Diastereo- and regioselective reactions of π-allyl molybdenum complexes with aldehydes

Article abstract of DOI:10.1016/S0040-4039(00)00066-6

Remote heteroatom coordination to molybdenum promotes unprecedented selectivity in aldehyde allylation using unsymmetrically substituted allylic metal complexes. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00066-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1335–1338
Diastereo- and regioselective reactions of π-allyl molybdenum
complexes with aldehydes
Marie E. Krafft ∗ and Martin J. Procter
Department of Chemistry, Florida State University, Tallahassee, FL 32306-4390, USA
Received 23 July 1999; accepted 26 July 1999
Abstract
Remote heteroatom coordination to molybdenum promotes unprecedented selectivity in aldehyde allylation
using unsymmetrically substituted allylic metal complexes. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: alcohols; allylation; allyl complexes; heteroatom-directed; molybdenum.
Allylation of nucleophiles or electrophiles using both main group and transition metal complexes is an
important and synthetically useful transformation that is applicable to the stereocontrolled synthesis of
acyclic systems.1,2 For example, η3-allyl Pd(+2) complexes stand out as highly versatile intermediates for
the effective allylation of nucleophiles.3 On the other hand, η1-allylstannanes and allylboranes have been
found to be particularly useful in the nucleophilic allylation of aldehydes and ketones.4 Faller has shown
that aldehydes undergo condensation with monosubstituted η3-allylmolybdenum5 complexes to yield
homoallylic alcohols with regio- and diastereocontrol (Eq. (1)).6–8 As part of our continuing investigation
into substrate directable reactions,9 we investigated the effect of tethered amines and sulfides on the
condensation of η3-allylmolybdenum complexes with carbonyl compounds. We now report a high degree
of selectivity in the reaction of aldehydes with both mono- and 1,3-disubstituted η3-allylmolybdenum
complexes (Eq. (2)).
(1)
(2)
∗
Corresponding author. E-mail: mek@chem.fsu.edu (M. E. Krafft)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00066-6
tetl 16346

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Molybdenum complexes 3–6 bearing tethered homoallylic amines and sulfides were synthesized by
heating a solution of allylic acetates 1 or 2, Mo(CO)6 and the sodium salt of diethyl malonate, 2,4-
pentanedione or 1,1,1-trifluoro-2,4-pentanedione in THF.10 The complexes were air stable, prepared in
one step compared to Faller’s multistep procedure, and could be purified by silica gel chromatography.
The structure of 3 was determined crystallographically. 10
Table 1
Treatment of molybdenum complex 3 with benzaldehyde in CH2Cl2 in the presence of MeOH for
48 h at ambient temperature yielded homoallylic alcohol 7 as a single regioisomer (Table 1). No other
1
regioisomeric alcohols could be detected by H NMR spectroscopy (500 MHz). In contrast to prior
results6 (Eq. (1)) where carbon–carbon bond formation occurred only at the more substituted terminus,
with these examples, carbon–carbon bond formation occurred exclusively at the unsubstituted terminus.
As demonstrated by the results in Table 1, both aliphatic and aromatic aldehydes underwent allylation
efficiently and selectively. Heating a solution of complex 3 with benzaldehyde in THF in the presence
of MeOH led to a slight decrease in yield, but shortened the reaction time to 24 h (Table 1, entry 2).
Changes in the nature of the bidentate ligand had a significant effect on the efficacy of the aldehyde
condensation. Substitution of the 2,4-pentanedionate ligand with 1,1,1-trifluoro-2,4-pentanedionate gave
complex 4 (1:1 mixture of isomers due to different orientations of the fluoro-substituted pentanedionate
ligand) which was completely unreactive toward benzaldehyde even upon heating. Unlike the reactions
with amino complex 3, which were very selective, reaction of sulfide complex 5 with benzaldehyde
resulted in the formation of a 6:1 mixture of regioisomers (Table 1, entry 5). The sulfide complex 6 was
relatively unstable compared to the corresponding 2,4-pentanedionate derivative 5, and, while it could be
isolated and characterized, it decomposed in situ before significant amounts of aldehyde allylation could
be observed (Table 1, entry 6).
The explanation for the diastereoselectivity observed by Faller6 (Eq. (1)) incorporates a π to σ allyl
shift and coordination of the aldehyde to the metal followed by reaction via a chair-like conformation of
a six-membered transition state (A in Scheme 1). Preferential formation of the less substituted molyb-
denum–carbon bond provides a rationale for the high regiochemical preference. Using an analogous
mechanistic argument for the reactions of 3 and 5, coordination of the heteroatom to the metal center
dictates the formation of a secondary metal–carbon bond with the reaction now being driven to occur at
the less substituted allylic terminus (B in Scheme 1, R=H).

1337
Scheme 1.
Reaction of the 1,3-unsymmetrically disubstituted π-allyl molybdenum complexes 11 or 12 (prepared
from the corresponding amino allylic acetate as described above) with aliphatic or aromatic aldehydes
gave rise to the anti diastereomer in addition to small amounts of the corresponding regioisomer (Table 2).
The examples in Table 2 are noteworthy because of the high levels of regioselective aldehyde allylation
observed using unsymmetrically substituted allylic moieties, even when the groups at the allylic termini
are similar in size. It is remarkable that complex 11, which is a 1:1 mixture of isomers, reacts with
high regio- and diastereoselectivity. Reaction of complex 12, bearing a 2,4-pentanedionate chelating
ligand rather than a malonate ligand, with benzaldehyde led to the expected homoallylic alcohol 14
in 60% yield, but the reaction required 72 h at ambient temperature. Complex 13 (1:1 mixture of
isomers due to different orientations of the fluoro-substituted pentanedionate ligand), bearing the electron
deficient 1,1,1-trifluoro-2,4-pentanedionate ligand, failed to react with aldehydes and decomposition of
the complex occurred under the reaction conditions. The stereochemical outcome of the reactions in
Table 2 can be rationalized as resulting from reaction via a chair-like conformation of a six-membered
transition state (Scheme 1, B (R=CH3)). The relative stereochemistry of alcohol 14 was determined by
conversion to ketal 1711 (Scheme 2).
Table 2
We have demonstrated that a chelating heteroatom can be used to control the regioselectivity of
molybdenum mediated aldehyde allylation thus providing excellent acyclic stereocontrol. The residual
amine or sulfide is ideally positioned for further functional group manipulations. Further work in this
area is in progress.

1338
Scheme 2.
Acknowledgements
Support of this work from the National Science Foundation is gratefully acknowledged.
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