Synthesis of transition metal isocyanide compounds from carbonyl complexes via reaction with Li[Me3SiNR]

Article abstract of DOI:10.1039/b917156h

The reaction between a transition metal carbonyl compound, L _nMCO, and Li[Me₃SiNR] yields the corresponding isocyanide derivative, L_nMCNR, thereby providing a new route to transition metal isocyanide compounds that does no

Full text of DOI:10.1039/b917156h

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Synthesis of transition metal isocyanide compounds from carbonyl
complexes via reaction with Li[Me3SiNR]wz
Wesley Sattler and Gerard Parkin*
Received (in Berkeley, CA, USA) 19th August 2009, Accepted 12th October 2009
First published as an Advance Article on the web 12th November 2009
DOI: 10.1039/b917156h
The reaction between a transition metal carbonyl compound,
LnMCO, and Li[Me3SiNR] yields the corresponding isocyanide
derivative, LnMCNR, thereby providing a new route to transi-
tion metal isocyanide compounds that does not require the use of
free isocyanides as reagents.
we report that the conversion of LnMCO to LnMCNR may
be achieved via treatment of the metal carbonyl with
Li[Me3SiNR], a reagent that may be generated by, for
example, treatment of RNH2 with Me3SiCl followed by
BunLi.18 Furthermore, since primary amines are commonly
used precursors for free isocyanides,1f,19 it is evident that this
method enables the synthesis of metal isocyanide compounds
from RNH2 without having to synthesize first the free
isocyanide. In addition to the synthetic simplicity, this method
has the advantage of eliminating problems associated with the
putrid smell of the free isocyanide compounds.19b
Isocyanides (RNC) feature prominently in transition metal
chemistry by virtue of their ability to serve as combined
s-donor and p-acceptor ligands.1–3 While this dual feature
of the bonding is common to CO, an important distinction
between RNC and CO is that the steric and electronic properties
of isocyanides may be effectively tuned by modifying the
substituent on nitrogen. Indeed, this aspect has been of benefit
in the application of transition metal isocyanide complexes
as catalysts for organic transformations.4–7 Despite their
widespread use, however, the majority of transition metal
isocyanide complexes feature rather simple substituents on
nitrogen, a reflection of the paucity of readily available
commercial isocyanide compounds. Progress in transition
metal isocyanide chemistry would, therefore, be facilitated
by the development of new synthetic methods that do not
require the use of the isocyanide as a reagent. Two such
approaches include (i) the formal addition of R+ to the
nitrogen atom of a metal cyanide ligand1b,c,f and (ii) the formal
replacement of the oxygen atom of a metal carbonyl ligand by
an NR group.8–10 The latter is a particularly appealing
approach in view of the ubiquity of transition metal carbonyl
compounds, and here we describe a new method that utilizes
Li[Me3SiNR] as a reagent for converting a carbonyl compound
into its isocyanide counterpart.
For example, M(CO)6 reacts with Li[Me3SiNBut]20 to yield
mono, bis, tris and tetrakis derivatives, namely M(CO)5(CNBut)
(M = Cr, Mo, W), cis-M(CO)4(CNBut)2 (M = Cr, Mo, W),
fac-M(CO)3(CNBut)3 (M = Cr, Mo, W), and cis-M(CO)2-
(CNBut)4 (M = Mo), as illustrated in Scheme 1. The syntheses
21
of the molybdenum compounds Mo(CO)6ꢀn(CNBut)n is
of note in view of their recent use as catalysts for
organic transformations.5 While the isocyanide complexes
M(CO)6ꢀn(CNBut)n have been previously synthesized by
reactions employing ButNC as
a
reagent,22 we have
determined the molecular structures of Cr(CO)5(CNBut),
cis-M(CO)4(CNBut)2 (M = Cr, Mo, W), fac-W(CO)3(CNBut)3,23
and cis-Mo(CO)2(CNBut)4 by X-ray diffraction.
The synthetic method is not restricted to the Group 6
metals, as illustrated by the fact that Li[Me3SiNBut] also
reacts with Fe(CO)5 to give axial-Fe(CO)4(CNBut) and
trans-Fe(CO)3(CNBut)2 (Scheme 1). While
a variety of
compounds of the class Fe(CO)5ꢀn(CNR)n are known,24 the
only structurally characterized heteroleptic examples of which
we are aware are disubstituted derivatives Fe(CO)3(CNR)2,
25
The carbon atom within a metal carbonyl moiety, LnMCO,
is well known to be susceptible to nucleophilic attack by
external reagents.11 For example, attack by carbon nucleo-
philes has been employed in the syntheses of Fischer carbene
complexes,12 while attack by oxygen nucleophiles features
prominently in the mechanisms of (i) the water-gas shift
reaction,13 (ii) the exchange of oxygen atoms between a
carbonyl ligand and H2O,14 and (iii) the Me3NO induced
dissociation of a carbonyl ligand as CO2.15 The carbon atom
of LnMCO is also susceptible to attack by nitrogen based
nucleophiles16,17 which provides, in principle, a first step for
converting a carbonyl to an isocyanide ligand. In this regard,
namely Fe(CO)3(CNMe)2
and Fe(CO)3(CNBut)2.26 The
molecular structure of monosubstituted Fe(CO)4(CNBut) is,
therefore, presented in Fig. 1, thereby demonstrating that the
isocyanide ligand adopts an axial position, with an Fe–C bond
length (1.89 A) that is distinctly longer than those of the
carbonyl ligands (1.79–1.81 A).
In addition to the synthesis of ButNC complexes, the
method may also be employed to obtain complexes that
feature other isocyanide ligands. For example, the arylisocyanide
derivatives Mo(CO)5(CNAr) (Ar = p-C6H4Me,27 p-C6H4OMe3)
and Mo(CO)4(CNAr)2 (Ar = 2,4,6-C6H2Me3) have been synthe-
sized by this approach, of which Mo(CO)5(CN-p-C6H4OMe)
and Mo(CO)4(CN-2,4,6-C6H2Me3)2 have been structurally
characterized by X-ray diffraction.
Department of Chemistry, Columbia University, New York,
NY 10027, USA. E-mail: parkin@columbia.edu
w Dedicated to Professor Jenny Green on the occasion of her retirement.
z Electronic supplementary information (ESI) available: Experimental
details and crystallographic data. CCDC 744953–744964. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/b917156h
Since attention has recently been given to the application of
isocyanide ligands that feature bulky6,28,29 and chiral8
substituents, we have evaluated the ability to synthesize such
complexes via Li[Me3SiNR]. For example, the adamantyl
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Fig. 2 Molecular structure of Mo(CO)5(CN-1-Ad).
Scheme 1
Fig. 3 Molecular structure of Mo(CO)5(CN-1-NpH ).
4
known to react with CO to generate [R2NC(O)Li]35 and that
silyl derivatives of the type [(R3Si)N(R)C(O)Li] decompose to
give RNC and R3SiOLi.34
The overall carbonyl–isocyanide conversion employing
RNH2 is related to the observation that thiocarbonyl
compounds react with RNH2 to generate the isocyanide
derivative, accompanied by elimination of H2S.36 An
important distinction between the reactivity of carbonyl and
thiocarbonyl ligands, however, is that the required elimination
of H2O to convert a carbonyl to an isocyanide derivative is not
a facile process. Thus, carbonyl compounds simply react with
RNH2 (but not aryl amines) to give carbamoyl derivatives of
the type {LnM[C(O)NHR]}[RNH3]16 and conversion to
LnMCNR requires treatment with dehydrating agents such
as C(O)Cl2 and [C(O)Cl]2.10 For example, CpFe(CO)2-
[C(O)NHMe] reacts with C(O)Cl2 in the presence of Et3N to
give [CpFe(CNMe)]Cl.10a The ability of Li[Me3SiNR] to
achieve deoxygenation of the carbonyl ligand may be attributed
to the strength of the Si–O bond which provides a driving
Fig. 1 Molecular structure of axial-Fe(CO)4(CNBut).
complexes Mo(CO)5(CN-1-Ad) and cis-Mo(CO)4(CN-1-Ad)2
may be synthesized by this approach (Scheme 1) and have
been structurally characterized by X-ray diffraction (see, for
example, Fig. 2).30 Likewise, the enantiomerically pure (R)-
1,2,3,4-tetrahydro-1-naphthyl derivative, Mo(CO)5(CN-1-NpH ),
4
has also been isolated and structurally characterized (Fig. 3).
Compared to tertiary phosphine counterparts, enantio-
merically pure isocyanide ligands have been little employed
in transition metal chemistry,8 and the synthetic transformation
described here provides a simple means to access transition
metal compounds of such ligands from the chiral amine.
The mechanism proposed for the conversion of the
carbonyl ligand to an isocyanide ligand is illustrated in
Scheme 2, with the essential features involving nucleophilic
attack by [Me3SiNR]ꢀ on the carbonyl carbon of LnMCO to
generate a silylcarbamoyl intermediate LnM[C(O)N(R)SiMe3]ꢀ,
followed by elimination of Me3SiOꢀ.31–34 Precedent for this
mechanism is provided by the fact that lithium amides are
Scheme 2
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force for the reaction. In this regard, the use of Li[Me3SiNR]
to effect the CO–CNR conversion of a carbonyl compound is
analogous to the formation of isocyanide compounds by using
phosphorimidates, Li[(EtO)2(O)PNR],8 and phosphinimines,
R3PNR0,9 for which P–O bond formation presumably
provides the driving force.
12 (a) E. O. Fischer, Adv. Organomet. Chem., 1976, 14, 1; (b) P. de
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14 D. J. Darensbourg and J. A. Froelich, J. Am. Chem. Soc., 1977, 99,
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While the formation of an isocyanide complex by reaction of
a thiocarbonyl derivative with RNH2 may be considered a more
appealing direct method of synthesis than is the reaction of a
carbonyl complex with Li[Me3SiNR], a downside of the former
route is that compounds which feature thiocarbonyl ligands are
much less common than carbonyl derivatives.37 Therefore, the
reaction of a carbonyl derivative with Li[Me3SiNR] provides
a convenient alternative for the synthesis of isocyanide
compounds from carbonyl derivatives. In addition, another
advantage of using Li[Me3SiNR] to synthesize transition metal
isocyanide compounds is that Li[Me3SiNR] derivatives
are obtained from primary amines, which are more readily
available commercially than are isocyanides.
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We thank the US Department of Energy, Office of Basic
Energy Sciences (DE-FG02-93ER14339) for support of this
research. The National Science Foundation (CHE-0619638) is
thanked for acquisition of an X-ray diffractometer. Aaron
Sattler is thanked for helpful discussions.
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7568 | Chem. Commun., 2009, 7566–7568