphy over silica gel with a solvent mixture of hexane/CH2Cl2 (1/1)
as eluent to give RhIII(ttp)Bn(4-Cl) 3c1d (7.1 mg, 0.0079 mmol,
0.0077 mmol, 62%). Rf = 0.52 (hexane/CH2Cl2 = 1/1). H NMR
3
(CDCl3, 400 MHz): δ −3.74 (d, 2 H, JRh−H = 3.7 Hz), 2.73 (s, 12
H), 2.95 (d, 2 H, J = 8.6 Hz), 3.45 (s, 3 H), 5.44 (d, 2 H, J = 8.6 Hz),
7.57−7.55 (m, 8 H), 8.02−7.98 (m, 4 H), 8.10−8.08 (m, 4 H), 8.70
(s, 8 H).
1
63%). Rf = 0.58 (hexane/CH2Cl2 = 1/1). H NMR (CDCl3, 400
MHz): δ −3.74 (d, 2 H, 3JRh−H = 3.7 Hz), 2.73 (s, 12 H), 2.90 (d, 2
H, J = 7.9 Hz), 5.68 (d, 2 H, J = 7.8 Hz), 7.56 (t, 8 H, J = 11.2 Hz),
8.00−7.97 (m, 4 H), 8.10−8.07 (m, 4 H), 8.69 (s, 8 H).
Reaction of RhIII(ttp)Cl and 4-Nitrobenzyl Alcohol. RhIII(ttp)
Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125 mmol), and
4-nitrobenzyl alcohol (2i) (191.3 mg, 1.25 mmol) were added to
benzonitrile (1.5 mL) in a Teflon screw-capped tube and degassed for
three freeze−pump−thaw cycles, refilled with N2, and heated at 120
°C for 0.5 h. After benzonitrile was removed by vacuum distillation,
the residue was quickly purified by column chromatography over silica
gel with a solvent mixture of hexane/CH2Cl2 (1/1) as eluent to give
RhIII(ttp)Bn(4-NO2) (3i)20 (2.1 mg, 0.0023 mmol, 19%). Rf = 0.55
(hexane/CH2Cl2 = 1/1). 1H NMR (CDCl3, 400 MHz): δ −3.81 (d, 2
H, 3JRh−H = 3.9 Hz), 2.73 (s, 12 H), 2.92 (d, 2 H, J = 8.6 Hz), 6.71 (d,
2 H, J = 8.6 Hz), 7.58 (t, 8 H, J = 17.8 Hz), 7.98 (d, 4 H, J = 8.8 Hz),
8.07 (d, 4 H, J = 7.6 Hz), 8.74 (s, 8 H).
Reaction of RhIII(ttp)Cl and 4-Trifluoromethylbenzyl Alco-
hol. RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg,
0.125 mmol) and 4-trifluoromethylbenzyl alcohol (2d) (220.2 mg,
1.25 mmol) were added to benzonitrile (1.5 mL) in a Teflon screw-
capped tube and degassed for three freeze−pump−thaw cycles,
refilled with N2, and heated at 120 °C for 3 h. After benzonitrile was
removed by vacuum distillation, the residue was quickly purified by
column chromatography over silica gel with a solvent mixture of
hexane/CH2Cl2 (1/1) as eluent to give RhIII(ttp)Bn(4-CF3) 3d1d
1
(7.6 mg, 0.0081 mmol, 65%). Rf = 0.58 (hexane/CH2Cl2 = 1/1). H
NMR (CDCl3, 400 MHz): δ −3.82 (d, 2 H, 3JRh−H = 4.0 Hz), 2.71 (s,
12 H), 2.96 (d, 2 H, J = 8.0 Hz), 6.08 (d, 2 H, J = 8.1 Hz), 7.54 (d, 4
H, J = 7.8 Hz), 7.55 (d, 4 H, J = 7.6 Hz), 7.94 (d, 4 H, J = 8.0 Hz),
8.05 (d, 4 H, J = 8.0 Hz), 8.70 (s, 8 H).
Reaction of RhIII(ttp)Cl and Methanol. RhIII(ttp)Cl (1) (10.1
mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125 mmol), and methanol
(2j) (40.0 mg, 1.25 mmol) were added to benzonitrile (1.5 mL) in a
Teflon screw-capped tube and degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 120 °C for 24 h. After
benzonitrile was removed by vacuum distillation, the residue was
quickly purified by column chromatography over silica gel with a
solvent mixture of hexane/CH2Cl2 (1/1) as eluent to give
RhIII(ttp)Me (3j)17 (6.8 mg, 0.0086 mmol, 69%). Rf = 0.74
(hexane/CH2Cl2 = 1/1). 1H NMR (CDCl3, 400 MHz): δ −5.79
Reaction of RhIII(ttp)Cl and 4-tert-Butylbenzyl Alcohol.
RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125
mmol), and 4-tert-butylbenzyl alcohol (2e) (205.3 mg, 1.25 mmol)
were added to benzonitrile (1.5 mL) in a Teflon screw-capped tube
and degassed for three freeze−pump−thaw cycles, refilled with N2,
and heated at 120 °C for 2 h. After benzonitrile was removed by
vacuum distillation, the residue was quickly purified by column
chromatography over silica gel with a solvent mixture of hexane/
CH2Cl2 (1/1) as eluent to give RhIII(ttp)Bn(4-tBu)1d (3e) (10.5 mg,
3
(d, 3 H, JRh−H = 2.9 Hz), 2.72 (s, 12 H), 7.56 (t, 8 H, J = 5.1 Hz),
8.05 (d, 4 H, J = 7.3 Hz), 8.10 (d, 4 H, J = 7.2 Hz), 8.73 (s, 8 H).
Reaction of RhIII(ttp)Cl and Ethanol. RhIII(ttp)Cl (1) (10.1 mg,
0.0125 mmol), K2CO3 (17.3 mg, 0.125 mmol), and ethanol (2k)
(57.5 mg, 1.25 mmol) were added to benzonitrile (1.5 mL) in a
Teflon screw-capped tube and degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 120 °C for 24 h. After
benzonitrile was removed by vacuum distillation, the residue was
quickly purified by column chromatography over silica gel with a
solvent mixture of hexane/CH2Cl2 (1/1) as eluent to give RhIII(ttp)Et
(3k)27 (3.7 mg, 0.0046 mmol, 37%). Rf = 0.72 (hexane/CH2Cl2 = 1/
1). 1H NMR (CDCl3, 400 MHz): δ −4.84 (qd, 2 H, J = 7.3, 3.0 Hz),
−4.42 (td, 3 H, J = 7.4, 1.6 Hz), 2.72 (s, 12 H), 7.55 (t, 8 H, J = 7.3
Hz), 8.02 (d, 4 H, J = 7.6 Hz), 8.10 (d, 4 H, J = 7.6 Hz), 8.76 (s, 8
H).
1
0.0114 mmol, 91%). Rf = 0.77 (hexane/CH2Cl2 = 1/1). H NMR
(CDCl3, 400 MHz): δ −3.75 (d, 2 H, 3JRh−H = 3.7 Hz), 0.99 (s, 9 H),
2.73 (s, 12 H), 2.97 (d, 2 H, J = 8.2 Hz), 5.92 (d, 2 H, J = 8.2 Hz),
7.57 (t, 8 H, J = 6.6 Hz), 8.10−8.05 (m, 8 H), 8.68 (s, 8 H).
Reaction of RhIII(ttp)Cl and 4-Methylbenzyl Alcohol.
RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125
mmol), and 4-methylbenzyl alcohol (2f) (152.5 mg, 1.25 mmol) were
added to benzonitrile (1.5 mL) in a Teflon screw-capped tube and
degassed for three freeze−pump−thaw cycles, refilled with N2, and
heated at 120 °C for 2 h. After benzonitrile was removed by vacuum
distillation, the residue was quickly purified by column chromatog-
raphy over silica gel with a solvent mixture of hexane/CH2Cl2 (1/1)
as eluent to give RhIII(ttp)Bn(4-Me) (3f)1d (8.5 mg, 0.0093 mmol,
1
Reaction of RhIII(ttp)Cl and n-Propanol. Rh(ttp)Cl 1 (10.1 mg,
0.0125 mmol), K2CO3 (17.3 mg, 0.125 mmol), and n-propanol (2l)
(75.1 mg, 1.25 mmol) were added to benzonitrile (1.5 mL) in a
Teflon screw-capped tube and degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 120 °C for 24 h. After
benzonitrile was removed by vacuum distillation, the residue was
quickly purified by column chromatography over silica gel with a
solvent mixture of hexane/CH2Cl2 (1/1) as eluent to give
RhIII(ttp)nPr (3l)27 (1.2 mg, 0.0015 mmol, 12%). Rf = 0.60
78%). Rf = 0.72 (hexane/CH2Cl2 = 1/1). H NMR (CDCl3, 400
3
MHz): δ −3.74 (d, 2 H, JRh−H = 3.7 Hz), 1.71 (s, 3 H), 2.73 (s, 12
H), 2.90 (d, 2 H, J = 7.9 Hz), 5.68 (d, 2 H, J = 7.8 Hz), 7.57 (t, 8 H, J
= 5.8 Hz), 8.02−7.98 (m, 4 H), 8.11−8.08 (m, 4 H), 8.70 (s, 8 H).
Reaction of RhIII(ttp)Cl and 2-Methylbenzyl Alcohol.
RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125
mmol), and 2-methylbenzyl alcohol (2g) (152.5 mg, 1.25 mmol)
were added into benzonitrile (1.5 mL) in a Teflon screw-capped tube
and degassed for three freeze−pump−thaw cycles, refilled with N2,
and heated at 120 °C for 4 h. After benzonitrile was removed by
vacuum distillation, the residue was quickly purified by column
chromatography over silica gel with a solvent mixture of hexane/
CH2Cl2 (1/1) as eluent to give RhIII(ttp)Bn(2-CH3) (3g)1d (8.0 mg,
1
(hexane/CH2Cl2 4/1). H NMR (CDCl3, 400 MHz): δ −4.94 to
−4.98 (m, 2 H), −4.37 to −4.47 (m, 2 H), −1.72 (t, 2 H, J = 7.4 Hz),
2.72 (s, 12 H), 7.55 (t, 8 H, J = 6.4 Hz), 8.03 (d, 4 H, J = 7.4 Hz),
8.09 (d, 4 H, J = 7.4 Hz), 8.73 (s, 8 H).
1
0.0091 mmol, 73%). Rf = 0.69 (hexane/CH2Cl2 = 1/1). H NMR
Reaction of RhIII(ttp)Cl and n-Butanol. RhIII(ttp)Cl (1) (10.1
mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125 mmol), and n-butanol
(2m) (92.6 mg, 1.25 mmol) were added to benzonitrile (1.5 mL) in a
Teflon screw-capped tube and degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 120 °C for 24 h. After
benzonitrile was removed by vacuum distillation, the residue was
quickly purified by column chromatography over silica gel with a
solvent mixture of hexane/CH2Cl2 (1/1) as eluent to give
RhIII(ttp)nBu (3m)27 (0.9 mg, 0.0011 mmol, 9%). Rf = 0.84
3
(CDCl3, 400 MHz): δ −3.54 (d, 2 H, JRh−H = 3.7 Hz), −0.88 (s, 3
H), 2.58 (d, 1 H, J = 7.4 Hz), 2.73 (s, 12 H), 5.68 (t, 1 H, J = 7.4 Hz),
5.75 (d, 1 H, J = 7.4 Hz), 6.33 (t, 1 H, J = 7.3 Hz), 7.57 (t, 8 H, J =
11.6 Hz), 8.00−7.98 (m, 4 H), 8.10−8.07 (m, 4 H), 8.71 (s, 8 H).
Reaction of RhIII(ttp)Cl and 4-Methoxylbenzyl Alcohol.
RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125
mmol), and 4-methoxylbenzyl alcohol (2h) (172.5 mg, 1.25 mmol)
were added to benzonitrile (1.5 mL) in a Teflon screw-capped tube
and degassed for three freeze−pump−thaw cycles, refilled with N2,
and heated at 120 °C for 2 h. After benzonitrile was removed by
vacuum distillation, the residue was quickly purified by column
chromatography over silica gel with a solvent mixture of hexane/
CH2Cl2 (1/1) as eluent to give RhIII(ttp)Bn(4-OMe) (3h)20 (6.9 mg,
1
(hexane/CH2Cl2 1/1). H NMR (CDCl3, 400 MHz): δ −4.91 to
−4.95 (m, 2 H), −4.44 to −4.52 (m, 2 H), −1.56 (q, 2 H, J = 7.4
Hz), −0..81 (t, 3 H, J = 7.3 Hz), 2.72 (s, 12 H), 7.55 (t, 8 H, J = 7.3
Hz), 8.01 (d, 4 H, J = 7.6 Hz), 8.10 (d, 4 H, J = 7.4 Hz), 8.73 (s, 8
H).
G
DOI: 10.1021/acs.organomet.9b00454
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Organometallics
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Reaction of RhIII(ttp)Cl and 2-Cyanoethanol. RhIII(ttp)Cl (1)
(10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125 mmol), and 2-
cyanoethanol (2n) (88.8 mg, 1.25 mmol) were added to benzonitrile
(1.5 mL) in a Teflon screw-capped tube and degassed for three
freeze−pump−thaw cycles, refilled with N2, and heated at 120 °C for
1 h. After benzonitrile was removed by vacuum distillation, the
residue was quickly purified by column chromatography over silica gel
with a solvent mixture of hexane/CH2Cl2 (1/1) as eluent to give
RhIII(ttp)CH2CH2CN (3n)28 (6.5 mg, 0.0078 mmol, 63%). Rf = 0.46
added to PhCN (1.5 mL) in a Teflon screw-capped tube and degassed
for three freeze−pump−thaw cycles, refilled with N2, and heated at 80
°C for 13 h. The reaction aliquot was analyzed by GC-MS. After the
removal of PhCN by vacuum distillation, the crude mixture was
worked up by extraction with CH2Cl2 and H2O. The organic extract
was dried by anhydrous Na2SO4, filtered, and rotary evaporated. The
residue was purified by column chromatography over silica gel with a
solvent mixture of hexane/CH2Cl2 (1/1) as eluent to give
RhIII(ttp)Bn (3a) (4.3 mg, 0.0050 mmol, 40%).
General Procedures for the Competition Experiments.
RhIII(ttp)Cl (1) (10.0 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125
mmol), benzyl alcohol (2a) (135.2 mg, 1.25 mmol), and substituted
benzyl alcohol (1.25 mmol) were added to PhCN (1.5 mL) in a
Teflon screw-capped tube and degassed for three freeze−pump−thaw
cycles, refilled with N2, and heated at 80 °C for 15 h. After the
completion of the reaction and removal of solvent, the 1H NMR
spectrum of the crude reaction mixture was taken and the yields were
measured using DCE as the internal standard.
1
(hexane/CH2Cl2 1/2). H NMR (CDCl3, 400 MHz): δ −4.97 to
−5.02 (m, 2 H), −3.69 (t, 2 H, J = 8.4 Hz), 2.73 (s, 12 H), 7.58 (d, 8
H, J = 7.4 Hz), 8.07 (t, 8 H, J = 9.4 Hz), 8.81 (s, 8 H).
Reaction of RhIII(ttp)Cl and 2-Phenylethanol. RhIII(ttp)Cl (1)
(10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125 mmol), and 2-
phenylethanol (2o) (152.5 mg, 1.25 mmol) were added to
benzonitrile (1.5 mL) in a Teflon screw-capped tube and degassed
for three freeze−pump−thaw cycles, refilled with N2, and heated at
120 °C for 24 h. After benzonitrile was removed by vacuum
distillation, the residue was quickly purified by column chromatog-
raphy over silica gel with a solvent mixture of hexane/CH2Cl2 (1/1)
as eluent to give a mixture of RhIII(ttp)CH2CH2Ph (3o)29 and
RhIII(ttp)CH(CH3)Ph (3p)29 in a total yield of 29%, and the ratio of
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.S.C.: ksc@cuhk.edu.hk.
■
1
3n to 3o was measured to be 2.2:1 by an H NMR spectrum.
Reaction of RhIII(ttp)Cl and 1-Phenylethanol. RhIII(ttp)Cl (1)
(10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125 mmol), and 1-
phenylethanol (2p) (152.5 mg, 1.25 mmol) were added to
benzonitrile (1.5 mL) in a Teflon screw-capped tube and degassed
for three freeze−pump−thaw cycles, refilled with N2, and heated at
120 °C for 75 h. After benzonitrile was removed by vacuum
distillation, the residue was quickly purified by column chromatog-
raphy over silica gel with a solvent mixture of hexane/CH2Cl2 (1/1)
as eluent to give a mixture of RhIII(ttp)CH2CH2Ph (3o) and
RhIII(ttp)CH(CH3)Ph (3p) in a total yield of 8%, and the ratio of 3o
ORCID
Ching Tat To: 0000-0003-4272-6881
Kin Shing Chan: 0000-0002-1112-6819
Notes
The authors declare no competing financial interest.
†Y.B. is visiting scholar from Jinzhong University, Shanxi
Province, People’s Republic of China.
ACKNOWLEDGMENTS
1
■
to 3p was measured to be 1.2:1 by an H NMR spectrum.
Reaction of RhIII(ttp)Cl and Benzyl Alcohol in CD3CN.
RhIII(ttp)Cl (1) (10.1 mg, 0.0125 mmol), K2CO3 (17.3 mg, 0.125
mmol), and benzyl alcohol (2a) (135.2 mg, 1.25 mmol) were added
to CD3CN (1.5 mL) in a Teflon screw-capped tube and degassed for
three freeze−pump−thaw cycles, refilled with N2, and heated at 120
°C for 4 h. After CD3CN was removed by vacuum distillation, CDCl3
and DCE as an internal standard were added to measure the NMR
yields. The yields of RhIII(ttp)Bn (3a) and benzaldehyde (4) were
determined to be 67% and 61% based on metal Rh, respectively.
Reaction of RhIII(ttp)H and Benzyl Alcohol in CD3CN.
RhIII(ttp)H (5) (9.7 mg, 0.0125 mmol) and benzyl alcohol (2a)
(135.2 mg, 1.25 mmol) were added to CD3CN (1.5 mL) in a Teflon
screw-capped tube and degassed for three freeze−pump−thaw cycles,
refilled with N2, and heated at 120 °C for 4 h. After CD3CN was
removed by vacuum distillation, the mixture was worked up by
extraction with CH2Cl2 and H2O. The organic extract was dried by
anhydrous Na2SO4, filtered, and rotary evaporated. The residue was
purified by column chromatography over silica gel with a solvent
mixture of hexane/CH2Cl2 (1/1) as eluent to give RhIII(ttp)Bn (3a)
(<1%).
We thank the Research Grants Council of Hong Kong for
financial support (GRF No. 14300115).
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