Redox-switchable phase tags - Facile Mitsunobu reactions using ferrocenyl-tagged triphenylphosphine

Article abstract of DOI:10.1002/adsc.200606024

The use of redox-switched phase tags in ferrocenyl-substituted triphenylphosphine combined with DBAD (di-tert-butyl azodicarboxylate) allows high yield (> 90%) Mitsunobu transformations without the need for the Chromatographic purification of the products. The redox-switchable phosphine can be easily synthesized in two steps from 4-bromoaniline, ferrocene and chlorodiphenylphosphine. It is separated from the reaction mixture by oxidation with iron(III) chloride and can be recycled efficiently by reductive treatment.

Full text of DOI:10.1002/adsc.200606024

FULL PAPERS
DOI: 10.1002/adsc.200606024
Redox-Switchable Phase Tags – Facile Mitsunobu Reactions using
Ferrocenyl-Tagged Triphenylphosphine
Christoph A. Fleckensteina and Herbert Plenioa,*
a
Anorganische Chemie im Zintl-Institut, TU Darmstadt, Petersenstr. 18, 64287 Darmstadt, Germany
Fax : (+49)-6151-16-6040; e-mail: plenio@tu-darmstadt.de
Received: January 23, 2006; Accepted: April 15, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The use of redox-switched phase tags in ferrocene and chlorodiphenylphosphine. It is separat-
ferrocenyl-substituted triphenylphosphine combined ed from the reaction mixture by oxidation with
with DBAD (di-tert-butyl azodicarboxylate) allows iron(III) chloride and can be recycled efficiently by
A
high yield (>90%) Mitsunobu transformations with- reductive treatment.
out the need for the chromatographic purification of
the products. The redox-switchable phosphine can be Keywords: ferrocene; Mitsunobu reaction; phos-
easily synthesized in two steps from 4-bromoaniline, phanes; redox reaction; redox-switchable phase tag
Introduction
chromatography, which is especially important for
combinatorial library synthesis.
The implementation of efficient purification strategies
For the separation of organometallic catalyst com-
in chemical synthesis is now considered to be an im- plexes we have recently introduced redox-switchable
portant issue for the applicability of chemical trans- phase tags, which are composed of ferrocenyl groups
formations. In this respect an enlightening contribu- whose oxidation state can be changed reversibly.[13] In
tion from Curran, reminded chemists that even in the the reduced state such phase tags are neutral and as
presence of powerful (though time-consuming) sepa- such are lipophilic units with a good solubility in non-
ration techniques such as chromatography, both syn- polar solvents. However, on oxidizing the ferrocenyl
thesis and separation determine the practical value of groups with a mild oxidation reagent, the neutral
a reaction.[1]
phase tag is converted into a cationic group, which
However, there are a number of chemical transfor- immediately precipitates from non-polar solvents. A
mations that are highly useful from a synthetic point significant advantage in the separation of reagents/
of view, but are often plagued by purification prob- catalysts with attached redox-active phase tags is the
lems. Infamous in this respect are reactions in which orthogonality of the solubility determining reductive
triphenylphosphine is used as a stoichiometric re- or oxidative transformation of the phase tag. Further-
agent[2] such as the Mitsunobu[3] and the Staudinger more, the redox potential of such phase tags can be
reactions[4] or the reduction of primary ozonides.[5]
Nonetheless, the Mitsunobu reaction is a powerful and substrates.
adjusted easily to match different reaction conditions
synthetic tool for the condensation of an acidic pronu-
We now describe the synthesis and the redox-
cleophile (RXH) and an alcohol (R’OH), due to its switched separation of ferrocenyl-tagged triphenyl-
wide applicability, stereospecificity and mild reaction phosphine which is used as a stoichiometric reagent in
conditions. Several strategies have been developed to the Mitsunobu reaction.
deal with purification problems in the Mitsunobu re-
action such as polymer-supported reagents,[6] basic
phosphines,[7] tagged phosphines, various azodicar- Results and Discussion
boxylate reagents,[8,9] fluorous reagents,[10] or phase
switching approaches,[11] as described in detail in a To be attractive for the user, the synthesis of a redox-
recent review by Dandapani and Curran, dealing ex- switchable triphenylphosphine should be as simple
clusively with Mitsunobu purification strategies.[12] as possible. We have thus devised a two-step proce-
Ideally such approaches yield pure products obviating dure (Scheme 1) starting from relatively inexpensive,
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Adv. Synth. Catal. 2006, 348, 1058 – 1062

Mitsunobu Reactions using Ferrocenyl-Tagged Triphenylphosphine
FULL PAPERS
Furthermore, when switching stoichiometric reagents
such as 3, a cheap, easily available and non-toxic oxi-
dant would significantly enhance the impact of this
approach. This oxidant should be soluble in the or-
ganic solvent used for the Mitsunobu reaction, to
effect rapid oxidation of the phase tag.
With these limitations in mind, we decided to use
anhydrous iron(III) chloride, which is cheap, readily
A
available and soluble in THF. Most importantly, its
oxidizing power is sufficient to quantitatively convert
2 and 3 into the corresponding ferrocenium salts.
After the Mitsunobu reaction, solid iron(III) chloride
A
or a solution in THF was added to the reaction mix-
ture in the same solvent to oxidize the phase tag. The
various iron salts can now be extracted with water. In
this manner>98% of the ferrocenyl-tagged phos-
phine oxide 3+ are removed. When using di-tert-butyl
Scheme 1. Synthesis of ferrocenyl-tagged PPh3 (2).
commercially available materials such as ferrocene, 4- azodicarboxylate (DBAD) 4M HCl is next added to
bromoaniline and diphenylchlorophosphine. destroy the spent Mitsunobu reagent. This highly
Following the diazotation of 4-bromoaniline, the re- acidic approach may impose limits with respect to cer-
action of the diazonium salt with ferrocene was per- tain acid-labile substrates. However, the redox-switch-
formed under phase-transfer conditions,[14] to easily ed separation described here is also compatible with
yield deca-gram amounts of 4-bromophenylferrocene other removal strategies which have been applied for
(1) in 74% yield. Next, the Grignard reagent generat- modified DEAD (diethyl azodicarboxylate) or DIAD
ed from 1 is reacted with diphenylchlorophosphine to (diisopropyl azodicarboxylate).[12] It is also possible
result in the formation of the ferrocenyl-tagged tri- to attach redox-switchable phase tags to the azodicar-
phenylphosphine (2) in 83% yield.
boxylate by generating the respective ester of ferroce-
The redox potentials of the ferrocenes such as 1 nylmethanol. However, this could lead to complica-
(E1/2 =0.491 V; DE=78 mV), 2 (E1/2 =0.477 V; DE= tions in the recovery of reagent 2.
82 mV) and 3 (E1/2 =0.517 V; DE=74 mV) were de-
termined by cyclic voltammetry (Figure 1).
Following the Mitsunobu procedure listed in detail
in the Experimental Section the respective products
The choice of the oxidation reagent used to switch can be isolated in excellent yields (Table 1), with a
the phase tag is critical for the usefulness of the high purity of the crude products not requiring chro-
redox-switchable phase tag approach described here. matographic purification.
In principle, numerous oxidants are able to convert
Furthermore, it is very easy to recover 3+ from the
ferrocene into the respective ferrocene cation, howev- aqueous phase by simply adding the reductant sodium
er, the strength of the oxidizer needs to be adjusted thiosulfate which converts 3+ to 3 (Scheme 2). Fol-
to enable quantitative oxidation and to avoid over-ox- lowing the addition of THF and diethyl ether, ferro-
idation, i.e., decomposition of the ferrocene cation.[15] cene 3 can be re-extracted into the organic phase,
from which it is isolated after evaporation. Reduction
(R3PO ! R3P) of residual 3 with trichlorosilane
allows the re-isolation of more than 80% of the ini-
tially added ferrocenylphosphine 2.
Finally, we were also interested whether the ferro-
cenyl-group has a significant influence on the Mitsu-
nobu reactivity. Therefore we have monitored the
conversion-time curve for the Mitsunobu reactions of
chloroacetic acid and 2-propanol using the two differ-
ent phosphines triphenylphosphine and 2, which dis-
play roughly identical rates of product formation
(Figure 2).
Conclusions
In conclusion, we have demonstrated that the use of
Figure 1. Cyclic voltammogram of 3, referenced vs. FcMe8.
redox-switchable phase tags allows us to reversibly
Adv. Synth. Catal. 2006, 348, 1058 – 1062
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www.asc.wiley-vch.de
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Christoph A. Fleckenstein and Herbert Plenio
FULL PAPERS
Table 1. General scheme of the Mitsunobu reaction and a
list of the products synthesized (conversion/isolated yield),
wavy lines denote the bonds formed.
Figure 2. Comparison of reactivity for PPh3 and the ferro-
cenyl-tagged PPh3 (2); reaction temperature 08C; determi-
nation of yield via GC using isooctane as an internal stan-
dard.
DBAD, Mitsunobu products can be synthesized in
typically>90% yields without the need for chromato-
graphic purification.
Experimental Section
General Remarks
All chemicals were purchased as reagent grade from com-
mercial suppliers and used without further purification,
unless otherwise noted. THF was distilled over potassium
and benzophenone under an argon atmosphere, Toluene
was distilled over sodium and benzophenone under an
argon atmosphere. Proton (1H NMR), carbon (13C NMR)
and phosphorus (31P NMR) nuclear magnetic resonance
spectra were recorded on a Bruker DRX 500 at 500 MHz,
125.75 MHz and 202.46, respectively, and are referenced to
tetramethylsilane (d=0.0 ppm), for 1H and 13C NMR, and
to H3PO4 (d=0.0 ppm) for 31P NMR. Thin layer chromatog-
raphy: Fluka silica gel 60 F 254 (0.2 mm) on aluminum
plates. Column chromatography: E. Merck silica gel 60
(0.063–0.20 mesh ASTM). Mass spectra (MS) were recorded
on an Agilent 1100 HPLC-MS (column: YMC J’Sphere
ODS H80, 4 mm, 20”2.1 mm; flow: 1.0 mLminꢀ1; T=308C;
eluent: (A: water+0.05% TFA/B: acetonitrile+0.05%
TFA) 0.00 min: 4% B ! 2.00 min: 95% B ! 2.45 min: 4%
B); detection: UV+MS (ESI/quadrupole). GC experiments
were run on a Perkin–Elmer AutoSystem, CP-Sil (CB, l=
15 m, di =0.25 mm, dF =1 mm), N2 (flow: 17 cm/sec; split
1:20), FID. Cyclic voltammetry: EG&G 263 A-2 potentio-
stat. All cyclic voltammograms were recorded in dry CH2Cl2
under an argon atmosphere at ambient temperature. A
three-electrode configuration was employed. The working
electrode was a Pt disk (diameter 1 mm) sealed in soft glass
with a Pt wire as counterelectrode. The pseudoreference
electrode was an Ag wire. Potentials were calibrated inter-
Scheme 2. Recycling of ferrocenyl-tagged PPh3.
modify the solubility properties of a substituted tri-
phenylphosphine enabling the facile separation of the
corresponding phosphine oxide. In combination with nally against the formal potential of octamethylferrocene
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Mitsunobu Reactions using Ferrocenyl-Tagged Triphenylphosphine
[10 mV (CH2Cl2) vs. Ag/AgCl]. NBu4PF6 (0.1 mol/L) was
FULL PAPERS
NMR (acetone-d6): d=141.9, 139.0 (d, JP, C =12.3 Hz), 138.9
used as supporting electrolyte. p-Bromophenylferrocene was
(d,
JP, C =9.9 Hz), 134.9, 134.7, 130.1, 129.8 (d, JP, C =
first prepared by Rosenblum et al.[16]
11.9 Hz), 127.4 (d, JP, C =6.8 Hz), 85.6, 70.8, 70.5, 67.8;
31P NMR (acetone-d6): d=ꢀ7.81; anal. calcd. for C28H23FeP
(446.3): C 75.4, H 5.20; found: C 75.3, H 4.95.
p-Bromophenylferrocene (1)
In
a 1 L round-bottom flask 4-bromoaniline (52.5 g,
General Mitsunobu Procedure
339 mmol) was suspended in 300 mL half-concentrated sul-
furic acid. Within 30 min. 135 mL of an aqueous solution of
NaNO2 (2.5 mol/L) were added at 08C. After stirring for an
additional 30 min at 08C and securing an excess of HNO2
by use of KI-starch paper, the resulting clear yellow diazoni-
um solution was added to a vigorously stirred solution of
ferrocene (21.0 g, 113 mmol) in 600 mL Et2O and Aliquat
336 (9 g) within 30 min. The reaction mixture was stirred
vigorously for 1.5 h at 08C and for 30 min at ambient tem-
perature. The reaction mixture was transferred in a separa-
tion funnel and the aqueous phase was extracted with Et2O
(3”200 mL). The combined organic phases were washed
subsequently with 200 mL water, then 200 mL brine and fi-
nally dried with MgSO4. Filtration and concentration under
vacuum gave the crude product which was dissolved in
25 mL ethyl acetate and adsorbed on silica gel. Column
chromatography (silica, cyclohexane) afforded pure 1 (CAS
58482–65–8) as brown crystals; yield: 28.5 g (74%); Rf 0.44
To a stirred solution of the nucleophile (carbonic acid or
phenol or phthalimide) (1.3 mmol), FcTPP 2 (1.3 mmol) and
the alcohol (1.0 mmol) in dry THF (3 mL), di-tert-butyl azo-
dicarboxylate (1.3 mmol) dissolved in dry THF (2 mL) was
added. The resulting solution was stirred overnight at ambi-
ent temperature. The reaction mixture was treated with
FeCl3 (500 mg) and stirred for 5 min, then Et2O (10 mL)
and water (10 mL) were added. The separated organic
phase was treated again with FeCl3 (200 mg), stirred for
5 min and washed with water. Then 5 mL HCl (4.0 mol/L in
dioxane, 20 mmol) were added to the slightly yellow organic
phase and stirred for 1 h. The solution was washed with
water (2”20 mL), dried with MgSO4 and filtered. Volatiles
were removed under vacuum. The crude product was fil-
tered through a pad of silica gel (5 cm, eluent: cyclohexane/
ethyl acetate, 10:1). Volatile compounds of the filtrate were
removed under vacuum, affording the respective products as
slightly yellow oils (>90% isolated yield).
1
3
(cyclohexane). H NMR (500 MHz, CDCl3): d=7.39 (d, J=
3
9.0 Hz, 2H, arom), 7.32 (d, J=8.5 Hz, 2H, arom), 4.60 (t,
Cyclopentyl 2-chlorobenzoate (CAS 501356–76–9):
1H NMR (CDCl3): d=7.77 (dd, 3J=5 Hz, 4J=1.5 Hz, 1H,
o-CH, arom), 7.44–7.37 (m, 3H, CH, arom), 5.44 (m, 1H,
COOCH), 1.97–1.64 (m, 8H, CH2); 13C{1H} NMR
(125.77 MHz, CDCl3): d=165.7, 133.4, 132.2, 131.2, 131.0,
130.9, 126.5, 78.6, 32.8, 23.8.
J=2.0 Hz, 2H, Fc), 4.32 (t, J=2.0 Hz, 2H, Fc), 4.03 (s, 5H,
Fc); 13C{1H} NMR (125.77 MHz, CDCl3): d=138.5, 131.4,
127.6, 119.4, 84.1, 69.7, 69.2, 66.4.
Diphenyl(p-ferrocenylphenyl)phosphine (FcTPP) (2)
n-Butyl 3-tert-butylphenyl ether (CAS 136–60–7):
3
1H NMR (CDCl3): d=7.14 (t, J=8 Hz, 1H, m-CH, arom),
Grignard route: In a 250 mL round-bottom flask Mg turn-
ings (158 mg, 6.5 mmol) were placed under an argon atmos-
phere. p-Bromophenylferrocene (2.0 g, 5.9 mmol), dissolved
in 80 mL dry THF, was added at ambient temperature and
sonicated for 12 h. The Grignard solution was transferred
into a 250 mL Schlenk-flask, containing Ph2PCl (1.24 mL,
6.9 mmol), dissolved in dry THF (20 mL). The resulting re-
action mixture was stirred for 4 h at ambient temperature.
Silica gel (5 g) was added and volatiles were removed under
vacuum. Purification via column chromatography (silica gel,
100:2 cyclohexane/EtOAc) afforded 2 as a red viscous oil;
yield: 2.1 g (81%); Rf 0.60 (100:2 cyclohexane/EtOAc).
t-BuLi route: In a 100 mL Schlenk flask p-bromophenyl-
ferrocene (3.0 g, 8.8 mmol) was dissolved in 50 mL absolute
THF. Under an argon-atmosphere t-BuLi, 1.5m in pentane
(6.0 mL, 8.9 mmol) was added at ꢀ788C within 10 min. The
now brownish solution was stirred for additional 2 h at that
temperature before PhPCl2 (1.75 mL, 2.15 g, 9.75 mmol) was
added. The reaction mixture was stirred for 1 h at ꢀ788C
and further 2 h at ambient temperature. Silica gel (5 g) was
added and volatiles were removed under vacuum. Purifica-
tion via column chromatography (silica gel, 100:2 cyclohex-
ane/EtOAc) afforded 2 as a red viscous oil; yield: 3.25 g
6.90–6.86 (m, 2H, CH, arom), 6.65–6.62 (m, 1H, CH, arom),
3.89 (t, 3J=6.5 Hz, 2H, O-CH2), 1.73–1.67 (m, 2H, CH2),
3
1.47–1.39 (m, 2H, CH2), 1.24 (s, 9H, t-Bu), 0.91 (t, J=8 Hz,
3H, CH2CH3); 13C{1H} NMR (CDCl3): d=158.0, 151.9,
127.9, 116.6, 111.6, 109.5, 66.5, 33.7, 30.5, 30.3, 18.3, 12.9;
MS: m/z=207 [M+H]+.
3-Prop-2-ynyl
3-tert-butylphenyl
ether:
1H NMR
(CDCl3): d=7.16 (t, 3J=7.5 Hz, 1H, m-CH, arom), 6.96–
6.94 (m, 2H, CH, arom), 6.73–6.70 (m, 1H, CH, arom), 4.61
(d, 4J=2.5 Hz, 2H, O-CH2), 2.43 (t, 4J=2.5 Hz, 1H,
CꢁCH), 1.23 (s, 9H, t-Bu); 13C{1H} NMR (CDCl3): d=
156.4, 152.1, 127.9, 117.7, 112.0, 109.9, 77.8, 74.3, 54.7, 33.7,
30.2; MS: m/z=189 [M+H]+.
sec-Hexyl 3-tert-butylphenyl ether: 1H NMR (CDCl3):
3
d=7.12 (t, J=7.5 Hz, 1H, m-CH, arom), 6.89–6.84 (m, 2H,
CH, arom), 6.63–6.61 (m, 1H, CH, arom), 4.27 (tq, 3J=
6 Hz, 1H, O-CH), 1.71–1.65 (m, 1H, O-CHCH2), 1.52–1.46
(m, 1H, O-CHCH2), 1.41–1.29 (m, 4H, CH2), 1.23 (s, 9H, t-
3
3
Bu), 1.22 (d, J=7 Hz, 3H, O-CHCH3), 0.84 (t, J=7.5 Hz,
3H, CH2CH3); 13C{1H} NMR (CDCl3): d=157.0, 151.9,
127.8, 116.5, 113.0, 110.9, 72.6, 35.3, 33.7, 30.3, 26.8, 21.7,
18.8, 13.0; MS: m/z=235 [M+H]+.
1
1
4-Bromobenzyl chloroacetate: H NMR (CDCl3): d=7.42
(83%); Rf 0.60 (100:2 cyclohexane/EtOAc). H NMR (ace-
3
3
(d, J=8.5 Hz, 2H, CH, arom), 7.16 (d, J=8.5 Hz, 2H, CH,
tone-d6): d=7.43–7.07 (m, 14H, arom), 4.62 (t, J=1.8 Hz,
2H, Fc), 4.21 (t, J=1.8, 2H, Fc), 3.88 (s, 5H, Fc); 13C{1H}
arom), 5.08 (s, 2H, O-CH2), 4.01 (s, 2H, COCH2); 13C{1H}
Adv. Synth. Catal. 2006, 348, 1058 – 1062
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Christoph A. Fleckenstein and Herbert Plenio
FULL PAPERS
NMR (CDCl3): d=166.0, 132.9, 130.8, 129.1, 121.8, 66.0,
39.8; MS: m/z=212, 214 [M-CH3Cl]+.
Reduction of FcTPPO (3)
n-Propyl p-sec-butoxybenzoate: 1H NMR (CDCl3): d=
FcTPPO (3) (450 mg, 0.97 mmol) was dissolved in absolute
toluene (7 mL) and placed in a pressure tube under an
argon atmosphere. TEA (1.5 mL) and HSiCl3 (1.0 mL) were
added and the reaction mixture was stirred at 1208C for
12 h. The mixture was cooled to room temperature, 20 mL
H2O were added before extraction with Et2O (3”20 mL).
The combined organic phases were washed subsequently
with water (15 mL) and brine (15 mL), dried with MgSO4
and filtered. Removal of volatiles under vacuum afforded 2
as a red viscous oil; yield: 431 mg (100%). 1H, 13C and
31P NMR spectra were found to be identical with those de-
scribed above for FcTPP.
3
3
7.98 (d, J=9.3 Hz, 2H, CH, arom), 6.89 (d, J=9.0 Hz, 2H,
CH, arom), 4.38 (tq, 3J=6 Hz, 1H, O-CH), 4.24 (t, 3J=
6.3 Hz, 2H, COOCH2), 1.83–1.30 (m, 4H, CH2), 1.31 (d,
3J=6 Hz, 3H, CHCH3), 1.04–0.94 (m, 6H, CH2CH3);
13C{1H} NMR (CDCl3): d=166.5, 162.1, 131.6, 122.5, 115.0,
75.1, 66.2, 29.1, 22.2, 19.1, 10.6, 9.7; MS: m/z=237 [M+H]+.
N-sec-Hexylphthalimide (CAS 221155–51–7): 1H NMR
(CDCl3): d=7.74–7.72 (m, 2H, CH, arom), 7.63–7.61 (m,
2H, CH, arom), 4.26 (tq, 3J=7 Hz, 1H, N-CH), 2.02–1.94
(m, 1H, CHCH2), 1.69–1.62 (m, 1H, CHCH2), 1.38 (d, 3 J=
7 Hz, 3H, CHCH3), 1.29–1.07 (m, 4H, CH2), 0.77 (t, 3J=
7.5 Hz, 3H, CH2CH3); 13C{1H} NMR (CDCl3): d=167.5,
132.7, 131.0, 122.0, 46.5, 32.4, 27.9, 21.3, 17.7, 12.9; MS:
m/z=233 [M+H]+.
Acknowledgements
N-2-Ethylhexylphthalimide:1H NMR (CDCl3): d=7.85–
7.82 (m, 2H, CH, arom), 7.72–7.70 (m, 2H, CH, arom), 3.58
(d, 3J=7.5 Hz, 2H, N-CH2), 1.84 (tt, 3J=6.5 Hz, 1H, N-
CH2CH), 1.41–1.25 (m, 8H, CH2), 0.92 (t, 3J=7.5 Hz, 3H,
ethyl-CH3), 0.88 (t, 3J=7.5 Hz, 3H, hexyl-CH3); 13C{1H}
NMR (CDCl3): d=167.7, 132.8, 131.1, 122.1, 40.9, 37.3, 29.5,
27.5, 22.9, 22.0, 13.0, 9.4; MS: m/z=260 [M+H]+.
This work was supported by the “Fonds der Chemischen In-
dustrie” and the “Studienstiftung des Deutschen Volkes”
through a fellowship to C. F. as well as by the TU Darmstadt
and the Provadis, Partner für Bildung und Beratung GmbH.
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pure FcTPPO as
a red-orange solid; yield: 500 mg
(1.08 mmol, 83%); Rf 0.38. (1:9 cyclohexane/EtOAc).
1H NMR (benzene-d6): d=7.86–7.82 (m, 4H, arom), 7.76–
7.72 (m, 2H, arom), 7.30–7.27 (m, 2H, arom), 7.08–7.02 (m,
6H, arom), 4.41 (t, J=2.0 Hz, 2H, Fc), 4.11 (t, J=2.0, 2H,
Fc), 3,82 (s, 5H, Fc); 13C{1H} NMR (DMSO-d6): d=144.1,
133.8, 133.0, 132.3, 132.0, 131.84 (d, JP, C =9.3 Hz), 129.1 (d,
J
C,B =11.1 Hz), 126.3 (d, JP, C =12.3 Hz), 83.3, 70.0, 69.9,
67.2; 31P NMR (acetone-d6): d=25.70; 31P NMR (DMSO-
d6): d=23.99; anal. calcd. for C28H23FeOP (462.3): C 72.8,
H 5.02; found: C 73.9, H 5.55.
(S) 2000, 491–492.
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877–910.
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82, 4206–4208.
1062
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Adv. Synth. Catal. 2006, 348, 1058 – 1062