Minimizing side reactions in chemoenzymatic dynamic kinetic resolution: Organometallic and material strategies

Article abstract of DOI:10.1039/c2dt31781h

Chemoenzymatic dynamic kinetic resolution (DKR) of rac-1-phenyl ethanol into R-1-phenylethanol acetate was investigated with emphasis on the minimization of side reactions. The organometallic hydrogen transfer (racemization) catalyst was varied, and this

Full text of DOI:10.1039/c2dt31781h

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PAPER
Minimizing side reactions in chemoenzymatic dynamic kinetic resolution:
organometallic and material strategies†
Ciara L. Pollock, Kevin J. Fox, Sophie D. Lacroix, James McDonagh, Patricia C. Marr,*
Alanna M. Nethercott, Annie Pennycook, Shimeng Qian, Linda Robinson, Graham C. Saunders* and
Andrew C. Marr*
Received 3rd August 2012, Accepted 6th September 2012
DOI: 10.1039/c2dt31781h
Chemoenzymatic dynamic kinetic resolution (DKR) of rac-1-phenyl ethanol into R-1-phenylethanol
acetate was investigated with emphasis on the minimization of side reactions. The organometallic
hydrogen transfer (racemization) catalyst was varied, and this was observed to alter the rate and extent of
oxidation of the alcohol to form ketone side products. The performance of highly active catalyst
[(pentamethylcyclopentadienyl)IrCl2(1-benzyl,3-methyl-imidazol-2-ylidene)] was found to depend on the
batch of lipase B used. The interaction between the bio- and chemo-catalysts was reduced by employing
physical entrapment of the enzyme in silica using a sol–gel process. The nature of the gelation method
was found to be important, with an alkaline method preferred, as an acidic method was found to initiate a
further side reaction, the acid catalyzed dehydration of the secondary alcohol. The acidic gel was found to
be a heterogeneous solid acid.
Introduction
Combining bio- and chemo-catalysis provides powerful methods
for chemical synthesis.1 Chemoenzymatic dynamic kinetic resol-
ution (DKR) of secondary alcohols has received considerable
attention. In kinetic resolution an enzyme-catalyzed reaction is
used to kinetically discriminate between two enantiomers in a
racemic mixture and convert one but not the other into the
product. In DKR a chemical catalyst is added in order to race-
mize the enantiomers, ensuring that all the substrate can be uti-
Scheme 1 Dynamic kinetic resolution of rac-1-phenyl ethanol into
R-1-phenylethanol acetate.
lized. The combination of lipase B and an organometallic
hydrogen transfer catalyst for the conversion of secondary alco-
hols into enantiopure acetates has been particularly effective, for
example the DKR of rac-1-phenyl ethanol into R-1-phenyletha-
nol acetate (Scheme 1).2,3 The success of this combination has
led to commercialization.4 Lipase B is frequently added as an
immobilized enzyme aggregate such as Novozym® 435, a
macroporous acrylic resin containing CalB (Candida antarctica
lipase B). Highly successful catalysts are often Ru(II) com-
plexes,5,6 the most recognizable of which is Shvo’s catalyst7
(Fig. 1), but examples of efficient Rh(III)8 and Ir(III)8,9 catalysts
have also been reported. The metal complex catalyses racemiza-
tion by a hydrogen transfer mechanism (Scheme 2).
The chemoenzymatic DKR of secondary alcohols to yield
chiral esters is susceptible to oxidation of the alcohol by de-
hydrogenation or Oppenauer oxidation10–12 to form ketone side
products. This side reaction is expected to vary as the organo-
metallic catalyst varies since the relative rate of oxidation versus
racemization should be chemocatalyst dependent.
A further side reaction that may occur in chemoenzymatic
DKR is the reaction between the chemical catalyst and biocata-
lyst. Enzymes such as CalB have a wide variety of ionic
domains and regions rich in groups that may act as donors for
the dissolved metal ion. In instances in which the chemocatalyst
becomes associated with the protein structure racemization will
be prevented, or at least, severely reduced; and the interaction
may also disrupt the enzyme’s activity by altering the protein
structure. The prevalence of this type of interaction explains why
School of Chemistry and Chemical Engineering, Queen’s University
Belfast, David Keir Building, Belfast, BT9 5AG, UK.
E-mail: a.marr@qub.ac.uk
†This paper is dedicated to Prof. David J. Cole-Hamilton on occasion of
his retirement in celebration of his distinguished (and ongoing) contri-
bution to transition metal catalysis, and his role in introducing ACM to
organometallic chemistry for homogeneous catalysis, his enthusiasm
was truly infectious.
This journal is © The Royal Society of Chemistry 2012
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Fig. 1 Chemocatalysts for chemoenzymatic DKR.
Table 1 DKR of rac-1-phenyl ethanol co-catalyzed by Ru(II)
complexes
Catalyst
% Estera (% ee) % Ketone
12[(η6-Mesitylene)RuCl(μ-Cl)]2 + DPEN
12[(η6-p-Cymene)RuCl(μ-Cl)]2 + DPEN
12[(η6-Mesitylene)RuCl(μ-Cl)]2 + PHEN
12[(η6-p-Cymene)RuCl(μ-Cl)]2 + PHEN
[(η6-Mesitylene)RuCl(dppe)]BF4
[(η6-Mesitylene)RuCl(dppp)]BF4
[(η6-Mesitylene)RuCl(dfppm)]BF4
[(η6-p-Cymene)RuCl(dfppm)]BF4
1
70 (97)
73 (97)
80 (97)
86 (94)
94 (85)
83 (98)
89 (92)
75 (91)
>99 (96)
>99 (98)
3
2
1
<1
<1
<1
—
<1
<1
<1
1b
Scheme 2 Hydrogen transfer mechanism for the racemization of a sec-
ondary alcohol by catalyst MLn.
Toluene (2.4 mL), catalyst (0.0072 mmol), rac-1-phenyl ethanol
(7.2 mmol), isopropenyl acetate (7.2 mmol), Novozym® 435 (40.5 mg),
potassium carbonate (0.26 g), temperature 70 °C, reaction time 18 h. 1 is
12[(η6-mesitylene)Ru(μ-Cl)3RuCl(dfppm)].a Percentage conversion to 1-
phenylethanol acetate by NMR. b 5 h reaction. Mesitylene is 1,3,5-
trimethylbenzene, p-cymene is 1-methyl-4-(1-methylethyl)benzene,
DPEN is 1,2-diphenyl-1,2-ethylenediamine (R,R), PHEN is 2-
phenylglycinol (S), dppe is bis(diphenylphosphino)ethane, dppp is 1,3-
bis(diphenylphosphino)propane, dfppm is (C6F5)2PCH2P(C6F5)2.
many active hydrogen transfer catalysts exist which are inactive
in the presence of lipase.5
Results and discussion
Varying the organometallic chemocatalyst
The hydrogen transfer activity of a range of piano stool com-
plexes or Ru(II), Rh(III) and Ir(III) has been reported previously.13
Here we report the application of these catalysts to the chemoen-
zymatic dynamic kinetic resolution (DKR) of secondary alco-
hols. Activity was screened for DKR of secondary alcohols with
Candida antarctica lipase B (CalB). Pentamethylcyclopentadie-
nyl (Cp*) iridium N-heterocyclic carbene (NHC) complexes,
have already been shown to a promote DKR.8
the presence of the metal in many cases, affording lower enan-
tioselectivity than obtained in the absence of the complex. Of
the Ru(II) complexes tested bimetallic complex 113 gave
optimum activity and fully conserved selectivity. The synthesis
of similar complexes with non-fluorinated ligands has been
reported.14 Comparing 1 and Shvo’s catalyst (Fig. 1)7 both these
active catalysts have hindered bimetallic cores and produce two
different complexes on dissociation.
In order to validate the work-up and analysis methods kinetic
resolutions were first carried out in the absence of an organo-
metallic complex. As expected the procedure gave 50% conversion
to R-1-phenylethanol acetate, the percentage enantiomeric excess
(% ee) was 94% at 20 °C. DKR results were obtained for a
variety of Ru(II) (Table 1), Rh(III) and Ir(III) (Table 2) complexes.
The structure of 1, 2 and 3 is given (Fig. 1).
Loss of stereoselectivity was more common for Rh(III) and
Ir(III) complexes (Table 2), and this points towards an increased
interaction between the metal and lipase B. Complex 2 was
selective, but exhibited lower activity, requiring longer reaction
times; 97% conversion and good enantioselectivity could be
obtained after 48 h. The slow rate of reaction could be attributed
to steric crowding as the metal ion is surrounded by fluorinated
groups. Such bulk is expected to retard interactions between the
bio- and chemo-catalyst, but this must be balanced with the rate
Most of the organometallic complexes tested afforded
reasonable conversion over 18 h. The enzyme was affected by
13424 | Dalton Trans., 2012, 41, 13423–13428
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Table 2 DKR of rac-1-phenyl ethanol co-catalyzed by Rh(III) or Ir(III)
complexes
chemocatalyst associates with the enzyme, this will prevent
further racemization and can alter the selectivity of the enzyme
by distorting the protein’s structure. One way of retarding associa-
tion, which has been very effectively applied by Bäckvall and
co-workers, is to add bulk to the organometallic chemocatalyst,
Shvo’s catalyst and Bäckvall’s catalyst contain bulky phenyl sub-
stituents on the cyclopentadienyl ring (Fig. 1).5 An alternative
approach is to associate the catalyst with a solid material.
A materials approach to the prevention of mutual poisoning of
bio- and chemo-catalysts has been reported by Blum and
Avnir.16 The catalysts were compartmentalized by entrapping
them inside porous silica by sol–gel processes. This method
physically imprisoned the catalyst preventing free diffusion
through the solvent and therefore making processes that result
from the interaction between the bio- and chemo-catalyst un-
likely.17 Entrapping both catalysts in chemoenzymatic DKR is
likely to lead to a highly diffusion limited process, therefore we
investigated the entrapment of just the biocatalyst (CalB) in
silica gels.
Catalyst
% Ester (% ee)
% Ketone
12[Cp*IrCl(μ-Cl)]2 + DPEN
12[Cp*IrCl(μ-Cl)]2 + PHEN
[Cp*IrCl(dfppm)]BF4
12[Cp*RhCl(μ-Cl)]2 + DPEN
12[Cp*RhCl(μ-Cl)]2 + PHEN
[Cp*RhCl(dppe)]BF4
[Cp*RhCl(dfppm)]BF4
[Cp*RhCl2(PPh3)]
[Cp*RhCl2(p-C6H4F)3]
2
82 (84)
86 (91)
84 (83)
71 (96)
88 (97)
70 (83)
36 (86)
>99 (90)
87 (98)
63 (96)
97 (96)
>99 (97)
2
3
1
12
4
2
5
<1
2
—
<3
<1
2a
3
Toluene (2.4 mL), catalyst (0.0072 mmol), rac-1-phenyl ethanol
(7.2 mmol), isopropenyl acetate (7.2 mmol), Novozym® 435
(40.5 mg), potassium carbonate (0.26 g), temperature 70 °C, reaction
time 18 h.
2
is [{η5,κP-C5Me4CH2C6F4-2-P(C6F5)CH2P(C6F5)2)}-
RhCl2]. 3 is [Cp*IrCl2(1-benzyl,3-methyl-imidazol-2-ylidene)].8 a 48 h
reaction.
The activity of a batch (different from that above) of lipase
acrylic resin from Candida Antarctica (Novozym® 435) and 3
was evaluated. Kinetic resolution gave the expected result of
50% 1-rac-phenyl ethanol conversion to the R-1-phenylethyl
acetate. The reaction was repeated as a DKR in the presence of
3, after 48 h only 47% conversion to 1-rac-phenyl acetate was
observed. This drop in performance is indicative of an inter-
action between the chemocatalyst and enzyme. In order to com-
partmentalize the catalyst and reduce poisoning the Novozym®
435 was entrapped in silica by sol–gel methods. This provides a
porous silica coating around the lipase acrylic resin. Entrapment
methods have been successfully applied by Hoyos et al.18 as an
enzyme stabilising method in the dynamic DKR of benzoins by
Pseudomonas stutzeri employing Shvo’s catalyst. In order to
entrap the enzyme a basic sol–gel method was applied catalyzed
by benzylamine; this method was based on an organogel-
templated method previously described.19 Upon moderate
heating and cooling organogelator 2,5-di-O-methanesulfonyl-
1,4:3,6-dianhydro-D-sorbitol gels a range of solvents, including
alcohols. In the presence of a silica precursor and promoter the
organogel formed acts as a template transcribing silica formation
and dictating structure on the micro- and nano-scale. This
method was capable of providing a stable matrix that entrapped
the enzyme. The distribution of the enzyme throughout the
matrix is important to the activity of the resultant material. A
base catalysed method of gelation and transcription was used as
base promotes the hydrogen transfer reaction. AOT (dioctyl
sodium sulfosuccinate) and PEG (polyethylene glycol) 400 were
used to stabilize the enzyme and create the correct environment
within the matrix.20 The charged head group on AOT has the
potential to interact with the enzyme, and the PEG forms stable
microemulsions within a gel matrix.21
of racemization, as very bulky ligands will slow the approach of
the organic substrate to the metal centre.
Levels of ketone formation were low for Ru(II) and Ir(III) cata-
lysts and higher for Rh(III) catalysts. [Cp*RhCl(μ-Cl)]2 + DPEN
was the most active system for the dehydrogenative oxidation of
1-phenyl ethanol under these conditions.
Overall the results suggest that the activity and selectivity can
be tuned, we suggest that the key to a more active chemoenzy-
matic system is the positioning of enough steric bulk to prevent
interaction between the enzyme and chemocatalyst, but not too
much, as this will slow down the overall rate of DKR by retard-
ing racemization.
The most active Ru(II) catalyst 1 was tested as a racemization
catalyst on the aliphatic substrate rac-pinacolyl alcohol (3,3-
dimethyl-2-butanol). Under similar conditions 81% ester at 82%
ee was obtained with 3% ketone, Ir(III) catalyst 3 achieved 99%
ester, 99% ee.8
The activity of 3 was compared to that of [(η5-Ph5C5)RuCl
(CO)2], a highly active racemization catalyst (Fig. 1),15 both cat-
alysts facilitated good conversion (>99%) and good enantio-
selectivity (>97% ee) in the presence of base, but only 3 was
found to be active in the absence of base. Liquid sampling of a
scaled up reaction employing 3 (containing base) revealed the
reaction was complete after 2 h 30 min. Isolation of the ester by
distillation from a repeated reaction gave a 70% isolated yield of
R-1-phenylethanol acetate (97% ee).
Changing the enzyme and bio-chemo compatibility, entrapping
the enzyme
An acidic sol–gel method was also used to entrap lipase B
employing PEG 400 to stabilise the enzyme and phosphoric acid
to catalyse the gelation.
In order to investigate the effects of the silica support, the
entrapped enzyme catalysts were tested in kinetic resolution. The
activity of the doubly immobilized basic gel was approximately
1/10th that of the acrylic resin Novozym® 435 (Table 3). The
activity of the acidic gel could not be measured as the resulting
Results obtained from a single batch of the enzyme exhibit
excellent reproducibility, with results repeatable multiple times,
with little variation, independent of the experimenter. However,
in our investigations, we found that the results of organometallic
chemoenzymatic DKR can vary markedly as the batch of lipase
B is changed.1 These problems are assumed to arise from
the interaction between the bio- and chemo-catalysts. If the
This journal is © The Royal Society of Chemistry 2012
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Table 3 Kinetic resolution of rac-phenyl ethanol with entrapped
enzymes
Table 5 KR and DKR of rac-phenyl ethanol, the effect of 3 on
enzyme activity
Catalyst
Active/g
% Ester
Catalyst
Active/g
0.041
Time/h % Conversion % Ester
Novozym® 435
Novozym® 435
Basic gel
0.02
0.04
0.33
0.35
36
50
39
NA
Novozym® 435
Novozym® 435 + 3 0.020 + 0.0022 48
Basic gel + 3 0.15 + 0.0022 48
20
50
47
52
50
47
52
Acidic gel
Toluene (2.4 mL), rac-1-phenyl ethanol (7.2 mmol), isopropenyl acetate
(7.2 mmol), Novozym® 435 (41 mg), potassium carbonate (0.26 g),
temperature 70 °C.
Toluene (2.4 mL), rac-1-phenyl ethanol (7.2 mmol), isopropenyl acetate
(7.2 mmol), temperature 70 °C, reaction time 4 h, NA = not applicable
due to side reactions.
Table 4 Recycling CalB-containing materials
Initial
conversion/%
Filtrate
conversion/%
Recycled
conversion/%
Catalyst
Scheme 3 The dehydration of rac-1-phenyl ethanol.
Novozym® 435
Basic gel
32
10
36
15
9
27
Table 6 Dehydration of rac-1-phenylethanol by the acidic gel
Toluene (2.4 mL), rac-1-phenyl ethanol (7.2 mmol), isopropenyl acetate
(7.2 mmol), temperature 70 °C.
Time/h
Substrate/%
Ether/%
Styrene/%
2 (2 h on gel)
4 (2 h on gel)
6 (4 h on gel)
81
81
27
14
14
52
5
5
21
1H NMR spectrum contained many additional resonances, indi-
cating that new side reactions had occurred; these were further
investigated and the results reported below.
Toluene (2.0 mL), rac-1-phenyl ethanol (5.6 mmol), temperature
105 °C.
In order to assess the extent to which the lipase B in the basic
gel and in Novozym® 435 was heterogeneous and recyclable,
the following procedure was carried out. The enzyme-containing
material was exposed to reaction conditions for 2 h, the solution
was then filtered off and returned to reaction conditions for a
further 2 h, the lipase-containing material was added to a fresh
reactant solution and run for 4 h (Table 4).
Under these conditions this batch of Novozym® 435 did not
recycle well. A significant further conversion (from 32 to 36%)
was observed when the filtrate was exposed to reaction con-
ditions for a further 2 h, indicating leaching of active material.
Furthermore the used enzyme aggregate gave only 9% conver-
sion after 4 h, indicating little active material was left. The basic
gel performed better but not ideally. Initial conversion was
reduced relative to Novozym® 435, this was expected as the
reactant must now diffuse through the silica gel to reach the
entrapped lipase. Upon separation the filtrate was still active, and
this reveals that active enzyme was leaching, the activity was
50% of that observed in the presence of the gel. The used and
recycled entrapped enzyme gave 27% conversion after 4 h, con-
sistent with conservation of enzyme activity.
Comparing the DKR activity of the entrapped enzyme with 3
and the activity of the parent Novozym® 435 with 3, the
entrapped enzyme enabled a 5% improvement in conversion,
affording 52% ester (Table 5).
We propose that the DKR reaction of the basic gel and 3
failed to achieve full conversion for two reasons. Firstly there is
a reduction in the activity of the enzyme due to the additional
diffusion processes necessary to approach the active site. Sec-
ondly there is measureable leaching from the gel, the active
enzyme in solution is believed to interact with the chemocatalyst,
retarding activity and preventing full conversion of the substrate.
As the entrapment of lipase B was only partially successful and
the basic gel exhibited some leaching, higher but not complete
conversion was observed in the resultant DKR. We suggest that
a better entrapment method would lead to a more robust and
recyclable DKR.
In order to investigate the side-reactions occurring when the
acidic gel was employed, a gel was prepared consisting of PEG
400, phosphoric acid and silica, with no entrapped lipase. The
gel was heated to 105 °C (unstirred) in a toluene solution con-
taining rac-1-phenylethanol. The products detected correlated
with the side products during kinetic resolution and were ident-
ified as the products of dehydration [1-(1-phenylethoxylethyl)-
benzene] and styrene (Scheme 3). In order to test whether the
catalyst behaved as a homogeneous or heterogeneous Brønsted
acid catalyst, the following procedure was carried out. The dehy-
dration was performed for 2 h and the solution and gel separated
by filtration. The filtrate was returned to the reaction conditions
for a further 2 h. For the final period of 2 h the gel was returned
to the solution. The reaction was followed by 1H NMR and
showed straightforward heterogeneous behaviour (Table 6,
Chart 1).
Conclusion
There are a number of side reactions that can reduce the
efficiency of the chemoenzymatic DKR of secondary alcohols to
esters. Base-catalyzed DKR is susceptible to alcohol dehydro-
genation to form ketones due to the Oppenauer oxidation activity
of the chemocatalyst. As the relative amount of this side reaction
depends on the organometallic catalyst, it can be largely avoided
13426 | Dalton Trans., 2012, 41, 13423–13428
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[Cp*RhCl{(C6F5)2PCH2CH2(C6F5)2}]BF4
dfppm with the equivalent amount of dppe.25
by
substituting
[Cp*RhCl2(PPh3)] was prepared by a method analogous to
[Cp*RhCl2P(C6H4F-4)].
[Cp*RhCl(μ-Cl)]2 (0.071 g, 0.11 mmol) and P(C6H4F-4)3
(0.073 g, 0.23 mmol) were dissolved in dichloromethane
(20 mL). The resulting red solution was kept at ambient temp-
erature overnight. Concentration by rotary evaporation afforded
the product as a red solid, which was filtered and dried in vacuo.
Yield 0.116 g (81%).
1H NMR (CDCl3): δ 7.80 (6H, m, Ho or Hm), 7.08 (6H, m,
Ho or Hm), 1.38 [15H, d, 4J(P–H) 3.5, CH3]. 19F NMR (CDCl3):
δ −108.43 (2F, br), −110.45 (1F, br). 31P NMR (CDCl3): δ 28.9
[d, J(Rh–P) 146]. EI-MS: m/z 589 [M − Cl]+. (Found: C, 53.4;
1
Chart 1 Heterogeneous solid acid behaviour in the dehydration of
rac-1-phenyl ethanol.
H, 4.2. Calc. for C28H27Cl2F3PRh: C, 53.8; H, 4.35%).
3 [Cp*IrCl2(1-benzyl,3-methyl-imidazol-2-ylidene)] was pre-
pared as previously reported.26,27 2,5-Di-O-methanesulfonyl-
1,4:3,6-dianhydro-D-sorbitol was prepared as previously
reported.19
The following reagents were used as supplied; Novozym®
435, isopropenyl acetate, DPEN, PHEN, benzylamine, Tetraethyl-
orthosilicate (TEOS), PEG 400 and AOT (Sigma-Aldrich), Rac-
1-phenyl ethanol (Fluka and Sigma-Aldrich), potassium carbon-
ate (Lancaster), phosphoric acid (85% aqueous solution
Lancaster).
by picking a suitable catalyst precursor. In the absence of base
an acid catalyzed dehydration is possible to form ethers and
alkenes. These side reactions are catalyzed by acids and can be
excluded through sensible choice of catalysts, reagents and
conditions.
The most detrimental and unpredictable side reaction in che-
moenzymatic DKR is the interaction between the chemo-catalyst
and bio-catalyst, this reduces the activity of the enzyme, can
hamper enantioselectivity, and frequently stops the racemization
catalyst working altogether. The application of bulky groups on
the metal catalyst can help to avoid this interaction, but this can
be at the detriment of the overall rate, as steric bulk will slow
down racemization. In instances where considerable poisoning
occurs it is difficult to prevent, and extremely active hydrogen
transfer catalysts can be rendered inactive. We suggest that the
further protection of the enzyme in a porous material, or alterna-
tively, the adoption of a heterogeneous hydrogen transfer cata-
lyst, can reduce poisoning. A sol–gel entrapped dehydrogenation
catalyst was recently reported by Oded et al.22
KR/DKR to convert rac-1-phenyl ethanol to R-1-phenylethanol
acetate
The catalyst precursor(s) (if required), isopropenyl acetate and
rac-1-phenyl ethanol were dissolved in toluene and degassed
in vacuo. Potassium carbonate was added and the suspension
stirred at 70 °C under nitrogen for 15 min. Novozym® 435 was
added and the reaction stirred gently at 70 °C under nitrogen for
the reaction time. The product solution was filtered through a
silica plug eluted with hexane–diethyl ether (10 : 1). The filtrate
was reduced by rotary evaporation to yield a colourless liquid.
1
The product was analysed by H NMR to determine conversion
and HPLC to determine enantiopurity. 1H NMR was used to
determine conversion as the spectrum can be recorded in the
presence of the metal complex (the presence of the metal catalyst
in solution would be damaging to the HPLC column). This
enables the degree of conversion to be measured before and after
work-up in order to check the validity of the work-up method.
Experimental
NMR spectra were recorded on 400 MHz and 300 MHz Bruker
NMR spectrometers. HPLC was performed on an Agilent 1100
with a chiral AD-H column (25 × 0.46 cm), wavelength 190 nm,
column flow 1 cm3 min−1, solvents hexane–propan-2-ol (95 : 5),
injection volume 5 μl.
Solvents were dried and degassed using standard methods.
Entrapping lipase in porous silica, basic method
Complexes [(η6-mesitylene)RuCl(μ-Cl)]2, [(η6-p-cymene)-
23
24
RuCl(μ-Cl)]2 [Cp*IrCl(μ-Cl)]2, [Cp*RhCl(μ-Cl)]2 and [(η5-
Ph5C5)RuCl(CO)2]15 were prepared by literature methods. The
synthesis of [(η6-mesitylene)RuCl(dfppm)]BF4, [(η6-p-cymene)-
RuCl(dfppm)]BF4, 1 [(η6-mesitylene)Ru(μ-Cl)3RuCl(dfppm)],
Ethanol (1.0 mL) was added to ground Novozym® 435 (0.04 g)
in a glass vial. AOT (0.0445 g, 0.100 mmol) was added and the
mixture stirred until it dissolved. 2,5-Di-O-methanesulfonyl-
1,4:3,6-dianhydro-D-sorbitol (0.10 g, 0.33 mmol), PEG 400
(2.5 mL), ethanol (1.25 mL) and water (0.25 mL) were heated in
a separate vial in a water bath to 60 °C and stirred to obtain a
homogeneous solution. The solutions were combined maintain-
ing heating and stirring. Benzylamine (0.14 g, 1.3 mmol) and
TEOS (0.475 g, 2.38 mmol) were added and the mixture heated
to 60 °C and stirred for a further 10 min. The gel was aged for
2–3 weeks before use.
[Cp*IrCl(dfppm)]BF4,
[Cp*RhCl(dfppm)]BF4
and
2
[{η5,κP-C5Me4CH2C6F4-2-P(C6F5)CH2P(C6F5)2)}RhCl2] were
conducted as previously reported.13 [(η6-Mesitylene)RuCl-
(dppe)]BF4, [(η6-mesitylene)RuCl(dppp)]BF4 were prepared by
methods analogous to [(η6-mesitylene)RuCl(dfppm)]BF4 by sub-
stituting dfppm with the equivalent amount of dppe and dppp.
[Cp*RhCl(dppe)]BF4 was prepared in a method analogous to
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Ground Novozym® 435 (0.04 g) was weighed into a glass vial.
PEG 400 (1.0 mL) and TEOS (1.0 mL) were added, followed by
phosphoric acid (0.40 mL, 85%). The mixture was stirred until
the liquids homogenized, then left to gel. The gel was aged for
2–3 weeks before use.
Dehydration of rac-1-phenylethanol by an acidic gel
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The dehydration was performed under 1 atm N2, filtrations and
work-up were performed in air. Acidic gel (0.0433 g), prepared
as above but omitting Novozym® 435, was heated with anhy-
drous toluene (2 mL) and rac-1-phenyl ethanol (0.67 mL,
5.6 mmol) without stirring, heating oil temperature 105 °C, for
2 h. The solution was filtered from the gel under gravity and a
sample taken for 1H NMR analysis. The solution was returned to
the heat for a further 2 h and another sample taken and analyzed
1
by H NMR. The gel and solution were recombined and heated
for a final 2 h and then a final 1H NMR sample was analysed.
Acknowledgements
Thanks to Shifang Liu (CCM research), Peter Goodrich, QUILL,
ASEP (QUB) and the European Social Fund.
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M. A. Bennett, J. P. Ennett, T. W. Matheson, L. Sawyer and V. K. Shah,
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