Ionic hydrogenation of acylferrocenes using zinc borohydride: An efficient, mild method for the preparation of alkylferrocenes

Article abstract of DOI:10.1021/om950751n

An effective mild procedure for the reductive deoxygenation of α-ferrocenyl aldehydes, ketones, and alcohols into the corresponding alkylferrocenes is described using a combination of zinc borohydride and zinc chloride. This is the first example of such reactivity of zinc borohydride. The present method allows the synthesis of alkylferrocenes bearing terminally functionalized pendant chains.

Full text of DOI:10.1021/om950751n

Organometallics 1996, 15, 1065-1066
1065
Ion ic Hyd r ogen a tion of Acylfer r ocen es Usin g Zin c
Bor oh yd r id e: An Efficien t, Mild Meth od for th e
P r ep a r a tion of Alk ylfer r ocen es
Sukanta Bhattacharyya*
Department of Chemistry, Vijoygarh College, Calcutta 700032, India
Received September 22, 1995X
Summary: An effective mild procedure for the reductive
deoxygenation of R-ferrocenyl aldehydes, ketones, and
alcohols into the corresponding alkylferrocenes is de-
scribed using a combination of zinc borohydride and zinc
chloride. This is the first example of such reactivity of
zinc borohydride. The present method allows the syn-
thesis of alkylferrocenes bearing terminally functional-
ized pendant chains.
transformation of acylferrocenes into the corresponding
alkylferrocenes constitutes the most frequently encoun-
tered reaction5 in the preparation of both simple alkyl-
ferrocenes and those bearing terminally functionalized
pendant alkane chains. The prominent protocols for
this transformation include catalytic hydrogenolysis,6
Clemensen reduction,7 and reductive deoxygenation
employing lithium aluminum hydride8 in the presence
of anhydrous aluminum chloride. However, the pres-
ence of other functionalities is limited under these
reaction conditions.
As part of our efforts in the development of mild
borohydride-based reagent systems9 for selective trans-
formations, we have recently utilized10 a combination
of zinc chloride and zinc borohydride in the reductive
methylation of amines. In a continuing study, this
paper reports on the results for the novel application of
this reagent system in the ionic hydrogenation of
acylferrocenes at room temperature, an effective mild
method for the preparation of alkylferrocenes (Scheme
1). The use of zinc borohydride, prepared from sodium
borohydride and zinc chloride, as a uniquely mild
reducing agent has been amply demonstrated in the
literature.11
Stirring a mixture of acetylferrocene, 2 mol equiv of
zinc chloride, and 2 mol equiv of zinc borohydride in
anhydrous THF at room temperature for 8 h provided
ethylferrocene in 92% yield after quenching of the
reaction with aqueous ammonia, extraction with diethyl
ether, and concentration. The reaction proceeds under
mild conditions, the procedure is easy to perform, and
no special handling technique is needed. The method
is general for a variety of R-ferrocenyl aldehydes,
ketones, and alcohols. All reactions were carried out
In recent years ferrocene derivatives have emerged
as the most versatile reagent for materials and cata-
lysts1 because of their unique optical, thermal, and
redox properties. The design and synthesis of ferrocene
derivatives possessing terminally functionalized alkane
chains are of particular interest in view of their poten-
tial utilizations2 as electrode surface modifiers, in the
constructions of dendrimers, in molecular recognition,
and in multielectron redox catalysis, among other
applications.
The Friedel-Crafts alkylation3 of ferrocene, which is,
in principle, the most direct route for the preparation
of alkylferrocenes, however, proceeds in poor yields and
invariably produces intractable mixtures of mono- and
polyalkylated derivatives. In contrast, the Friedel-
Crafts acylation reactions4 of ferrocene afford excellent
yields of mono- and diacylated ferrocenes with a re-
markable degree of regioselectivity. Accordingly, the
* Current address: Department of Chemistry, University of Mis-
sissippi, University, MS 38677.
X Abstract published in Advance ACS Abstracts, J anuary 1, 1996.
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0276-7333/96/2315-1065$12.00/0 © 1996 American Chemical Society

1066 Organometallics, Vol. 15, No. 3, 1996
Notes
Sch em e 1
ferrocenophane in high yield. Unlike the existing
procedures, the mild reaction conditions employed in
this method allow syntheses of alkylferrocenes bearing
terminally functionalized pendant chains; e.g., ((3-
carbomethoxy)propyl)ferrocene (entry 5) and (3-carboxy-
propyl)ferrocene (entry 6) are obtained in high yields
from the corresponding acylferrocenes. It is noteworthy
that attempts14 to perform Wolff-Kishner or Clem-
mensen reductions on similar acylferrocenes were un-
successful.
Ta ble 1. Ion ic Hyd r ogen a tion of Acylfer r ocen es
a n d r-F er r ocen yl Alcoh ols Usin g Zin c Bor oh yd r id e
a n d Zin c Ch lor id e
Because of the neutral reaction conditions, easy
workup, high yields of pure products, and compat-
ibility10,11 of zinc borohydride with a variety of normally
reducible functional groups, this procedure promises to
be the method of choice for the preparation of alkylfer-
rocenes, particularly for those containing functionalized
alkane chains.
In conclusion, alkylferrocenes are prepared in high
yields by ionic hydrogenation of ferrocenyl aldehydes,
ketones, and alcohols using zinc chloride and zinc
borohydride. Further utilization of this reagent system
in reductive deoxygenation of other types of ketones is
currently being explored.
Exp er im en ta l Section
The commercially available starting ferrocenes (entries 1-3
and 9-11)) were used as received from their respective
suppliers. The bridged diketone (entry 7) was prepared from
commercially available 1,1′-diacetylferrocene by following a
known procedure.13 The remaining substrates were prepared4
by Friedel-Crafts acylation of ferrocene. THF was freshly
distilled from LAH before use. A stock solution of zinc
borohydride in dry THF prepared from sodium borohydride
and zinc chloride according to the reported11a procedure can
be stored in a sealed container at 0-5 °C for 3-4 weeks.
Gen er a l P r oced u r e for th e P r ep a r a tion of Alk ylfer -
r ocen es fr om Acylfer r ocen es. To a magnetically stirred
mixture of the acylferrocene (5 mmol) and zinc chloride (10
mmol) in anhydrous THF (15 mL) was added a solution of zinc
borohydride (10 mmol) in THF dropwise over 15 min. The
contents were further stirred at room temperature for 8-10
h. The reaction mixture was then diluted with diethyl ether
(50 mL) and quenched with aqueous ammonia (30 mL, 2 N).
The organic layer was separated, and the aqueous part was
extracted with diethyl ether (2 × 20 mL). Drying of the
combined organic extracts over Na2SO4, solvent removal, and
flash chromatography of the residue over silica gel using
hexanes-diethyl ether (9:1) as the eluent afforded the pure
alkylferrocenes.
a
The ratios starting compound:zinc chloride:zinc borohydride
are as follows: entries 1-6, 1:2:2; entries 7-9, 1:4:4; entry 10,
1:1:1; entry 11, 1:2:2. Spectroscopic and physical constant data
b
for all compounds were in complete agreement with the literature
data. c Yields are of isolated and purified products.
at room temperature, and THF was the solvent of
choice. The molar ratio of the reactants and the results
obtained for a representative group of compounds are
shown in Table 1. It seems likely that the reaction
proceeds through the formation of the corresponding
R-ferrocenyl alcohol as the intermediate, which is then
reduced via the facile formation of a remarkably stable
R-ferrocenylalkyl carbocation12 in the presence of zinc
chloride. The conversion of the R-ferrocenyl alcohols
(entries 10 and 11) into the corresponding alkylfer-
rocenes following the same procedure further cor-
roborates the intermediacy of R-ferrocenyl alcohols in
these reactions. As shown in Table 1, the method is
equally efficient for the deoxygenation of diacylated
ferrocenes (entries 7-9). The bridged ferrocenyl dike-
tone13 (entry 7) can be converted into the corresponding
In the case of diacylated ferrocenes the same general
procedure was followed, except that double the amounts of zinc
chloride and zinc borohydride were used.
Su p p or tin g In for m a tion Ava ila ble: Figures giving 1H
NMR spectra of all the products (9 pages). Ordering informa-
tion is given on any current masthead page.
OM950751N
(13) Ito, Y.; Konoike, T.; Harada, T.; Saegusa, T. J . Am. Chem. Soc.
1977, 99, 1487.
(14) Graham, P. J .; Lindsey, R. V.; Parshall. G. W.; Peterson, M.
L.; Whitman, G. M. J . Am. Chem. Soc. 1957, 79, 3416.
(12) (a) Cais, M. Organomet. Chem. Rev. 1966, 1, 435. (b) Feinberg,
J .; Rosenblum, M. J . Am. Chem. Soc. 1969, 91, 4324. (c) Turbitt, T.
D.; Watts, W. E. J . Chem. Soc., Chem. Commum. 1973, 182.