Phosphoramidate-mediated conversion of carbonyl ligands into isocyanide ligands: A new approach to chiral isocyanide ligands

Article abstract of DOI:10.1002/1521-3765(20020104)8:1<269::AID-CHEM269>3.0.CO;2-9

Metal isocyanides have been used and studied by organometallic chemists for many years and, as a result, they have a rich and interesting chemistry. The nature of metal-free isocyanides and the methods of making isocyanide complexes, however, has resulted

Full text of DOI:10.1002/1521-3765(20020104)8:1<269::AID-CHEM269>3.0.CO;2-9

FULL PAPER
Phosphoramidate-Mediated Conversion of Carbonyl Ligands into Isocyanide
Ligands: A New Approach to Chiral Isocyanide Ligands
Susan E. Gibson,*[a] Hasim Ibrahim,[a] Corinne Pasquier,[a] Mark A. Peplow,[b]
Julia M. Rushton,[a] Jonathan W. Steed,[a] and Surojit Sur[a]
Abstract: Metal isocyanides have been used and studied by organometallic chemists
for many years and, as a result, they have a rich and interesting chemistry. The nature
of metal-free isocyanides and the methods of making isocyanide complexes, however,
has resulted in the vast majority of studies to date being performed with structurally
simple isocyanides. We report here a new approach to the synthesis of isocyanide
ligands that involves the reaction of a metal carbonyl ligand with the anion of a
phosphoramidate. As phosphoramidates can be synthesised in one step from amines,
our method means that the structural diversity of readily available amines,
particularly chiral amines, can now be incorporated into isocyanide ligands.
Keywords: amines
synthesis ¥ carbonyl ligands ¥ iron
¥ isocyanide ligands
¥
asymmetric
multiple substitutions resulting in low yields, as exemplified
by the synthesis of the chromium complex 1,[2] but in some
cases, such as those detailed in a recent report on the use of
Introduction
Isocyanide ligands are known to form complexes with many
transition metals.[1] Difficulties associated with the synthesis
of isocyanide ligands, however, mean that the vast majority of
isocyanide complexes synthesised and studied to date have
been complexes of relatively simple aryl and alkyl isocyanides.
Complexes of more complicated isocyanides, such as chiral
isocyanides, are surprisingly rare, a situation which contrasts
sharply of course with the number of complexes bearing chiral
phosphine ligands that have been prepared and studied. In
this paper we demonstrate how a new method of synthesising
isocyanide ligands can be used to generate enantiomerically
pure isocyanide complexes in a relatively straightforward
manner.
The synthesis of isocyanide complexes is most commonly
achieved by direct combination of a metal complex with an
isocyanide. In the case of metal carbonyl complexes, ther-
molysis or photolysis is used to remove a carbonyl ligand from
the parent complex before the free ligand site is occupied by
the isocyanide. This approach is generally complicated by
dicarbonyl(isocyanide)chromium(0) linkers for attaching fluo-
roarenes to solid supports,[3] yields can be very good. Creation
of a vacant coordination site may also be achieved by the
removal of an iodide ligand with a silver(i) salt as illustrated by
the synthesis of iron cation 2.[4] There are several
[a] Prof. S. E. Gibson, H. Ibrahim, Dr. C. Pasquier, J. M. Rushton,
Dr. J. W. Steed, Dr. S. Sur
Department of Chemistry, King×s College London
Strand, London WC2R 2LS (UK)
Fax : (‡44)171-8482810
drawbacks to the direct synthesis approaches outlined above
which perhaps explain why isocyanides other than simple aryl
and alkyl isocyanides have not featured more prominently in
organometallic chemistry to date: few free isocyanides are
commercially available and the synthesis of other isocyanides
E-mail: susan.gibson@kcl.ac.uk
[b] M. A. Peplow
Department of Chemistry
Imperial College of Science, Technology and Engineering
South Kensington, London SW7 2AY (UK)
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269

S. E. Gibson et al.
FULL PAPER
is frequently arduous; some isocyanides are toxic and many
are malodorous.
pentacarbonyliron(o) with a phosphinimine.[7] This reaction
has occasionally been exploited in a similar manner,[8] (see
below) although it is of note that phosphinimines are
frequently moisture- and/or air-sensitive.[9]
The challenges associated with the present approaches to
isocyanide ligand synthesis have led to the vast majority of
isocyanide ligands reported to date having very simple
structures. The following notable exceptions, however, illus-
trate the potential interest in more sophisticated isocyanide
ligands.
Alternative methods for creating isocyanide ligands have
been developed, including alkylation of cyanide ligands and
the reaction of phosphinimines with carbonyl ligands. The
production of cobalt complex 3 was achieved by a double
alkylation reaction.[5] Despite the fact that this approach
Isocyanides separated from a stereogenic centre by a
promesogenic core have been used as monomers to create
chiral helical poly(isocyanides). More recently isocyanides of
this type have been used as ligands to generate chiral
organometallic liquid crystals with helical mesophases.[10]
The palladium and gold complexes examined, exemplified
by 5, were synthesised by direct displacement of labile ligands,
requires the use of KCN to create the necessary cyanide
ligands, it continues to find application as indicated by its
recent use in the synthesis of a range of alkyl, allyl and
propargyl isocyanide complexes by treatment of trans-
[FeH(CN)(dppe)2] with suitable electrophiles.[6] The iron
isocyanide complex 4 has been synthesised by reaction of
such as benzonitrile and tetrahydrothiophene, from appro-
priate metal halides by pre-formed isocyanides. To date, the
only example of the use of a chiral isocyanide ligand in a
catalytic context is provided by the demonstration that a
palladium(0) isocyanide complex, formed from pre-formed
isocyanide 6 and Pd(acac)2, catalyses an intramolecular bis-
silylation of an alkene. This pioneering reaction gave the
Editorial Board Member:[*] Sue Gib-
son, born in 1960, studied Natural
Sciences at Cambridge University be-
fore undertaking doctoral research
work with Professor Stephen Davies
at Oxford University. A Research
Fellowship awarded by the Royal
Society enabled her to study with
Professor Albert Eschenmoser at the
ETH Z¸rich, after which she returned
to the UK to take up a Lectureship at the University of Warwick
in 1985. In 1990 she moved to Imperial College, London, and
in 1998, she took up the Daniell Chair of Chemistry at King×s
College London. Her research interests are dominated by the
application of transition metals in organic synthesis. Current
projects include the synthesis and application of chiral isonitrile
complexes, the use of the Heck reaction to synthesis amino
acids embedded in macrocycles, transition metals as linkers in
solid-phase chemistry, development of new transition metal
catalysts, including immobilised catalysts, the biological and
catalytic applications of conformationally constrained amino
acids, and the application of chiral base chemistry and
tricarbonylchromium(0) complexes of arenes to natural prod-
uct synthesis and catalyst design.
cyclic product in a very encouraging enantiomeric excess.[11]
A
range of complexes of an isocyanide bearing a pendant
phosphine have been prepared.[12a] A typical example is
provided by the dirhenium complex 7, which was prepared
from the corresponding phosphinimine-phosphine and deca-
carbonyldirhenium(0). The synthesis and reactivity of com-
[*] Members of the Editorial Board will be introduced to readers with their
first manuscript.
270
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Isocyanide Ligands
269 276
Table 1. Synthesis of phosphoramidates 8.
plexes of 2-hydroxyphenyl isocyanide and its derivatives has
been reviewed.[12b]
In view of the paucity of studies to date on the synthesis and
reactivity of complex isocyanide ligands, we have developed a
new synthesis of isocyanide ligands that is designed to provide
relatively straightforward access to such ligands. We describe
herein 1) our new approach to isocyanide ligands, an outline
of which has been published as a preliminary communica-
tion,[13] and 2) how our method can be used to generate
complexes of structurally interesting isocyanides such as
chiral isocyanides.
Product
Yield [%]
method A[a]
Yield [%]
method B[b]
Results and Discussion
89
Our interest in tricarbonyl(vinylketene)iron(0) complexes and
their nitrogen analogues, tricarbonyl(vinylketenimine)iron(0)
complexes, led us to devise a method for the conversion of the
former into the latter, central to which were the anions of
diethyl N-alkylphosphoramidates.[14] We subsequently ques-
tioned whether or not this approach to the conversion of
carbon oxygen bonds into carbon nitrogen bonds could be
used to convert metal carbonyls into metal isocyanides. Given
that phosphoramidates may be synthesised from the corre-
sponding amines in one step, and that many chiral amines are
readily available, it was reasoned that this approach, if
successful, should provide access to a wide range of structur-
ally diverse isocyanide ligands.
60
72
86
74
59
78
86
Five phosphoramidates have been employed in the course
of this study and their preparation is summarised at this point.
The first method that we used for the synthesis of known
phosphoramidates 8a c and novel phosphoramidates 8d
e
involved treating the appropriate amine with diethyl phos-
phite in the presence of carbon tetrachloride.[15, 16, 17] Although
this gave satisfactory yields of the required phosphoramidates
(Table 1, Method A), we were concerned about the use of
environmentally unfriendly carbon tetrachloride. We were
therefore pleased to find that a method using diethyl
chlorophosphate,[18] (a somewhat more expensive reagent
than diethyl phosphite) gave reasonable yields of the phos-
phoramidates (Table 1, Method B).
The first experiment we carried out to test whether or not
metal carbonyls react with anions of phosphoramidates to
give metal isocyanides involved the tert-butyl phosphorami-
date 8a and the iron carbonyl cation 9. The carbonyl-
containing complex 9 was selected for this study as it is
readily synthesised,[19] very stable, and, looking to the future,
16-electron species derived from cations closely related to 9
have been shown to act as Lewis acid catalysts in, for example,
the Diels Alder reaction[20] and the Mukaiyama aldol
reaction.[21] Phosphoramidate 8a was chosen for our initial
exploratory experiment, as successful conversion of a carbon-
yl ligand of 9 to an isocyanide ligand with this phosphorami-
date would produce the known complex 10.[4] Thus phosphor-
amidate 8a was dissolved in THF, cooled to À788C and
treated with one equivalent of n-butyllithium (Scheme 1). A
stirred suspension of one equivalent of cation 9 in THF was
added to this solution, and the resulting mixture was held at
À788C for four hours. Work up of the product mixture
[a] Amine (or hydrochloride salt)/diethyl phosphite/CCl4. [b] Amine/Et3N/
diethyl chlorophosphate.
produced light yellow crystals that were identified as the
dicarbonyl isonitrile complex 10 by comparison of their
1H NMR and IR spectra with literature values.[4] Cation 10
was generated in an acceptable 58% yield on a 1.0 mmol
scale, thus indicating that the phosphoramidate-mediated
conversion of carbonyl ligands to isocyanide ligands is indeed
feasible.
Having established that a phosphoramidate anion may be
used to convert a metal carbonyl ligand into an isocyanide
ligand, we turned our attention to the creation of a chiral
isocyanide ligand by this approach. Pleasingly, reaction of
cation 9 with the anion of phosphoramidate 8b, derived from
commercially available (S)-a-methylbenzylamine, gave the
novel complex 11 containing a chiral isocyanide ligand
(Scheme 1).
Aware that many of the best chiral environments created by
phosphine ligands around metal centres are generated by two
phosphine ligands or diphosphines, we wished to determine
whether or not our carbonyl isocyanide conversion method
could be used to introduce two isocyanide ligands. Initially we
attempted to introduce achiral ethyl and tert-butyl groups in
order to determine to what extent steric bulk affected the
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271

S. E. Gibson et al.
FULL PAPER
Scheme 1.
creation of two isocyanide ligands. Reaction of two equiv-
alents of the anions of phosphoramidates 8c and 8a with
cation 9 gave the novel diisocyanide cation 12 and the known
diisocyanide cation 13[4] in 77% and 23% yield, respectively
(Scheme 1). Interestingly the tert-butylisocyanide containing
cation 10 reacted with the anion of phosphoramidate 8c to
give the novel mixed diisocyanide cation 14 in 63% yield.
From these experiments it was predicted that it should be
possible to create two isocyanide ligands at the iron centre by
using phosphoramidates derived from primary amines attach-
ed to dialkyl- or alkyl/aryl-substituted carbon atoms. Indeed
reaction of cation 9 with two equivalents of anions derived
from phosphoramidates 8d and 8e gave the novel diisocya-
nide complexes 15 and 16 in good yield (Scheme 1). The X-ray
crystal structure of complex 16 revealed the expected ™piano
À
stool∫ geometry (Figure 1). The Fe CNR bond lengths of
272
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Isocyanide Ligands
269 276
acid in THF.[26] Melting points were recorded in
open capillaries on a B¸chi 510 melting point
apparatus, and are uncorrected. IR spectra were
recorded on
a Perkin Elmer 1600 FT IR
spectrometer and in CHCl3 unless otherwise
stated. NMR spectra were recorded at room
temperature on JEOL GSX270, Bruker
AM360 and Bruker DRX400 instruments in
CDCl3 unless otherwise stated. Chemical shifts
are reported in ppm relative to residual undeu-
terated solvent as the reference and J values are
reported in Hz. Mass spectra were recorded on
JEOL AX505W, VG Micromass 7070E and
Kratos MS890MS mass spectrometers, and all
elemental analyses were performed by the
North London University microanalytical serv-
ices. Flash column chromatography was per-
formed over Merck silica gel 60 (230
400 mesh).
Synthesis of phosphoramidates by using meth-
od A
Figure 1. ORTEP view of the cationic part of carbonylcyclopentadienyldi[(S)-1,2,2-trimethylpropyl-
isocyanide]iron(ii) hexafluorophosphate, 16. Ellipsoids at the 50% probability level.
Diethyl N-tert-butylphosphoramidate (8a):[15]
tert-Butylamine (17.9 mL, 170.7 mmol) was add-
ed to a cooled solution of diethyl phosphite
1.855(4) ä (av) are similar to other unhindered isonitrile
complexes (e.g., [Fe(CO)3(CNtBu)2] 1.865 ä av,[22] [Fe2Cp2-
(CO)3(CNCH2CHMe2)] 1.846 ä,[23] and [FeCp(CO)2(CNMe)]-
[BF4] 1.900 ä[24]) indicating an absence of strain at the metal
(10.0 mL, 77.6 mmol) in petroleum ether (200 mL) and CCl4 (120 mL), and
the reaction mixture stirred for 24 h at room temperature. The reaction
mixture was washed with aq. HCl (2m, 2 Â 100 mL), the organic fraction
was dried over MgSO4, and the solvent removed in vacuo to afford 8a as a
À1
À
ˆ
colourless oil (14.5 g, 89%). IR: nÄ ˆ 3398 (N H stretch), 1266 cm (P O);
1H NMR (400 MHz): d ˆ 4.00 (quin, J ˆ 7.2 Hz, 4H; OCH2), 1.31 (dt, J ˆ
1.2, 7.2 Hz, 6H; OCH2CH3), 1.26 (s, 9H; tBu); CI-MS: m/z (%): 227 (20)
[M‡NH4] , 210 (100) [M‡H] .
À
centre. The C-N-C angles are essentially linear and the C N
bond lengths are consistent with a triple-bond description.
Interestingly, the bulky CHMetBu substituents are inter-
locked in such a way as to render the static complex chiral
(Figure 1). This is necessary in order to avoid unfavourable
steric interactions between the substituents and augurs well
for the design of chirality at the metal centre.
‡
‡
Diethyl N-[(S)-a-methylbenzyl]phosphoramidate (8b):[16, 17] (S)-a-Methyl-
benzylamine (8.52 mL, 66.1 mmol), was added to a cooled solution of
diethyl phosphite (7.10 mL, 55.1 mmol) in petroleum ether (150 mL) and
CCl4 (100 mL), and the reaction mixture stirred for 24 h at room
temperature. The solution was washed with HCl (2m, 2 Â 50 mL), the
organic fraction collected and dried over MgSO4, and the solvent removed
in vacuo to afford 8b as a white solid (8.40 g, 60%). [a]3D2 ˆ À51.3 (c ˆ 0.87
Having established that iron cations containing one and two
chiral isocyanide ligands may be created by using phosphor-
amidate anions, we were interested to determine whether or
not the third carbonyl ligand of the tricarbonyl cation 9 could
be replaced with a chiral isocyanide ligand. Attempts to treat
cation 9 with three equivalents of phosphoramidate anions
were unsuccessful. Reaction of cation 16, however, with
trimethylamine-N-oxide led to decarbonylation and the iso-
lation of the triisocyanide complex 17 in 62% yield (theoret-
ical maximum 67%) (Scheme 1).
À
À
in CH2Cl2); m.p. 42 438C; IR: nÄ ˆ 3383 (N H stretch), 1602 (N H bend),
1234 cmÀ1 (P O); H NMR (270 MHz): d ˆ 7.34 7.30 (m, 5H; Ph), 4.32
1
ˆ
4.29 (m, 1H; CH), 4.09 3.64 (m, 4H; OCH2), 3.19 (brs, 1H; NH), 1.47 (dd,
J ˆ 0.9, 6.9 Hz, 3H; CHCH3), 1.30 (dt, J ˆ 0.9, 6.9 Hz, 3H; OCH2CH3), 1.09
(dt, J ˆ 0.9, 6.9 Hz, 3H; OCH2CH3); 13C NMR (100.6 MHz): d ˆ 145.6 (s,
ipso-Ph), 129.1 (s, o-Ph), 127.9 (s, p-Ph), 126.3 (s, m-Ph), 62.7 (d, J ˆ 5 Hz,
OCH2), 62.6 (d, J ˆ 5 Hz, OCH2), 51.9 (s, CH), 25.6 (s, CHCH3), 16.7 (d, J ˆ
8 Hz, OCH2CH3), 16.4 (d, J ˆ 8 Hz, OCH2CH3); EI-MS: m/z (%): 257 (25)
‡
‡
‡
[M] , 242 (100) [M À Me] , 228 (10) [M À CH2CH3] , 120 (25)
‡
‡
[PhCH(Me)NH] , 105 (30) [PhCHMe] .
Diethyl N-ethylphosphoramidate (8c):[17] Ethylamine hydrochloride
(8.15 g, 100 mmol), K2CO3 (27.6 g, 200 mmol), KHCO3 (20.0 g, 200 mmol)
and nBu4NBr (1.61 g, 5 mmol) were added to a cooled solution of diethyl
phosphite (12.9 mL, 100 mmol) in CH2Cl2 (40 mL) and CCl4 (60 mL), and
the reaction mixture stirred for 24 h at room temperature. The solution was
Conclusion
We have demonstrated that phosphoramidates, which are
easy to synthesise from amines, may be used to convert
carbonyl ligands into isocyanide ligands. As a wide variety of
amines are readily available, particularly chiral amines, this
new approach to isocyanide ligands provides access to a wide
range of structurally diverse isocyanide ligands.
filtered, and the solvents removed in vacuo to afford 8c as a pale yellow oil
À1
(13.1 g, 72%); IR: nÄ ˆ 3362 (N H stretch), 1267 cm (P O); 1H NMR
(400 MHz): d ˆ 3.97 (quin, J ˆ 7.2 Hz, 4H; OCH2), 2.86 (t, J ˆ 4.6 Hz, 2H;
NCH2), 1.24 (dt, J ˆ 7.2, 0.7 Hz, 6H; OCH2CH3), 1.06 (dt, J ˆ 7.2, 1.2 Hz,
À
ˆ
‡
‡
3H; NCH2CH3); CI-MS: m/z (%): 199 (30) [M‡NH4] , 182 (100) [M‡H] .
Diethyl N-[(R)-1,2,3,4-tetrahydro-1-naphthyl]phosphoramidate (8d): (R)-
1,2,3,4-Tetrahydro-1-naphthylamine (818 mg, 5.5 mmol) was added to a
cooled solution of diethyl phosphite (0.6 mL, 5.2 mmol) in hexane (10 mL)
and CCl4 (10 mL), and the reaction mixture stirred for 24 h at room
temperature. The solution was filtered, and the solvents removed in vacuo
to afford 8d as a yellow solid (1.084 g, 74%); [a]2D0 ˆ ‡25.5 (c ˆ 0.87 in
Experimental Section
À1
À
ˆ
CH2Cl2); m.p. 61 628C; IR: nÄ ˆ 3362 (N H stretch), 1267 cm (P O);
1H NMR (400 MHz): d ˆ 7.47 6.98 (m, 4H; Ph), 4.28 (m, 1H; CH), 4.08
(m, 4H; OCH2), 2.69 (m, 3H), 2.01 (m, 1H; NH), 1.85 1.68 (m, 3H), 1.34
(m, 6H; OCH2CH3); 13C NMR (400 MHz): d ˆ 138.8 (Ar), 137.5 (Ar), 129.4
(Ar), 129.0 (Ar), 127.5 (Ar), 126.5 (Ar), 62.8 (CH2O), 62.0 (CH2O), 50.4
General: All reactions were performed under an inert atmosphere of dry
nitrogen by using standard vacuum line and Schlenk tube techniques.[25]
THF was distilled over sodium/benzophenone. The concentration of
alkyllithium reagents was determined by titration against diphenylacetic
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S. E. Gibson et al.
FULL PAPER
‡
(CH), 33.0, 29.5, 20.2, 16.7 (CH3,CH2); MS: m/z (%): 283 (89) [M] , 254
(270 MHz): d ˆ 6.12 (s, 5H; Cp); 13C NMR (100.6 MHz): d ˆ 203.2 (CO),
‡
‡
‡
‡
‡
(68) [M À Et] , 154 (98) [M À naphthyl] , 146 (100) [M À 2EtO À PO] ;
elemental analysis calcd (%) for C14H22NO3P (283.29): C 59.30, H 7.80, N
4.94; found: C 59.4, H 7.8, N 4.9.
90.8 (Cp); FAB-MS: m/z (%): 205 (100) [M] , 177 (10) [M À CO] , 140 (5)
‡
[M À Cp] .
Synthesis of monoisocyanide cyclopentadienyliron dicarbonyl complexes
Diethyl N-[(S)-1,2,2-trimethylpropyl]phosphoramidate (8e): (S)-1,2,2-Tri-
methylpropylamine (556 mg, 5.5 mmol) was added to a cooled solution of
diethyl phosphite (0.64 mL, 5.0 mmol) in hexane (10 mL) and CCl4
(10 mL), and the reaction mixture stirred for 24 h at room temperature.
The solution was washed with HCl (2m, 2 Â 50 mL), the organic fraction
was collected and dried over MgSO4, and the solvent removed in vacuo to
afford 8e as a white solid. Crystallisation from hexane afforded white
(tert-Butylisocyanide)dicarbonylcyclopentadienyliron(ii) hexafluorophos-
phate (10):[4] A solution of 8a (209 mg, 1.00 mmol) in THF (10 mL) was
cooled to À788C and nBuLi (0.63 mL, 1.00 mmol of 1.6m solution in
diethyl ether) was added to it. The reaction mixture was allowed to warm to
room temperature before recooling to À788C. A suspension of 9 (350 mg,
1.00 mmol) in degassed THF (30 mL) was added through a cannula to the
reaction mixture. The mixture was then stirred at À788C for 4 h and the
solvent removed in vacuo. The dark brown residue was purified by
chromatography (SiO2; acetone/petroleum ether 1:1) to afford a yellow
solid. Crystallisation (acetone/dichloromethane) afforded 10 as cubic
20
crystals (0.696 g, 59%); [a]D ˆ ‡ 27.7 (c ˆ 0.87, CH2Cl2); m.p. 42 438C;
À1
IR: nÄ ˆ 3368 (N H stretch), 1267 cm (P O); 1H NMR (400 MHz): d ˆ
3.97 (m, 4H; OCH2), 2.86 (m, 1H; CH), 2.23 (t, J ˆ 9.7 Hz, 1H; NH), 1.24
(dt, J ˆ 7.2, 0.9 Hz, 6H; OCH2CH3), 1.06 (d, J ˆ 6.6 Hz, 3H; CH3-CH), 0.82
(s, 9H; C(CH3)3); 13C NMR (400 MHz): d ˆ 62.6 (OCH2), 56.7 (CH), 35.0
(C(CH3)3), 26.5 (C(CH3)3), 18.7 (CH3CH), 16.7 (CH3CH2); CI-MS: m/z
À
ˆ
ꢀ
yellow crystals (235 mg, 58%). M.p. 154 1558C; IR: nÄ ˆ 2206 (C N),
1
2081, 2040 cmÀ1 (C O); H NMR (270 MHz): d ˆ 5.38 (s, 5H; Cp), 1.60 (s,
ꢀ
‡
‡
9H; CMe3); 13C NMR (100.6 MHz) d ˆ 205.8 (CO), 87.0 (Cp), 61.1 (CMe3),
(%): 255 (11) [M‡NH4] , 238 (100) [M‡H] ; elemental analysis calcd (%)
‡
‡
29.4 (CMe3); FAB-MS: m/z (%): 260 (100) [M] , 204 (30) [M À 2CO] .
for C10H24NO3P (237.27): C 50.62, H 10.10, N 5.90; found: C 50.6, H 10.2, N
5.8.
Dicarbonylcyclopentadienyl[(S)-a-methylbenzylisocyanide)iron(ii) hexa-
fluorophosphate (11): A solution of 8b (257 mg, 1.00 mmol) in THF
(10 mL) was cooled to À788C and nBuLi (0.63 mL, 1.00 mmol of 1.6m
solution in diethyl ether) was added to it. The reaction mixture was allowed
to warm to room temperature before recooling to À788C. A suspension of
9 (350 mg, 1.00 mmol) in degassed THF (30 mL) was added through a
cannula to the reaction mixture. The mixture was then stirred at À788C for
6 h and solvent removed in vacuo. The dark brown residue was purified by
chromatography (SiO2; acetone/petroleum ether 1:2) to yield an orange oil.
Crystallisation (dichloromethane/diethyl ether 1:1) afforded 11 as yellow
crystals (250 mg, 55%). [a]D29 ˆ À2.26 (c ˆ 1.55 in CH2Cl2); M.p. 62 638C;
Synthesis of phosphoramidates by using method B
Diethyl N-[(S)-a-methylbenzyl]phosphoramidate (8b):
Dry dichloromethane (15 mL) was added to (S)-a-methylbenzylamine
(0.61 g, 5 mmol) and dry triethylamine (2.09 mL, 15 mmol), and the
solution was cooled to À108C in an ice/sodium chloride bath. Diethyl
chlorophosphate (0.76 mL, 5.25 mmol) was added dropwise, and the
solution was left in the cooling bath to reach room temperature overnight.
After 14 h, the precipitate was filtered off and washed with diethyl ether.
The solvents were removed in vacuo and the residue purified by
chromatography (SiO2; diethyl ether/hexane/triethylamine 80:17:3) to give
8b as a white solid (1.10 g, 86%). Data corresponded with those obtained
from 8b generated by using method A.
À1
1
ꢀ
ꢀ
IR: nÄ ˆ 2214 (C N), 2082, 2041 cm (C O); H NMR (270 MHz): d ˆ 7.48
(m, 5H; Ph), 5.81 (s, 5H; Cp), 5.54 (q, J ˆ 6.8 Hz, 1H; CHMe), 1.81 (d, J ˆ
6.8 Hz, 3H; Me); 13C NMR (100.6 MHz): d ˆ 206.3 (CO), 137.8 (ipso-Ph),
129.8 (o-/m-Ph), 129.4 (p-Ph), 126.0 (o-/m-Ph), 87.8 (Cp), 59.2 (CHMe),
Diethyl N-[(R)-1,2,3,4-tetrahydro-1-naphthyl]phosphoramidate (8d): Dry
dichloromethane (15 mL) was added to (R)-1,2,3,4-tetrahydronaphthyl-
amine (740 mg, 5 mmol) and dry triethylamine (2.09 mL, 15 mmol), and the
solution was cooled to À108C in an ice/sodium chloride bath. Diethyl
chlorophosphate (0.76 mL, 5.25 mmol) was added dropwise and the
solution left in the cooling bath to reach room temperature overnight.
After 14 h, the precipitate was filtered off and washed with diethyl ether.
The solvents were removed in vacuo, and the residue was purified by
chromatography (SiO2; diethyl ether/hexane/triethylamine 80:17:3) to give
8d as a white solid (1.11 g, 78%). Data corresponded with those obtained
from 8d generated by using method A.
‡
‡
23.9 (Me); FAB-MS: m/z (%): 308 (100) [M] , 280 (5) [M À CO] , 252 (15)
‡
‡
[M À 2CO] , 204 (20) [FpCNH] ; HRMS calcd for C16H14NF6FeO2P:
308.0374; found: 308.0358.
Synthesis of diisocyanide cyclopentadienyliron dicarbonyl complexes
Carbonylcyclopentadienyldi(ethylisocyanide)iron(ii) hexafluorophosphate
(12): Phosphoramidate 8c (362 mg, 2.00 mmol) was dissolved in THF
(10 mL), the solution degassed and cooled to À788C. nBuLi (1.6m in
hexane, 1.25 mL, 2.00 mmol) was added dropwise, and the mixture allowed
to warm to room temperature before recooling to À788C. A suspension of
9 (350 mg, 1.00 mmol) in degassed THF (20 mL) was cooled to À788C and
added through a cannula to the phosphoramidate anion solution. The
mixture was stirred at À788C for 6 h, and solvent removed in vacuo. The
dark brown residue was dissolved in acetone, and preadsorbed onto silica
gel before purification by column chromatography (SiO2; acetone/light
petroleum 1:2) to afford 12 as a yellow oil (310 mg, 77%). IR: nÄ ˆ 2218,
Diethyl N-[(S)-1,2,2-trimethylpropyl]phosphoramidate (8e): Dry dichloro-
methane (15 mL) was added to (S)-1,2,2-trimethylpropylamine (510 mg,
5 mmol) and dry triethylamine (2.09 mL, 15 mmol), and the solution was
cooled to À108C in an ice/sodium chloride bath. Diethyl chlorophosphate
(0.76 mL, 5.25 mmol) was added dropwise, and the solution left in the
cooling bath to reach room temperature overnight. After 14 h, the
precipitate was filtered off and washed with diethyl ether. The solvents
were removed in vacuo, and the residue purified by chromatography (SiO2;
diethyl ether/hexane/triethylamine 80:17:3) to give 8e as a white solid
(1.02 g, 86%). Data corresponded with those obtained from 8e generated
by using method A.
À1
1
ꢀ
ꢀ
2191 (C N), 2021 cm (C O); H NMR (270 MHz): d ˆ 5.32 (s, 5H; Cp),
3.91 (q, J ˆ 7.2 Hz, 4H; CH2), 1.42 (t, J ˆ 7.2 Hz, 6H; Me); 13C NMR
(100.6 MHz): d ˆ 213.8 (CO), 86.2 (Cp), 42.0 (NCH2), 15.5 (Me).
Di(tert-butylisocyanide)carbonylcyclopentadienyliron(ii) hexafluorophos-
phate (13):[4] Phosphoramidate 8a (500 mg, 2.39 mmol) was dissolved in
THF (10 mL), and the solution degassed and cooled to À788C. MeLi
(1.35m in hexane, 1.81 mL, 2.44 mmol) was added dropwise, and the
mixture allowed to warm to room temperature before recooling to À788C.
A suspension of 9 (400 mg, 1.14 mmol) in degassed THF (40 mL) was
cooled to À788C and added through a cannula to the phosphoramidate
anion solution. The mixture was stirred at À788C for 5 h, and then allowed
to warm to room temperature for a further 19 h, before solvent was
removed in vacuo. The dark brown residue was dissolved in acetone and
preadsorbed onto silica gel before purification by column chromatography
(SiO2; acetone/petroleum ether 1:2). The product was crystallised from
dichloromethane to afford 13 as yellow crystals (120 mg, 23%). M.p. 123 À
Synthesis of iron the tricarbonyl substrate
Tricarbonylcyclopentadienyliron(ii) hexafluorophosphate (9):[18] A solution
of dicarbonylcyclopentadienyliron(ii) dimer (4.0 g, 11.3 mmol) in THF
(200 mL) was added to 2% Na/Hg, and the mixture was allowed to stir at
room temperature for 24 h. The supernatent liquid was carefully trans-
ferred into a Schlenk flask and the solution cooled to À788C. Ethyl
chloroformate (10.0 mL, 104 mmol) was added dropwise, and the reaction
mixture was stirred for 1 h at À788C and slowly warmed to room
temperature. Solvent was removed in vacuo, and the resultant solid
triturated with petroleum ether (4 Â 40 mL) to yield a yellow solution. HCl
gas was then bubbled through the solution for a period of 20 minutes at
room temperature; this resulted in the precipitation of a yellow solid. This
was dissolved in MeOH (40 mL) and added to NH4PF6 (3.7 g, 22.7 mmol).
After stirring at room temperature for 1 h, the solvent was evaporated and
À1
1
ꢀ
ꢀ
1258C; IR: nÄ ˆ 2197, 2171 (C N), 2021 cm (C O); H NMR (300 MHz):
d ˆ 5.24 (s, 5H; Cp), 1.45 (s, 18H; 2 CMe3); 13C NMR (75.4 MHz): d ˆ 206.5
(CO), 86.4 (Cp), 61.5 (CMe3), 30.7 ( CMe3); FAB-MS: m/z (%): 315 (100)
‡
‡
‡
‡
‡
the orange solid was crystallised from acetone/diethyl ether (6.24 g, 81%).
[M] , 287 (25) [M À CO] , 259 (5) [MH À CMe3] , 231 (5) [MH À CO À
À1
‡
‡
ꢀ
M.p. >2508C (decomp); IR: nÄ ˆ 2124, 2074 (C O)cm
;
1H NMR
CMe3] , 148 (15) [CpFeCNH] ; elemental analysis calcd (%) for
274
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0801-0274 $ 17.50+.50/0 Chem. Eur. J. 2002, 8, No. 1

Isocyanide Ligands
269 276
C16H23N2F6FeOP (477.97): C 41.76, H 5.04 N 6.09; found: C 41.58, H 5.02, N
5.89.
correction. The structure was solved using SHELXS-97[29] and refined by
alternating least-squares cycles and difference Fourier synthesis
(SHELXL-97[29]) with the aid of the program XSeed.[30] In general all
non-hydrogen atoms were modelled anisotropically, while hydrogen atoms
were assigned an isotropic thermal parameter 1.2 times that of the parent
atom (1.5 for terminal atoms) and allowed to ride, except for acidic protons
which were located on the final difference Fourier map and refined freely.
All calculations were carried out with either a Silicon Graphics Indy
workstation or an IBM compatible PC. Crystallographic data (excluding
structure factors) for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as supple-
mentary publication no. CCDC-168587. Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
(tert-Butylisocyanide)carbonylcyclopentadienyl(ethylisocyanide)iron(ii)
hexafluorophosphate 14: Diethyl N-ethylphosphoramidate 8c (92 mg,
0.51 mmol) was dissolved in THF (10 mL), and the solution was degassed
and cooled to À788C. nBuLi (1.6m in hexane, 0.32 mL, 0.51 mmol) was
added dropwise, and the mixture allowed to warm to room temperature
before recooling to À788C. A solution of 10 (205 mg, 0.51 mmol) in
degassed THF (10 mL) was cooled to À788C and added through a cannula
to the phosphoramidate anion solution. The mixture was stirred at À788C
for 7 h, and solvent removed in vacuo. The dark brown residue was
dissolved in acetone and preadsorbed onto silica gel before purification by
column chromatography (SiO2; acetone/petroleum ether 1:2). The product
was subsequently triturated (diethyl ether/petroleum ether 1:1) to afford 14
as a yellow powder (138 mg, 63%). M.p. 126 À 1288C; IR: nÄ ˆ 2212, 2179
Crystal data for 16: C20H31F6FeN2OP, Mr ˆ 516.29 gmolÀ1, orthorhombic,
À1
1
ꢀ
ꢀ
P212121,
a ˆ 11.3149(8),
b ˆ 12.1826(11),
c ˆ 17.9756(10) ä,
Vˆ
(C N), 2021 cm (C O); H NMR (270 MHz): d ˆ 5.33 (s, 5H; Cp), 3.92
(q, J ˆ 7.2 Hz, 2 H ; CH2), 1.54 (s, 9H; CMe3), 1.41 (t, J ˆ 7.2 Hz, 3H; Me);
13C NMR (100.6 MHz): d ˆ 213.5 (CO), 86.2 (Cp), 60.9 (CMe3), 41.9
2477.8(3) ä3, Z ˆ 4, MoKa, l ˆ 0.71073, 1calcd 1.384 MgmÀ3, m ˆ 0.731mmÀ1
,
max/min transmission 0.8105/0.7585, Tˆ 120(2) K, crystal size 0.40 Â
0.30 Â 0.30 mm3, theta range 2.71 27.498. Reflections collected: 12680,
independent reflections: 5634, parameters 289, R1 [I > 2s(I)] ˆ 0.0577, wR2
‡
(NCH2), 29.5 (CMe3), 15.4 (Me); FAB-MS: m/z (%): 287 (100) [M] , 259
‡
‡
‡
(30) [M À CO] , 231 (5) [CpFe(CNtBu)CNH] , 176 (10) [CpFeCNEt] ,
(F 2, all data) ˆ 0.1495, largest diff. peak/hole: 0.674/ À 0.660 eäÀ3
.
‡
148 (10) [CpFeCN] ; HRMS calcd for C14H19N2F6FeOP: 287.0847; found:
287.0853.
Synthesis of triisocyanide cyclopentadienyliron complex
Carbonylcyclopentadienyldi[(R)-1,2,3,4-tetrahydro-1-naphthylisocyanide)-
iron(ii) hexafluorophosphate (15): A solution of 8d (283 mg, 1 mmol) in
THF was cooled to À788C and MeLi (0.62 mL, 1 mmol of 1.6m solution in
diethyl ether) was added to it. The reaction mixture was allowed to warm to
room temperature before recooling to À788C. The solution was then
transferred through a cannula into a suspension of 9 (187 mg, 0.5 mmol) in
degassed THF. The mixture was then stirred at À788C for 6 h, after which
the reaction was quenched with MeOH and the solvent removed in vacuo.
The resultant dark brown residue was purified by chromatography (SiO2;
acetone/hexane 1:2) to yield a yellow solid. Crystallisation from diethyl
Cyclopentadienyltri[(S)-1,2,2-trimethylpropylisocyanide]iron(ii)
fluorophosphate (17):
hexa-
A solution of 16 (51.6 mg, 0.1 mmol) in THF was cooled to À308C and
added dropwise to a suspension of trimethylamine-N-oxide (30.0 mg,
0.4 mmol) in THF. The reaction mixture was stirred at À308C for 2 h and
then allowed to rise to room temperature. The solvent was removed in
vacuo, and the residue was extracted with chloroform and washed with
water. The organic layer was dried over Na2SO4. Removal of the solvent in
vacuo gave 17 as a yellow solid [37 mg, 62% based on 16 (67% theoretical
maximum)]; [a]2D0 ˆ ‡19.3 (c ˆ 0.15 in CH2Cl2); m.p. 111 1138C; IR
1
(CHCl3): nÄ ˆ 2187, 2145 cmÀ1 (C N); H NMR (400 MHz): d ˆ 4.77 (s, 5H;
ether afforded 15 as yellow crystals (250 mg, 78%). M.p. >2858C; IR: nÄ ˆ
ꢀ
À1
2180, 2146 (C N), 2021 cm (C O); 1H NMR (400 MHz): d ˆ 7.32 7.03
(m, 8H; Ar), 5.21 (q, 2H; J ˆ 5.5 Hz, CH), 5.02 (s, 5H; Cp), 2.79 2.62 (m,
4H), 2.13 2.01 (m, 4H), 1.84 1.73 (m, 4H); 13C NMR (CDCl3,
100.6 MHz): d ˆ 211.6 (CO), 136.9, 131.7, 131.75, 130.1, 129.2, 128.9,
128.8, 127.2 (Ar), 85.3 (Cp), 57.1 (CH), 30.5 (CH2), 28.8 (CH2), 19.5
ꢀ
ꢀ
Cp), 3.76 (q, J ˆ 6.8 Hz, 3H; CH), 1.33 (d, J ˆ 6.8 Hz, 9H; CH3), 0.99 (s,
27H; C(CH3)3); 13C NMR (100.6 MHz): d ˆ 81.9 (Cp), 63.8 (CH), 34.8
‡
(C(CH3)3), 25.6 (C(CH3)3), 16.6 (CH3); MS: m/z (%): 454 (100) [M À PF6] ,
‡
343 (57) [M À PF6 À CNCH(CH3)C(CH3)3] , 232 (27) [M À PF6 À
‡
CNCH(CH3)C(CH3)3] ; elemental analysis calcd (%) for C38H38N3F6FeP
‡
‡
(CH2); MS: m/z (% ˆ 548 (89) [M] , 520 (100) [M À CO] .
(737.55): C 52.09, H 7.40, N 7.01; found: C 52.0, H 7.5, N 6.9.
Carbonylcyclopentadienyldi[(S)-1,2,2-trimethylpropylisocyanide]iron(ii)
hexafluorophosphate (16): A solution of 8e (237 mg, 1 mmol) in THF was
cooled to À788C and MeLi (0.62 mL, 1 mmol of 1.6m solution in diethyl
ether) was added to it. The reaction mixture was allowed to warm to room
temperature before recooling to À788C. The solution was then transferred
through a cannula into a suspension of 9 (187 mg, 0.5 mmol) in degassed
THF. The mixture was then stirred at À788C for 7 h, after which the
reaction was quenched with MeOH and the solvent removed in vacuo. The
resultant dark brown residue was purified by chromatography (SiO2;
acetone/hexane 1:2) to yield an orange oil. Crystallisation from diethyl
Acknowledgement
CP and JMR wish to thank the EPSRC for a post-doctoral fellowship and
studentship, respectively, and HIand SS wish to thank King×s College
London for a studentship and post-doctoral fellowship, respectively. We
thank the EPSRC and King×s College London for the provision of the
X-ray diffractometer and the Nuffield Foundation for the provision of
computing equipment.
ether afforded 16 as yellow crystals (173 mg, 67%). [a]2d0 ˆ ‡12.6 (c ˆ 1.55
À1
ꢀ
in CH2Cl2); m.p. 274 2758C; IR (CHCl3): nÄ ˆ 2202, 2178 (C N), 2021 cm
1
ꢀ
(C O); H NMR (400 MHz): d ˆ 5.08 (s, 5H; Cp), 3.79 (q, 2H; J ˆ 6.7 Hz,
CHMe), 1.30 (t, 6H; J ˆ 6.7 Hz, CHCH3), 0.92 (s, 18H; tBu); 13C NMR
(100.6 MHz): d ˆ 211.0 (CO), 84.4 (Cp), 64.2 (CH), 34.3 (C(CH3)3), 25.2
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‡
(C(CH3)3), 15.8 (CHCH3); MS: m/z (%): 516 (59) [M] , 488 (100) [M À
‡
CO] ; elemental analysis calcd (%) for C20H31N2F6FeOP (516.29): C 46.50,
H 6.05, N 5.45; found: C 46.3, H 5.9, N 5.7.
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Received: August 20, 2001 [F3498]
276
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0801-0276 $ 17.50+.50/0
Chem. Eur. J. 2002, 8, No. 1