Bridged (β-alkoxyalkyl)CoIII(salen) complexes by intramolecular alkoxycobaltation of unactivated alkenes: New models for coenzyme B12

Article abstract of DOI:10.1039/b000196l

Co^II(salen) derivatives (salen = {N,N′-ethylenebis[salicylideneaminato]}) whose ethanediyl moiety carries an alkenyl side-chain R [R = prop-2-en-1-yl (6a), 2-methylprop-2-en-1-yl (6b), but-2-en-1-yl (6c), but-3-en-1-yl (6d)] react with oxygen a

Full text of DOI:10.1039/b000196l

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Bridged (ꢀ-alkoxyalkyl)CoIII(salen) complexes by intramolecular
alkoxycobaltation of unactivated alkenes: new models for
coenzyme B12
1
Rolf Blaauw,a Irene E. Kingma,a Jan H. Laan,a Juul L. van der Baan,*a Sijbe Balt,a Martinus
W. G. de Bolster,a Gerhard W. Klumpp,a Wilberth J. J. Smeetsb and Anthony L. Spekb
a Scheikundig Laboratorium, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam,
The Netherlands
b Bijvoet Centrum voor Biomoleculaire Research, Universiteit Utrecht, Padualaan 8,
3584 CH Utrecht, The Netherlands
Received (in Cambridge, UK) 11th January 2000, Accepted 23rd February 2000
CoII(salen) derivatives (salen = {N,NЈ-ethylenebis[salicylideneaminato]}) whose ethanediyl moiety carries an
alkenyl side-chain R [R = prop-2-en-1-yl (6a), 2-methylprop-2-en-1-yl (6b), but-2-en-1-yl (6c), but-3-en-1-yl (6d)]
react with oxygen and alcohols to give organocobalt() complexes containing a β-alkoxy-substituted three- or
four-carbon bridge between cobalt and the equatorial ligand. NMR and UV–VIS spectroscopic studies show that
product formation is a three-stage process involving (1) oxidation of cobalt() to produce an (alkoxo)cobalt()
complex, (2) intramolecular interaction of cobalt() with the alkenyl double bond to yield a carbocationic
intermediate, and (3) nucleophilic attack by the alcohol. In the case of cobalt() complex 6e (R = 3-methylbut-3-
en-1-yl), the major product is bridged β-methylene organocobalt() complex 10, demonstrating that proton loss
competes with addition of alcohols when the intermediate organocobalt() species has a substantial degree of
tertiary carbocation character. Application of the alkoxycobaltation reaction to 6d and ethane-1,2-diol afforded
bridged [β-(2-hydroxyethoxy)alkyl]Co(salen) complex 20, a simple model for coenzyme B12 with a built-in substrate.
The molecular structure of 20 has been determined by X-ray diffraction methods.
crystal structures of methylmalonyl-CoA mutase range from 6
Introduction
to 7 Å.5
For more than three decades now, the chemistry of coenzyme
We are interested in the formation and subsequent cleavage
B12-dependent (B12 = 5Ј-deoxyadenosylcobalamin) enzymatic
1,2-rearrangements has fascinated researchers in many fields of
chemistry. It is generally accepted that substrate rearrangement
is initiated by H-abstraction to give a substrate radical, followed
by the intramolecular migration of an electronegative group to
a neighbouring carbon atom and recapture of a hydrogen atom.
The role that cobalt–substrate interactions may play in the
actual mechanism of this migration is poorly understood. A
potentially useful strategy to study the importance of such
interactions is to force substrates, or models thereof, to stay in
close proximity to cobalt in order to see whether reactivity can
be induced which is otherwise not, or rarely, observed. Recent
studies by Murakami et al. have shown that the efficiency of
substrate rearrangement can be enhanced during photolysis of
a hydrophobic organocobalamin derivative by enclosing it
in a single-walled vesicle that acts as an artificial holoenzyme.1
Keese et al. found that rearrangement of methylmalonyl-CoA
in a protic solvent becomes an efficient process when both the
vitamin B12-derived catalyst and the substrate carry a long alkyl
chain.2 A considerable amount of methylsuccinic acid was
isolated by Rétey et al. after photolysis and saponification of a
methylmalonate bridged cobaloxime.3 These studies demon-
strate that one of the important functions of the holoenzyme
may be to keep the substrate close to the cobalt centre of the
coenzyme during reaction, thereby moderating the reactivity of
radical intermediates and allowing cobalt to assist in group
migration. Indeed, recent EPR studies of the B12-dependent
carbon skeleton rearrangement of glutamate to methyl-
aspartate have demonstrated ‘communication’ between CoII
and the 4-glutamyl radical over a distance of ca. 6 Å.4 This
accords with the finding that distances from Co to the putative
radical centres on the substrate or substrate analogues in
of the cobalt–carbon bond in model complexes6 for B12 in
which an alkenyl substituent is attached to the equatorial ligand
and thus forced to stay in close proximity to the cobalt centre
during reactions. The reaction of cobalt species with unacti-
vated alkenes is very unusual in the chemistry of B12 and struc-
tural models thereof. Generally, only alkenes with activat-
ing substituents are reactive towards cobalt(), cobalt(), or
cobalt() complexes to give, depending on the reaction condi-
tions, α- or β-substituted organocobalt derivatives7 or oxid-
ation products such as ketones and secondary alcohols.8 On the
other hand, alkenes are often formed by β-hydrogen elimination
after Co–C bond homolysis or heterolysis of B12 and deriva-
tives. Furthermore, alkenes, whether or not interacting with Co
species, have recently regained interest as potentially important
intermediates in B12-catalysed rearrangements.4,9 In order to
investigate whether the presence of cobalt in close proximity to
a C᎐C double bond enhances their mutual reactivity, we have
᎐
synthesized CoII(salen) derivatives whose ethanediyl moiety
carries an alkenyl side-chain. Here, we describe the ability of
these complexes to form cobalt–carbon bonds upon aeration
in alcoholic media. The products, bridged (β-alkoxyalkyl)-
CoIII(salen) complexes, are structural mimics of coenzyme B12.
Synthetic, structural, as well as mechanistic details will be pre-
sented. A preliminary report of part of this work has been
published.10
Results and discussion
Preparation of CoII(salen) complexes 6a–e with a pendant alkenyl
arm
H2salen ligands required for the synthesis of CoII(salen) deriv-
DOI: 10.1039/b000196l
J. Chem. Soc., Perkin Trans. 1, 2000, 1199–1210
This journal is © The Royal Society of Chemistry 2000
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Co(OAc)2 in hot deaerated THF.‡ Evaporation of the
solvent and repeated washings with deaerated water and diethyl
ether, respectively, furnished 6a–e as air-sensitive red-brown
paramagnetic solids which were identified by the 1H NMR
spectra of the corresponding diamagnetic iodocobalt()
derivatives after in situ conversion with I2 in DMSO-d6 or
pyridine-d5.§
Oxygenation of cobalt(III) complexes 6a–e in alcohols
Red solutions of CoII(salen) derivatives 6a–e in methanol
quickly turn brown upon exposure to air. Further changes then
follow, the course of which depends on the structure of the
alkenyl side chain. For 6a–d, the brown solutions eventually
turn dark red. UV–VIS spectral changes that accompany the
oxidation process are quite similar in shape and extinction, and
indicate the rapid conversion of a CoII(salen) into a transient
CoIII(salen) species, followed by a further slower reaction that
eventually leads to the spectrum characteristic of (alkyl)-
CoIII(salen) compounds.15 Starting material has disappeared
completely after ca. 3 hours (6a), 14 days (6b,c) or 8 hours (6d),
respectively.
Red diamagnetic solids were obtained after solvent evapor-
ation and purification by column chromatography (Al2O3, 10%
MeOH in CH2Cl2) or preparative TLC (silica, 5% MeOH in
CHCl3). In non-coordinating solvents like chloroform or
dichloromethane, these red products dissolve to give an intense
green colour and display UV–VIS absorption at ca. 650 nm
(CH2Cl2, ε = 1.6 × 103 dm3 molϪ1 cmϪ1) which is characteristic
for five-coordinate (alkyl)CoIII(salen) complexes.6,15 The green
solids, obtained after extensive drying of the red products in
vacuo, were dissolved in CDCl3 and identified by correlated
NMR spectroscopic techniques as bridged (β-methoxyalkyl)-
CoIII(salen) complexes 7–9 (Scheme 2).
Interestingly, no signals of diastereomers are observed in the
NMR spectra of any of the bridged complexes except 7b,
indicating that the addition of cobalt and a methoxy group to
the C᎐C double bond (methoxycobaltation) is a stereospecific
᎐
process. For complex 7b, two diastereomers I and II (Fig. 1) are
formed in a ca. 1:2 ratio.
In order to determine the bridge conformations of the com-
plexes 7a–c, the Karplus equation was applied to the vicinal
coupling constants of H(9) with H(17a) and H(17b), respect-
ively (Fig. 1). From the scalar values of the coupling constants
it was concluded that compounds 7a, 7b(I), and 7c have a
geometry in which the Co–N–C(9)–C(17)–C(18)–C(19) ring
adopts a chair-like conformation, while the corresponding six-
membered ring in diastereomer II of 7b has a boat-like con-
formation. NOE experiments were carried out to reveal the
configuration around the β-carbon atom of the bridge. In com-
plexes 7a and 7c, the OCH3 substituent occupies the pseudo-
equatorial position i.e. OCH3 is antiperiplanar with respect to
cobalt. By contrast, it is the methyl group that takes this pos-
ition in 7b(I) and 7b(II). In both I and II, the OCH3 substituent
is proximal to the equatorial salen ligand, probably because the
OCH3-oxygen atom is less bulky than the CH3 substituent. This
assignment is further corroborated by the anisotropic highfield
shift of the OCH3-protons (δ 2.67 and 2.68 ppm for I and II,
respectively) compared to those of 7a and 7c (δ 3.33 and 3.17
Scheme 1 Reagents and conditions: i, PhCHO, Et3N, CH2Cl2, MgSO4;
ii, Method I (a, b): R1CH᎐CHR2CH2OC(O)OEt, (Ph3P)4Pd(0), THF;
᎐
Method II (c, d, e): 1) LiN(i-Pr)2, THF, HMPA, Ϫ60 ЊC, 2) R1CH᎐
᎐
CHR2(CH2)nBr, RT; iii, 1.5 M HCl; iv, NH3, MeOH; v, LiAlH4, THF,
55 ЊC; vi, salicylaldehyde, EtOH, 60 ЊC; vii, Co(OAc)2, THF, 60 ЊC.
atives are generally prepared by condensation of salicylalde-
hyde with the appropriate vicinal diamines, which, in their turn,
are available via several routes. 1,2-Diaminoalkenes 4 used for
the construction of CoII(salen) complexes 6 are conveniently
prepared in a five-step procedure (without purification of the
intermediates) in which alkylation of methyl N-benzylidene-
glycinate, either by Pd(0)-catalysed reaction with allyl carbon-
ates11 (→6a,b) or by reaction of the (LDA-generated) lithium
enolate with bromoalkenes12 (→6c,d,e), is the crucial step
(Scheme 1). The crude alkylation products 1 (which in the case
of 1c,d,e contained varying amounts of dialkylation products)†
were hydrolysed to aminoesters 2 and then converted to amino
amides 3 by NH3–MeOH. Reduction with a large excess of
LiAlH4 in THF13 afforded the 1,2-diamines 4, which could
easily be purified by distillation. Heating of 4 with two equiv-
alents of salicylaldehyde in ethanol14 gave H2salen ligands 5 as
very viscous liquids in high yield and reasonable purity. Since
purification by chromatography often led to partial decom-
position, the crude products were used in the final conversion
of 5 to the desired CoII(salen) complexes 6 by reaction with
‡ Previously,6,10 we had used DMF, which was difficult to remove.
Solubility of Co(OAc)2 is sufficiently high in boiling THF to accom-
plish complete complexation.
§ Although, in principle, the formation of two diastereomeric ICoIII
-
(salen) derivatives is expected in this reaction, the in situ 1H NMR
spectra in DMSO-d6 and pyridine-d5 show the presence of only one
species, probably because of extensive dissociation of the Co–I bond
in these solvents. In chloroform-d1, the complexes are paramagnetic.
However, after evaporation of this solvent and dissolution in DMSO-d6
or pyridine-d5, the original spectra are restored (cf. 18 and 19 in Scheme
6 and accompanying text).
† Preparation of 1c was also tried via Pd(0)-catalysed alkylation, but
in that case also a considerable amount of an isomer that was difficult
to separate was formed by reaction at C-3 of the but-2-en-1-yl moiety
of the starting material.
1200
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like conformation of the Co–N–C(9)–C(17)–C(18)–C(19) ring
with a pseudo-equatorial methoxymethyl group and a zigzag
orientation from C(17) to C(20) is considered most likely. Its
1H NMR spectrum displays a specific highfield signal at Ϫ0.46
ppm, similar to the signal at Ϫ0.49 ppm reported for an (alkyl)-
Co(salen) complex with a trimethylene bridge,6 and assigned
unambiguously by NOE experiments to the pseudo-equatorial
β-proton of the bridge [H(18)] which is antiperiplanar with
respect to cobalt.
Oxidation of cobalt() complex 6e in methanol follows a
somewhat different course. During the reaction a green-brown
precipitate forms as the major product which was identified by
NMR (see Experimental section) as bridged (β-methylene-
butyl)Co(salen) complex 10 (Scheme 3). Minor amounts of one
Fig. 1 Presentations of parts of 7a–c showing bridge conformations
and configurations around C(18), Newman projections along C(17)–
C(9) and vicinal coupling constants of 7b(I) and 7b(II) (CH3 and OCH3
are omitted for clarity).
Scheme 3
or more bridged (β-methoxyalkyl)cobalt() complexes are also
1
formed, as suggested by characteristic signals in the H NMR
spectrum (for instance, a signal at Ϫ0.42 ppm, indicative of a
bridged (α-methoxymethylalkyl)cobalt() derivative similar to
9). The formation of 10 demonstrates that proton loss can
compete with methanol addition.
Initially, it was assumed that a similar product would not be
formed upon oxidation of 6b because of too much strain. How-
ever, when 6b was oxidised in methanol-d4 containing a small
amount of deuterium chloride ([Dϩ] = 10Ϫ5 M), a precipitate
slowly formed, which was separated by filtration and identified
by NMR as bridged (β-methylenepropyl)Co(salen) complex 11
(Scheme 3). The filtrate contained the two diastereomers 7b(I)
and (II).
The intramolecular alkoxycobaltation is not limited to
methanol; (β-oxyalkyl)cobalt()salen complexes with a bridge
geometry similar to 7a are obtained when 6a is oxidised in e.g.
ethanol, phenol (20 equiv. in THF), water (H2O–THF = 1:1),
or ethane-1,2-diol (50% in THF, vide infra). Unidentified
Scheme 2
ppm, respectively). Ring current effects cause the H(18)-signal
of 7a and 7c to be shifted to higher field (δ 3.25 and 2.82 ppm,
respectively). For complex 8, separated from its isomer 9 by
chromatography (8:9 ca. 2:1), the antiperiplanar orientation
of the β-OCH3 substituent and cobalt, and a zigzag bridge con-
formation is confirmed by its crystal structure reported earlier.10
The exact geometry of complex 9 was not determined; however,
in view of the results mentioned above, a structure with a chair-
J. Chem. Soc., Perkin Trans. 1, 2000, 1199–1210
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2,2,2-trichloroethanol-d1. Very probably, the trichloroethoxy
group of 13 is rapidly exchanged by methoxy to give 12a which
then cyclises to 7a (Scheme 4).
Fig. 2
paramagnetic material was obtained when propan-2-ol or tert-
butyl alcohol were used as reaction solvents.¶
Mechanistic observations
Mechanistic experiments mainly focused on the methoxy-
cobaltation of 6a, since this reaction is sufficiently fast and
furnishes only one intramolecularly bridged product (7a) in
high yield.
Analogous to the oxygenation of CoII(salen), the first step
of the reaction of 6a with O2 is very probably the formation of
the corresponding µ-peroxo- and/or superoxocobalt() com-
plex. This type of reaction is known to be rapid and reversible,16
and several monomeric and dimeric cobalt–dioxygen complexes
of CoII(salen) and derivatives have been isolated.17 In coordin-
ating solvents such as pyridine, DMSO or DMF, aeration of
CoII(salen) leads to precipitation of diamagnetic [(L)CoIII-
(salen)]2O2 complexes (L = solvent).16a These complexes, when
dissolved in alcohols, are irreversibly transformed into the
corresponding alkoxocobalt() derivatives.18
Probably because of higher solubility, no precipitate was
formed when air was bubbled through solutions of 6a in these
solvents. Furthermore, the solid obtained by evaporation of the
solvent in a stream of air was found to be paramagnetic in
pyridine-d5 or DMSO-d6 solution. On the other hand, as
described before, aeration of 6a in methanol initially induces
UV–VIS spectral changes which are very similar under these
conditions to those characterizing the conversion of CoII(salen)
into (methoxo)CoIII(salen) via the corresponding peroxo com-
plexes.18b However, further, slower changes are observed for 6a
until eventually the spectrum of 7a is obtained. Similarly, when
Scheme 4
Like trichloroethoxo complex 13, tert-butylperoxo complex
14 (Fig. 2) (prepared in the same way as (tert-butylperoxo)-
CoIII(salen))19 reacts in deaerated methanol to give 7a. It was
not investigated whether formation of 7a from 14 involves fast
replacement of the tert-BuOO moiety by a MeO group to give
12a followed by methoxycobaltation, or more complicated
alkyl peroxide decomposition pathways are followed.
1
the oxidation of 6a in methanol-d4 was followed by H NMR
spectroscopy, a diamagnetic CoIII-intermediate was observed
initially whose carbon–carbon double bond is still intact. This
intermediate spectrum (which already contained signals of 7a-
d3) then changed further to give the spectrum of 7a-d3 only.
These results indicate that the first detectable CoIII-inter-
mediate in the methoxycobaltation of 6a is the corresponding
methoxocobalt() complex 12a (Fig. 2). However, its isolation
proved to be cumbersome. Concentration of a methanol solu-
In methanol, the rate of formation of 7a from 6a is about 100
times faster at pH 4 than at pH 7, suggesting that protonated
species are involved in the rate-determining step. The pH–rate
profile of the reaction of 6a in buffered 9:1 methanol–water
mixtures follows a titration curve and gives a pKa value (of the
conjugate acid of 6a) of 7.6.10 In alkaline solutions formation
of 7a is very slow. Thus, when air was admitted to a solution of
6a in methanol-d4 containing a small amount of a solution of
NaOD (40%) in D2O such that [ODϪ] is 10Ϫ2 M, fast oxidation
to (CD3O)CoIII(salen) derivative 12a-d3 was observed by 1H
NMR spectroscopy. Further reaction to give 7a-d3 was slowed
down dramatically by the presence of the base. In a control
experiment, 7a was shown to be stable in alkaline methanol-d4,
ruling out the possibility that 7a cannot exist in basic solution.
In an attempt to observe a resonance for the cobalt-bound
methoxy group of 12a by 13C NMR spectroscopy, the above
experiment was repeated in an alkaline mixture of CH3OH and
CD3OD (4:1). However, even at Ϫ75 ЊC, such a signal was
not found. Since an analogous signal was also not present in
spectra of (MeO)CoIII(salen), it is concluded that exchange of
the axial methoxo ligand by CH3OH is too fast to be observed
on the NMR time scale. In DMSO-d6, however, the resonance
of the methoxo ligand of (MeO)CoIII(salen) is clearly present
(δ 13C = 51.9 and δ 1H = 1.21 ppm).
1
tion of 6a after short exposure to air was shown by H NMR
spectroscopy in DMSO-d6 to furnish a mixture of 7a and
paramagnetic material. The precipitate isolated after addition
of diethyl ether to a saturated solution of oxidised 6a in meth-
anol turned out to be a similar mixture. Apparently, equilibria
are involved which make it difficult to isolate intermediates.
When 6a was oxidised in the presence of the weakly nucleo-
philic 2,2,2-trichloroethanol (20 equiv. in dichloromethane),
only (2,2,2-trichloroethoxo)cobalt()salen derivative 13 (Fig.
2) was isolated and identified by NMR spectroscopy as a mix-
ture of two diastereomers (ratio ca. 1:1). It is the end-product of
the reaction, i.e., no subsequent cobalt–carbon bond formation
occurred. Trichloroethoxo complex 13 reacts anaerobically
1
when dissolved in methanol-d4 (monitored by H NMR spec-
troscopy) to form bridged product 7a-d3 and one equivalent of
¶ Recently we found that the expected β-isopropoxy- and tert-butoxy-
substituted intramolecularly alkylated salen derivatives are formed
when cationic complex 19 (Scheme 6) is dissolved in propan-2-ol and
tert-butyl alcohol, respectively.
Methoxocobalt() complexes 12b and 12c (Fig. 2) are
certainly intermediates in the oxidation of 6b and 6c, respect-
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ively. Because oxidation is fast whilst Co–C bond formation is
very slow, they could be isolated (although contaminated with
small amounts of 7b and 7c, respectively) and characterised
by 1H NMR spectroscopy in DMSO-d6 as mixtures of two
diastereomers (12b: ratio ca. 2:1; 12c: ratio ca. 1:1). However,
these complexes are rather unstable and decompose to para-
magnetic material at room temperature.
addition of a solution of 18 in acetonitrile to a solution of
silver tetraphenylborate in the same solvent, a yellowish precipi-
tate and a green-brown solution formed. Removal of the solid
(AgI) and concentration of the filtrate gave 19 as a green-brown
solid, identified by its 1H NMR spectrum in DMSO-d6. Because
the spectrum of 19 is fully identical to that of 18 (except for the
aromatic signals of the anion) and because no diastereomers
were detected, we conclude that both compounds are exten-
sively ionized in DMSO (and in pyridine).
CoII(salen) was found not to react with (a large excess of)
hex-1-ene in methanol. The only cobalt() product isolated
after two weeks was (MeO)CoIII(salen). Similarly, CoII(salen)
complex 15 with a hydroxymethyl group attached to the
equatorial ligand (Scheme 5) did not furnish an alkoxycobalt-
In strongly coordinating solvents like DMSO and pyridine,
an interaction between the C᎐C double bond and Co() is not
᎐
expected due to full occupation of the coordination sites.21
This is in accord with the fact that 19 undergoes fast anaerobic
methoxycobaltation to give 7a when dissolved in methanol,
whereas (similar to 6a) neither 18 nor 19 cyclizes to 7a in mix-
tures of methanol and DMSO (or pyridine). On the other hand,
in non-coordinating solvents like dichloromethane and chloro-
form, both 18 and 19 appear to be paramagnetic. With the
exception of the aromatic region, only very broad indistinct
NMR signals are observed. Decomposition can be excluded,
since removal of the chlorinated solvent and dissolution in
DMSO-d6 (or pyridine-d5) results in restoration of the original
NMR spectra found for diamagnetic 18 and 19. A possible
explanation is that, in non-coordinating solvents, both 18 and
19 have a geometry in which one of the coordinating oxygens of
the salen ligand is displaced from the equatorial plane to an
axial position causing transition from a low-spin to a high-spin
state. Attempts to crystallize 19, in order to observe a possible
Co()–olefin interaction in the solid state, have so far been
unsuccessful.
Mechanism of intramolecular alkoxycobaltation
Scheme 5
In our preliminary account10 we proposed the mechanism given
in Scheme 7 for the methoxycobaltation of 6a and 6d. The
results of the present work provide additional support for this
mechanism which also explains the results obtained with 6b, 6c,
and 6e, and thus may be general for these reactions.
ation product when exposed to hex-1-ene in aerated methanol.
In contrast, we recently reported that 16, provided with both a
pendant alkenyl and an ω-hydroxyalkyl side-chain, undergoes
a very fast reaction to give 17 when dissolved in aerated
methanol.20 The intramolecular presence or close proximity of
the unactivated C᎐C double bond, therefore, seems to be essen-
᎐
tial for Co–C bond formation to occur by alkoxycobaltation.
A bonding interaction between a C᎐C group and Co() in
᎐
salen derivatives carrying an alkenyl side-chain can only be
established and detected if free coordination sites on Co() are
available. Hence, we synthesized cationic CoIII(salen) derivative
19 neutralized by the large counterion tetraphenylborate
(Scheme 6). The starting material, iodocobalt() complex 18,
was prepared by reaction of iodine with 6a in THF. Upon
Scheme 6
Scheme 7
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The first step in this mechanism is most likely identical to the
oxygenation of the parent unsubstituted CoII(salen) in meth-
anol and produces methoxocobalt() derivatives via (transient)
µ-peroxo-, superoxo- and hydroxocobalt() intermediates.18 As
described above for 6b and 6c, the methoxocobalt() com-
plexes 12b and 12c could be isolated and identified by NMR
due to the fact that subsequent steps in the reaction sequence
are much slower.
The following step is probably the acid-catalyzed dissociation
of the methoxo ligand to yield a cationic organocobalt()
intermediate. This is evidenced by the fact that methoxycobalt-
ation is more rapid in mildly acidic solution (optimum at ca.
pH = 4), whereas in alkaline medium the reaction essentially
stalls after formation of the methoxocobalt() complex.
Moreover, cationic salen derivative 19 is rapidly converted
(anaerobically!) to 7a on dissolution in methanol.
The nature and fate of this cationic organocobalt()
intermediate may be rather similar to those of several related
intermediates of reactions in the B12 field. These include the
addition of vinyl ethers to cob()alamin and cob()aloximes,
respectively, in alcohols to give (2,2-dialkoxyalkyl)cobalt()
complexes,22 and the acid-catalysed Co–C bond heterolysis
of coenzyme B12,23 of (β-hydroxyalkyl)- and (β-alkoxyalkyl)-
cobalamins and -cobinamides,24 and of (β-hydroxyalkyl)- and
(β-alkoxyalkyl)cobaloximes25 to give Co(), alkene, and alco-
hol or water.
Mechanisms that have been proposed for these reactions
involve Co()–olefin π-cations,22,25 β-cobaltoethyl cations
stabilised by cobalt–carbon hyperconjugation,25 and a con-
certed elimination of alcohol or water and cobalt().24 More
recently, cobaloxime π-cations have also been suggested as
intermediates in the reaction of (β-hydroxyalkyl)cobaloximes
with nucleophiles26 and in cobalt-mediated carbocyclic ring
constructions.27 Considering these proposals, it seems that a
structure-dependent mechanistic continuum exists, similar to
that implied for the solvomercuration–demercuration reaction
for which arguments in favour and against bridged mercur-
inium and unsymmetrical mercury-stabilised carbocations,
respectively, have been advanced.28 Depending on the substitu-
tion pattern of the alkenyl side-chain of the starting CoII(salen)
derivative, delocalization of the positive charge in the cationic
Fig. 3 Proposed transition state structures for formation of 7a (A,
R = H), 7c (A, R = CH3), 8 (B), 9 (C), and intermediate D for formation
of 7b (I and II) (n = 1), 10 (n = 2), and 11 (n = 1).
in methanol, mainly produces methylene-substituted bridged
compound 10 (Scheme 3).
In principle, intramolecular alkoxycobaltation of 6a–e can
take place in two regioisomeric ways (cf. transition state struc-
tures B and C in Fig. 3). Indeed, for 6d both isomers are found
of which the major component 8 has a four-carbon bridge with
a β-methoxy substituent and the minor constituent 9 a three-
carbon bridge with an α-methoxymethyl substituent. For 6a–c,
bond formation between Co() and the internal sp2-carbon
would lead to a CoIII(salen) derivative with a two-carbon bridge
which is too strained to exist as a stable compound.6
intermediate over Co and the C᎐C bond will be more or less
᎐
unsymmetrical. In the intermediates derived from 6b and 6e,
the methyl bearing carbon atom may have, to a large degree, the
characteristics of a tertiary carbocation.
An alternative mechanism for the formation of alkoxy-
cobaltation products in which the alkene inserts directly into
the Co–O bond of the intermediate alkoxocobalt() complex,
without formation of a cationic intermediate, can be ruled
out since it would lead to a syn-relation between cobalt and
the alkoxy substituent, whereas the actual arrangement is
anti.10
The last essential step in the alkoxycobaltation is nucleophilic
attack of an alcohol on the cationic intermediate. The fact that
2,2,2-trichloroethanol, a weak nucleophile, does react with
alkenyl-substituted CoII(salen) complexes and oxygen to pro-
duce (trichloroethoxo)CoIII(salen) derivatives but does not react
further to give alkoxycobaltation products, demonstrates that
the nucleophilicity of the alcohol is an important factor in Co–
C bond formation.
In view of the stereospecificity of the reaction it seems likely
that the incipient carbon-bridges in the transition states have
already adopted their favoured chair-like conformations (A, B
and C in Fig. 3) while methanol approaches the electrophilic
carbon centre on a stereoelectronically controlled path anti to
the final Co–C bond. The presumably planar structure of the
tertiary carbocationic part of the intermediate derived from 6b
allows methanol to close in from either side to furnish both
7b(I) and (II) (D in Fig. 3); alternatively, a proton is released to
give 11 (Scheme 3). The same applies to 6e which, on aeration
Application of intramolecular alkoxycobaltation to prepare a
bridged [ꢀ-(2-hydroxy)ethoxyalkyl]CoIII(salen) complex:
a simple model of coenzyme B12 with a built-in substrate
B12-Model complexes to which a substrate (or model thereof) is
connected, can be used to test for the possible occurrence
of cobalt–substrate interactions in coenzyme B12-dependent
enzymatic rearrangements. In that vein, intramolecular alkoxy-
cobaltation was used to prepare a bridged CoIII(salen) complex
with a built-in glycol unit positioned in such a way that, after
homolysis of the Co–C bond, a 1,5-H shift could initiate a 1,2-
radical rearrangement.29 Thus, but-3-en-1-yl-substituted CoII-
(salen) complex 6d was aerated in a 1:1 mixture of ethylene
glycol and THF. After stirring for five days in the dark, a green
solid was obtained, which was shown by NMR spectroscopy
to consist of a ca. 3:1 mixture of the bridged organocobalt
isomers 20 and 21 (Scheme 8). The desired isomer 20 (whose
four-carbon bridge is expected to undergo homolysis more
easily than the three-carbon bridge of 21)30 is less soluble in
dichloromethane than 21, enabling the selective isolation of
crystals of 20 suitable for X-ray analysis.
The molecular structure and atom numbering of 20 are
shown in Fig. 4. Selected bond lengths and angles are shown
in Table 1 and details of the X-ray structure determination
are described in the Experimental section. In the solid state,
compound 20 is a centrosymmetric dimer, half of which
1204
J. Chem. Soc., Perkin Trans. 1, 2000, 1199–1210

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Table 1 Selected bond lengths (Å) and bond angles (Њ) for bridged
[β-(2-hydroxyethoxy)butyl]CoIII(salen) complex 20
bridge and the antiperiplanar arrangement of Co and O(3) are
almost identical with those found in the solid state structures of
other bridged (β-alkoxyalkyl)Co(salen) complexes.10,20
Product formation after homolysis of the Co–C bond of
20 and related (β-alkoxyalkyl)Co(salen) complexes will be
reported separately.
Co(1)–C(20)
Co(1)–O(1)
1.964(6)
1.914(3)
1.887(4)
1.867(6)
1.863(5)
2.360(5)
C(18)–C(19)
C(19)–C(20)
C(19)–O(3)
C(7)–N(1)
1.514(10)
1.516(9)
1.457(8)
1.274(11)
1.293(10)
Co(1)–O(2)
Co(1)–N(1)
Co(1)–N(2)
C(10)–N(2)
Co(1)–O(1Ј) (dimer)
Conclusions
Intramolecularly bridged (β-alkoxyalkyl)CoIII(salen) complexes
are formed when CoII(salen) derivatives carrying an alkenyl
side-chain are oxidised in alcoholic media. Mechanistic studies
indicate that this reaction has three main separate stages: 1) fast
oxidation of cobalt() to give an (alkoxo)CoIII(salen) derivative,
2) acid-catalyzed dissociation of the alkoxo ligand enabling
intramolecular interaction of Co() with the alkenyl double
bond to yield a carbocationic intermediate and 3) nucleophilic
attack by alcohol. Whether the intermediate in the second
stage is better described as a Co(salen) π-cation than as a
β-cobaltoethyl cation stabilised by cobalt–carbon hyperconju-
Co(1)–C(20)–C(19)
C(17)–C(18)–C(19)
C(18)–C(19)–C(20)
C(20)–Co(1)–O(1)
C(20)–Co(1)–O(2)
120.2(4)
116.3(7)
114.3(6)
92.9(2)
C(20)–Co(1)–N(1)
C(20)–Co(1)–N(2)
O(3)–C(19)–C(20)
O(3)–C(19)–C(18)
90.7(3)
93.8(2)
109.0(5)
106.4(6)
91.2(2)
gation depends on the substitution pattern of the C᎐C bond.
᎐
The exclusive anti-arrangement of cobalt and the alkoxy sub-
stituent found in most complexes, and the fact that, in most
cases, the bridge in the organocobalt() products adopts the
favourable chair-like conformation, point to a mechanism that
proceeds via a product-like transition state in the third stage. In
cases where a methyl substituent enables formation of a tertiary
carbocation, addition of an alcohol can occur at either side of
the tertiary carbocation. Alternatively, a proton is released from
the neighbouring methyl group to give a methylene-substituted
bridged compound.
Co–C bond formation between cobalt() and unactivated
alkenes is highly unusual under such mild conditions, and seems
limited to cases where the reactants are formed or forced to stay
in close proximity. The present intramolecular alkoxycobalt-
ation gives access to (β-alkoxyalkyl)CoIII(salen) complexes
which are applicable as simple models for coenzyme B12.
Fig. 4 ORTEP diagram drawn at 50% probability level and atom
numbering scheme of half a dimer of 20. Solvent molecules have been
omitted for clarity.
Experimental
General
NMR spectra were obtained using a Bruker AC 200 (1H NMR:
200.1 MHz; 13C NMR: 50.29 MHz) or a Bruker MSL 400
spectrometer (1H NMR: 400.1 MHz; 13C NMR: 100.63 MHz).
Chemical shifts (δ) are reported in ppm relative to tetramethyl-
silane using the solvent signal as internal reference; J values are
quoted in Hz. Mass spectra were measured on a Finnigan MAT
90 spectrometer. Two ionization methods were used: Electron
Impact (EI) (70 eV ionization energy, source temperature
200 ЊC) and Fast Atom Bombardment (FAB) (8 keV xenon and
m-nitrobenzyl alcohol as matrix). UV–VIS spectra were
recorded on a Beckman DU-70 spectrophotometer. THF was
distilled from NaH and, subsequently, from sodium benzo-
phenone ketyl. Et2O was distilled from NaH. Other solvents
were dried over molecular sieves (3 or 4 Å).
Scheme 8
(for reasons of clarity) is shown in Fig. 4. The cobalt atom
of each molecule contained in the dimer is bonded to the
salen-O(1) atom of its enantiomeric partner (Co–OЈ = 2.360(5)
Å). This way of establishing hexacoordination of cobalt is
not uncommon; of the few examples of (alkyl)CoIII(salen)
complexes that have been structurally characterised, [(ethyl)-
All reactions were performed under a nitrogen atmosphere,
unless stated otherwise. In order to prevent cleavage of the
cobalt–carbon bond, all organocobalt complexes were handled
with minimal exposure to light.
31
Co(salen)]2 and two of the intramolecularly bridged organo-
cobalt(salen) complexes recently reported by us6 have the same
feature. The Co–C bond length of 1.964(6) Å lies in the range
usually found for (alkyl)CoIII(salen) complexes.6,10,20,32 The
length of the C(20)–C(19) bond is 1.516(9) Å, similar to the
corresponding bond in coenzyme B12.33 The C(19)–O(3) bond is
slightly longer than in the coenzyme (1.457(8) Å vs. 1.427 Å).
The Co–C–C angle of 120.2Њ, although not as large as in B12
(122.5Њ), deviates strongly from the ideal tetrahedral geometry.
This can be explained by the fact that the metal-bonded C(20)
has considerable sp2-character. A conspicuous feature is that,
like in other bridged (alkyl)Co(salen) compounds, the C–C–C
angles in the carbon bridge are quite large, probably because of
steric constraints. The zigzag conformation of the four-carbon
Methyl N-benzylideneglycinate,34 allyl ethyl carbonate,35
ethyl 2-methylprop-2-enyl carbonate,35 4-bromo-2-methylbut-2-
ene,36 4-bromobutene,37 and (MeO)Co(salen)18b were prepared
according to published procedures.
Methyl 2-(benzylideneamino)pent-4-enoate 1a (Method I)
To a solution of methyl N-benzylideneglycinate (33.2 g, 0.19
mol) and allyl ethyl carbonate (51.0 g, 0.39 mol) in dry THF
(190 cm3) was added (Ph3P)4Pd (9.16 g, 7.9 mmol). After stir-
ring at room temperature for five hours, the mixture was poured
J. Chem. Soc., Perkin Trans. 1, 2000, 1199–1210
1205

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into diethyl ether (800 cm3) and filtered through Celite. The
filtrate was concentrated in vacuo at 30 ЊC. H NMR analysis
obtained as yellow oils (75–90% yield) which were used as such
in the following step.
1
of the resulting oil (48.0 g) showed that 1a had been formed
almost quantitatively, but was contaminated with a small
amount of Ph3P which was removed in the next step; δH (200
Methyl 2-aminopent-4-enoate 2a. δH (200 MHz; CDCl3) 5.68
(1 H, m, CH᎐), 5.61 (2 H, m, ᎐CH ), 3.70 (3 H, s, OCH ), 3.51
᎐
᎐
2
3
MHz; CDCl ) 8.20 (1 H, s, CH᎐N), 7.72 (2 H, m, arom-H),
(1 H, dd, CHN), 2.38 (2 H, m, CH2), 1.52 (2 H, br s, NH2).
᎐
3
7.48 (3 H, m, arom-H), 5.70 (1 H, m, CH᎐), 5.10 (2 H, m,
᎐
CH ᎐), 4.00 (1 H, dd, CHN), 3.65 (3 H, s, OCH ), 2.70 (2 H, m,
Methyl 2-amino-4-methylpent-4-enoate 2b. δH (200 MHz;
᎐
2
3
CH2).
CDCl ) 4.82 (1 H, s, CHH᎐), 4.72 (1 H, s, CHH᎐), 3.67 (3 H, s,
᎐ ᎐
3
OCH3), 3.57 (1 H, dd, CHN), 2.44 (1 H, dd, CHH), 2.23 (1 H,
dd, CHH), 1.70 (3 H, s, CH3), 1.51 (2 H, br s, NH2).
Methyl 2-(benzylideneamino)-4-methylpent-4-enoate 1b
Compound 1b was prepared in almost quantitative yield from
methyl N-benzylideneglycinate and ethyl methallyl carbonate
similar to the method used for 1a; δH (200 MHz; CDCl3) 8.17
Methyl (E)-2-aminohex-4-enoate 2c. δH (200 MHz; CDCl3)
5.68–5.16 (2 H, m, CH᎐CH), 3.68 (3 H, s, OCH ), 3.45 (1 H, dd,
᎐
3
CHN), 2.50–2.05 (2 H, m, CH2), 1.6 (5 H, m, CH3/NH2).
(1 H, s, CH᎐N), 7.75–7.30 (5 H, m, arom-H), 4.78 (1 H, s,
᎐
CHH᎐), 4.14 (1 H, dd, CHN), 3.73 (3 H, s, OCH ), 2.65 (2 H,
m, CH2), 1.73 (3 H, s, CH3).
᎐
3
Methyl 2-aminohex-5-enoate 2d. δH (200 MHz; CDCl3) 5.75
(1 H, m, CH᎐), 4.98 (2 H, m, CH ᎐), 3.66 (3 H, s, OCH ), 3.40
᎐
᎐
2
3
(1 H, dd, CH), 2.11 (2 H, m, CH2), 1.9–1.45 (4 H, m, CH2/
NH2).
Methyl (E)-2-(benzylideneamino)hex-4-enoate 1c (Method II)
A solution of (iPr)2NH (8.5 cm3, 62 mmol) in dry THF (240
cm3) and HMPT (27 cm3) was cooled to 0 ЊC. A hexane solution
of nBuLi (37.5 cm3, 1.6 M, 60 mmol) was added in 10 min via a
syringe. The cooling bath was removed and the mixture was
stirred for 30 min at room temperature. After cooling to
Ϫ60 ЊC, methyl N-benzylideneglycinate (10.6 g, 60.0 mmol) was
added dropwise in 30 min and the mixture stirred for 1 h
at Ϫ60 ЊC. The orange solution was then transferred within 2 h
via a stainless steel tube to a vigorously stirred solution of
4-bromobut-2-ene (14.2 g, 90 mmol) in dry THF (55 cm3) at
room temperature and stirred overnight. The orange mixture
was poured into a stirred mixture of diethyl ether (250 cm3) and
a saturated NH4Cl solution (500 cm3). The organic layer was
washed with water and brine, dried over MgSO4 and concen-
Methyl 2-amino-5-methylhex-5-enoate 2e. δH (200 MHz;
CDCl ) 4.72 (1 H, s, ᎐CHH), 4.69 (1 H, s ᎐CHH), 3.70 (3 H, s,
᎐
᎐
3
OCH3), 3.44 (1 H, dd, CHN), 2.09 (2 H, m, CH2), 1.87 (1 H, m,
CHH), 1.71 (4 H, s/m, CH3/CHH), 1.61 (2 H, br s, NH2).
General procedure for the synthesis of 2-aminoalkenamides 3a–e
A solution of 2 in MeOH (2.3 cm3 per mmol of 2) was cooled to
0 ЊC and saturated with NH3. The resulting mixture was stirred
for 3 days at room temperature, repeating saturation at 0 ЊC
every 24 h. Removal of the solvent in vacuo gave 3a–e as yellow
semi-solids (ca. 90% yield) which were used as such in the
following step.
1
trated in vacuo at 30 ЊC. H NMR analysis of the resulting oil
2-Aminopent-4-enamide 3a. δH (200 MHz; CDCl3) 7.16 (1 H,
(15.4 g) showed that 1c had been formed but was contaminated
with the dialkylated product (35%) which was removed in the
final step of the reaction sequence; δH (200 MHz; CDCl3) 8.20
br s, NHHCO), 5.91 (1 H, br s, NHHCO), 5.74 (1 H, m, CH᎐),
᎐
5.11 (2 H, m, ᎐CH ), 3.45 (3 H, s, OCH ), 3.40 (1 H, dd, CHN),
᎐
2
3
2.52 (1 H, m, CHH), 2.22 (1 H, m, CHH), 1.61 (2 H, br s, NH2).
(1 H, s, CH᎐N), 7.75 (2 H, m, arom-H), 7.38 (3 H, m, arom-H),
᎐
5.45 (2 H, m, CH᎐CH), 3.98 (1 H, dd, CHN), 3.72 (3 H, s,
OCH3), 2.62 (2 H, m, CH2), 1.61 (3 H, d, CH3).
᎐
2-Amino-4-methylpent-4-enamide 3b. δH (200 MHz; CDCl3)
6.70 (1 H, br s, NHHCO), 5.50 (1 H, br s, NHHCO), 4.86 (1 H,
s, CHH᎐), 4.75 (1 H, s, CHH), 3.45 (1 H, dd, CHN), 2.61 (1 H,
dd, CHH), 2.07 (1 H, dd, CHH), 1.69 (3 H, s, CH3), 1.62 (2 H,
᎐
Methyl 2-(benzylideneamino)hex-5-enoate 1d
br s, NH2).
Compound 1d (contaminated with ca. 5% of the dialkylated
product) was prepared from methyl N-benzylideneglycinate and
4-bromobutene, similar to the method used for 1c (yield ca.
(E)-2-Aminohex-4-enamide 3c. δH (200 MHz; CDCl3) 7.10
(1 H, br s, NHHCO), 5.63–5.15 (3 H, m, CH᎐CH/NHHCO),
3.35 (1 H, dd, CHN), 2.48 (1 H, m, CHH), 2.18 (1 H, m, CHH),
1.70–1.45 (5 H, m, CH3/NH2).
᎐
90%); δ (200 MHz; CDCl ) 8.20 (1 H, s, CH᎐N), 7.70 (2 H, m,
᎐
H
3
arom-H), 7.49 (3 H, m, arom-H), 5.70 (1 H, m, CH᎐), 4.95
᎐
(2 H, m, CH ᎐), 4.00 (1 H, dd, CH), 3.70 (3 H, s, OCH ), 2.00
᎐
2
3
(4 H, m, CH2).
2-Aminohex-5-enamide 3d. δH (200 MHz; CDCl3) 7.05 (1 H,
br s, NHHCO), 5.80 (2 H, m, NHHCO/CH᎐), 5.01 (2 H, m,
᎐
Methyl 2-(benzylideneamino)-5-methylhex-5-enoate 1e
᎐CH ), 3.35 (1 H, dd, CHN), 2.15 (2 H, m, CH ), 1.94 (1 H, m,
᎐
2
2
Compound 1e (contaminated with ca. 5% of the dialkylated
product) was prepared from methyl N-benzylideneglycinate and
4-bromo-2-methylbutene, similar to the method used for 1c
CHH), 1.58 (1 H, m, CHH), 1.45 (2 H, br s, NH2).
2-Amino-5-methylhex-5-enamide 3e. δH (200 MHz; CDCl3)
7.05 (1 H, br s, NHHCO), 5.62 (1 H, br s, NHHCO), 4.71 (1 H,
s, ᎐CHH), 4.69 (1 H, s, ᎐CHH), 3.33 (1 H, dd, CHN), 2.10 (2 H,
(yield ca. 90%); δ (200 MHz; CDCl ) 8.25 (1 H, s, CH᎐N), 7.41
᎐
H
3
(2 H, m, arom-H), 7.26 (3 H, m, arom-H), 4.72 (1H, s, ᎐CHH),
᎐
᎐
᎐
4.65 (1H, s, ᎐CHH), 3.98 (1H, dd, CHN), 3.72 (3H, s, OCH ),
m, CH2), 2.00 (1 H, m, CHH), 1.72 (3 H, s, CH3), 1.60 (1 H, m,
CHH), 1.44 (2 H, br s, NH2).
᎐
3
2.10 (4H, br m, CH2CH2), 1.71 (3H, s, CH3).
General procedure for the preparation of methyl 2-aminoalkeno-
ates 2a–e
General procedure for the preparation of 1,2-diaminoalkenes
4a–e
Compound 1 (1 equiv.) was stirred for 2 h with 1.5 M HCl
(2 equiv.) at room temperature. After washing with Et2O (to
remove benzaldehyde), solid NaHCO3 was added to the aque-
ous phase until pH 8 was reached. After saturation with solid
NaCl, extraction with CHCl3 (3×), drying over MgSO4 and
removal of the solvent in vacuo at <30 ЊC, crude 2a–e were
Compound 3 (10 mmol) was added in small portions to a mix-
ture of LiAlH4 (30 mmol) in dry THF (2.0 cm3 per mmol of 3)
at room temperature. The mixture was vigorously stirred at
55 ЊC for 20 h, followed by cooling in an ice-bath and slow
addition of H2O (0.08 cm3 per cm3 of THF), a 15% NaOH
solution (0.08 cm3 per cm3 of THF) and H2O (0.16 cm3 per cm3
1206
J. Chem. Soc., Perkin Trans. 1, 2000, 1199–1210

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of THF), respectively. After vigorous stirring, the white sus-
pension was filtered, the residue extracted with boiling THF,
and the combined THF fractions dried over MgSO4 and con-
centrated using a 30 cm Vigreux column. Crude 4 was purified
by short path distillation under reduced pressure to give 4a–e
as colourless liquids (30–40% yield). For analytical purposes,
small portions of 4a–e were converted into the corresponding
dipicrates (4aЈ–eЈ).
(1 H, s, CHH᎐), 3.91 (1 H, m, CHN), 3.65 (1 H, m, CH N), 2.44
(2 H, m, CH2), 1.73 (3 H, s, CH3).
᎐
2
2,2Ј-{[1-(E)-But-2-enylethane-1,2-diyl]bis(nitrilomethyl-
idyne)}diphenol 5c. δH (200 MHz; CDCl3) 13.17 (2 H, br s,
2 × OH), 8.22 (1 H, s, CH᎐N), 8.20 (1 H, s, CH᎐N), 7.20 (4 H,
᎐
᎐
m, arom-H), 6.81 (4 H, m, arom-H), 5.43 (2 H, m, CH᎐CH),
᎐
3.87 (1 H, m, CHHN), 3.52 (2 H, m, NCHHCHN), 2.34 (2 H,
m, CH2), 1.58 (3 H, d, CH3).
1,2-Diaminopent-4-ene 4a. Collected at a bath temperature of
40–45 ЊC at 15 mmHg; δH (200 MHz; CDCl3) 5.80 (1 H, m
2,2Ј-[(1-But-3-enylethane-1,2-diyl)bis(nitrilomethylidyne)]-
diphenol 5d. δH (200 MHz; CDCl3) 13.20 (2 H, br s, 2 × OH),
CH᎐), 5.07 (2 H, m, ᎐CH ), 2.85–2.70 (3 H, m, CH N/CHN),
᎐
᎐
2
2
8.30 (2 H, s, 2 × CH᎐N), 7.20 (4 H, m, arom-H), 6.90 (4 H, m,
᎐
2.20 (2 H, m, CH2), 1.40 (4 H, s, 2 × NH2). 4aЈ (Found: C, 36.9;
H, 3.35; N, 19.9. Calc for C17H18N8O14: C, 36.6; H, 3.25; N,
20.1%).
arom-H), 5.80 (1 H, m, CH᎐), 5.00 (2 H, m, ᎐CH ), 3.90 (1 H,
᎐
᎐
2
m, CHHN), 3.70 (1 H, m, CHN), 3.55 (1 H, m, CHHN), 2.10
(2 H, m, CH2), 1.85 (2 H, m, CH2).
1,2-Diamino-4-methylpent-4-ene 4b. Collected at a bath tem-
perature of 50–55 ЊC at 15 mmHg; δH (200 MHz; CDCl3) 4.79
2,2Ј-{[1-(3-Methylbut-3-enyl)ethane-1,2-diyl]bis(nitrilo-
methylidyne)}diphenol 5e. δH (200 MHz; CDCl3) 13.21 (2 H, br
(1 H, s, CHH᎐), 4.71 (1 H, s, CHH᎐), 2.87 (1 H, m, CHN), 2.71
᎐
᎐
s, 2 × OH), 8.28 (2 H, s, 2 × CH᎐N), 7.25 (4 H, m, arom-H),
᎐
(1 H, dd, CHHN), 2.47 (1 H, dd, CHHN), 2.12 (1 H, dd,
CHH), 1.88 (1 H, dd, CHH), 1.69 (3 H, s, CH3), 1.27 (4 H, br s,
2 × NH2). 4bЈ (Found: C, 37.8; H, 3.3; N, 19.6. Calc for
C18H20N8O14: C, 37.75; H, 3.5; N, 19.6%).
6.87 (4 H, m, arom-H), 4.73 (1 H, s, CHH᎐), 4.63 (1 H, s,
᎐
CHH᎐), 3.93 (1 H, m, CHHN), 3.69 (1 H, m, CHHN), 3.52
᎐
(1 H, m, CHN), 2.06 (2 H, m, CH2), 1.88 (2 H, m, CH2), 1.69 (3
H, s, CH3).
(E)-1,2-Diaminohex-4-ene 4c. Collected at a bath temper-
ature of 50–60 ЊC at 15 mmHg; δH (200 MHz; CDCl3) 5.60–5.26
General procedure for the synthesis of cobalt(II) complexes 6a–e
(2 H, m, CH᎐CH), 2.75–2.58 (2 H, m, CHN/CHHN), 2.45
A solution of H2salen ligand 5 (1 mmol) in anhydrous THF
(15 cm3) was degassed by three freeze–pump–thaw cycles.
Co(OAc)2 (1 mmol) was added and the resulting mixture was
stirred at 60 ЊC for 2 h. A red solution had formed, which was
evaporated to dryness. The residue was washed thoroughly with
deaerated H2O and Et2O. Drying in vacuo gave the CoII(salen)
derivatives 6a–e as bright red microcrystalline solids (75–85%
᎐
(1 H, m, CHHN), 2.10 (1 H, m, CHH), 1.86 (1 H, m, CHH),
1.63 (3 H, d, CH3), 1.22 (4 H, br s, 2 × NH2). 4cЈ (Found: C,
37.7; H, 3.4; N, 19.7. Calc for C18H20N8O14: C, 37.75; H, 3.5;
N, 19.6%).
1,2-Diaminohex-5-ene 4d. Collected at a bath temperature of
50–55 ЊC at 15 mmHg; δH (200 MHz; CDCl3) 5.80 (1 H, m,
1
yield), which were characterised by H NMR spectroscopy as
CH᎐), 5.00 (2 H, m, ᎐CH ), 2.75–2.40 (3 H, m, CH N/CHN),
their corresponding iodocobalt() derivatives.
᎐
᎐
2
2
2.20 (2 H, m, CH2), 1.40 (6 H, m, CH2/2 × NH2). 4dЈ (Found: C,
37.8; H, 3.3; N, 19.6. Calc for C18H20N8O14: C, 37.75; H, 3.5; N,
19.6%).
{[2,2Ј-[(1-Prop-2-enylethane-1,2-diyl)bis(nitrilomethylidyne)]-
diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt 6a. (Found: C, 61.2; H,
5.1; N, 7.6; Co, 15.2. Calc for C19H18N2O2Coؒ0.5 H2O: C, 61.0;
H, 5.1; N, 7.5; Co, 15.7%); δH (200 MHz; C5D5N, I2) 8.72 (1 H,
1,2-Diamino-5-methylhex-5-ene 4e. Collected at a bath tem-
perature of 65–70 ЊC at 15 mmHg; δH (200 MHz; CDCl3) 4.65
(2 H, br s, CH ᎐), 2.79–2.55 (2H, m, CHN/CHHN), 2.44 (1 H,
s, CH᎐N), 8.67 (1 H, s, CH᎐N), 7.4 (6 H, m, arom-H), 6.57
᎐
᎐
(2 H, m, arom-H), 5.8 (1 H, m, CH᎐), 5.1 (2 H, m, ᎐CH ), 5.0
᎐
᎐
2
᎐
2
(2 H, m, NCHHCHN), 3.75 (1 H, m, NCHH), 2.90 (1 H, m,
CHH), 2.50 (1 H, m, CHH); δC (50.29 MHz, DMSO-d6) 168.3,
168.0 (C-7/C-10), 165.1, 164.8 (C-1/C-16), 134.8, 134.7 (C-3/
C-14), 134.4 (C-18), 134.2 (C-5/C-12), 122.6, 122.2 (C-2/C-15),
119.2, 118.8 (C-6/C-11), 117.9 (C-19), 115.1, 114.9 (C-4/C-13),
67.3 (C-9), 62.6 (C-8), 37.9 (C-17) [C-atom numbering is analo-
gous to 7a (Scheme 2)].
dd, CHHN), 2.09 (2 H, m, CH2), 1.69 (3 H, s, CH3), 1.58 (1 H,
m, CHH), 1.39 (1 H, m, CHH), 1.22 (4 H, br s, 2 × NH2). 4eЈ
(Found: C, 38.6; H, 3.7; N, 19.2. Calc for C19H22N8O14: C, 38.9;
H, 3.8; N, 19.1%).
General procedure for the synthesis of H2salen ligands 5a–e
At 60 ЊC, salicylaldehyde (2.1 mmol) was added to a stirred
solution of diamine 4 (1 mmol) in EtOH (3.5 cm3 per mmol of
4). After stirring for 2 h, the yellow solution was concentrated
and excess salicylaldehyde was removed in vacuo at 50 ЊC/10Ϫ3
mmHg during 3 h. The crude ligands were obtained as viscous
yellow oils in quantitative yield and used directly in the next
step.
{[2,2Ј-{[1-(2-Methylprop-2-enyl)ethane-1,2-diyl]bis(nitrilo-
methylidyne)}diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt
6b.
(Found: C, 59.9; H, 5.4; N, 6.8. Calc for C20H20N2O2Coؒ1 H2O:
C, 60.4; H, 5.6; N, 7.1%); δH (200 MHz; DMSO-d6, I2) 8.22
(1 H, s, CH᎐N), 8.09 (1 H, s, CH᎐N), 7.5 (6 H, m, arom-H),
᎐
᎐
6.68 (2 H, m, arom-H), 4.95 (1 H, br s, CHH᎐), 4.68 (1 H, br s,
᎐
CHH᎐), 4.2–3.9 (3 H, m, NCH CHN), 2.41 (2 H, m, CH ), 1.73
(3 H, s, CH3).
᎐
2
2
2,2Ј-[(1-Prop-2-enylethane-1,2-diyl)bis(nitrilomethylidyne)]-
diphenol 5a. This compound crystallized on prolonged stand-
ing, mp 70–75 ЊC (from hexane-ethanol) (Found: C, 74.1; H,
6.4; N, 9.0. Calc for C19H20N2O2: C, 74.0; H, 6.55; N, 9.1%); δH
(200 MHz; CDCl3) 13.60 (2 H, br s, 2 × OH), 8.25 (2 H, s,
{[2,2Ј-{[1-(E)-But-2-enylethane-1,2-diyl]bis(nitrilomethyl-
idyne)}diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt 6c. (Found: C,
62.6; H, 5.7; N, 7.2. Calc for C20H20N2O2Coؒ0.5 H2O: C, 61.9;
H, 5.5; N, 7.2%); δH (200 MHz; DMSO-d6, I2) 8.27 (1 H, s,
CH᎐N), 8.08 (1 H, s, CH᎐N), 7.44 (6 H, m, arom-H), 6.66
2 × CH᎐N), 7.20 (4 H, m, arom-H), 6.90 (4 H, m, arom-H),
᎐
5.80 (1 H, m, CH᎐), 5.10 (2 H, m, ᎐CH ), 3.90 (1 H, m, CHN),
᎐
᎐
2
᎐
᎐
3.70 (2 H, m, CH2N), 2.50 (2 H, m, CH2).
(2 H, m, arom-H), 5.47 (2 H, m, CH᎐CH), 4.4–3.9 (3 H, m,
᎐
NCH2CHN), 2.37 (2 H, m, CH2), 1.64 (3 H, d, CH3).
2,2Ј-{[1-(2-Methylprop-2-enyl)ethane-1,2-diyl]bis(nitrilo-
methylidyne)}diphenol 5b. δH (200 MHz; CDCl3) 13.18 (2 H, br
s, 2 × OH), 8.28 (1 H, s, CH᎐N), 8.23 (1 H, s, CH᎐N), 7.26 (4 H,
{[2,2Ј-[(1-But-3-enylethane-1,2-diyl)bis(nitrilomethylidyne)]-
diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt 6d. (Found: C, 60.2; H,
5.6; N, 6.8. Calc for C20H20N2O2Coؒ1 H2O: C, 60.4; H, 5.6; N,
᎐
᎐
m, arom-H), 6.86 (4 H, m, arom-H), 4.80 (1 H, s, CHH᎐), 4.74
᎐
J. Chem. Soc., Perkin Trans. 1, 2000, 1199–1210
1207

View Article Online
7.1%); δ (200 MHz; C D N, I ) 8.67 (1 H, s, CH᎐N), 8.62 (1 H,
123.4 (C-2/C-15), 119.8, 119.3 (C-6/C-11), 115.3, 115.0 (C-4/
᎐
H
5
5
2
s, CH᎐N), 7.4 (6 H, m, arom-H), 6.59 (2 H, m, arom-H), 5.8
C-13), 78.6 (C-18), 68.1 (C-8), 64.7 (C-9), 49.0 (C-17), 48.9
(C-21), 23.0 (C-20), 17.9 (C-19).
᎐
(1 H, m, CH᎐), 5.1 (2 H, m, ᎐CH ), 5.0 (2 H, m, NCHHCHN),
᎐
᎐
2
3.82 (1 H, m, NCHH), 2.2 (3 H, m, CH2CHH), 1.55 (1 H, m,
CHH).
(SPY-5-54)-{2,2Ј-[{[1-(2-Methoxy-3-methyltrimethylene-ꢁC3)-
ethane-1,2-diyl]bis(nitrilomethylidyne)}diphenolato](3؊)-ꢁ2N,
NЈ;ꢁ2O,OЈ}cobalt 7c
A solution of 6c (173 mg, 0.43 mmol) in MeOH (6 cm3) was
processed as described for 6b to yield 7c (93 mg, 53%) (Found:
C, 61.1; H, 5.7; N, 6.9; Co, 13.9. Calc for C21H23N2O3Co: C,
61.5; H, 5.65; N, 6.8; Co, 14.4%); δH (200 MHz; CDCl3) 7.82 (2
{[2,2Ј-{[1-(3-Methylbut-3-enyl)ethane-1,2-diyl]bis(nitrilo-
methylidyne)}diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt
6e.
(Found: C, 60.8; H, 5.7; N, 6.5. Calc for C21H22N2O2Coؒ1 H2O:
C, 61.3; H, 5.9; N, 6.8%); δH (200 MHz; DMSO-d6, I2) 8.33 (1 H,
s, CH᎐N), 8.24 (1 H, s, CH᎐N), 6.97 (6 H, m, arom-H), 6.40
᎐
᎐
(2 H, m, arom-H), 4.75 (1 H, br s, CHH᎐), 4.68 (1 H, br s,
᎐
H, s, CH᎐N), 7.2 (4 H, m, arom-H), 7.0 (2 H, m, arom-H), 6.51
᎐
CHH᎐), 4.4–4.0 (3 H, m, NCH CHN), 2.3–1.7 (4 H, m,
᎐
2
(2 H, m, arom-H), 4.13 (1 H, m, CHHN), 4.03 (1 H, dd,
CoCH), 3.85 (1 H, br s, CHN), 3.38 (1 H, m, CHHN), 3.17
(3 H, s, OCH3), 2.82 (1 H, m, CHO), 2.11 (1 H, m, CHH), 1.23
(1 H, m, CHH), 0.24 (3 H, d, CH3); δC (50.29 MHz; CDCl3)
166.1, 165.7 (C-1/C-16), 165.0 (C-7), 162.8 (C-10), 133.2, 133.1
(C-3/C-14), 132.7, 132.5 (C-5/C-12), 123.5, 123.0 (C-2/C-15),
119.9, 119.6 (C-6/C-11), 115.2, 114.8 (C-4/C-13), 83.2 (C-18),
67.4 (C-9), 62.5 (C-8), 57.2 (C-21), 42.0 (C-17), 30.8 (C-19), 23.7
(C-20).
CH2CH2), 1.70 (3 H, s, CH3).
(SPY-5-54)-{2,2Ј-[{[1-(2-Methoxytrimethylene-ꢁC3)ethane-1,2-
diyl]bis(nitrilomethylidyne)}diphenolato](3؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}-
cobalt 7a
A solution of 6a (0.50 g, 1.4 mmol) in MeOH (100 cm3) was
stirred in the dark for 3 h while being exposed to air. The result-
ing dark red solution was concentrated in vacuo at 30 ЊC and
furnished 7a as a green, microcrystalline solid. Traces of Co()
material were removed by flash chromatography over Al2O3
using 10% MeOH in CH2Cl2 as eluent (0.48 g, 87%) (Found: C,
60.3; H, 5.4; N, 6.8; O, 12.2. Calc for C20H21N2O3Co: C, 60.6; H,
5.3; N, 7.1; O, 12.1%) (Found: Mϩ, 396.0880. C20H21N2O3Co
requires M, 396.0884); δH (200 MHz; CDCl3) 8.17 (1 H, s,
(SPY-5-54)-{2,2Ј-[{[1-(3-Methoxytetramethylene-ꢁC4)ethane-
1,2-diyl]bis(nitrilomethylidyne)}diphenolato](3؊)-ꢁ2N,NЈ;ꢁ2O,
OЈ}cobalt 8 and (SPY-5-54)-{2,2Ј-[{[1-(3-methoxymethyl-
trimethylene-ꢁC3)ethane-1,2-diyl]bis(nitrilomethylidyne)}-
diphenolato](3؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt 9
A solution of 6d (0.78 g, 2.1 mmol) in MeOH (150 cm3) was
oxidised during 8 h as described for 6a, yielding a mixture of
isomers 8 and 9 (ratio 2:1) (0.76 g, 90%). Purification of 150 mg
of this mixture was achieved by preparative TLC (SiO2, 5%
MeOH in CH2Cl2).
CH᎐N), 8.02 (1 H, s, CH᎐N), 7.30 (4 H, m, arom-H), 7.19 (2 H,
᎐
᎐
dd, arom-H), 6.64 (2 H, m, arom-H), 4.66 (1 H, m, CoCHH),
4.25 (1 H, m, CHHN), 4.07 (1 H, m, CHN), 3.57 (1 H, m,
NCHH), 3.42 (1 H, m, CoCHH), 3.33 (3 H, s, OCH3), 3.25
(1 H, m, CHO), 2.25 (1 H, m, CHH), 1.51 (1 H, m, CHH);
δC (100.63 MHz; CDCl3) 166.1, 165.9 (s, s, C-1/C-16), 165.6 (d,
JCH 159, C-7), 163.1 (d, JCH 158, C-10), 133.6, 133.5 (d, d, JCH
158, C-3/C-14), 132.6 (d, JCH 155, C-5/C-12), 124.0, 123.9 (d, d,
JCH 161, C-2/C-15), 119.7, 119.6 (s, s, C-6/C-11), 115.5, 115.4 (d,
d, JCH 162, C-4/C-13), 78.0 (d, C-18), 67.9 (d, JCH 139, C-9),
62.9 (t, JCH 138, C-8), 56.3 (q, JCH 141, C-20), 44.5 (t, JCH 127,
C-17), 12.7 (br t, JCH 143, C-19).
8 (69 mg, 46%) (Found: C, 60.1; H, 5.7; N, 6.8; O, 12.7. Calc
for C21H23N2O3Coؒ0.5 H2O: C, 60.1; H, 5.8; N, 6.7; O, 13.35%);
δ (200 MHz; CDCl ) 8.07 (1 H, s, CH᎐N), 7.91 (1 H, s, CH᎐N),
᎐
᎐
H
3
7.26 (4 H, m, arom-H), 7.08 (2 H, dd, arom-H), 6.60 (2 H, m,
arom-H), 4.89 (1 H, m, CoCHH), 4.48 (1 H, m, NCHH), 4.10
(1 H, m, CHN), 3.60 (1 H, m, NCHH), 3.23 (3 H, s, OCH3),
2.70 (2 H, m, CoCHH/CHO), 1.85 (3 H, m, CH2CHH), 1.25
(1 H, m, CHH); δC (50.29 MHz, DMSO-d6) 165.8, 165.3 (C-1/
C-16), 164.1 (C-7), 162.6 (C-10), 133.4, 133.0 (C-3/C-14), 132.3,
132.1 (C-5/C-12), 121.9, 121.1 (C-2/C-15), 120.6, 119.9 (C-6/
C-11), 112.6, 111.9 (C-4/C-13), 84.5 (C-19), 65.3 (C-9), 62.4
(C-8), 54.5 (C-21), 35.8 (C-17), 31.4 (C-18), 22 (C-20). The
X-ray structure of 8 is described in ref. 10.
(SPY-5-54)-{2,2Ј-[{[1-(2-Methoxy-2-methyltrimethylene-ꢁC3)-
ethane-1,2-diyl]bis(nitrilomethylidyne)}diphenolato](3؊)-ꢁ2N,
NЈ;ꢁ2O,OЈ}cobalt 7b (I and II)
A solution of 6b (236 mg, 0.6 mmol) in MeOH (8 cm3) was
stirred in the dark during 14 days while being exposed to air.
After work-up as described for 6a, the two diastereomers 7b(I)
(52 mg, 21%) and 7b(II) (108 mg, 44%) were isolated by pre-
parative TLC (SiO2; 10% MeOH in CH2Cl2).
9 (36 mg, 24%): δH (200 MHz; CDCl ) 8.15 (1 H, s, CH᎐N),
᎐
3
7.97 (1 H, s, CH᎐N), 7.30 (4 H, m, arom-H), 7.15 (2 H, dd,
᎐
arom-H), 6.64 (2 H, m, arom-H), 4.25 (1 H, m, NCHH), 3.93
(1 H, m, CHN), 3.77 (1 H, m, CoCH), 3.53 (1 H, m, NCHH),
3.23 (2 H, d, CH2O), 3.18 (3 H, s, OCH3), 1.85 (2 H, m, CH2),
1.20 (1 H, m, CHH), Ϫ0.46 (1 H, m, CoCRHCHH).
7b(I) (Found: Mϩ, 410.104121. C21H23N2O3Co requires M,
410.104057); δH (200 MHz; CDCl3) 8.19 (1 H, s, H-10), 7.95
(1 H, s, H-7), 7.37 (2 H, d, H-2/H-15), 7.27 (2 H, m, H-3/H-14),
7.20 (1 H, d, H-12), 7.14 (1 H, d, H-5), 6.61 (2 H, m, H-4/H-13),
4.86 (1 H, dd, H-19a), 4.13 (1 H, dd, H-8a), 3.89 (1 H, m, H-9),
3.76 (1 H, s, H-8a), 3.73 (1 H, d, H-19b), 2.67 (3 H, s, OCH3),
2.11 (1 H, ddd, H-17a), 1.59 (1 H, dd, H-17b), 1.14 (3 H, s, Me);
δC (50.29 MHz; CDCl3) 166.0, 164.9 (C-1/C-16), 165.6 (C-7),
162.2 (C-10), 133.0, 132.7 (C-3/C-14), 132.4, 132.1 (C-5/C-12),
124.1, 123.2 (C-2/C-15), 120.3, 119.9 (C-6/C-11), 115.1, 115.0
(C-4/C-13), 77.1 (C-18), 67.3 (C-9), 62.2 (C-8), 50.9 (C-17), 49.0
(C-21), 22.0 (C-20), 16.5 (C-19).
(SPY-5-54)-{2,2Ј-[{[1-(3-Methylidenetetramethylene-ꢁC4)-
ethane-1,2-diyl]bis(nitrilomethylidyne)}diphenolato](3؊)-
ꢁ2N,NЈ; ꢁ2O,OЈ}cobalt 10
A solution of 6e (393 mg, 1 mmol) in MeOH (10 cm3) was
stirred in the dark for 2.5 h while being exposed to air. A brown
precipitate formed, which was isolated, washed with Et2O and
dried in vacuo to yield 10 as a dark green microcrystalline solid
(255 mg, 60%) (Found: Mϩ, 391.0857. [C21H20N2O2Co Ϫ H]ϩ
requires M, 391.0857); δH (200 MHz, DMSO-d6) 8.01 (2 H, s,
7b(II) (Found: C, 61.4; H, 5.6; N, 6.4; Co, 13.6. Calc for
C21H23N2O3Co: C, 61.5; H, 5.65; N, 6.8; Co, 14.4%); δH (200
MHz; CDCl ) 8.13 (1 H, s, CH᎐N), 7.89 (1 H, s, CH᎐N), 7.3–
CH᎐N), 7.2–7.0 (4 H, m, arom-H), 6.80 (2 H, dd, arom-H), 5.78
᎐
᎐
᎐
3
7.0 (6 H, m, arom-H), 6.55 (2 H, m, arom-H), 4.91 (1 H, dd,
CoCHH), 4.14 (1 H, dd), 3.85 (1 H, m, CHN), 3.26 (1 H, d,
CHHN), 3.17 (1 H, d, CoCHH), 2.68 (3 H, s, OCH3), 2.32
(1 H, m, CHH), 1.51 (1 H, dd, CHH), 1.06 (3 H, s, CH3);
δC (50.29 MHz; CDCl3) 166.0, 165.9 (C-1/C-16), 165.3 (C-7),
164.0 (C-10), 133.4, 133.0 (C-3/C-14), 132.7 (C-5/C-12), 123.9,
(2 H, d, ᎐CH ), 4.44 (1 H, d, CoCHH), 4.08 (2 H, m, NCH-
᎐
2
HCHN), 3.85 (1 H, d, CoCHH), 3.62 (1 H, m, NCHH), 2.28
(1 H, m, CHH), 1.93 (1 H, m, CHH), 1.83–1.53 (2 H, m, CH2);
δC (50.29 MHz, DMSO-d6) 165.4, 160.4 (C-1/C-16), 163.5,
162.8 (C-7/C-10), 133.3, 133.1 (C-3/C-14), 132.0 (C-5/C-12),
122.0, 121.2 (C-2/C-15), 120.7, 119.8 (C-6/C-11), 112.6, 111.9
1208
J. Chem. Soc., Perkin Trans. 1, 2000, 1199–1210

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(C-4/C-13), 104.4 (C-21), 66.2 (C-9), 64.7 (C-19), 62.8 (C-8),
30.7 (C-18); C-17 and C-20 were not observed.
lization failed due to instability; δH (200 MHz; DMSO-d6) 8.07
(s, CH᎐N, maj), 8.02 (s, CH᎐N, min), 7.91 (s, CH᎐N, min), 7.80
᎐
᎐
᎐
(s, CH᎐N, maj), 7.5–7.1 (6 H, m, arom-H), 6.45 (2 H, m,
᎐
(SPY-5-54)-{2,2Ј-[{[1-(2-Methylidenetrimethylene-ꢁC3)ethane-
1,2-diyl]bis(nitrilomethylidyne)}diphenolato](3؊)-ꢁ2N,NЈ;
ꢁ2O,OЈ}cobalt 11
arom-H), 4.88 (1 H, br s, CHH᎐, min/maj), 4.74 (br s, CHH=,
᎐
min), 4.65 (br s, CHH=, min), 4.29–3.63 (3 H, m, NCH2CHN),
2.6–2.4 (2 H, m, CH2), 1.87 (3 H, s, CH3), 1.25 (s, OCH3, maj),
1.14 (s, OCH3, min).
Oxygen was bubbled through a solution of 6b (11 mg, 29 µmol)
in CD3OD (0.45 cm3) containing 0.5 µL of a DCl solution in
D2O (11 mM, 5.5 × 10Ϫ3 µmol) for 12 min. After 7 days, a
precipitate was isolated, washed with cold MeOH and dry
Et2O, dried in vacuo (5.3 mg, 48%) and identified as 11 (Found:
C, 63.9; H, 5.3; N, 7.9; Co, 14.9. Calc for C20H19N2O2Co:
C, 63.5; H, 5.1; N, 7.4; Co, 15.6%); δH (200 MHz; DMSO-d6)
Methoxo{[2,2Ј-{[1-(3-methylprop-2-enyl)ethane-1,2-diyl]bis-
(nitrilomethylidyne)}diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt
12c
A solution of 6c (100 mg, 0.264 mmol) in MeOH (25 cm3) was
processed as for 6b to give crude product 12c in almost quanti-
tative yield (105 mg, 97%) as a mixture of two diastereomers
(ratio ca. 1:1). Attempts at purification by chromatography or
crystallization failed due to instability; δH (200 MHz; DMSO-
8.17 (1 H, s, CH᎐N), 7.83 (1 H, s, CH᎐N), 7.21–7.05 (4 H, m,
᎐
᎐
arom-H), 6.82 (2 H, dd, arom-H), 6.44–6.32 (2H, m, arom-H),
4.84 (1 H, s, ᎐CHH), 4.77 (1 H, s, ᎐CHH), 4.12 (2 H, s,
᎐
᎐
CoCH2), 4.01 (1 H, m, NCH), 3.78 (1 H, dd, NCHH), 3.43
(1 H, d, NCHH), 2.45 (1 H, m, CHH), 2.25 (1 H, m, CHH).
d ) 8.10, 8.02, 8.01, 7.91 (4 H, s, 4 × CH᎐N), 7.5–7.0 (6 H, m,
᎐
6
arom-H), 6.48 (2 H, m, arom-H), 5.44 (2 H, br s, CH᎐CH), 4.3–
᎐
3.4 (3 H, m, NCH2CHN), 2.6–2.4 (2 H, m, CH2), 1.68 (3 H, s,
CH3), 1.32 (s, OCH3), 1.27 (s, OCH3).
Methoxo{[2,2Ј-{[ethane-1,2-diylbis(nitrilomethylidyne)}-
diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt
In an NMR tube, CoII(salen) (16 mg, 49 µmol) was dissolved in
a mixture of CD3OD (0.10 cm3) and CH3OH (0.40 cm3). Oxy-
gen was bubbled through the red suspension for 15 min. A
{[2,2Ј-[(1-Prop-2-enylethane-1,2-diyl)bis(nitrilomethylidyne)]-
diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}(2,2,2-trichloroethoxo)cobalt
13
1
brown solution had formed. H and 13C NMR spectra were
A solution of 6a (0.17 g, 0.47 mmol) and CCl3CH2OH (0.90
cm3, 9.4 mmol, 20 equiv.) in CH2Cl2 was stirred for 8 h while
being exposed to air. The dark brown solution was concen-
trated and Et2O was added. A brown solid precipitated, which
was filtered, washed with dry Et2O and dried in vacuo (0.16 g,
66%). The 1H NMR spectrum showed the presence of two
diastereomers of 13 in a ratio of 1:1 (Found: C, 49.6; H, 4.1; N,
5.6; Cl, 20.5. Calc for C21H20Cl3N2O3Co: C, 49.1; H, 3.9; N,
5.45; Cl, 20.7%); δH (200 MHz; DMSO-d6) 8.11, 8.09, 7.96, 7.91
recorded at 25 and Ϫ80 ЊC. Spectra at both temperatures were
similar and showed that (MeO)CoIII(salen) had been formed; a
resonance for the methoxy ligand was not observed; δH (200
MHz) 8.10 (2 H, s, CH᎐N), 7.5–7.25 (6 H, m, arom-H),
᎐
6.60 (2 H, m, arom-H), 4.41 (4 H, s, NCH2CH2N); δC (100.63
MHz) 168.5 (CH᎐N), 167.7 (qC-O), 135.5, 135.4, 123.8, 115.6
᎐
(arom-CH), 120.2 (qC), 59.6 (NCH2). The following data for
(MeO)Co(salen) were not previously reported: δH (200 MHz;
DMSO-d ) 8.05 (2 H, s, CH᎐N), 7.35–7.10 (6 H, m, arom-H),
᎐
6
(2 H, 4 × s, CH᎐N), 7.4–7.15 (6 H, m, arom-H), 6.50 (2 H, m,
᎐
6.47 (2 H, m, arom-H), 3.87 (4 H, m, NCH2CH2N), 1.21 (3 H, s,
arom-H), 5.85 (1 H, m, CH᎐), 5.10 (2 H, m, ᎐CH ), 4.2–3.7
᎐
᎐
2
CoOCH3); δC (50.29 MHz; DMSO-d ) 166.7 (CH᎐N), 164.5
᎐
6
(3 H, m, NCH2CHN), 2.71 (2 H, s, CoOCH2), 2.50 (2 H, m,
CH2); δC (50.29 MHz; C5D5N) 167.3, 167.2 (C-1/C-16),
166.0, 165.3 (C-7/C-10), 133.9 (C-3/C-14), 133.8 (C-5/C12),
133.8 (C-18), 122.1, 122.0 (C-2/C-15), 119.4 (C-19), 118.0, 117.8
(C-6/C-11), 113.3, 113.1 (C-4/C-13), 81.1 (C-21), 75.6 (C-20),
66.4 (C-9), 63.1 (C-8), 38.8 (C-17).
(qC-O), 133.8, 132.9, 121.9, 112.5 (arom-CH), 119.2 (qC), 57.8
(NCH2), 51.9 (CoOCH3).
Oxidation of 6a in alkaline methanol: formation of methoxo-
{[2,2Ј-{(1-prop-2-enylethane-1,2-diyl)bis(nitrilomethylidyne)}-
diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt(III) 12a
In an NMR tube, Co(salen) complex 6a (18 mg, 49 µmol) was
dissolved in a mixture of CD3OD (0.10 cm3) and CH3OH (0.40
cm3) containing 2 µL of a solution of NaOD (40%, 30 µmol) in
D2O. Oxygen was bubbled through the red solution for 1 min by
which time the colour had changed to brown. 1H and 13C NMR
indicated that (MeO)CoIII(salen) complex 12a had been
formed. Neither at 25 ЊC nor at Ϫ75 ЊC was a resonance for the
expected cobalt-bound methoxy ligand observed; δH (200 MHz;
Iodo{[2,2Ј-[(1-prop-2-enylethane-1,2-diyl)bis(nitrilomethyl-
idyne)]diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt 18
Cobalt() complex 6a (528 mg, 1.44 mmol) and I2 (183 mg, 0.72
mmol) were dissolved in THF (20 cm3) and the resulting brown-
black solution was stirred at room temperature for 1.5 h. After
removal of the solvent in vacuo, the black residue was washed
thoroughly with dry Et2O and dried in vacuo to give 18 as a
black solid (639 mg, 90%). All NMR data were identical to
those of in situ oxidised 6a (vide supra) (Found: Mϩ, 365.0704.
[C19H18IN2O2Co Ϫ I]ϩ requires M, 365.0700).
CD OD–CH OH, 298 K) 8.02 (1 H, s, CH᎐N), 7.99 (1 H, s,
᎐
3
3
CH᎐N), 7.45–7.28 (6 H, m, arom-H), 6.56 (2 H, m, arom-H),
᎐
6.05 (1 H, m, CH᎐), 5.28 (2 H, m, ᎐CH ), 4.3–3.8 (3 H, m,
᎐
᎐
2
NCH2CHN), 3.1–2.65 (2 H, m, CH2); δC (100.63 MHz, 298 K)
168.3, 167.4 (C-7/C-10), 167.5 (C-1/C-16), 135.9 (C-18), 135.4
(C-5/C-12), 135.3, 135.0 (C-3/C-14), 123.8, 123.7 (C-2/C-15),
120.5, 120.2 (C-6/C-11), 119.2 (C-19), 115.4, 115.2 (C-4, C-13),
67.9 (C-9), 64.4 (C-8), 37.9 (C-17).
{[2,2Ј-[(1-Prop-2-enyl)ethane-1,2-diyl)bis(nitrilomethylidyne)]-
diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt tetraphenylborate 19
Compound 18 (383 mg, 0.777 mmol) was dissolved in aceto-
nitrile (15 cm3). A solution of AgBPh4 (1 equiv.) in acetonitrile
(15 cm3) was added, upon which the dark brown solution of the
cobalt() complex turned light green-brown and a light yellow
precipitate formed. After stirring for 30 min in the dark, the
mixture was filtered and the filtrate was evaporated to dryness
in vacuo to give 19 (431 mg, 81%) as a green-brown solid.
Attempts at crystallization in various solvents failed; δH (200
Methoxo{[2,2Ј-{[1-(2-methylprop-2-enyl)ethane-1,2-diyl]bis-
(nitrilomethylidyne)}diphenolato](2؊)-ꢁ2N,NЈ;ꢁ2O,OЈ}cobalt
12b
A solution of 6b (98 mg, 0.259 mmol) in MeOH (25 cm3) was
stirred under air for 1 h at room temperature. The resulting
brown solution was evaporated to dryness in vacuo at 30 ЊC.
Crude product 12b was obtained in almost quantitative yield
(106 mg, 100%) as a mixture of two diastereomers (ratio ca.
2:1). Attempts at purification by chromatography or crystal-
MHz; DMSO-d ) 8.35 (1 H, s, CH᎐N), 8.25 (1 H, s, CH᎐N),
᎐
᎐
6
7.9–7.0 (26 H, m, arom-H), 6.75 (2 H, m, arom-H), 5.9 (1 H, m,
CH᎐), 5.25 (2 H, m, ᎐CH ), 4.5–3.9 (3 H, m, NCH CHN), 2.50
᎐
᎐
2
2
(2 H, m, CH2).
J. Chem. Soc., Perkin Trans. 1, 2000, 1199–1210
1209

View Article Online
(SPY-5-54)-{2,2Ј-[{[1-(2-(2-Hydroxyethoxy)trimethylene-ꢁC3)-
ethane-1,2-diyl]bis(nitrilomethylidyne)}diphenolato](3؊)-ꢁ2N,
NЈ;ꢁ2O,OЈ}cobalt 20
6 B. van Arkel, J. L. van der Baan, S. Balt, F. Bickelhaupt, M. W. G. de
Bolster, I. E. Kingma, G. W. Klumpp, J. W. E. Moos and A. L. Spek,
J. Chem. Soc., Perkin Trans. 1, 1993, 3023.
7 P. J. Toscano and L. G. Marzilli, Prog. Inorg. Chem., 1984, 31, 105
and references cited therein.
8 A. Nishinaga, T. Yamada, H. Fujisawa, K. Ishizaki, H. Ihara and
T. Matsuura, J. Mol. Catal., 1988, 48, 249.
9 B. Beatrix, O. Zelder, F. Kroll, G. Örligsson, B. T. Golding and
W. Buckel, Angew. Chem., 1995, 107, 2573; Angew. Chem., Int. Ed.
Engl., 1995, 34, 2398; W. Buckel and B. T. Golding, Chem. Soc. Rev.,
1996, 329; D. M. Smith, B. T. Golding and L. Radom, J. Am. Chem.
Soc., 1999, 121, 9388.
10 I. E. Kingma, M. Wiersma, J. L. van der Baan, S. Balt,
F. Bickelhaupt, M. W. G. de Bolster, G. W. Klumpp and A. L. Spek,
J. Chem. Soc., Chem. Commun., 1993, 832.
Complex 6d (0.21 g, 0.55 mmol) was dissolved in a 1:1 mixture
of THF and ethylene glycol (10 cm3). The red-brown solution
was stirred for 5 days while being exposed to air. THF was
removed in vacuo and water was added to give a brown precipi-
tate, which was filtered and dried in vacuo. The crude product
was purified by column chromatography (Al2O3; 2% MeOH in
CH2Cl2) to give a green solid (0.14 g, 58%). This was shown by
NMR spectroscopy to consist of a mixture of isomers 20 and
21 in a ratio of ca. 3:1. Pure 20 was obtained by selective
crystallization from CH2Cl2; δH (400 MHz; DMSO-d6) 8.07
11 D. Ferroud, J. P. Genet and R. Kiolle, Tetrahedron Lett., 1986, 27,
(1 H, s, CH᎐N), 7.95 (1 H, s, CH᎐N), 7.14 (2 H, dd, H-3/H-14),
᎐
᎐
23.
7.08 (2 H, dd, H-5/H-12), 6.79 (2 H, dd, H-2/H-15), 6.40 (1 H,
dd, H-4), 6.36 (1 H, dd, H-13), 4.35 (1 H, t, OH), 4.18 (1 H, br s,
CoCHH), 3.93 (2 H, m, NCHHCHN), 3.48 (1 H, m, NCHH),
3.37–3.11 (3 H, m, HOCH2CHH), 2.85 (1 H, m, CHO), 2.47
(1 H, m, CoCHH), 1.85 (1 H, m, CHH), 1.69 (2 H, m, CHH-
CHH), 1.23 (1 H, m, CHHCHO); δC (100.63 MHz; DMSO-d6)
166.0, 165.6 (C-1/C-16), 164.2, 162.7 (C-7/C-10), 133.5, 133.2
(C-3/C-14), 132.4, 132.3 (C-5/C-12), 122.1, 121.3 (C-2/C-15),
120.9, 120.1 (C-6/C-11), 112.7, 112.1 (C-4/C-13), 83.5 (C-19),
69.1 (C-21), 65.5 (C-9), 62.7 (C-8), 60.6 (C-22), 36.4 (C-17), 31.9
(C-18), 22.7 (C-20).
12 D. Schirlin, F. Gerhart, J. M. Hornsperger, M. Hamon, J. Wagner
and M. J. Jung, J. Med. Chem., 1988, 31, 30.
13 A. R. Amundsen, J. Whelan and B. Bosnich, Inorg. Chem., 1979, 18,
206.
14 H. Diehl and C. C. Hach, Inorg. Synth., 1950, 3, 196.
15 E. G. Samsel and J. K. Kochi, J. Am. Chem. Soc., 1986, 108, 4790.
16 (a) C. Floriani and F. Calderazzo, J. Chem. Soc. A, 1969, 946;
(b) F. Calderazzo, C. Floriani and J.-J. Salzmann, Inorg. Nucl. Chem.
Lett., 1966, 2, 379; (c) M. Calligaris, G. Nardin, L. Randaccio and
A. Ripamonti, J. Chem. Soc. A, 1970, 1069.
17 R. D. Jones, D. A. Summerville and F. Basolo, Chem. Rev., 1979, 79,
139; E. C. Niederhoffer, J. H. Timmons and A. E. Martell, Chem.
Rev., 1984, 84, 137.
18 (a) G. Costa, G. Mestroni and G. Pellizer, J. Organomet. Chem.,
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Lett., 1985, 905.
19 A. Nishinaga, H. Tomita and H. Okara, Chem. Lett., 1983, 1751.
20 R. Blaauw, J. L. van der Baan, S. Balt, M. W. G. de Bolster, G. W.
Klumpp, H. Kooijman and A. L. Spek, Chem. Commun., 1998,
1295.
21 X.-H. Shi, X.-Z. You, C. Li, B.-L. Song and T.-H. Li, Acta
Crystallogr., Sect. C, 1995, 51, 206.
22 R. B. Silverman and D. Dolphin, J. Am. Chem. Soc., 1976, 98, 4626.
23 B. P. Hay and R. G. Finke, J. Am. Chem. Soc., 1986, 108, 4820 and
references cited therein; L. E. H. Gerards and S. Balt, Recl. Trav.
Chim. Pays-Bas, 1992, 111, 411.
Crystal structure determination of bridged [ꢀ-(2-hydroxyethoxy)-
butyl]Co(salen) complex 20||
Single crystals of 20 were grown selectively from a saturated
solution of 20 and 21 in dichloromethane. X-ray data were
collected on an Enraf-Nonius CAD4T diffractometer (Mo-Kα,
Rotating Anode, 150 K) for a needle shaped red crystal.
C22H25N2O4CoؒCH2Cl2, M = 525.32, monoclinic, P21/c,
a = 12.4467(14), b = 11.4978(7), c = 19.0943(18) Å, β =
125.340(7)Њ, Z = 4, µ(Mo-Kα) = 1.04 mmϪ1, 4578 reflections
scanned (θmax = 25Њ). 3932 Unique reflections (R(int) = 0.07) of
which 2223 greater than 2σ(i).
24 K. L. Brown, R. Salmon and J. A. Kirby, Organometallics, 1992, 11,
422.
The structure was solved by direct methods38 using
SHELXS86 and refined39 on F2 with SHELXL96. Hydrogen
atoms were introduced at calculated positions and refined
riding on their carrier atoms. The CH2Cl2 molecule of crystal-
lization was refined with a disorder model. Convergence was
reached at R = 0.0655 (wR2 = 0.133, S = 1.02).
25 B. T. Golding, H. L. Holland, U. Horn and S. Sakrikar, Angew.
Chem., 1970, 82, 983; B. T. Golding and S. Sakrikar, J. Chem. Soc.,
Chem. Commun., 1972, 1183; K. L. Brown, S. Ramamurthy and
D. S. Marynick, J. Organomet. Chem., 1985, 287, 377.
26 L. M. Grubb, K. A. Brown and B. P. Branchaud, Tetrahedron Lett.,
1998, 39, 3447; L. M. Grubb and B. P. Branchaud, J. Org. Chem.,
1997, 62, 242; J. L. Gage and B. P. Branchaud, J. Org. Chem., 1996,
61, 831; Y. Nishikubo and B. P. Branchaud, J. Am. Chem. Soc., 1999,
121, 10924.
27 G. Kettschau and G. Pattenden, Synlett, 1998, 783; G. Kettschau
and G. Pattenden, Tetrahedron Lett., 1998, 39, 2027; G. B. Gill,
G. Pattenden and G. A. Roan, Tetrahedron Lett., 1996, 37, 9369.
28 R. C. Larock, in Comprehensive Organometallic Chemistry II,
Editors-in-Chief E. W. Abel, F. G. A. Stone and G. Wilkinson,
Vol. Ed. A. McKillop, Elsevier Science Ltd, Oxford 1995, 11, 411.
29 R. J. Anderson, S. Ashwell, R. M. Dixon and B. T. Golding,
J. Chem. Soc., Chem. Commun., 1990, 70.
|| CCDCreferencenumber207/403. Seehttp://www.rsc.org/suppdata/p1/
b0/b000196l/ for crystallographic files in .cif format.
Acknowledgements
The investigations were supported in part (A. L. S.
and W. J. J. S.) by the Netherlands Foundation for Chemical
Research (SON) with financial aid from the Netherlands
Organization for Scientific Research (NWO).
30 R. Blaauw, I. E. Kingma, W. L. J. Werner, S. Wolowiec, J. L. van der
Baan, S. Balt, F. Bickelhaupt, M. W. G. de Bolster and G. W.
Klumpp, Inorg. Chim. Acta, 1998, 269, 203.
31 M. Calligaris, D. Minichelli, G. Nardin and L. Randaccio, J. Chem.
Soc. A, 1971, 2720.
32 M. Calligaris, G. Nardin and L. Randaccio, Coord. Chem. Rev.,
1972, 7, 385.
33 J. P. Bouquiere, J. L. Finney, M. S. Lehmann, P. F. Lindley and
H. F. J. Savage, Acta Crystallogr., Sect. B, 1993, 49, 79.
34 G. Stork, A. Y. W. Leong and A. M. Touzin, J. Org. Chem., 1976, 41,
3491.
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J. Chem. Soc., Perkin Trans. 1, 2000, 1199–1210