A strategy for the synthesis of well-defined iron catalysts and application to regioselective diene hydrosilylation

Article abstract of DOI:10.1021/ja106853y

We report the development of a well-defined Fe catalyst and its application to the regio-and stereoselective 1,4-hydrosilylation of 1,3-dienes. To the best of our knowledge, this is the first example of accessing a characterized low-valent Fe catalyst by controlled reductive elimination from a readily accessible Fe precatalyst.

Full text of DOI:10.1021/ja106853y

Published on Web 09/01/2010
A Strategy for the Synthesis of Well-Defined Iron Catalysts and Application to
Regioselective Diene Hydrosilylation
Jessica Y. Wu, Benjamin N. Stanzl, and Tobias Ritter*
Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford Street,
Cambridge, Massachusetts 02138
Received July 31, 2010; E-mail: ritter@chemistry.harvard.edu
When our evaluation of known bis(aryl) Fe complexes8 for
reductive elimination resulted in complexes that either were
unstable or did not afford low-valent Fe complexes, we designed
the new bis(aryl) Fe(II) complex 2, readily prepared from FeCl2,
Abstract: We report the development of a well-defined Fe
catalyst and its application to the regio- and stereoselective 1,4-
hydrosilylation of 1,3-dienes. To the best of our knowledge, this
is the first example of accessing a characterized low-valent Fe
pyridine, n-BuLi, and N,N-dimethylbenzylamine (Scheme 1).
catalyst by controlled reductive elimination from a readily acces-
sible Fe precatalyst.
Scheme 1. Synthesis of Well-Defined Fe Catalyst 1
Iron catalysis for organic synthesis has experienced intermittent
development since work by Kharasch in 1941.1,2 Many Fe-catalyzed
reactions use simple Fe compounds such as FeCl3 and Fe(CO)5,2
which is advantageous for simplicity but rarely offers the op-
portunity to rationally improve reactivity or selectivity, because the
identities of the active catalysts are typically unknown, ill-defined,
or cannot be rationally altered. Purified and well-characterized low-
valent Fe catalysts are rare and typically challenging to prepare
but, when available, can be used for rational reaction improvement.3,4
Herein we report the development of a well-defined Fe catalyst
and its application to regio- and stereoselective hydrosilylation of
1,3-dienes (eq 1). This report describes the first example of
accessing a characterized low-valent Fe catalyst by controlled
reductive elimination from a readily accessible Fe precatalyst. Our
strategy allowed us to quickly and reliably identify suitable ligands
to control regio- and stereoselectivity for the hydrosilylation reaction
at previously unreported levels.
Complex 2 is sufficiently stable for isolation, likely due to the
chelating amino aryl ligands that position both carbon-based
ligands mutually pseudo-trans and therefore prevent reductive
elimination. Upon coordination of an exogenous ligand, however,
ligand rearrangement can take place, possibly by nitrogen
dissociation, and reductive elimination can occur. Addition of
2 equiv of iminopyridine ligand 3 to 2 produced the bis(imi-
nopyridine)Fe pyridine complex 1, which could be purified and
characterized.
For catalysis, we selected redox-active ligands on Fe,9 such
as the iminopyridine 3, to potentially lower activation barriers
due to metal-ligand redox chemistry. Redox-active ligands may
function as electron reservoirs and compensate the electronic
demand of the Fe at different stages of the catalytic cycle, similar
to metal-metal cooperation in bimetallic catalysis.10 Evidence
for metal-ligand redox participation in 1 was obtained spec-
troscopically (see Supporting Information) and suggests that Fe
complex 1 should be more correctly represented as Fe(II),
coordinated by two radical anion ligands, as previously observed
for related complexes.9
Complex 1 is a precatalyst for the 1,4-hydrosilylation of 1,3-
dienes (eq 1). A proposed catalytic cycle for the Fe-catalyzed
hydrosilylation is shown in Scheme 2. In contrast to our previous
work with ill-defined Fe species in heterogeneous reactions,6
reaction with 1 is homogeneous, is well behaved, and can be
followed kinetically. From the kinetic profiles shown in the insert
of Scheme 2, we identified that addition of exogenous pyridine and
Catalysis development with low-valent late transition metals
benefits from the availability of useful precatalysts such as Pd2dba3
and Ni(cod)2; no analogous complex has been available for Fe.
Fe(0)(CO)n and Fe(0)(cyclooctatetraene)2 complexes are accessible;
however, the increasing difficulty to displace all CO or COT ligands
only allows access to CO- or COT-containing Fe catalysts.5
Previously, we used magnesium metal to reduce Fe(II) complexes
to afford ill-defined catalysts.6 Reduction of Fe(II) complexes is
an established method to access low-valent Fe complexes but
requires optimization of reaction conditions for every individual
complex.7 The strategy reported here demonstrates access to low-
valent Fe catalysts by controlled reductive elimination from a single
Fe(II) bis(aryl) precatalyst upon exogenous ligand coordination.
Well-defined two-electron reductive elimination reactions, well-
known for other late transition metals such as Pd or Ni, are
uncommon for Fe.
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C O M M U N I C A T I O N S
Scheme 2. Proposed Fe-Catalyzed Hydrosilylation Mechanisma
Table 1. Regio- and Stereoselective Fe-Catalyzed Hydrosilylation
a Graph shows kinetic profiles for hydrosilylation catalyzed by 1, with
no added ligand (top), with added pyridine (middle), and with added 3
(bottom).
iminopyridine ligand 3 retarded the rate of hydrosilylation. Addition
of only 1 equiv of iminopyridine ligand to 2 generated a catalyst
more active than when 2 equiv of iminopyridine were added. While
no complex could be isolated and characterized, presumably due
to instability of a catalytically active, coordinatively unsaturated
Fe center, hydrosilylation efficiency increased. This procedure had
the added advantage that 2 could be used directly, without
conversion to and purification of 1.
Allylsilane synthesis by hydrosilylation can avoid allylmetal
addition to chlorosilanes, which is not compatible with several
electrophilic functional groups. Several regioselective diene hy-
drosilylation reactions catalyzed by various transition metals have
been reported.11 None, however, has been shown to generally afford
the allylsilane products in as high regioselectivity as we report here,
and often other constitutional isomers are observed as major
products.
Availability of the Fe precatalyst 2 allowed us to readily
evaluate different ligands to control regioselectivity. Catalyst 1
affords allylsilanes as a 1:2 mixture of constitutional isomers
(linear/branched, see Supporting Information), but we identified
iminopyridine ligands 4 and 5 that controlled regioselectivity
to afford the linear product predominantly, typically in a ratio
of 94:6 or greater (Table 1). Hydrosilylation proceeded for 1-,
2-, and 2,3-substituted dienes. Double bond geometry was
controlled in all cases with >99:1 selectivity, possibly a
consequence of the stereospecific mechanism suggested in
Scheme 2, without double bond isomerization. Several functional
groups, including epoxides, esters, and amines, are tolerated,
and different silanes can be used (7, 8, 9).
In conclusion, we report the synthesis and use of a well-defined
Fe complex as a precatalyst for 1,4-hydrosilylation of 1,3-dienes
to afford allylsilanes in high selectivities that have not been reported
with other methods. Reductive elimination from 2 can generate
several Fe catalysts that control the regio- and stereoselectivity of
hydrosilylation. Access to a low-valent Fe precatalyst can support
the development of Fe catalysis.
a Isolated yield; average of two runs. b 5 mol % ligand 5. c 15 mol %
2 and 15 mol % 4. d 10 mol % 2 and 10 mol % 4.
Supporting Information Available: Detailed experimental proce-
dures and spectroscopic data for all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
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