Oxidative addition of the C-O bond of amino acid esters to Rh(I) forming chelating acyl complexes

Article abstract of DOI:10.1016/j.ica.2004.05.028

Esters of N-acylated amino acids and the sterically demanding phosphine 2-(di-ortho-tolylphosphino)phenol react within 1 h at room temperature with the Rh(I) centers of [Cl(μ-Cl)Rh(cyclooctene)₂]₂ to give products of oxidative addition of the ester carbonyl-O bond. The N-acyl carbonyl oxygen is bound to the Rh in these initial adducts, but is displaced upon addition of PMe₃, PhPMe₂, NH₂NMe₂, or the thioether function of a methionine derivative. Remarkably, both initial products from achiral amino acids and their ligand adducts are formed as single five-coordinate diastereomers in essentially quantitative yields. However, asymmetric induction by chiral amino acid derivatives of proline and phenylalanine on the stereochemistry at Rh was modest. Finally, the identities of infrared absorptions of acyl and amide groups in the complexes were established unequivocally by synthesis and spectroscopy of N-acetylglycine esters with a ¹³C label at either the ester or amide carbonyl group.

Full text of DOI:10.1016/j.ica.2004.05.028

Inorganica Chimica Acta 357 (2004) 3047–3056
www.elsevier.com/locate/ica
Oxidative addition of the C–O bond of amino acid esters to
Rh(I) forming chelating acyl complexes
a,
b,1
Douglas B. Grotjahn *, Camil Joubran
a
Department of Chemistry, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-1030, USA
Department of Chemistry and Biochemistry, Arizona State University, Box 871604, Tempe, AZ 85287-1604, USA
b
Received 24 February 2004; accepted 8 May 2004
Available online 15 June 2004
Abstract
Esters of N-acylated amino acids and the sterically demanding phosphine 2-(di-ortho-tolylphosphino)phenol react within 1 h at
room temperature with the Rh(I) centers of [Cl(l-Cl)Rh(cyclooctene)2]2 to give products of oxidative addition of the ester carbonyl–
O bond. The N-acyl carbonyl oxygen is bound to the Rh in these initial adducts, but is displaced upon addition of PMe3, PhPMe2,
NH2NMe2, or the thioether function of a methionine derivative. Remarkably, both initial products from achiral amino acids and
their ligand adducts are formed as single five-coordinate diastereomers in essentially quantitative yields. However, asymmetric
induction by chiral amino acid derivatives of proline and phenylalanine on the stereochemistry at Rh was modest. Finally, the
identities of infrared absorptions of acyl and amide groups in the complexes were established unequivocally by synthesis and
spectroscopy of N-acetylglycine esters with a 13C label at either the ester or amide carbonyl group.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Amino acids; Peptides; Oxidative addition; Rhodium; C–O bond activation; Isotopic labeling
1. Introduction
acid anhydride has been the key step. Oxidative addition
of Ni(0) to cyclic anhydrides of glutamic acid and sub-
sequent decarbonylation give nickelacycles [9–12]. Nu-
cleophilic attack of [CpFe(CO)2]ꢀ on mixed anhydrides
of amino acid derivatives produces chiral acyl complexes
for further asymmetric transformations [13]. One excit-
ing application of oxidative addition chemistry of amino
acid anhydride derivatives is in polymer chemistry [14].
In contrast, in our previous communication [8], we
reported that oxidative addition was a key step in clean
organometallic transformations of amino acid esters,
illustrated in Scheme 1. Esterification of an N-acylated
amino acid 1 with the phosphine-containing alcohol 2
gave amino acid ester 3. Addition of a soft, low-valent
Rh(I) precursor to 3 presumably led to Rh(I)–phosphine
complexation (as in 4), followed by chelate-driven oxi-
dative addition [15] (to give 5) and amide coordination,
to give 6, which was the only product besides free cyc-
looctene detectable during the reaction. These reactions
with o-tol groups on P and a Rh(I) center proceeded in
at least 95% yields to form single stereoisomers. Con-
sidering oxidative additions of organometallics to esters,
The key role of amino acids in protein chemistry and
medicine has inspired the ongoing study of both coor-
dination [1] and organometallic [2] complexes of amino
acids and peptides. Examples include model complexes
in bioinorganic chemistry [1], the creation of chiral
catalysts using the stereocenter(s) of the natural amino
acid [2–5], or attachment of an organometallic fragment
as a tag [6,7]. Here, we present a full report of a fun-
damentally different study, in which a metal center is
used to break a key bond of the amino acid framework
[8].
Transition-metal organometallics have been used on
rare occasions in order to transform amino acids, and in
these cases, oxidative addition of a metal to an amino
* Corresponding author. Former address: Arizona State University.
Tel.: +1-6195940231; fax: +1-6195944634.
E-mail address: grotjahn@chemistry.sdsu.edu (D.B. Grotjahn).
1
Present address: AstraZeneca RD Boston, 35 Gatehouse Drive,
Waltham, MA 02451, USA.
0020-1693/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.05.028

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D.B. Grotjahn, C. Joubran / Inorganica Chimica Acta 357 (2004) 3047–3056
Scheme 1.
the effectiveness of the chemistry reported here is sig-
nificant because [16] that whereas addition of the alkyl
C–O bond in allylic esters (a, Scheme 2) is well known
and leads to useful organic chemistry [17], the alterna-
tive addition of the carbonyl–O bond in esters (b,
Scheme 2) is a rare and usually messy reaction [18–
21].
In the case of 6, IR evidence suggested that the amide
group was coordinated, but that addition of PMe3 led to
a free amide in 7. Here, we report the synthesis and use

D.B. Grotjahn, C. Joubran / Inorganica Chimica Acta 357 (2004) 3047–3056
3049
Scheme 2.
of specifically labeled 13C isotopomers of 3 in order to
conclusively identify the IR absorptions in 3, 6, and li-
gand adducts 7. In addition, in this full report, we ex-
plore the ability of a chiral amino acid (proline or
phenylalanine) to direct the formation of the Rh ste-
reocenter, and the ability of a variety of ligands other
than PMe3 to break the chelation in 6 and stereoselec-
tively from 7.
2.3. 2-(Di-ortho-tolylphosphino)phenol esters of N-ace-
tyl- -phenylalanine (3d) and of N-(4-chlorophenylace-
tyl)proline (3e)
D
Using the general procedure, phosphine 2 (0.250 g,
0.819 mmol) in dichloromethane (10 mL) and DMF (10
mL) was treated with DMAP (0.010 g, 0.0819 mmol)
and N-acetyl-D-phenylalanine (0.187 g, 0.901 mmol) at
0 °C, followed by a solution of DCC (0.186 g, 0.901
mmol) in dichloromethane (10 mL) added over 30 min.
The reaction was stirred and allowed to warm to room
temperature overnight. Workup followed by chroma-
tography (hexanes/ethyl acetate) gave a white solid
2. Experimental
2.1. General remarks
1
(0.120 g, 30%). H NMR (C6D6, 500 MHz) d 1.28 (s,
In addition to the general considerations reported in
[16], some NMR spectra reported here were obtained
using Varian Gemini 2000 200 MHz or INOVA 500
MHz spectrometers. 2-(Di-ortho-tolylphosphino)phenol
(2) was prepared as reported previously [8], as were es-
ters 3a–c and 3f. N-acetyl-C1-13C-glycine was made by
acetylating commercially available C1-13C-glycine
(Cambridge Isotope Labs) as for the unlabeled com-
pound [22–24].
3H), 2.34 (s, 3H), 2.36 (s, 3H), 2.81 (dd, J ¼ 14:0, 7.5
Hz, 1H), 3.13 (dd, J ¼ 14:0, 5.5 Hz, 1H), 5.03 (dd,
J ¼ 12:5, 7.0 Hz, 1H), 5.18 (d, J ¼ 8:0 Hz, 1H), 6.72 (dt,
J ¼ 7:0, 1.0 Hz, 1H), 6.84–7.05 (m, 15H), 7.15 ppm
(ddd, J ¼ 7:5, 4.5, 1.0 Hz, 1H); 31P NMR (C6D6, 202.3
MHz) d )32.03 ppm; IR(NaCl, C6D6, cmꢀ1) 3430 (N–
H), 1769 (C@O, ester), 1690 (C@O, amide). Anal. Calc.
for C31H30NO3P (495.55): C, 75.14; H, 6.10; N, 2.83.
Found: C, 74.75; H, 6.50; N, 3.28%.
Similarly, the proline ester 3e (0.300 g, 69% yield) was
prepared from N-acylated derivative 1e (0.209 g, 0.786
mmol), 2 (0.240 g, 7.86 mmol) using DCC (0.180 g,
0.865 mmol) and DMAP (0.010 g, 0.0786 mmol). Its
NMR spectra were extremely complicated because of
the presence of two rotamers in a ratio of 10:1 in C6D6,
and because of diastereotopic protons in similar envi-
2.2. 2-(Di-ortho-tolylphosphino)phenol esters of N-acyl
amino acids (3) – general procedure
Specific procedures and analytical data for esters 3a–c
and 3f have been reported previously [8].
Phosphine 2 (1–2 mmol) was placed in a flask with
deoxygenated dichloromethane and DMF (10–20 mL
each). DMAP (typically 0.1 equiv.) and N-acyl amino
acid (1 equiv.) were added to the flask and the solution
was stirred at 0 °C. A solution of DCC (1.1–1.2 equiv.)
in dichloromethane (10 mL) was added via addition
funnel over 30 min. The reaction was stirred and al-
lowed to warm to room temperature overnight. The
solution was filtered to remove DCU, dried over mag-
nesium sulfate, filtered, and the solvents were removed
on the high vacuum line. The resulting white solid was
purified by chromatography (dichloromethane or di-
chloromethane/ethyl acetate) yielding 3 as a white solid
(30–69%).
1
ronments. Partial H NMR (C6D6, 499.9 MHz) d 7.38
(ddd, J ¼ 1:1, 4.5, 8.0 Hz, 1H, major rotamer), 6.71 (dt,
J ¼ 1:1, 7.5 Hz, 1H, major rotamer), 4.81 (dd, J ¼ 4:6,
8.1 Hz, 1H, CHN of minor rotamer), 4.53 (dd, J ¼ 4:1,
8.2 Hz, 1H, CHN of major rotamer), 3.16 (s, 2H, ma-
jor), 2.36 and 2.35 (two s, 3H each, diastereotopic tol-
CH3 of major rotamer), 2.31 and 2.29 ppm (two s, 3H
each, diastereotopic tol-CH3 of minor rotamer). Partial
13C NMR (C6D6, 125.7 MHz) d 170.5 and 169.0 (ester
and amide carbonyls of major rotamer), 170.8 and 168.9
(amide and ester carbonyls of minor rotamer), 154.8 (d,
J ¼ 18:5 Hz), 124.0 (d, J ¼ 2:0 Hz, major rotamer),
123.0 (d, J ¼ 2:0 Hz, minor rotamer), 60.0 (minor rot-

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D.B. Grotjahn, C. Joubran / Inorganica Chimica Acta 357 (2004) 3047–3056
Table 1
NMR and selected IR data for Rh acyl complexes 6 and 7 in C6D6 (shifts d in ppm, coupling constants J in Hz)a
1H NMR
Complex
31P{1H}NMR
Methylenes
Ar–CH3 (s, 3H each)
Ar–H
N–CH3COCH3 or L
6a
3.43 (d, J ¼ 11:0, 1H)
4.04 (d, J ¼ 11:0, 1H)
2.55
2.86
6.47 (t, J ¼ 7:5, 1H)
6.63 (t, J ¼ 5:7, 1H)
6.73 (t, J ¼ 7:5, 1H)
6.82 (t, J ¼ 7:8, 2H)
6.95–7.20 (m, 4H)
7.38 (t, J ¼ 10:8, 2H)
8.12 (dd, J ¼ 12:3, 7.5,
1H)
1.92 (s, 3H, COCH3) +52.48 (d, J ¼ 174)
6b
3.42 (d, J ¼ 12:0, 1H)
3.89 (d, J ¼ 12:0, 1H)
2.61
2.93
6.47 (t, J ¼ 6:6, 1H)
1.71 (s, 3H) and 1.75 +53.88 (d, J ¼ 176)
(s, 3H) (N–CH3 and
COCH3)
6.63 (t, J ¼ 6:0, 1H)
6.74 (t, J ¼ 7:2, 1H)
6.82 (t, J ¼ 7:2, 1H)
6.97–7.24 (m, 6H)
7.42 (t, J ¼ 6:9, 1H)
8.22 (dd, J ¼ 13:2, 7.5,
1H)
6c
3.18 (d, J ¼ 16:0, 1H)
3.39 (d, J ¼ 16:0, 1H)
3.49 (d, J ¼ 12:5, 1H)
3.91 (d, J ¼ 12:5, 1H)
2.60
2.86
6.48 (t, J ¼ 7:0, 1H)
6.66 (t, J ¼ 6:5, 1H)
6.75 (t, J ¼ 7:5, 1H)
6.83 (t, J ¼ 7:5, 1H)
6.85–7.21 (m, 10H)
7.42 (t, J ¼ 8:0, 1H)
8.23 (dd, J ¼ 13:5, 8.0,
1H)
1.89 (s, 3H, N–CH3)
+53.81 (d, J ¼ 177)
6db;c
NDa
2.49 and 2.89 (major),
NDa
1.56 (s, 3H, COCH3
major)
+53.38 (d, J ¼ 171,
major) and +52.90
(d, J ¼ 177, minor)
2.57 and 2.85 (minor)
2.58 and 2.93 (major),
1.69 (s, 3H, COCH3
minor)
6eb
5.20 (dd, J ¼ 4:3, 7.3,
NDa
3.34 and 3.24 (two d, +53.87 (d, J ¼ 174,
1H of major),
J ¼ 16:5, 1H each,
COCH2 (4-ClC6H4)
major) and +53.57
(d, J ¼ 180, minor)
4.79 (dd, J ¼ 5:0, 8.0,
1H of minor)
2.59 and 2.81 (minor)
7a-PMe3
3.83 (dd, J ¼ 18:1, 5.7,
1H)
2.34
2.69
6.51 (t, J ¼ 6:6, 1H)
1.28 (s, 3H)
+32.46 (dd,
J ¼ 389, 111)
+6.04 (dd, J ¼ 389,
111)
4.34 (dd, J ¼ 18:1, 7.2,
1H)
6.78–6.87 (m, 3H)
1.46 (dd, J ¼ 11:4,
2.7, 9H)
6.94–7.24 (m, 5H)
7.36 (t, J ¼ 8:1, 1H)
7.61 (dd, J ¼ 11:1, 8.4,
1H)
7a-PhPMe2
3.63 (d, J ¼ 11:6, 1H)
4.57 (d, J ¼ 11:6, 1H)
2.64 (bs, 6H)d
6.49 (t, J ¼ 7:2, 1H)
1.28 (s, 3H,
NCOCH3)
+33.12 (dd,
J ¼ 384, 112)
6.60–6.80 (m, 3H)
1.89 (d, J ¼ 10:4, 6H, +9.55 (dd, J ¼ 384,
PMe2)
113)
6.88 (t, J ¼ 6:8, 1H)
6.90–7.10 (m, 6H)
7.20–7.80 (m, 6H)
6.48 (t, J ¼ 7:2, 1H)
7a-BocMetOMe
4.46–4.56 (m, 2H)
2.15–3.20 (broad, 9H,
tolyl and O–Me)d
1.40 (s, 3H)
+54.26 (d, J ¼ 174)
6.70–7.20 (m, 10H)
7.30–7.40 (m, 1H)
1.67 (s, 9H, BOC)
2.06 (s, 3H)
1.80–2.00 and 2.10–
2.30 (two m, each 2H,
CH2CH2S)
c
7a-NH2NMe2
3.68 (d, J ¼ 15:0, 1H)
4.78 (d, J ¼ 15:0, 1H)
2.60
2.62
6.47 (t, J ¼ 6:0, 1H)
6.81–7.41 (m, 10H)
7.65 (m, 1H)
1.26 (s, 3H)
2.80 (s, 6H)
ND

D.B. Grotjahn, C. Joubran / Inorganica Chimica Acta 357 (2004) 3047–3056
3051
Table 1 (continued)
1H NMR
Complex
31P{1H}NMR
Methylenes
Ar–CH3 (s, 3H each)
Ar–H
N–CH3COCH3 or L
7b-PMe3
3.86 (d, J ¼ 17:7, 1H)
2.36
6.51 (t, J ¼ 6:6, 1H)
1.54 (s, 3H)
+32.10 (dd,
J ¼ 388, 113)
4.45 (d, J ¼ 17:7, 1H)
2.71
6.74–7.25 (m, 9H)
1.58 (dd, J ¼ 11:4,
3.0, 9H)
1.64 (s, 3H)
+6.55 (dd, J ¼ 388,
113)
7.34(t, J ¼ 6:9, 1H)
7.60 (dd, J ¼ 10:8, 7.8,
1H)
7c-PMe3
3.10 (s, 2H)
2.33 (s, 3H)
2.70 (s, 3H)
6.50 (t, J ¼ 7:2, 1H)
1.53 (dd, J ¼ 10:8,
2.7, 9H)
1.67 (s, 3H)
+32.19 (dd,
J ¼ 386, 113)
3.87 (d, J ¼ 17:0, 1H)
4.48 (d, J ¼ 17:0, 1H)
6.70–7.25 (m, 13H)
7.31 (t, J ¼ 8:1, 1H)
+6.49 (dd, J ¼ 386,
113)
7.58 (dd, J ¼ 11:1, 7.8,
1H)
ND
c
7d-PMe3
ND
ND
ND
+34.79 (dd,
J ¼ 392, 114) and
+7.24 (dd, J ¼ 392,
117) (major) and
+29.83 (dd,
J ¼ 389, 114) and
+7.38 (dd, J ¼ 389,
112) (minor)
a ND, not determined, usually because of the complexity of the spectra.
b Where possible, peaks are assigned to major and minor rotamers.
c IR data (NaCl cells, C6D6, cmꢀ1) for 6d: 1724 (acyl), 1584 (coordinated amide); 7a-NH2NMe2: 1713 (acyl), 1686 (uncoordinated amide); and 7d-
PMe3: 1724 (acyl), 1682 (uncoordinated amide).
d Broad or featureless peak, difficult to assign.
amer), 59.8 (major rotamer), 47.3 (major rotamer), 47.1
(minor rotamer), 41.5 (minor rotamer), 41.4 ppm (major
rotamer). 31P NMR (C6D6, 202.3 MHz) d)32.28 (ma-
jor), )32.24 ppm (minor). IR(NaCl, C6D6, cmꢀ1) 1771
(ester) and 1655 (amide). Anal. Calc. for C33H31ClNO3P
(556.03): C, 71.28; H, 5.62; N, 2.52. Found: C, 71.48; H,
5.87; N, 2.93%.
2.5. Preparation of ligand adducts 7 – exemplary proce-
dure on NMR tube scale
In the glovebox, 3a (4.4 mg, 0.0109 mmol) and tet-
rakis(trimethylsilyl)methane (0.8 mg, internal standard)
were dissolved in C6D6 (10 mL) in resealable NMR tube
1
(J. Young). The H NMR spectrum was acquired and
the integrals of peaks from 3a and the internal standard
were measured. The NMR tube was returned to the
glovebox and the dimer, [Cl(l-Cl)Rh(cyclooctene)2]2
(3.9 mg, 0.0054 mmol), was added. After 1 h, PhPMe2
2.4. Preparation of ligand adducts 7 – exemplary proce-
dure on larger scale
1
In the glovebox, 3a (0.0500 g, 0.124 mmol) was added
to benzene (10 mL) in a 50-mL round bottom flask. The
dimer, [Cl(l-Cl)Rh(cyclooctene)2]2 (0.0440 g, 0.0618
mmol), was added and the dark orange colored reaction
was stirred for 30 min until a yellow color had devel-
oped. The yellow reaction mixture was stirred for an
additional 30 min to ensure complete reaction before
PMe3 (0.0094 g, 0.12 mmol) was added in one portion
and the solution immediately turned orange. The solvent
volume was reduced to 5 mL and hexane was added to
induce precipitation. The precipitate was filtered and
washed with hexane (3 ꢁ 5 mL) yielding 7a-PMe3 as a
bright yellow solid (0.072 g, 94%). NMR data: see Table
1 and text. IR(C6D6, cmꢀ1) 3432 (N–H), 1709 (C@O,
acyl), 1686 (C@O, amide). Anal. Calc. for
C26H33ClNO3P2Rh (618.81): C, 52.31, H, 5.38, N, 2.26.
Found: C, 52.54; H, 5.45; N, 2.28%.
(1.5 mg, 0.0109 mmol) was added and the H and 31P
NMR spectra were recorded to determine the yield of
product (7a-PhPMe2) and give the values shown in
Table 1.
2.6. Synthesis of N-acetyl-C10-13C-glycine via its benzyl
ester
Sodium acetate (1-13C, 0.300 g, 3.61 mmol) and (+)-
camphor-10-sulfonic acid (0.420 g, 3.61 mmol) were
placed in a flask with CH2Cl2 (60 mL) and stirred at
0 °C. A solution of DCC (0.819 g, 3.97 mmol) and
DMAP (0.440 g, 3.61 mmol) in CH2Cl2 (30 mL) was
added dropwise over 15 min. After 2 h, a solution of N-
ethylmorpholine (0.420 g, 3.61 mmol) and glycine benzyl
ester hydrochloride salt (1.22 g, 3.61 mmol) in CH2Cl2
(30 mL) was added. The mixture was allowed to reach

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D.B. Grotjahn, C. Joubran / Inorganica Chimica Acta 357 (2004) 3047–3056
room temperature overnight (16 h) before it was filtered.
The filtrate was washed with aqueous sodium hydroxide
(1 M, 3 ꢁ 30 mL) and the organic phase was dried over
magnesium sulfate, filtered, and the filtrate concen-
trated. The residue was purified by radial chromatog-
raphy (CH2Cl2/MeOH 50:10) to give a clear oil (0.300 g,
41%). 1H NMR (CDCl3, 300 MHz) d 2.01 (d, 2JCH ¼ 6:3
bound phosphine [30]. The IR absorptions of 6a at 1726
and 1584 cmꢀ1 were consistent with metal-acyl [15c,15d]
and metal-coordinated amide [31], respectively. For
comparison, the amide stretching frequency in starting
material 3a is 1678 cmꢀ1. Below, additional evidence for
these assignments is provided by use of 13C-labeled
compounds. In the 13C{1H} NMR spectrum of 6a, the
3
3
1
Hz, 3H), 4.05 (dd, JHHJ ¼ 5:8, JCH ¼ 3:5 Hz, 2H),
5.17 (s, 2 H), 6.00 (bs, 1H), 7.34–7.35 ppm (m, 5H).
These data are similar to those reported for the corre-
sponding ethyl ester [25,26].
most downfield resonance at d 209.35 (dd, JRhC ¼ 34:0
Hz, 2JPC ¼ 4:4 Hz) also confirms the existence of a metal
acyl, and hence oxidative addition of the ester acyl–ox-
ygen bond to Rh [16]. Although the precise stereo-
chemistry of 6 cannot be assigned, the small value of
2JPC shows unequivocally that the acyl and phosphine
ligands are mutually cis [16,32,33]. The methylene pro-
tons (Ha, Hb) are diastereotopic (2JHH ¼ 11:0 Hz), in-
dicating that a stereogenic element is present in 6a. Both
the sarcosine esters 3b and 3c reacted with [Cl(l-
Cl)Rh(cyclooctene)2]2 to give single products 6b and 6c
in quantitative yield. Very similar NMR and IR spectral
data were obtained (Table 1).
A 1-liter hydrogenation bottle was charged with the
benzyl ester (0.300 g, 1.44 mmol), Pd–C (10% by weight,
0.030 g), and methanol (20 mL). The mixture was sha-
ken under hydrogen (40 psi) overnight before it was
filtered through Celite and the filtrate concentrated by
rotary evaporation. The residue was dried over P4O10
under high vacuum overnight, leaving the acid as a
white solid (0.116 g, 70%). 1H NMR (D2O, 300 MHz) d
2
3
1.89 (d, JCH ¼ 6:3 Hz, 3H), 3.82 ppm (d, JCH ¼ 3:9
Hz, 2H). These data are very similar to those reported
for a sample dissolved in CD3OD [25,26].
Unfortunately, the available data do not allow us to
determine the geometry of 6a–6c. The literature contains
many examples of stable five coordinate rhodium acyl
complexes, whether square pyramidal or trigonal bipy-
ramidal [15c,15d,34]; alternatively, cyclooctene could
occupy a sixth site, making 6a–6c octahedral. Some
evidence for alkene involvement when a smaller alkene
(and presumably better ligand) was involved came from
reactions of the ethylene analog [Cl(l-Cl)Rh(ethyl-
ene)2]2 with 3a; the resulting spectrum contained sharp
resonances for 6a, but a somewhat broadened resonance
for ethylene. Upon standing in C6D6 for 12–15 h, the 1H
and 31P NMR spectra of acyl complex 6a degrade, and
no identifiable features appear in replacement. Potential
decomposition pathways include alkyl migration fol-
lowed by b-hydride elimination involving the N–H
group. In support of this idea, rhodium acyl complexes
6b and 6c featuring N-methyl groups were more stable in
solution, persisting for 24–48 h. Nonetheless, complexes
6 could not be obtained in analytically pure form.
Two strategies were used to characterize complexes 6
further: in one set of studies to be described immediately,
ligands were added to 6a to prevent decomposition by
saturating the coordination sites; in the other, 13C-la-
beled amino acid derivatives were used to identify un-
equivocally the positions of IR absorptions for acyl and
amide groups. For ligand addition studies, the small,
strongly coordinating phosphine PMe3 was added to a
solution of 6a in benzene, changing the color from yellow
to orange. Clear changes in spectral data (Table 1) im-
plicate formation of a single product 7a-PMe3 in greater
than 95% yield. Similar results were obtained for 6b–d,
giving 7b-PMe3, 7c-PMe3, and 7d-PMe3, respectively.
Significantly, the PMe3 adduct 7a-PMe3 remains stable
in solution for a week and could be characterized by
combustion analysis. The incoming trimethylphosphine
3. Results and discussion
Preliminary studies [8] showed that in order to
maintain the 1:1 stoichiometry of metal and phosphine
shown in Scheme 1, the bulky phosphine 2 and its esters
3 were necessary. As previously described [8], novel
phosphine 2 was made in 81% yield by deprotonation of
PhOCH2OCH3 with t-BuLi followed by addition of
ClP(o-tolyl)2, acid-catalyzed deprotection of the acetal,
and neutralization.
A selection of six N-acylated amino acid esters 3
(Scheme 1, lower part) was made by DCC-mediated
coupling of N-acylated amino acids 1 and the phenol 2.
Different types of amino acids were represented by esters
from the achiral acid glycine (3a), its N-alkylated de-
rivative sarcosine (3b and 3c), the chiral amino acids
phenylalanine (3d) and proline (3e), and the dipeptide
glycylglycine (3f) protected with a N-terminal carbo-
benzoxy (Cbz) group. The esters 3 were formed using
DCC and a catalytic amount of DMAP [27,28], because
the attack of hindered nucleophile 2 on the activated N-
acylurea intermediate was expected to be slow.
To a room-temperature solution of the glycine ester
3a and internal standard [(CH3)3Si]4C in C6D6 was
added 1 equiv. (0.5 mol) of the rhodium (I) dimer [Cl(l-
Cl)Rh(cyclooctene)2]2 [29]. Remarkably, within 30–60
min, one product formed in quantitative yield as deter-
mined by NMR integrations [8]. The structure shown
for 6a (Scheme 1) is consistent with all spectroscopic
data (Table 1). The 31P{1H} NMR spectrum of 6a
contains
a doublet that is coupled to rhodium
(1JRhP ¼ 174 Hz) and provides the evidence for rhodium

D.B. Grotjahn, C. Joubran / Inorganica Chimica Acta 357 (2004) 3047–3056
3053
ligand seemed to take up the site of the coordinated
amide as revealed by the IR absorption at 1686 cmꢀ1 (cf.
1678 cmꢀ1 for free ligand 3a). Also, the 31P spectrum
contained two separate resonances, both doublets of
oxygen and the two diastereotopic tolyl groups. The
latter feature was also seen in the spectrum of 7a-
PhPMe2. Finally, whereas ligand NH2NMe2 seemed to
give a single adduct (7a-NH2NMe2), the bulky phos-
phine 2 failed to react. Changes in infrared data on
addition of ligands were consistent with amide decoor-
dination, see footnote of Table 1. Although these studies
suggest the range of ligands which may add to 6, the
complexity of the NMR spectra prompted us to move
on to examine isotopically labeled species.
1
doublets (2JPP ¼ 389 Hz, JRhP ¼ 111 Hz). Large split-
tings of each resonance indicate that the phosphines are
mutually trans [30], with the acyl mutually cis to both
phosphines: the 13C{1H} NMR data for the acyl carbon
showed a larger splitting by Rh (1JRhC ¼ 34:0 Hz) and
two smaller and equal splittings by the P nuclei
(2JPC ¼ 2JP C ¼ 6:3 Hz). The complexes are five coordi-
Comparing the reduced masses of 12C@O and 13C@O
groups, one can predict that the ratio of infrared ab-
sorption frequencies for two isotopomers containing
12C@O and 13C@O groups in either the amide or acyl
position would be 1.022:1. Thus, two isotopomers of
N-acetylglycine were made (Scheme 3), one from com-
mercially purchased C1-13C-glycine and acetic anhy-
dride, the other by conversion of sodium C1-13C-acetate
to acetic acid in situ using 1.0 equiv. of anhydrous
camphorsulfonic acid, followed by DCC-mediated and
DMAP-catalyzed coupling with the benzyl ester of gly-
cine, followed by catalytic hydrogenation. The two iso-
topically labeled glycine derivatives were coupled with 2
as done for unlabeled material, giving 3a-C1-13C and 3a-
N-Ac-13C (Scheme 3). Table 2 compares NMR and IR
data for the three isotopomers studied. The effects of
label on the IR absorptions of ester and amide carbonyls
are clear, amounting to 1765/1734 ¼ 1.018 and 1678/
1651 ¼ 1.016, respectively. We note that the positions of
the IR bands should probably be cited to within ca. 5
cmꢀ1, thus the experimentally determined ratios for
m(12C@O)/m(13C@O) will vary somewhat from the cal-
culated value of 1.022:1. Regardless, the observed
magnitudes of the shifts (on the order of 30 cmꢀ1 or
more) are unmistakably larger than estimated experi-
0
nate, although at this point we do not know if they are
square pyramidal or trigonal bipyramidal. Like com-
plexes 7-PMe3, complex 7f-PMe3 also gives correct ele-
mental analysis data [8], and its 31P{1H} NMR and IR
data [8] are very similar to those of the simpler analogs,
1
but its H NMR spectrum exhibited several broadened
resonances, perhaps because of hydrogen bonding of the
three carbonyl groups and two NH groups in either an
intramolecular or intermolecular sense.
Complex 6a was used to test the ability of ligands
other than trimethylphosphine to add to the Rh center.
The slightly larger ligand PhPMe2 added to give a single
adduct, assigned structure 7a-PhPMe2 (Scheme 1), be-
cause its 31P{1H} NMR data (Table 1) were very similar
to those of the PMe3 analog. In addition, the sulfur-
based ligand N-tert-butyloxycarbonyl methionine me-
thyl ester (BOC–Met–OMe, Scheme 1) produced a
single adduct, assigned structure 7a-BOCMetOMe, be-
cause of its single sharp 31P NMR resonance (Table 1).
NMR data do not allow us to assign the binding mode
of the methionine derivative, but it is almost certainly
bound through the thioether function. The complexity
of the proton NMR spectrum precluded a rigorous as-
signment of the resonances. In particular, broad and
overlapping peaks were seen for the methyl groups on
1
mental uncertainties. Interestingly, in H NMR spectra
Scheme 3.

3054
D.B. Grotjahn, C. Joubran / Inorganica Chimica Acta 357 (2004) 3047–3056
Table 2
Spectral data for isotopomers of 3a and complexes derived therefrom (all in C6D6)a
Compound
1H for CH3CONHCH2
13C
31P
IR(NaCl cells, cmꢀ1
)
3
3a
3.85 (d, JHH ¼ 5:4, 2H)
168.05 (ester)
169.99 (amide)
)31.82
3437 (N–H)
1.37 (s, 3H)
1765 (C@O, ester)
1678 (C@O, amide)
3434 (N–H)
3
3a-C1-13
C
3.81 (t, JHH ¼ 2JCH ¼ 6:0, 2H) 168.39 (ester)
1.33 (s, 3H)
1734 (C@O, ester)
1692 (C@O, amide)
3434 (N–H, amide)
3a-NAc-C10-13
C
3.83 (dd, 3JHH ¼ 5:7, 3JCH ¼ 3:3, 168.90 (amide)
2H)
1.35 (d, JCH ¼ 6.3, 3H)
2
1775 (C@O, ester)
1651 (C@O, amide)
3394 (N–H)
1
6a
3.43 (d, J ¼ 11:0, 1H) 4.04
(d, J ¼ 11:0, 1H)
209.1 (dd, JRhC ¼ 33,
+53.20 (d, J ¼ 177)
+52.33 (d, J ¼ 168)
ND
2JPC ¼ 4:8, acyl)
1726 (C@O, acyl)
1584 (C@O, amide)
3396 (N–H)
1.92 (s, 3H)
ND
6a-C1-13
C
209.35 (dd, JRhC ¼ 34:6,
1
2JPC ¼ 4:4, acyl)
176.64 (amide)
ND
1688 (C@O, acyl)
1584 (C@O, amide)
3399 (N–H)
2
6a-NAc-C10-13
C
3.43 (d, JHH ¼ 11:0, 1H)
2
4.04 (d, JHH ¼ 11:0, 1H)
1726 (C@O, acyl)
1554 (C@O, amide)
3432 (N–H)
2
1.92 (d, JCH ¼ 6:0, 3H)
2
2
7a
3.83 (dd, JHH ¼ 18:1,
+32.46 (dd, JPP ¼ 389,
3JHH ¼ 5:7, 1H)
1JRhP ¼ 110)
2
2
4.34 (dd, JHH ¼ 18:1,
+6.04 (dd, JPP ¼ 389,
1709 (C@O, acyl)
3JHH ¼ 7:2, 1H)
1.28 (s, 3H)
ND
1JRhP ¼ 113)
1686 (C@O, amide)
3432 (N–H)
7a-PMe3-C1-13
C
204.15 (dt, JRhC ¼ 34:0,
+32.47 (ddd, JPP ¼ 389,
1
2
2JPC ¼ 6:3, acyl)
1JRhP ¼ 110, JCP ¼ 5:2)
2
2
+6.08 (ddd, JPP ¼ 389,
1674 (C@O, acyl)
1JRhP ¼ 113, JCP ¼ 7:6)
2
1686 (C@O, amide)
3432 (N–H)
7a-PMe3-NAc-C10-13
C
3.83 (dt, JHH ¼ 18,
169.11 (amide)
ND
2
3JHH ¼ 3JCH ¼ 5:2, 1H)
2
4.34 (ddd, JHH ¼ 18,
1709 (C@O, acyl)
3JHH ¼ 6:8, JCH ¼ 3:0, 1H)
3
2
1.27 (d, JCH ¼ 6.0)
1645 (C@O, amide)
ND, not determined.
a For labeled species, 13C data refer to the labeled carbon only.
for both labeled esters, the signals for the glycine
methylene protons show splitting by the nearby 13C
nucleus. Reaction with [Cl(l-Cl)Rh(cyclooctene)2]2 gave
the corresponding isotopomers of 6a, also presented in
Table 2. Comparing 3a and its isotopomer with label at
the acyl position, it is clear that the 1726 cmꢀ1 absorp-
tion in the spectrum of 3a is due to the acyl moiety.
Similarly, in comparing the data for 3a and its analog
labeled at the N-acetyl group shows that the absorption
at 1584 cmꢀ1 is due to the amide C@O, which must be
coordinated. Table 2 also shows data for the PMe3 ad-
ducts, in which the amide is not coordinated, since iso-
topic labeling of the amide carbonyl clearly identifies its
acids to chiral organic products, it was of great interest
to see if the chiral center of an amino acid other than
glycine would influence the configuration at the newly
formed Rh stereocenter in 6. Thus, esters 3 were made
from N-acylated proline and phenylalanine. Both spe-
cies reacted quantitatively with [Cl(l-Cl)Rh(cyclooc-
tene)2]2 at room temperature within an hour. The
phenylalanine ester 3d formed 6d as a 2.3:1 mixture of
major and minor diastereomers, which both reacted
with PMe3 to give a 3:1 mixture of PMe3 adducts 7d-
PMe3. Similarly, the proline ester 3e formed 6e as a 5:1
mixture of major and minor diastereomers, which both
reacted with PMe3 to give a mixture of PMe3 adducts.
Because the ratios seen were not extremely high, these
mixtures were not characterized further, but they show
the potential for asymmetric induction and chirality
transfer in oxidative addition chemistry of amino acid
derivatives.
stretching frequency in 7a-PMe3 as 1686 cmꢀ1
.
Remarkably, oxidative addition of Rh(I) to the four
achiral amino acid esters and amide coordination gave
6 as a single diastereomer. Because this chemistry
could have the potential to transform chiral amino

D.B. Grotjahn, C. Joubran / Inorganica Chimica Acta 357 (2004) 3047–3056
3055
(p) D. Carmona, A. Mendoza, F.J. Lahoz, L.A. Oro, M.P.
Lamata, E. San Jose, J. Organometal. Chem. 396 (1990) C17;
(q) D.F. Dersnah, M.C. Baird, J. Organomet. Chem. 127 (1977)
C55;
4. Conclusions
The oxidative addition chemistry of Rh(I) on achiral
esters of N-acylated amino acids (3) is controlled by the
use of a new, sterically hindered phosphine (2). Within
an hour at room temperature, single products 6 were
obtained from 3 in essentially quantitative yields. It is
clear that in conversion of 3 to 6, the ester CO–O bond
has been added to the Rh center to form five-coordinate
phenoxy acyl complexes featuring a coordinated amide
group. Although complexes 6 were too unstable to be
characterized by combustion analysis or X-ray diffrac-
tion, 13C-isotopically labeled species were used to fully
characterize the coordination of the amino acid N-acyl
group in 6. Although achiral amino acid esters gave 6 as
single diastereomers in which the phosphine and acyl
ligands are cis, use of a chiral amino acids gave mixtures
of diastereomers, showing that effective asymmetric in-
duction from the amino acid chiral center to the newly
formed Rh stereocenter will require further develop-
ments. Addition of PMe3, PhPMe2, NH2NMe2, or a
methionine derivative displaces the amide, giving five-
coordinate acyls 7 in which the new ligand appears trans
to the bulky phosphine. The decoordination of the
amide group in going from 6 to 7 was verified by
changes in infrared spectral data on both natural
abundance and 13C-labeled materials.
€
(r) R. Kramer, K. Polborn, C. Robl, W. Beck, Inorg. Chim. Acta
198-200 (1992) 415;
(s) D. Carmona, F.J. Lahoz, R. Atencio, L.A. Oro, M.P. Lamata,
ꢀ
E. San Jose, Tetrahedron: Asymmetry (1993) 1425;
(t) W.S. Sheldrick, E. Hauck, S. Korn, J. Organomet. Chem. 467
(1994) 283;
(u) W.S. Sheldrick, K. Brandt, Inorg. Chim. Acta 217 (1994) 51;
€
(v) H. Werner, T. Daniel, O. Nurnberg, W. Knaup, U. Meyer, J.
Organomet. Chem. 445 (1993) 229;
(w) W.S. Sheldrick, A. Gleichmann, J. Organomet. Chem. 470
(1994) 183;
(x) W.S. Sheldrick, R. Exner, Inorg. Chim. Acta 166 (1989)
213;
(y) D.B. Grotjahn, T.L. Groy, J. Am. Chem. Soc. 116 (1994)
6969;
(z) D.B. Grotjahn, T.L. Groy, Organometallics 14 (1995) 3669;
D.B. Grotjahn, C. Joubran, J.L. Hubbard, Organometallics 15
(1996) 1230.
[4] S.R. Gilbertson, X. Wang, J. Org. Chem. 61 (1996) 434.
[5] S.R. Gilbertson, R.V. Pawlick, Angew. Chem., Int. Ed. Engl. 35
(1996) 902.
[6] (a) B. El Mouatassim, H. El Amouri, M. Salmain, G. Jaouen, J.
Organomet. Chem. 479 (1994) C18;
ꢀ ꢀ
(b) K. Malisza, S. Top, J. Vaissermann, B. Caro, M.-C. Senechal-
ꢀ ꢀ
Tocquer, D. Senechal, J.-Y. Saillard, S. Triki, S. Kahlal, J.F.
Britten, M.J. McGlinchey, G. Jaouen, Organometallics 14 (1995)
5273;
(c) B. El Mouatassim, H. El Amouri, J. Vaissermann, G. Jaouen,
Organometallics 14 (1995) 3296;
(d) A. Gorfti, M. Salmain, G. Jaouen, M.J. McGlinchey, A.
Bennouna, A. Mousser, Organometallics 15 (1996) 142;
(e) H. El Amouri, Y. Besace, J. Vaissermann, G. Jaouen, J.
Organomet. Chem. 515 (1996) 103;
References
[1] (a) S.H. Laurie, Amino acids, peptides and proteins, in: G.
Wilkinson (Ed.), Comprehensive Coordination Chemistry, vol. 2,
Pergamon, Oxford, 1987, p. 739;
(f) H. El Amouri, S. Canceil, Y. Besace, L. Ricard, Organomet-
allics 15 (1996) 2303;
(g) B. Rudolf, J. Zakrewski, J. Organomet. Chem 522 (1996) 313;
€
(h) M.J. Schweiger, T. Ederer, K. Sunkel, W. Beck, J. Organomet.
(b) A.A. Ioganson, Russ. Chem. Rev. 54 (1985) 277;
(c) H. Sigel, R.B. Martin, Chem. Rev. 82 (1982) 385.
[2] K. Severin, R. Bergs, W. Beck, Angew. Chem., Int. Ed. Engl. 37
(1998) 1634.
Chem. 545-546 (1997) 17;
(i) B. Rudolf, J. Zakrewski, M. Salmain, G. Jaouen, Tetrahedron
Lett. 39 (1998) 4281.
[3] (a) D. Dowerah, M.M. Singh, J. Ind. Chem. Soc. 57 (1980) 368;
(b) D. Dowerah, M.M. Singh, J. Chem. Res. (S) (1979) 38;
(c) D. Dowerah, M.M. Singh, Trans. Met. Chem. 1 (1976) 294;
[7] (a) D.B. Grotjahn, C. Joubran, D. Combs, D. Brune, J. Am.
Chem. Soc. 120 (1998) 11814;
(b) D.B. Grotjahn, Coord. Chem. Rev. 190-192 (1999) 1125.
[8] Part of this work has been communicated: D.B. Grotjahn, C.
Joubran, D. Combs, J. Organomet. Chem. 589 (1999) 115.
ꢀ
(d) Z. Nagy-Magos, P. Kvintovics, L. Marko, Trans. Met. Chem.
5 (1980) 186;
(e) S. Shinoda, N. Inoue, K. Takita, Y. Saito, Inorg. Chim. Acta
65 (1982) L21;
~
[9] A.M. Echavarren, A.M. Castano, Adv. Metal-Organic Chem. 6
(1998) 1.
~
[10] A.M. Echavarren, A.M. Castano, Tetrahedron 51 (1995) 2369.
(f) W.S. Sheldrick, S. Heeb, J. Organomet. Chem. 377 (1989) 357;
(g) W.S. Sheldrick, R. Exner, Inorg. Chim Acta 175 (1990) 261;
~
[11] A.M. Castano, A.M. Echavarren, Tetrahedron Lett. 31 (1990)
€
(h) E. Lippmann, R. Kramer, W. Beck, J. Organomet. Chem. 466
4783.
[12] A.M. Castano, A.M. Echavarren, Tetrahedron Lett. 34 (1993)
~
(1994) 167;
(i) R. Bergs, K. Sunkel, W. Beck, Chem. Ber. 126 (1993) 2429;
€
4361.
[13] (a) R.W. Hungate, F. Miller, M.S. Goodman, Tetrahedron Lett.
29 (1988) 4273;
(j) D.J. Darensbourg, E.V. Atnip, K.K. Klausmeyer, J.H.
Reibenspies, Inorg. Chem. 33 (1994) 5230;
ꢀ
(b) C. Amiens, G. Balavoine, F. Guilbe, J. Organomet. Chem. 443
(1993) 207.
€
(k) K. Severin, K. Sunkel, W. Beck, Chem. Ber. 127 (1994) 615;
€
€
(l) T.M. Klapotke, H. Kopf, I.C. Tornieporth-Oetting, P.S. White,
Angew. Chem., Int. Ed. Engl. 33 (1994) 1518;
[14] T.J. Deming, J. Polym. Sci. Part A: Polym. Chem. 38 (2000) 3011.
[15] (a) Selected examples: (a) chelate-driven additions of Rh(I) and
Ir(I) to aldehydes: J.W. Suggs, J. Am. Chem. Soc. 100 (1978) 640;
(b) E.F. Landvatter, T.B. Rauchfuss, Organometallics 1 (1982)
506;
€
(m) T.M. Klapotke, H. Kopf, I.C. Tornieporth-Oetting, P.S.
White, Organometallics 13 (1994) 3628;
€
(n) I.C. Tornieporth-Oetting, P.S. White, Organometallics 14
(1995) 1632;
€
(o) R. Kramer, K. Polborn, H. Wanjek, I. Zahn, W. Beck, Chem.
Ber. 123 (1990) 767;
(c) Cleavage of ketone C–C bonds directed by quinoline N: J.W.
Suggs, C.-H. Jun, J. Am. Chem. Soc. 106 (1984) 3054;

3056
D.B. Grotjahn, C. Joubran / Inorganica Chimica Acta 357 (2004) 3047–3056
(d) J.W. Suggs, M.J. Wovkulich, P.G. Williard, K.S. Lee, J.
Organomet. Chem. 307 (1987) 71;
[24] The acetylation has also been carried out in unspecified yield using
AcCl: X.G. Chen, P. Li, J.S.W. Holtz, Z. Chi, V. Pajcini, S.A.
Asher, L.A. Kelly, J. Am. Chem. Soc. 118 (1996) 9705.
[25] J.A. Wiles, S.H. Bergens, Organometallics 18 (1999)
3709.
(e) Chelate-driven oxidative additions to N–H bonds: D. Hedden,
D.M. Roundhill, W.C. Fultz, A.L. Rheingold, J. Am. Chem. Soc.
106 (1984) 5014;
(f) D. Hedden, D.M. Roundhill, Inorg. Chem. 25 (1986) 9.
[16] D.B. Grotjahn, C. Joubran, Organometallics 14 (1995) 5171.
[17] (a) P.J. Harrington, in: E.W. Abel, F.G.A. Stone, G. Wilkinson
(Eds.), Comprehensive Organometallic Chemistry II, vol. 12,
Pergamon, Oxford, 1995;
[26] The only other reference is devoid of experimental detail: X.
He, A.F. Bell, P.J. Tonge, J. Phys. Chem.
6056.
B 106 (2002)
[27] B. Neises, W. Steglich, Angew. Chem., Int. Ed. Engl. 17 (1978)
522.
€
€
[28] G. Hofle, W. Steglich, H. Vorbruggen, Angew. Chem., Int. Ed.
(b) W. Oppolzer, in: E.W. Abel, F.G.A. Stone, G. Wilkinson
(Eds.), Comprehensive Organometallic Chemistry II, vol. 12,
Pergamon, Oxford, 1995.
Engl. 17 (1978) 569.
[29] A. Van der Ent, A.L. Onderdelinden, Inorg. Synth. 14 (1973) 92.
[30] P.S. Pregosin, R.W. Kunz, 31P and 13C NMR of Transition Metal
Phosphine Complexes, Springer, Berlin, 1979.
[18] Review of C–O bond activation in general: A. Yamamoto, Adv.
Organomet. Chem. 34 (1992) 111.
€ €
[31] D.B. Grotjahn, F.E.K. Kroll, T. Schafer, K. Harms, K.-H. Dotz,
[19] Pioneering work on Ni(0): T. Yamamoto, J. Ishizu, T. Kohara, S.
Komiya, A. Yamamoto, J. Am. Chem. Soc. 102 (1980) 3758.
[20] Additions to strained butyrolactone are clean but appear to
involve cleavage of the ester alkyl–O bond: A.A. Zlota, F. Frolow,
D. Milstein, Organometallics 9 (1990) 1300.
Organometallics 11 (1992) 298.
[32] E. Lindner, Q. Wang, H.A. Mayer, R. Fawzi, M. Steimann,
Organometallics 12 (1993) 1865.
2
[33] Observed JPC values for cis- and trans-oriented CH3-Pt-PR3
[21] K.-T. Aye, D. Colpitts, G. Ferguson, R.J. Puddhepatt, Organo-
metallics 7 (1988) 1454.
systems were 5–9 and 75 Hz, respectively: M.H. Chisolm, H.C.
Clark, L.E. Manzer, J.B. Stothers, J.E.H. Ward, J. Am. Chem.
Soc. 95 (1973) 8574.
[22] R.D. Herbst, D. Shemin, Org. Synth. 19 (1939) 4.
[23] Cited in T.G. Oas, C.J. Hartzell, T.J. McMahon, G.P. Drobny,
F.W. Dahlquist, J. Am. Chem. Soc. 109 (1987) 5956.
[34] D.L. Egglestone, M.C. Baird, C.J.L. Lock, G. Turner, J. Chem.
Soc., Dalton Trans. (1977) 1576.