Synthesis and Lewis acid properties of a ferrocene-based planar-chiral borenium cation

Article abstract of DOI:10.1039/c3cc41556b

The first planar chiral ferrocenylborenium species (pR)-3⁺ is obtained in the enantiopure form by halide abstraction from the corresponding chloroborane adduct (pR)-2 using Krossing's salt. Competition experiments suggest that the Lewis acidity of (pR)-3⁺ is higher than that of B(C₆F₅)₃ towards anions and slightly lower towards neutral Lewis bases. The ferrocenylborenium species (pR)-3⁺ is examined as a catalyst for the stereoselective hydrosilylation of ketones. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc41556b

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Synthesis and Lewis acid properties of a
ferrocene-based planar-chiral borenium cation†
Cite this: DOI: 10.1039/c3cc41556b
Jiawei Chen, Roger A. Lalancette and Frieder Jakle*
¨
Received 28th February 2013,
Accepted 26th March 2013
DOI: 10.1039/c3cc41556b
www.rsc.org/chemcomm
The first planar chiral ferrocenylborenium species (pR)-3+ is obtained in
the enantiopure form by halide abstraction from the corresponding
chloroborane adduct (pR)-2 using Krossing’s salt. Competition experi-
ments suggest that the Lewis acidity of (pR)-3+ is higher than that of
B(C6F5)3 towards anions and slightly lower towards neutral Lewis
bases. The ferrocenylborenium species (pR)-3+ is examined as a catalyst
for the stereoselective hydrosilylation of ketones.
Chart 1 Examples of highly Lewis acidic organoboranes.
Highly Lewis acidic organoboranes continue to attract tremendous
interest and current applications range from Lewis acid catalysis of
organic transformations1 and polymerization reactions2 to small
molecule activation,3,4 sensors for anions,5 and the development of
new and unusual electronic materials.6 Many of the earlier studies
have focused on installing electron withdrawing fluorine substituents
onto organoboranes as a means of increasing the electron-deficient
character of boron, and B(C6F5)3 is nowadays broadly applied not
only in academia but has also found its way into a wide range of
industrial processes.7 An alternative approach involves the incorpora-
tion of tricoordinate boron into antiaromatic cyclics as in borole and
diborabenzene derivatives. In this case, Lewis acid–base adduct
formation is energetically favored by conversion to non-aromatic
species.8 Most recently, highly reactive borenium species9,10 have
been introduced, in which the cationic charge further enhances the
potency of tricoordinate borane Lewis acids (Chart 1).11
a bifunctional ferrocene-based Lewis acid as a stereoselective reagent
for the allylation of ketones with ee’s of up to 80%.15 We have also
introduced fluorinated arylboranes attached to a chiral naphthylferro-
cenyl framework.18 Related planar-chiral Z5-1,2-azaborolyl iron com-
plexes have been utilized by Fu and coworkers in Mukayama
aldol reactions and in stereoselective additions to imines.20
Interestingly, planar chiral borenium cations were proposed as
intermediates in these processes. We describe herein the first
synthesis of an isolable planar-chiral ferrocenylborenium cation,
discuss the Lewis acid properties and present a proof-of-principle
investigation of its utility in the hydrosilylation of ketones.
The chloroborane adduct (pR)-2 was prepared by treatment of
the organotin compound (pS)-121 with an excess of PhBCl2 in a
hexanes–toluene mixture (Scheme 1).22,23 Based on 1H NMR studies
(Fig. 1a) two different species (ratio 2.5 : 1) are present in solution,
which are attributed to the exo-Ph and endo-Ph isomers of (pR)-2.
Borenium species have proven to be highly advantageous
especially in the promotion of electrophilic aromatic substitution
chemistries.10,12 Very recently the first studies of their use in hydro-
genation and hydroboration catalysis have also been reported.13 For
broad applications in organic synthesis it is desirable to develop and
utilize chiral borenium Lewis acids. Chiral alkyl- and arylboranes,
including fluorinated derivatives, are widely applied as reagents
and catalysts in stereoselective synthesis.14–16 We have, over
the past several years, pursued planar chiral ferrocene-based
Lewis acids.15,17–19 We have demonstrated, for example, the utility of
Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark,
NJ, USA. E-mail: fjaekle@rutgers.edu; Fax: +1 973 353 1264; Tel: +1 973 353 5064
† Electronic supplementary information (ESI) available: Experimental details.
CCDC 907178 and 907179. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3cc41556b
Scheme 1 Synthesis of (pR)-3+. (i) 2 PhBCl2, toluene–hexanes, ꢀ37 1C to rt.
(ii) Method A: [Ag(CH2Cl2)]{Al[OC(CF3)3]4}, CHCl3, ꢀ37 1C to rt; Method B:
Li{Al[OC(CF3)3]4}, CDCl3, rt.
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Fig. 2 (a) Ortep plot of (pR)-3+ (50% thermal ellipsoids); counteranions and H atoms
are omitted. Selected distances (Å) and angles (1) [2nd independent molecule]:
B1–N1 1.544(7) [1.550(9)], B1–C2 1.519(9) [1.539(9)], B1–C18 1.524(9) [1.504(10)],
C2–B1–N1 102.2(5) [100.6(5)], C2–B1–C18 131.4(5) [132.2(6)], C18–B1–N1 126.3(5)
[127.1(6)]. (b) Ortep plot of (pR)-3+(acetophenone) (50% thermal ellipsoids); counter-
anions and H atoms are omitted. Selected distances (Å) and angles (1) [2nd indepen-
dent molecule]: B1–N1 1.603(9) [1.592(9)], B1–O1 1.560(8) [1.577(8)], B1–C2 1.580(11)
[1.593(10)], B1–C18 1.603(11) [1.594(10)], O1–C24 1.257(8) [1.255(8)].
Fig. 1 Expansion of the Cp region of the 1H NMR spectra for (a) (pR)-2 (exo and
endo-isomers), (b) (pR)-2 + B(C6F5)3 and (c) (pR)-3+ (CDCl3) and photographs
illustrating the associated color change.
According to a 2D NOESY experiment,† the major isomer features
the Ph group in the exo-position and the minor isomer in the endo-
position. Coalescence was not observed in the 1D 1H NMR spectra
even at temperatures up to 70 1C. This indicates that isomerization amount of B(C6F5)3 led to only a very modest downfield shift of the Cp
is slow on the NMR time scale. resonances (see Fig. 1b). This implies that only partial abstraction
We initially attempted to prepare the corresponding borenium ({50%) of chloride from (pR)-2 occurs and suggests that (pR)-3+ as a
cation by Clꢀ abstraction with B(C6F5)3. Indeed, when compound cation has a higher affinity than B(C6F5)3 towards anionic Lewis bases
(pR)-2 was mixed with one equivalent of B(C6F5)3, coalescence into a such as Clꢀ. To also explore the Lewis acid properties of (pR)-3+ toward
single set of broad 1H NMR resonances was observed (Fig. 1b). neutral substrates we performed a competitive binding study, in which
However, the Lewis acid strength of B(C6F5)3 proved to be far too low a mixture of the borenium species (pR)-3+ and B(C6F5)3 was treated with
to bring about complete abstraction of Clꢀ. In contrast, treatment of an equimolar amount of the neutral Lewis base Et3PO. The complexes
(pR)-2 with 1 equiv. of Krossing’s salt24 [Ag(CH2Cl2)]{Al[OC(CF3)3]4} or Et3POꢁ(pR)-3+ and Et3POꢁB(C6F5)3 are present in a ratio of 1 : 3.6 at room
Li{Al[OC(CF3)3]4} in CHCl3 led to quantitative chloride abstraction with temperature, which shows that slightly more Et3PO binds to B(C6F5)3
the formation of the tricoordinate borenium cation (pR)-3+ (Scheme 1). than to (pR)-3+.† We conclude that (pR)-3+ has a higher affinity than
1
The H NMR spectrum showed strongly downfield shifted signals at B(C6F5)3 towards anionic Lewis bases such as Clꢀ and is only slightly
5.75, 5.58 and 5.48 ppm for the substituted Cp ring, consistent with less Lewis acidic than B(C6F5)3 towards neutral Lewis bases such as
attachment of a highly electron-deficient borenium cation (Fig. 1c). In Et3PO. In short, (pR)-3+ proved to be a remarkably strong Lewis acid.
the 11B NMR spectrum a broad signal was observed at 45.4 ppm for the
An application of Lewis acids in organic synthesis that has
cation and in the 19F NMR spectrum a sharp signal at ꢀ75.4 ppm for attracted considerable interest is the metal-free hydrosilylation
the anion. Another characteristic feature of the borenium cation (pR)-3+ of ketones. This process was first studied for B(C6F5)3 by Parks
is its deep purple color (Fig. 1, lmax = 566 nm, e = 2200 Mꢀ1 cmꢀ1). This and Piers.31 Piers et al. demonstrated that activation of the Si–H
unusual color is likely due to enhanced charge transfer from iron to functionality in silanes with B(C6F5)3 plays a key role4 and more
the more extended and highly electron-deficient p-system of the recent studies by Mewald and Oestreich further support this
pyridylborenium heterocycle in (pR)-3+.25
A single crystal X-ray diffraction analysis confirmed retention of sions of this process continues to be a challenge.14
the planar chirality upon halide abstraction from (pR)-2 to give (pR)-3+ We first examined the reactivity of (pR)-3+ toward ketones and
notion.32 However, the development of enantioselective ver-
(Fig. 2a). As expected, the highly electron-deficient nature of the silanes, respectively. When (pR)-3+ was mixed with 1 equiv. of HSiEt3
borenium moiety results in a strong interaction between B and in CDCl3 at RT, the 1H NMR spectra of both components were
the electron-rich Fe, which is reflected in pronounced bending of unchanged. However, at elevated temperatures a loss of H–Si–CH2
the boryl group out of the Cp plane, towards the iron center. The tilt coupling was clearly observed,† which is consistent with an R03Si–
angles for the boryl groups in two independent cations are 14.11 HꢁꢁꢁBR3 interaction according to studies by Koller and Bergman33
(FeꢁꢁꢁB 2.925 Å) and 14.51 (FeꢁꢁꢁB 2.907 Å), respectively, which is on related Al-based systems. Treatment of compound (pR)-3+ with
similar to Piers’ perfluorophenyl-substituted FcB(C6F5)226 (161, FeꢁꢁꢁB acetophenone resulted in a distinct color change from deep purple
2.924 Å) and FcBBr227 (17.7, 18.91; FeꢁꢁꢁB 2.856/2.840 Å).28 It is to orange-red due to adduct formation.34 The complex (pR)-3+(aceto-
noteworthy that the CpCENTꢁꢁꢁCpCENT distance of 3.316 Å is consistent phenone) can be readily isolated as a crystalline solid and its single
with a neutral ferrocene,29 supporting the assignment of (pR)-3+ as a crystal X-ray structure shows binding of the acetophenone from the
borenium-substituted ferrocene rather than a borane-substituted endo-side of the ferrocene (B–O = 1.560(8) [1.577(8)] Å, Fig. 2b). When
ferrocenium species.30 Another interesting aspect is that the planar a sample of this complex was treated with an excess of HSiEt3, the
p-conjugated structure of (pR)-3+ favors formation of inter-molecular acetophenone was reduced and fully converted to the hydrosilylation
p–p dimers that consist of two independent cations.†
product. The color of the solution changed back from orange-red to
A first indication of the high Lewis acidity of (pR)-3+ came dark purple, and the 1H NMR data were consistent with re-formation
from the fact that the reaction of (pR)-2 with an equimolar of the free borenium ion (pR)-3+. The fact that the lower basicity of the
c
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In conclusion, halide abstraction from the Lewis acid–base
pair (pR)-2 was accomplished using the Ag or Li derivative of
Krossing’s salt, resulting in a novel planar-chiral borenium-type
Lewis acid. The Lewis acid strength of chiral (pR)-3+ proved to
be comparable to that of B(C6F5)3. While modest enantiomeric
excess was achieved in the hydrosilylation of ketones with
(pR)-3+ as a catalyst, this study represents the first application
of a planar-chiral borenium Lewis acid in the stereoselective
reduction of ketones. Further modification of the steric demand
of the pyridyl ligand and the exocyclic boron substituent is
expected to allow for significantly enhanced selectivities, while
the activity can likely be optimized through tuning of the
electronic effect of the exocyclic B-aryl group.
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We thank the Petroleum Research Fund administered by the
American Chemical Society for financial support. The X-ray
diffractometer was purchased with support from the National
Science Foundation (NSF-CRIF 0116066) and Rutgers University
(Academic Excellence Fund). We thank Fei Cheng for assistance
with HPLC measurements.
21 The IUPAC-recommended Cahn–Ingold–Prelog (CIP) descriptors pS
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¨
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borodestannylation reaction. Mechanistic details of this unusual
rearrangement will be reported elsewhere.
23 A single crystal X-ray structure of (pR)-2 confirmed the stereo-
chemical assignment and revealed the exo-Ph isomer as the only
stereoisomer in the solid state.
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This journal is The Royal Society of Chemistry 2013
Chem. Commun.