Reactivity and kinetic-mechanistic studies of regioselective reactions of rhodium porphyrins with unactivated olefins in water that form β-hydroxyalkyl complexes and conversion to ketones and epoxides

Article abstract of DOI:10.1039/b912219b

This article reports on the selective oxidation of unactivated alkenes to ketones and epoxides through the intermediacy of β-hydroxyalkyl rhodium porphyrin complexes which are formed by reactions of terminal alkenes with tetra(p-sulfonatophenyl)porphyrin

Full text of DOI:10.1039/b912219b

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Reactivity and kinetic–mechanistic studies of regioselective reactions of
rhodium porphyrins with unactivated olefins in water that form b-hydroxyalkyl
complexes and conversion to ketones and epoxides†
Jiadi Zhang,a Bradford B. Wayland,b Lin Yun,a Shan Lib and Xuefeng Fu*a
Received 22nd June 2009, Accepted 19th September 2009
First published as an Advance Article on the web 22nd October 2009
DOI: 10.1039/b912219b
This article reports on the selective oxidation of unactivated alkenes to ketones and epoxides through
the intermediacy of b-hydroxyalkyl rhodium porphyrin complexes which are formed by reactions of
terminal alkenes with tetra(p-sulfonatophenyl)porphyrin rhodium(III) complex. The b-hydroxyalkyl
rhodium porphyrin complexes in water undergo b-C–H elimination to produce ketones in aqueous
pH 9.0 solutions and O–H deprotonation in KOH/DMSO solutions resulting in the rapid and
quantitative intramolecular nucleophilic displacement to form 1,2-epoxyalkanes.
Introduction
Results and discussion
The activation of alkenes by coordination to metal centers is
one of the most widely exploited synthetic methodologies in the
functionalization of organic molecules,1-5 and has contributed
significantly to advances in selective organic transformations.6-10
Catalytic air oxidation of ethene to acetaldehyde by an aqueous
Pd(II)/Cu(II) catalyst system is known as the Wacker process.
Mechanistic studies for the Wacker reaction and closely related
processes have shown that alkene binding with palladium(II)
activates reactions of olefins with nucleophiles including water,
alcohols, and amines to form b-substituted alkyl complexes.11 Sub-
sequent C–H and O–H elimination reactions of the b-substituted
alkyl complexes produce organic carbonyls, epoxides, heterocyclic
molecules, and hydrofunctionalized alkenes.12-15 Several rhodium
and iridium complexes have also been reported to mediate
oxidation of olefins in hydrocarbon media,16-19 but relatively little
attention has been given to complexes of transition metals other
than palladium(II). The importance of olefin transformations and
objectives of green chemistry justify the continuing search for
new classes of catalyst materials that give oxidations in water and
use dioxygen as the oxidant. This article reports on the observed
stepwise reactions of rhodium porphyrins with olefins in water to
form b-hydroxyalkyl complexes that react on to give stoichiometric
oxidation of alkenes to ketones in aqueous media and epoxides
in DMSO. Several objectives in olefin activation and oxidation
have been advanced in this study through observation of product
inhibited catalytic oxidation of olefins to organic carbonyls in
water using dioxygen as the oxidant.
Formation of b-hydroxyalkyl rhodium complexes in water
Tetra(p-sulfonatophenyl)porphyrin rhodium(III) diaquo complex
[(TSPP)RhIII(D2O)2]3- (1) is a convenient entry point for aque-
ous (D2O) solution reactivity studies of rhodium porphyrins.
The rhodium(III) diaquo complex occurs in D2O solution
as a pH dependent equilibrium distribution with the mono-
and bis-hydroxo complexes ([(TSPP)RhIII(OD)(D2O)]4- (2) and
[(TSPP)RhIII(OD)2]5- (3)) where K1 = 1.4 0.2 ¥ 10-8 and K2 =
2.8 0.3 ¥ 10-12 (eqn (1) and (2)).20
[(TSPP)RhIII(D2O)2]3- ꢀ [(TSPP)RhIII(OD)(D2O)]4- + D+ (1)
[(TSPP)RhIII(OD)(D2O)]4- ꢀ [(TSPP)RhIII(OD)2]5- + D+ (2)
Reactions of (TSPP)RhIII complexes with a series of olefins
produce b-hydroxyalkyl complexes in water.‡
Reaction of (TSPP)RhIII with 2-substituted olefins. Ethene
and larger terminal alkene hydrocarbons (CH2=CHR) react
regioselectively with (TSPP)RhIII in D2O to form b-hydroxy alkyl
rhodium complexes ([(TSPP)Rh–CH2CH(OD)R(D2O)]4-) where
rhodium is attached to the terminal primary CH2 unit (eqn (3)).21
[(TSPP)Rh–OD(D2O)]4- + CH2 CHR ꢀ
=
[(TSPP)Rh–CH2CH(OD)R(D2O)]4-
(3)
The b-hydroxyalkyl complexes are easily identified by the
porphyrin ring current induced high-field 1H NMR reso-
nances of the organic group bonded to the rhodium cen-
ter. The 1H NMR resonances for the diastereotopic a-CH2
group in (TSPP)Rh–CH2CH(OD)CH2CH2CH3 in D2O are
centered at -5.93 and -5.74 ppm. Isolating ((TSPP)Rh–
CH2CH(OH)CH2CH2CH3) formed in water and dissolving in an-
hydrous DMSO-d6 permitted observing the doublet hydroxyl pro-
ton resonance ((TSPP)RhCH2CH(OH)CH2CH2CH3) at 0.01 ppm
aBeijing National Laboratory for Molecular Sciences, State Key Lab of Rare
Earth Materials Chemistry and Applications, Peking University, Beijing,
100871, China. E-mail: fuxf@pku.edu.cn; Fax: +86 10 6275 1708; Tel: +86
10 6275 6035
bDepartment of Chemistry, Temple University, Philadelphia, PA, 19122,
USA. E-mail: BWayland@Temple.edu; Fax: +1 215 204 1532; Tel: +1 215
204 7875
‡ Tetra(p-sulfonatophenyl)porphyrin rhodium(III) monohydroxide com-
plex ([(TSPP)RhIII(D2O)(OD)]4-) and its alkyl derivatives [(TSPP)RhIII
–
† Electronic supplementary information (ESI) available: Experimental
procedures and NMR spectra. See DOI: 10.1039/b912219b
R(D2O)]4- are 4- charged complexes and abbreviated as (TSPP)Rh–OD
and (TSPP)Rh–R, respectively.
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 477–483 | 477
©

Table 1 Formation of b-hydroxyalkyl rhodium porphyrin complexes in
borate buffer aqueous solution (pH = 9.0) at 298 Ka
Entry Substrate
1
t
Product
Conv. (%)
92 (3)c
3 h
2
3
4
6.7 h
80 (6)c
84 (2)c
>95
1.5 hb
<5 min
5
6
<5 min
<5 min
>95
>95
Fig. 1 High-field 1H NMR (400 MHz) for (TSPP)Rh–CH2aC(OHb)-
(CH3g )(CH2dCH3e) (a) in D2O, (b) in CD3OD, and (c) in DMSO-d6.
7
8
82 h
>95
1.4 hb
94
is substantially improved from the broad resonance observed in
D2O at -5.72 ppm (Fig. 1(a)). The hydroxyl proton resonance for
(TSPP)Rh–CH2C(OH)(CH3)(CH2CH3) was also identified from
the 1H NMR spectrum (s, -0.16 ppm) in anhydrous DMSO-d6 by
preparation of the sample in H2O and transferring into DMSO-d6
(Fig. 1(c)).
9
34 hb
34 hb
89
91
10
Kinetics of alkene reaction with (TSPP)RhIII in water
Evaluation of rate and equilibrium constants (kf, kr, Keq).
Formations of (TSPP)Rh–CH2CH(OD)R as a function of time
from reactions of (TSPP)RhIII species with pentene and hexene in
D2O at 298 K using a pH = 9.0 buffer are illustrated in Fig. 2.
Expressions were derived for several plausible mechanisms. The
kinetics for this reaction are potentially complicated by having
three different aquo/hydroxo complexes, but the monohydroxo
complex is the dominant species (>93%) at pH = 9.0 so that
effectively all of the b-hydroxyalkyl products were formed through
the monohydroxo complex ([(TSPP)RhIII(D2O)(OD)]4- (2)). Thus,
the rate of formation of [(TSPP)Rh–CH2CH(OD)R(D2O)]4- is
derived as d[Rh–CH2CH(OD)R]/dt = kf[2][alkene] - kr[Rh–
CH2CH(OD)R]. The best fit from nonlinear least squares curve
fitting is obtained for the mechanism where effectively all of the
product forms from reaction of 2 with alkenes (see Experimental
section for detailed derivation). The forward rate constant for the
pentene reaction at 298 K (kf = (2.3 0.1) ¥ 10-1 L mol-1 s-1) and
the reverse reaction rate constant (kr = (10.0 1.0) ¥ 10-5 s-1),
along with the corresponding values for the hexene reaction (298
K) (kf = (8.0 0.1) ¥ 10-2 L mol-1 s-1, kr = (7.8 1.0) ¥ 10-5 s-1)
are obtained from the kinetic simulation (Fig. 2). The equilibrium
constant for pentene Keq (Keq = kf/kr = (2.3 0.1) ¥ 103) that is
derived from the kinetic analysis is close to the value obtained from
direct equilibrium thermodynamic measurement by integration of
the 1H NMR spectrum (Keq = (4.1 0.6) ¥ 103).
a The initial concentration of (TSPP)RhIII was 1.9 ¥ 10-3 M. The yield
of the reaction was measured by 1H NMR spectroscopy. b The reaction
was run in a mixed-solvent composed of 1,4-dioxane-d8 and D2O (1/4
v/v). c The values in parentheses are the overall percentage of the formed
(TSPP)Rh–CH2C(O)R as by-product.
3
(d, JHH = 5.6 Hz). Reactions of (TSPP)RhIII with a series
of terminal alkenes in a borate buffer with pH 9.0 produce
b-hydroxyalkyl complexes [(TSPP)Rh–CH2CH(OD)R(D2O)]4- (4)
that reach equilibrium in time periods of several minutes to hours
depending inversely on the olefin solubility (Table 1). Highly
soluble alkenes (Table 1, entries 4–6) cleanly react to completion
within minutes but poorly soluble alkenes (Table 1, entries 1–3)
react much slower which allows time to produce some b-carbonyl
organometallic derivatives ([(TSPP)Rh–CH2C(O)R)(D2O)]4-) by
b-C–H elimination of 4.22
Reaction of (TSPP)RhIII with 2,2-disubstituted alkenes in water.
Reaction of (TSPP)RhIII with a 2,2-disubstituted alkene (2-methyl-
1-butene) also forms a b-hydroxyalkyl complex [(TSPP)Rh–
CH2C(OD)(CH3)(CH2CH3)(D2O)]4- (5) (eqn (4)).
[(TSPP)Rh–OD(D2O)]4- + CH2 C(CH3)CH2CH3 ꢀ
=
[(TSPP)Rh–CH2C(OD)(CH3)(CH2CH3)(D2O)]4-
(4)
1
The H–1H and 103Rh–1H coupling patterns that are partially
obscured by line broadening in water became clearly resolved when
5 was isolated from water and redissolved in CD3OD (Fig. 1(b)).
The characteristic AB pattern for the a-CH2 unit (d(ppm): -5.55
(HA), -5.48 (HB), JAB = 9.9 Hz, JRhH = 2.9 Hz) in 5 in CD3OD
Reaction (3) occurs through formation of p complexes
=
((TSPP)Rh(CH2 CHR)) by alkene substitution for a coordinated
water molecule followed by nucleophilic attack on the olefin p
complex and a proton transfer.
478 | Dalton Trans., 2010, 39, 477–483
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The Royal Society of Chemistry 2010
©

1
by H NMR at 243 K. The b-hydroxyalkyl complex of rhodium
octaethylporphyrin ((OEP)Rh–CH2CH2OH) was formed in
hydrocarbon media through hydroxide nucleophilic addition to
+
25
((OEP)Rh(CH2 CH2)) . Olefin p complexes of (TSPP)RhIII in
water at 298 K have not yet been observed by 1H NMR even when
the conditions for p complex formation are optimized by using
high concentrations of water-soluble olefins and slightly acidic
aqueous media where the diaquo complex 1 dominates.
=
Thermal dissociation of b-hydroxyalkyl rhodium porphyrin
complexes
b-Hydroxyalkyl rhodium porphyrin complexes are intermediate
organometallic complexes for both b-C–H elimination reac-
tions that produce ketones in pH 9.0 aqueous solution and
intramolecular C–O elimination to form epoxides quantitatively
in KOH/DMSO (Scheme 1).
Fig.
2
Formation of [(TSPP)Rh–CH2CH(OD)R(D2O)]4- from
[(TSPP)RhIII(OD)(D2O)]4- with pentene and hexene at 298 K with pH =
9.0 buffer. The solid line is the nonlinear least-square best fit to the equation
d[(TSPP)Rh–CH2CH(OD)R]/dt = b - a[(TSPP)Rh–CH2CH(OD)R],
Scheme 1 Thermal dissociation pathways of b-hydroxyalkyl rhodium
porphyrin complexes.
b = kfc(Rh )c0/(([D+]/K1) + (K2/[D+]) + 1), a = (b/c(Rh )) + kr using
T
T
OriginPro 7.5 software. The initial concentration of (TSPP)RhIII, c(Rh
,
T
)
equals 1.9 ¥ 10-3 M. The saturated solubility of pentene in water, c0, is
2.1 ¥ 10-3 M, and hexane is 1.9 ¥10-3 M.23 (A) Reaction of pentene gives
b-Hydrogen elimination of b-hydroxyalkyl rhodium porphyrin
complexes in water. b-Hydrogen elimination reactions are among
the most important fundamental organometallic transformations
in a variety of transition metal alkyl and alkoxide complexes. The
hydrogen migration or elimination usually occurs in coordinately
unsaturated complexes through use of a vacant cis coordination
site. However, b-hydrogen elimination processes for alkyl rhodium
porphyrin complexes must utilize a different mechanism because
all of the cis coordination sites are occupied by the porphyrin
pyrrole nitrogen donors.26 The b-hydroxyalkyl complexes formed
by reaction (3) in water spontaneously transform to (TSPP)Rh–H
and ketones in the absence of air by an effective b-hydrogen
elimination (eqn (5), Table 2).22
a = (5.5 0.1) ¥ 10-4, b = (8.5 0.1) ¥ 10-7, and kf = (2.3 0.1) ¥ 10-1
L
mol-1 s-1, kr = (10.0 1.0) ¥ 10-5 s-1, and Keq = (2.3 0.1) ¥ 103 are
derived. (B) Reaction of hexane gives a = (2.2 0.1) ¥ 10-4, b = (2.7
0.1) ¥ 10-7 and kf = (8.0 0.1) ¥ 10-2 L mol-1 s-1, kr = (7.8 1.0) ¥ 10-5 s-1,
and Keq = (1.0 0.1) ¥ 103 are derived.
The regioselectivity for the initial nucleophilic attack step is
controlled by the sterically bulky porphyrin ligand which favors
placing rhodium on the less hindered terminal primary carbon as
the kinetic product. In the case of unactivated alkenes the kinetic
and thermodynamic products have rhodium on the primary
–CH2 ((TSPP)Rh–CH2CH(OD)R), but olefins with electron with-
drawing groups (R = phenyl, CO2Na) initially form (TSPP)Rh–
CH2CH(OD)R and then rearrange to (TSPP)Rh–CH(R)CH2OD
as the thermodynamic products.
Electron withdrawing groups better stabilize the negative charge
and provide an electronic energy term that favors placing the metal
on the same carbon as the electron withdrawing group which for
the phenyl and CO2Na derivatives more than compensates for the
unfavorable steric effect.21,24
[(TSPP)Rh–CH2CH(OD)R(D2O)]4-
[(TSPP)Rh(H)(D2O)]4- + CH2DC(O)R
ꢀ
(5)
Thermal reaction of (TSPP)Rh–CH2CH(OD)(CH2)2CH3 in
D2O results in the formation of (TSPP)Rh–H and a mono-
deuterated 2-pentanone (CH2(D)C(O)CH2CH2CH3) where deu-
terium is selectively incorporated into the methyl group. The selec-
tive deuteration indicates that b-hydrogen elimination produces an
enol containing an OD unit (Scheme 2). This mechanistic feature is
different from the Pd(II) Wacker oxidation where deuterium from
D2O does not occur in the oxidation product. Isomerization of
b-hydroxyalkyl to the a-hydroxy isomer that occurs prior to the
product forming step in the Wacker reaction cannot occur for the
rhodium porphyrin because the cis coordination sites are blocked.
An octaethylporphyrin rhodium ethene
p
complex
+
=
((OEP)Rh(CH2 CH2)) has been directly observed in toluene
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 477–483 | 479
©

Aerobic oxidation of (TSPP)Rh–H to (TSPP)RhIII in water.
Table 2 Formation of ketones through b-hydrogen elimination of
b-hydroxyalkyl rhodium porphyrin complexes in pH = 9.0 buffera
The [(TSPP)Rh(H)(D2O)]4- species formed in reaction (5) in a
rapid dissociation equilibrium with [(TSPP)RhI(D2O)]5- and H+
undergo an immediate color change to a clear dark red solution
when exposed to air. The distinctive pyrrole hydrogen resonance
associated with (TSPP)RhIII species is observed by 1H NMR. The
sequence of reactions with dioxygen to convert (TSPP)Rh–H/
(TSPP)RhI to (TSPP)RhIII is depicted by Scheme 3.
Entry Substrate
1
t/h
Solvent
H2O
Product
9b
D2O
H2O
D2O
H2O
2
3
16.5b
3c
9c
D2O
H2O
4
5
3c
3c
Scheme
3
Aerobic oxidations of (TSPP)Rh–H/(TSPP)RhI to
(TSPP)RhIII in water.
Rapid air oxidation of Rh(I) to Rh(III) completes the cycle which
provides the potential for catalytic oxidation of olefins.
H2O
Formation of b-carbonyl alkyl rhodium porphyrin complexes in
water
a The reactions were performed in borate buffers with pH = 9.0 in
the absence of air, and the ketone products were nearly quantitatively
Aqueous buffer solutions of (TSPP)RhIII species react with ketones
and aldehydes to produce b-carbonyl derivatives [(TSPP)Rh–
CH2C(O)R(D2O)]4- (6), which has a precedent in reaction of
[(OEP)RhIII]+ with ketones.28 This electrophilic C–H activation
by the cationic metal center is thought to proceed through a
stepwise mechanism similar to that of reaction (3), in which the
enol intermediate reacts with (TSPP)RhIII to give (TSPP)Rh–
CH2C(OD)2R, followed by the dehydration step of the unstable
gem-diol complex.
1
formed as measured by H NMR spectroscopy although small amounts
of (TSPP)RhIII were also observed. b 333 K. c 353K.
Reaction of the acetaldehyde methyl C–H unit occurs with
high regioselectivity to form (TSPP)Rh–CH2CHO (eqn (6)). No
evidence is found for reaction of the weaker aldehydic C–H bond,
which is analogous to reactions of (por)RhII complexes with
CH3CHO in benzene.29
Scheme 2 Formation of ketones through (i) thermal b-hydrogen elimina-
tion of 4 and (ii) keto–enol tautomerism in water.
The observed fast reactions of rhodium porphyrin hydrides
with olefins in water compared to benzene is ascribed to water
supporting ionic reaction pathways.27 In aqueous solution the
hydride complex [(TSPP)RhIII(D)(D2O)]4- functions as a weak acid
and partially dissociates into D+ and [(TSPP)RhI(D2O)]5-. The
rhodium(I) porphyrin complex in water is proposed to activate
olefins which subsequently protonate to form rhodium alkyl
complexes. The b-C–H elimination or migration process is the
microscopic reverse of the addition of the rhodium hydride to
olefins in water which is proposed to occur by dissociation of
Rh–H into H+ and Rh(I)- and subsequent stepwise addition.27
The complex (TSPP)Rh–CH2C(OD)(CH3)(CH2CH3) 5 lacks a
b-hydrogen atom and thus is not capable of producing carbonyl
compounds.
[(TSPP)Rh–OD(D2O)]4- + CH3CHO ꢀ
[(TSPP)Rh–CH2CHO(D2O)]4- + HOD
(6)
The compound formed from reaction of (TSPP)RhIII with
acetone (eqn (7)) has an identical 1H NMR spectrum with
the complex formed from reaction of (TSPP)RhI in water with
ClCH2C(O)CH3 that produces (TSPP)Rh–CH2C(O)CH3.
[(TSPP)Rh–OD(D2O)]4- + CH3C(O)CH3 ꢀ
[(TSPP)Rh–CH2C(O)CH3(D2O)]4- + HOD
(7)
Addition of 300 equiv. of 2-pentanone to an aqueous solution
of (TSPP)RhIII in pH 9.0 D2O buffer resulted in an equi-
librium distribution of (TSPP)Rh–CH2C(O)CH2CH2CH3 and
480 | Dalton Trans., 2010, 39, 477–483
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The Royal Society of Chemistry 2010
©

(TSPP)RhIII which permitted evaluation of the equilibrium con-
stant (Keq(298 K) = 33). The relatively slow reaction rates of ke-
tones compared to alkenes may result from the very low enol con-
centration. The enolization equilibrium constant KE for acetone30
is (4.69 0.19) ¥ 10-9 and the resulting low concentration of enol
place limits on the rate of b-carbonyl formation. 2-Pentanone
can enolize in two ways resulting in regio-isomeric enols,
A brownish solution characteristic of (TSPP)RhI is observed
to form immediately after (TSPP)Rh–CH2CH(OD)(CH2)nCH3 is
dissolved in KOH/DMSO-d6. C–O reductive elimination from
the d6 metal center usually occurs through an SN2 route and
a direct C–O elimination pathway.31 Groves reported a facile
C–O reductive elimination from b-hydroxyalkyl complexes of
rhodium tetraphenyl porphyrin in KOtBu/C6D6 by a proposed
SN2 pathway.19
The origin of the selective formation of ketone in water and
epoxide in KOH/DMSO may be that the nucleophilicity of the
alkoxy group is dramatically reduced in water due to the solva-
tion. b-Hydroxyalkyl rhodium porphyrin complexes thus proceed
through the alternate usually higher energy b-H elimination
pathway which produces ketones. The favored formation of three-
membered oxygen heterocycles through intramolecular C–O bond
formation is driven by the strong nucleophilicity of the alkoxy
group formed in strongly basic conditions of the (KOH/DMSO)
medium.
=
=
CH2 C(OD)CH2CH2CH3 (A) and CH3C(OD) CHCH2CH3 (B),
but the reaction of (TSPP)RhIII with 2-pentanone exclusively
produces (TSPP)Rh–CH2C(O)CH2CH2CH3. The origin of this
result may be the thermodynamic preference for primary alkyl
rhodium complexes.
Conclusions
(TSPP)RhIII reacts with terminal alkenes to form b-hydroxy alkyl
rhodium porphyrin complexes with high conversion in pH 9.0
aqueous buffer solution at room temperature. Regioselectivity of
the reaction is controlled by the sterically bulky porphyrin ligand
and the less-substituted alkyl rhodium complexes are formed as
the kinetic products. The equilibrium constant of reaction of
(TSPP)RhIII with pentene was evaluated as K = (2.3 0.1) ¥ 103
from kinetic simulation. The b-hydroxyalkyl rhodium porphyrin
complexes underwent the b-hydrogen elimination reaction to
produce ketones in pH 9.0 aqueous solution and near quantitative
intramolecular formation of epoxides in KOH/DMSO. Product
inhibited catalytic oxidation of olefins to ketones by (TSPP)RhIII
is accomplished by using dioxygen as the oxidant.
The ketone products react with the (TSPP)Rh(III) oxidation
catalyst resulting in a product inhibited catalytic oxidation of
olefins which limits the numbers of turnovers.
Formation of epoxides from b-hydroxyalkyl rhodium porphyrin
complexes
The b-hydroxyalkyl rhodium porphyrin complexes, (TSPP)Rh–
CH2CH(OD)(CH2)nCH3 (n = 2–4) (eqn (8)) undergo immediate
(<5 min) epoxide forming elimination reactions in KOH/DMSO-
d6 (cKOH = 3.3 mg mL-1) at room temperature to give quantitative
formation of (TSPP)Rh–D which is rapidly deprotonated to form
1
(TSPP)RhI and 1,2-epoxyalkanes as observed by both H NMR
Experimental
and GC-MS (Table 3).
Typical procedure for preparation of (TSPP)Rh–CH2CH(OD)R in
water
(8)
Alkenes (0.01 mmol) and 1 (1.1 mg, 0.001 mmol) were dissolved
in 0.4 mL borate buffer D2O solution (pH = 9.0) in J. Young valve
adapted NMR tubes at room temperature. The progress of the
reaction was monitored by 1H NMR spectroscopy.
Table 3 Formation of 1,2-epoxyalkanes through epoxide-forming elimi-
nation of b-hydroxyalkyl rhodium porphyrin complexes in KOH/DMSO
Entry
1
Substrate
t
Product
Typical procedure for b-hydrogen elimination of
(TSPP)Rh–CH2CH(OD)R in water
<5 min
The (TSPP)Rh–CH2CH(OD)R complexes were prepared accord-
ing to the procedure given above, which exclusively converted
(TSPP)RhIII into (TSPP)Rh–CH2CH(OD)R. The excesses of
alkenes and solvent D2O were pumped out. Fresh D2O was added
into the NMR tube and subjected to three freeze–pump–thaw
cycles. The initial 1H NMR spectrum was recorded to show
the formation of Rh–CH2CH(OD)R and a clean range from
0 to 4 ppm. The sample (TSPP)Rh–CH2CH(OD)R was heated
in a water-bath at 60 ◦C (or 80 ◦C) for a period of hours,
and the progress of the reaction was monitored by 1H NMR
spectroscopy. When the reactions reached completion after all
(TSPP)Rh–CH2CH(OD)R complexes were converted to ketones
2
3
4
<5 min
<5 min
<5 min
The reactions were performed in KOH/DMSO (cKOH = 3.3 mg mL-1) and
the product epoxides were formed quantitatively as measured by 1H NMR
spectroscopy. The resulting 1,2-epoxyalkanes were extracted into Et2O and
examined by GC-MS.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 477–483 | 481
©

and (TSPP)Rh–D (which was rapidly deprotonated to form
(TSPP)RhI), the product ketones were extracted by CDCl3. A
parallel sample of (TSPP)Rh–CH2CH(OH)R dissolved in H2O
was also heated under the same reaction conditions, and extracted
by CDCl3. Both 1H NMR and GC-MS were used to characterize
the product ketones.
Department of Energy, Division of Chemical Sciences, Office of
Science DE-FG02–09ER16000.
Notes and references
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Kinetic simulation for reaction of (TSPP)RhIII with pentene
K
[(TSPP)RhIII (D2O)2 ]3− (1)
[(TSPP)RhIII (OD)(D2O)]4− (2) + D+
1
ꢀꢀꢀꢁ
ꢂꢀꢀꢀ
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[(TSPP)RhIII (OD)2 ]5− (3) + D+
2
ꢀꢀꢀꢁ
ꢂꢀꢀꢀ
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(10)
K
3
[(TSPP)Rh−OD(D2O)]4− + CH2 = CHCH2CH2CH3
[(TSPP)Rh−CH2CH(OD)(CH2 )2CH3(D2O)]4− (4)
ꢀꢀꢀꢁ
ꢂꢀꢀꢀ
(11)
We then have:
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[3] = [2]K2/[D+],
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[D+ ]
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=
T
)
3663.
1.9 ¥ 10-3 M and c0 = [CH2 CHCH2CH2CH3] = 2.1 ¥ 10-3 M are
=
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Acknowledgements
This work was supported by a starter grant from Peking Uni-
versity, NSFC (grants 20841002 and 20801002), and the U.S.
482 | Dalton Trans., 2010, 39, 477–483
This journal is
The Royal Society of Chemistry 2010
©

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The Royal Society of Chemistry 2010
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Porphyrin rhodium(i) reacts with terminal alkenes to
form β-hydroxy alkyl rhodium porphyrin complexes
which undergo βC-H elimination to produce ketones
in water and intramolecular C-O elimination to form
epoxides in KOH/DMSO. Rapid aerobic oxidation of
Rh(i) to Rh(iii) provides the possibility for catalytic
oxidation of olefins in water.
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