Au Nanoparticle-Catalyzed Silaboration of Aryl-Substituted Cyclopropyl Aldehydes Forming Rearranged β-Boronate Silyl Enol Ethers

Article abstract of DOI:10.1002/ejoc.201901408

2-Aryl-substituted cyclopropyl aldehydes undergo an unprecedented Au nanoparticle-catalyzed silaboration leading to rearranged linear β-boronate-bearing silyl enol ethers. Formation of these products is attributed to the ring opening radical clock rearran

Full text of DOI:10.1002/ejoc.201901408

A Journal of
Accepted Article
Title: Au Nanoparticle-Catalyzed Silaboration of Aryl-Substituted
Cyclopropyl Aldehydes Forming Rearranged β-Boronate Silyl
Enol Ethers
Authors: Vasiliki Kotzabasaki, Marios Kidonakis, Eleni Vasilikogiannaki,
and Manolis Stratakis
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201901408
Link to VoR: http://dx.doi.org/10.1002/ejoc.201901408
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10.1002/ejoc.201901408
European Journal of Organic Chemistry
COMMUNICATION
Au Nanoparticle-Catalyzed Silaboration of Aryl-Substituted
Cyclopropyl Aldehydes Forming Rearranged -Boronate Silyl
Enol Ethers
Vasiliki Kotzabasaki, Marios Kidonakis, Eleni Vassilikogiannaki and Manolis Stratakis*[a]
Abstract: 2-Aryl-substituted cyclopropyl aldehydes undergo an
unprecedented Au nanoparticle-catalyzed silaboration leading to
rearranged linear boronate-bearing silyl enol ethers. Formation of
these products is attributed to the ring opening radical clock
rearrangement of the intermediate -cyclopropyl radical into a
benzyl radical.
Introduction
Scheme 1. Reaction of silylborane pinB-SiMe2Ph with aromatic
Silylboranes, a class of compounds that possess a  Si-B bond
are fairly stable, especially if they bear electron rich substituents
on the boron atom. Their chemistry has attracted the interest of
organic chemists, as under certain reaction conditions they are
capable of transferring their interelement bonded silicon and
boron partners to organic substrates forming C-Si and/or C-B
bonds.[1] The catalytic activation of the Si-B bond of silylboranes
occurs either through oxidative addition on Pd(0) or Pt(0)
complexes,[1] or via formation of the silyl bearing complex LCu-
SiR3, in case the catalyst is Cu(I) or Cu(II).[2]
Our group was the first to report a few years ago the
smooth activation of silylborane pinB-SiMe2Ph (pin: pinacolato)
on Au nanoparticles supported on titania (Au/TiO2), and its
subsequent cis-1,2-addition to alkynes[3] or allenes,[4] and on the
 C-O bond of strained cyclic ether.[5] More recently we have
presented its Au/TiO2-catalyzed reaction with aromatic
aldehydes and acetophenones which results to a C-C bond
forming pathway, instead.[6] The activation of pinB-SiMe2Ph on
Au/TiO2 most probably involves the chemisorption of the
interelement Si-B linkage on the electron deficient low-
coordinated Au atoms, primarily at the corners of the Au
nanoparticle,[7] and then follows the delivery of the two
chemisorbed partners on the proximal physisorbed substrate.[8]
While pinB-SiMe2Ph adds to alkynes, allenes, epoxides and
oxetanes,[3]-[5] in the case of carbonyl compounds, a silylative
pinacol-type reductive dimerization pathway is exclusively
observed[6] (Scheme 1). It was clearly established that
dimerization occurs through a radical-chain process, as the
postulated intermediate -silyloxy radical that eventually
dimerizes was almost quantitatively captured in the presence of
the free radical TEMPO.
carbonyl compounds catalyzed by Au/TiO2, and the proposed
intermediate -silyloxy radical.[6]
Given the radical nature of the transformation shown in
Scheme 1, we sought to study the Au/TiO2-catalyzed reaction of
pinB-SiMe2Ph with cyclopropyl aldehydes. -Cyclopropyl
carbinyl radicals, the anticipated reaction intermediates, are
well-known to undergo on certain cases ring-opening
rearrangement into homoallylic radicals, given their suitable
substitution and lifetime (Scheme 2).[9] This radical-clock type
rearrangement has been used in the past on many occasions
either as an evidence of a radical mechanism, or as a
mechanistic probe, to distinguish between polar or radical
mechanisms in cyclopropyl substrates bearing an additional
carbocation stabilizing methoxy group.[10]
Scheme 2. The radical clock rearrangement of an -cyclopropyl
carbinyl radical.
Results and Discussion
In this work we attempted to exploit synthetically such a
possible radical clock rearrangement, by examining the Au
nanoparticle-catalyzed silaboration of cyclopropyl-substituted
aldehydes. For this purpose we synthesized a series of 2-aryl-
substituted cyclopropyl aldehydes as a mixture of trans/cis
isomers through the Cu(II)-catalyzed reaction between ethyl
diazoacetate and alkenes followed by reduction of the produced
esters and finally oxidizing the resulting alcohols (Scheme 3).
The phenyl-substituted aldehyde 1, whose possible silaboration
was initially studied as a model-substrate (see Scheme 4), was
synthesized as a single trans-diastereoisomer starting from the
corresponding commercially available trans-carboxylic acid.
[a]
Dr. Vasiliki Kotzabasaki, Dr. Marios Kidonakis, Dr. Eleni
Vasilikogiannaki, Prof. Dr. Manolis Stratakis
Department of Chemistry, University of Crete,
Voutes 71003 Iraklion, Greece
E-mail: stratakis@uoc.gr
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201901408
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Scheme 4. Silaboration of aldehyde 1-trans catalyzed by Au/TiO2 and
the structural proof of product 1a.
As a further structural proof, the mixture of products 1a-Z
and 1a-E was treated with NaBO3, a reaction that transforms C-
B bonds into C-OH. The only isolated product was hemiacetal
12,[11] as a thermodynamic mixture of two epimers in a relative
ratio trans/cis = 1.3/1 (Scheme 4). Apparently under the reaction
conditions the silyl enol ethers were deprotected and the
hydroxyl group installed at the former C-Bpin moiety reacted
intramolecularly with the aldehyde forming lactol 12. To the best
of our knowledge, this is the first example in the literature of
silaboration of cyclopropyl aldehydes. Somehow relevant could
be considered the Rh(I)[12] or Pd(0)[13]-catalyzed hydrosilylation
of cyclopropyl ketones leading to enol ethers, as well as, their
Ni(0)-catalyzed borylation.[14] In addition, so far there is only one
example in the literature involving a nano Au(0)-catalyzed
cleavage of a  cyclopropane bond, in the reaction between
methylenecyclopropanes and diboron pinB-Bpin.[15] In this
transformation occurs a direct 1,2-addition on a  bond, while in
our case we present a formal 1,4-O,C-addition.
It is reasonable that the initially formed -silyloxy
cyclopropyl radical (intermediate II, Scheme 5) from the
cleavage of I undergoes ring-opening rearrangement into a
benzyl radical (intermediate III), that is captured by the
chemisorbed Bpin moiety forming the final silaboration product.
It’s also possible that coupling may occur in intermediate I, via
the radical-type bond-forming and breaking concerted-like route
shown in the bottom part of Scheme 5, as the  cyclopropane
bonds are rather loose.[16]
Scheme 3. General route for the synthesis of the 2-substituted
cyclopropyl aldehydes.
Although, aliphatic aldehydes are unreactive against pinB-
SiMe2Ph,[6] the phenyl-substituted cyclopropyl aldehyde trans-1
reacted smoothly with 1.5 equiv of pinB-SiMe2Ph in the
presence of Au/TiO2 (1 mol%), within 1 h at 70 oC, in anhydrous
benzene as solvent. To our delight, no dimeric pinacol-type
products are formed, as in the case of aryl aldehydes, but we
isolated instead the ring-opened silaboration product 1a
(Scheme 4) as a mixture of geometrical isomers in relative ratio
cis/trans = 72/28, and in 62% yield. The structural assignment of
the products was based on 2D-NMR data. It is possible that the
prevailing cis-isomer is the thermodynamically more stable due
to an intramolecular boron – silyloxy interaction. Simple MM3
calculations indicate that the cis isomer is more stable relative to
the trans by 0.7 kcal/mol, while the more advanced MP2 (RI-
MP2/def2-TZVP) calculations show that the trans isomer is more
stable by merely ~0.5 kcal/mole. With the MP2 method there is
not a very good agreement, nevertheless, calculations reveal
that cis isomer is relatively more stable than anticipated. Notably,
exactly the same stereochemical product outcome (1a-Z/1a-E ~
70/30) was observed in the silaboration of a mixture of
diastereomers of aldehyde 1 (trans/cis = 60/40), which were
prepared based on the general route shown in Scheme 3,
indicative that both cis and trans cyclopropane isomers lead to
the same products (apparently via a common intermediate, as it
will be shown below). The silaboration products 1a are somehow
labile, as they easily undergo hydrolytic deprotection into the
corresponding aldehyde,
a
process that is presumably
accelerated by the Lewis acidic character of the boryl moiety.
The resulting aldehyde in turn, transforms into a mixture of
unidentified products, possibly through reaction of the aldehyde
functionality with the pinacolato moiety. The silaboration product
can be storred in CDCl3 without decomposition by adding a base
(e.g. pyridine).
Scheme 5. Possible mechanisms for the formation of rearranged
silaboration product 1a.
Gratified by the results of silaboration of 1, and given that
the stereoselectivity of product formation is independent of the
cis/trans arrangement of the phenyl group in 1, we synthesized a
series of para, meta or ortho 2-aryl-substituted cyclopropyl
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aldehydes as a mixture of cis/trans isomers based on the route
shown in Scheme 3, and studied their reaction with pinB-
SiMe2Ph in the presence of Au/TiO2. The conditions in all
experiments were the same (1 mol% catalyst, 1.5 equiv
silylborane, benzene as solvent, 70 oC, 1 h). The results are
Scheme 6. Lack of reactivity in the attempted Au/TiO2-catalyzed
silaboration of cyclopropanecarboxaldehyde (11).
collectively summarized in Figure
transformation is quite general yielding -boronate-substituted
1 and show that the
silyl enol ethers in moderate to good isolated yields, and as a
mixture of geometrical isomers (cis/trans
~ 2-3/1). The
Conclusions
transformation is quantitative, but the isolated product yield is
somehow diminished because the silaboration products are in
general labile, as it has been indicated aldready for the case of
1a.
In conclusion, we present herein an unprecedented mode of
reactivity catalyzed by supported Au nanoparticles, namely the
silaboration of 2-aryl-substituted cyclopropyl aldehydes, which
yields -boronate-bearing silyl enol ethers. This pathway is
explained in terms of a radical clock rearrangement of the
initially formed -carbinyloxy cyclopropyl radicals. This novel
mode of reactivity show once more the powerful and unique
nature of heterogeneous nano Au materials in catalysis.[17]
Experimental Section
Typical procedure of the Au/TiO2-catalyzed silaboration of aryl-
substituted cyclopropyl aldehydes: To a dried sealed tube containing
trans-2-phenylcyclopropanecarbaldehyde, 1-trans (15 mg, 0.1 mmol),
Au/TiO2 (20 mg, 1 mol%) in 0.3 mL dry benzene were added via syringe
40 L (0.15 mmol) of pinB-SiMe2Ph. The mixture was heated to 70 oC,
and after 1 h (100% conversion as monitored by TLC) the slurry was
filtered under reduced pressure through a short pad of silica gel with the
aid of dichloromethane (~2 mL) to withhold the supported catalyst. The
filtrate was evaporated under vacuum and the residue was carefully
chromatographed using hexane/ethyl acetate = 50/1 as eluent to afford
25 mg of 1a-Z and 1a-E (62% yield) in a relative ratio 72/28.
Additional experimental details and product characterization are found in
the Supporting Information.
Acknowledgements
This research has been co-financed by the Operational Program
"Human Resources Development, Education and Lifelong
Learning" and is co-financed by the European Union (European
Social Fund) and Greek national funds (program MIS 5005942).
ProFI (FORTH, Heraklion, Greece) is acknowledged for
obtaining the HRMS spectra of the unknown compounds. We
also thank Dr. Taxiarchis Stergiannakos for performing the
theoretical calculations.
Figure 1. Products from the silaboration of 2-aryl-substituted
cyclopropyl aldehydes catalyzed by Au/TiO2.
Notably,
the
unsubstituted
parent
Keywords: Au nanoparticles • Catalysis • Silaboration •
Cyclopropyl aldehydes • Radical clock rearrangement
cyclopropanecarboxaldehyde (11) does not undergo any
silaboration and remains intact, even under more forcing
reaction conditions and excess of silyl borane (Scheme 6).
Obviously, an aryl group at the 2-position is essential to drive the
reaction, because it is capable of stabilizing the rearranged
radical.
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Silaboration
COMMUNICATION
Vasiliki Kotzabasaki, Marios Kidonakis,
Eleni Vasilikogiannaki and Manolis
Stratakis*
Page No. – Page No.
Title: Au Nanoparticle-Catalyzed
Silaboration of Aryl-Substituted
Cyclopropyl Aldehydes Forming
Rearranged -Boronate Silyl Enol
Ethers
2-Aryl-substituted cyclopropyl aldehydes undergo an unprecedented Au
nanoparticle-catalyzed ring-opening silaboration forming -boronate-bearing silyl
enol ethers, through a ring opening radical clock rearrangement of the intermediate
cyclopropyl radical.
This article is protected by copyright. All rights reserved.