Reductive arene ortho-silanolization of aromatic esters with hydridosilyl acetals

Article abstract of DOI:10.1039/c4cc09850a

This work describes the design and application of a single-pot, reductive arene C-H silanolization of aromatic esters for synthesis of ortho-formyl arylsilanols. This strategy involves a sequence of two transition metal (Ir and Rh)-catalyzed reactions for reductive arene ortho-silylation directed by hydridosilyl acetals and hydrolysis.

Full text of DOI:10.1039/c4cc09850a

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Journal Name
RSCPublishing
COMMUNICATION
Reductive Arene ortho-Silanolization of Aromatic
Esters with Hydridosilyl Acetals
Cite this: DOI: 10.1039/x0xx00000x
Yuanda Hua, Parham Asgari, Udaya Sree Dakarapu, and Junha Jeon*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
i) Ir-catalyzed ester hydrosilylation;
ii) Rh-catalyzed C–H bond silylation;
H
O
HOR32Si
R1
O
This work describes the design and application of a single-
pot, reductive arene C–H silanolization of aromatic esters
for synthesis of ortho-formyl arylsilanols. This strategy
involves a sequence of two transition metal (Ir and Rh)-
catalyzed reactions for reductive arene ortho-silylation
directed by hydridosilyl acetals and hydrolysis.
OR2
H
R1
iii) hydrolysis
2
3
R3=Et
R3=iPr
1
HR32Si
R32Si
R1
O
i) IrLn
O
iii) H3O+
H2SiR3
ii) RhLn
2
OR2
OR2
R1
Organosilanols1 are, in general, environmentally benign, and
their use has been significantly increased with wide application
for silicon-based materials2 and biomedically relevant agents.3,4
They are also useful synthetic agents for a variety of chemical
transformations, including silicon-based cross-coupling
reactions,5 oxidations,6 silanol hydrogen bond donor catalysts,7
directing groups for C–H bond functionalization.8 In particular,
arylsilanol synthesis often involves a two-step sequence of
silylation and hydrolysis. Several silanolization methods have
been developed, such as: 1) metal-halogen exchange/silylation,9
2) hydrolytic oxidation of hydrosilanes,10 3) metal-catalyzed
silylation of haloarenes followed by hydrolysis,11 and 4) a
sequence of directed arene ortho-metalation, silylation, and
hydrolysis.12 These methods offer excellent site-selectivity, yet
often require strongly basic and cryogenic conditions or a
stoichiometric amount of reagents, thereby displaying poor
functional group compatibility. Alternatively to direct
– H2
4
5
R3 = Et
R3 = iPr
6
7
R3 = Et
R3 = iPr
■ direct formation of silanols
■ direct formation of a functionalizable C-Si bond
■ controlled reduction of esters to aldehyde for subsequent reactions
Scheme 1 Reductive arene ortho-silanolization of esters via a
sequence of Ir and Rh-catalyzed reactions and hydrolysis
demonstrated that hydridosilyl ether-directed, Ir-catalyzed Csp2
–
H and Csp3–H silylation of alcohols, aldehydes, ketones, and
amines could provide a variety of cyclic silyl ethers.13f,21
Our approach to the development of a reductive arene
ortho-silanolization reaction, which could directly prepare
arylsilanols, features the sequence of two transition metal-
catalyzed reactions and facile hydrolysis in a single vessel
(Scheme 1). Specifically, in situ generation of hydridosilyl
acetals 4/5 via Ir-catalyzed hydrosilylation of esters 120 could
direct C–H silylation under suitable catalytic conditions to
afford cyclic silyl acetals 6/7. Upon simple aqueous work-up,
ortho-formyl arylsilanols 2/3 could be produced. Notably, the
versatile, yet labile silyl acetals are shown to act as directing
groups for catalytic C–H silylation, which has not been reported
to date. Nonetheless, our strategy for the single-pot reductive
ortho-silanolization of arenes required the resolution of two
challenges: First, the compatibility of the two catalysts toward a
combined single-pot reaction sequence had to be established.
Secondly, the discovery of a catalytic system suitable for C–H
functionalization directed by labile hydridosilyl acetals was
silylation,
prefunctionalized
moieties
[e.g.,
aryl
(pseudo)halides] are demanded within substrates.
Metal-catalyzed arene dehydrogenative silylation have
emerged as a powerful method for preparation of useful
organosilanes.13-19 To access diverse, functionalized organo-
silanols, which were previously difficult to access in an atom-
and step-economical fashion, we envisioned dehydrogenative
silanolization via catalytic C–H activation. This strategy
consolidates two remarkable methods, developed by the
Brookhart and Hartwig laboratories. Brookhart reported a
method for highly controlled Ir-catalyzed ester reduction via
hydrosilylation to afford corresponding aldehydes.20a Hartwig
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required. Herein, we report a regio- and chemoselective, single- within 10 min. These results showed that the two catalytic
pot reductive arene ortho-silanolization of aromatic esters via a systems (Ir and Rh) are compatible, and that a single-pot
sequential Ir-catalyzed ester hydrosilylation and Rh-catalyzed reductive arene ortho-silylation directed by hydridosilyl acetals
C–H silylation, directed by versatile hydridosilyl acetals, is feasible.
followed by hydrolysis of acetals.
Upon determination of optimal reaction conditions for the
From the outset, diethylhydridosilyl acetals were prepared hydrosilylation of esters and concomitant C–H silylation via Ir
via a controlled hydrosilylation of aromatic esters utilizing and Rh sequential catalyses,22 we investigated the substrate
[Ir(coe)2Cl]2 (0.1 mol %) and diethylsilane (2 equiv).20a The scope of the reductive arene ortho-silanolization of esters 1.
resultant diethylhydridosilyl acetals did not require additional The sequential processes utilizing diethylsilane produce cyclic
purification for the next steps. We then investigated arene silyl acetals 6 in generally good yields, regardless of electronic
ortho-silylation of hydridosilyl acetal 4a-Me. Although we and sterics of arenes. However, the propensity for silanol
aimed to utilize a single Ir catalyst for both processes, neither condensation to afford disiloxanes during hydrolysis resulted in
the Ir/phen catalytic system (or phen derivatives as ligands), nor inconsistent yields of ortho-formyl arylsilanols 2. Denmark’s
other screened Ir/ligand complexes efficiently afforded the method resolved this issue by utilizing a buffer solution (pH 5)
cyclic silyl acetal 6a-Me (ca. 30% yield upon complete to reliably produce 2 (Table 2).11a Under the conditions, the
conversion). Steric and electronic differences of hydridosilyl tandem reactions with electron-rich and deficient esters yielded
ether and hydridosilyl acetal directing groups had considerable the corresponding arylsilanols (2a-2e) in good yields. A
impact on reactivity. This obstacle was overcome by utilizing boronic ester, silyl blocking group, and trisubstituted alkene23
Rh-catalyzed C–H bond silylation (Table 1).14 Takai had shown were tolerated in the reaction system to afford 2f-2h. We
that Rh-catalyzed C–H silylation could be used to prepare observed chemoselective silanolization of aryl Csp2–H over
silafluorenes, by employing monodentate triphenylphosphine benzylic Csp3–H within 2d. Highly regioselective C–H
ligands.14d Gratifyingly, monodentate phosphine ligands silanolization of 1-naphthoate was achieved, where the
effectively promoted the hydridosilyl acetal-directed corresponding hydridosilyl acetal exclusively triggers C–H
dehydrogenative cyclization in the presence of norbornene as a activation of hydrogen at C2 over the hydrogen at C8 to afford
hydrogen acceptor. For instance, triphenylphosphine afforded 6i. Hydrolysis of 6i in a wide range of pH buffer solutions,
6a-Me in excellent yield (entry 1). However, sterically hindered however, provided either the recovered starting material or
and alkyl substituted phosphines, PPh2(o-tol) and PPh2Me, significant desilylation product. In 2-naphthoate, silanolization
were not effective ligands (entries 2-3). To systematically study occurred at the C3 position regioselectively to provide 2j.
the influence of electronic perturbation of phosphine ligands in Lastly, the reaction of 1a on 12 mmol scale provided 2a in 72%
the C–H silylation, we examined a series of electronically tuned isolation yield.
phosphine ligands (entries 4-7). Electron-donating ligand P(4-
Silicon groups bearing larger substituents, such as
MeOPh)3 efficiently promotes the cyclization to afford 6a-Me isopropyl groups, greatly suppressed the silanol condensation,
in excellent yield (98%) within only 10 min. In comparison, thereby consistently improving yields (Table 3). The reaction
well-established hydridosilyl ether-directed arene C–H with electron-rich and deficient esters provided the
corresponding silanols (3b-l) in good yields. The reactions
Table 2 Substrate scope of aromatic esters using diethyllsilanea
i) [Ir(coe)2Cl]2 (0.1 mol %)
silylation took 11-48 h at 80-120 °C employing 1 mol % of
[Ir(cod)OMe]2/phen.13f However, other phosphines such as P(4-
Me2NPh)3 and P(C6F5)3 drastically reduced the overall reaction
efficiency (entry 5-6). Upon addition of P(2-furyl)3 to the
reaction, the yield was improved, but not comparable to P(4-
MeOPh)3. Moreover, the Rh/(4-MeOPh)3 catalytic system
achieved the reaction with sterically hindered 2- and 3-methyl
benzoates to afford the corresponding cyclic silyl acetals (98%
and 92% yields vis-à-vis 63% and 54% with PPh3, respectively)
O
SiEt2OH
CHO
H2SiEt2 (2 equiv), CH2Cl2, rt
OMe
R1
R1
ii) [Rh(nbd)Cl]2 (0.4 mol %)
P(4-OMePh)3 (2.4 mol %)
1
2
nbe (2 eqiuv), THF, 120 °C, 10 min
iii) pH 5 buffer solution, rt
SiEt2OH
SiEt2OH
CHO
SiEt2OH
SiEt2OH
Table 1 Evaluation of ligands for Rh-catalyzed hydridosilyl acetal-
directed arene ortho-silylation.a
CHO
CHO
CHO
H
Me
[Rh(nbd)Cl]2 (0.4 mol %)
Et2
Me
Si
ligand (2.4 mol %)
2a
78% (98%),b 72%c
2b
72% (91%)b
2c
63% (92%)b
2d
86% (98%)b
H
O
Et2Si
O
nbe (2 equiv)
OMe
OMe
SiEt2OH
CHO
SiEt2OH
SiEt2OH
CHO
SiEt2OH
CHO
THF, 120 °C
time
CHO
4a-Me
6a-Me
TBSO
Cl
Bpin
O
Me
Me
2e
74% (92%)b
2g
78% (92%)b
Yield (%)b
2f
Entry
Ligand
Time (min)
75% (96%)b
MeO
O
1
2
3
4
5
6
7
PPh3
10
60
60
10
60
60
60
98
10
20
98
20
20
2h
78% (96%)b
H
SiEt2
CHO
SiEt2OH
PPh2(o-tol)
PPh2Me
P(4-MeOPh)3
P(4-Me2NPh)3
P(C6F5)3
6i
2j
91% (98%)d
72% (95%)b
a Conditions: 1 (1.0 mmol), CH2Cl2 (3.3 M); THF (1 M). b Yield of
isolated product 2. Yield of cyclic silyl acetals 6 determined by 1H
NMR spectroscopy utilizing an internal standard (CH2Br2) is shown in
parentheses. c Reaction of 1a on 12 mmol (1.63 g) scale yielded 2a in
72% isolation yield. d Isolated yield of 6i.
P(2-furyl)3
95
b
a
Conditions: Silyl acetal 4a-Me (0.2 mmol), THF (1 M). Determined by
1H NMR spectroscopy utilizing an internal standard (CH2Br2).
2 | J. Name., 2012, 00, 1-3
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i) [Ir] (0.1 mol %)
H2SiiPr2,CH2Cl2
rt (100% conv.)
Table
3
Substrate scope of aromatic esters using
a)
O
D
O
D
O
i-Pr2Si
diisopropylsilanea
D
D
OCD3
OCD3
OMe
+
+
7a-H
i) [Ir(coe)2Cl]2 (0.1 mol %)
H2SiiPr2 (2 equiv), CH2Cl2, rt
O
SiiPr2OH
CHO
ii) [Rh]/P(4-OMePh)3
(0.4 mol %), nbe
THF, 120 °C
D
H
D
D
D
D
OMe
R1
R1
1a-D8
1a
7a-D7
ii) [Rh(nbd)Cl]2 (0.4 mol %)
P(4-OMePh)3 (2.4 mol %)
kH/kD = 1.3 ± 0.1 (9% yield)
1
2
nbe (2 eqiuv), THF, 120 °C, 10 min
iii) pH 5 buffer solution, rt
b)
i) [Ir] (0.1 mol %)
H2SiiPr2 (2 equiv),CH2Cl2, rt
(100% conv.)
O
H
O
H
i-Pr2Si
OMe
O
OMe
OMe
SiiPr2OH
CHO
SiiPr2OH
CHO
SiiPr2OH
CHO
SiiPr2OH
+
Si
iPr2
ii) [Rh]/P(4-OMePh)3 (0.4 mol %)
nbe, THF, 120 °C
D
D
CHO
R
N
O
1a-D
7a-D1 (68%)
7a-H (32%)
kH/kD = 2.1 ± 0.1
Me
O
Me
Me
yield of
3 (%)b
3p
41% (54%)b
3q
62% (73%)b
R
Scheme 2 Studies on a) intermolecular and b) intramolecular
kinetic isotope effects.
3a
H
91 (98)
90 (98)
67 (86)
90c
3o 71% (82%)b
SiiPr2OH
MeO
3b 4-Me
3c 5-Me
7d 6-Me
3e 4-F
3f 4-Cl
3g 5-Cl
3h 4-CF3
O
CHO
CHO
Cyclic silyl acetals 6/7 and ortho-formyl arylsilanols 2/3
are versatile intermediates for a number of transformations
(Scheme 3): a) Nucleophilic addition to 6a was achieved using
MeMgBr to furnish silane 8; b) Oxidation of 6a salicylic
aldehyde 9;13f c) Fleming-Tamao/Dakin oxidation cascade of
6a employing the flavin-type catalyst 11 afforded catechol 10;27
d) Lewis acid-catalyzed allylation of 6a yielded homoallylic
methyl ether 12;28 e) Iodo ipso-desilylation of 2a/6a or 3a/7a
installed halogen to afford 13. f) IBX-mediated oxidation of 3a
furnished benzosilalactone 14; and g) Horner-Wadsworth-
Emmons reaction of 2a/3a gave enoates 15/16.
SiiPr2
SiiPr2OH
iPrO
85 (96)
75 (86)
78 (93)
72 (98)
O
O
7t 91%e
SiiPr2OH
3r 71% (87%)b
3s 57% (71%)b,d
3i 4-OMe 56 (92)
3j 5-OMe 77 (91)
3k 6-OMe 36 (48)
3l 5-NMe2 91 (98)
CHO
CHO
SiiPr2OH
OHC
SiiPr2OH
3m 4-Bpin
3n 5-Bpin
77 (91)
72 (97)
3u 77% (96%)b
3v 75% (91%)b,f
a Conditions: 1 (1.0 mmol), CH2Cl2 (3.3 M); THF (1 M). b Yield of
isolated product 3. Yield of cyclic silyl acetals 7 determined by 1H
NMR spectroscopy utilizing an internal standard (CH2Br2) is shown in
parentheses. c Isolated yield of cyclic silyl acetal 7d. d H2SiiPr2 (1.1
CHO
MeEt2Si
OH
Me
a
R12Si
O
e
OMe
I
equiv), rt, 48 h. e Isolated yield of 7t. H2SiiPr2 (4 equiv); [Rh(nbd)Cl]2
f
13 (94/90% from 2a/6a,
89/85% from 3a/7a)
8 (91%)
OH
(0.8 mol %), P(4-OMePh)3 (4.8 mol %).
6a R = Et
7a R = iPr
O
demonstrated reasonably good functional group tolerance in the
presence of an amine, boronic ester, or trisubstituted alkene23
(3l-o). Heterocyclic indolyl and furanyl esters were tolerated by
the reaction conditions to provide 3-silanol indole 2-
carbaldehyde 3p and silanol furanals 3q-r. In particular, 3r was
the sole product with excellent regioselectivity. Remarkably,
chemoselective arene C–H ortho-silanolization (methyl vs.
isopropyl esters) within diester 1s was viable, exclusively
affording 3s. As seen in Table 2, reactions using naphthoates
and diisopropylsilane exhibited complete regioselectivity to
afford 7t, which did productively undergo, and 3u. Dual
reductive C–H silanolization was also achieved with additional
reagents to yield doubly functionalized disilanol 3v. It is worth
noting that silyl hemi-acetal formation was only observed when
diisopropylsilane was used. Presumably, this is due to
substantial structure and reactivity difference of silanols and
alcohols as well as conformational preference by diisopropyl
silane substituents (cf., diethylsilane).24,25
f
R2
Me
H
N
O
Si
iPr2
N
O
b
c
N
R2 = CHO (88%)
10 R2 = OH (66%)
N
Me
9
14 (91%)
Et
O
11
CO2Me
SiR12OH
CHO
(TMSO)Et2Si
OMe
d
g
OTMS
Si
R1
2
2a R = Et
3a R = iPr
15 R1 = Et (93%)
16 R1 = iPr (82%)
12 (72%)
Reagents and conditions: (a) MeMgBr, THF, 60 °C. (b) t-BuO2H,
TBAF, CsOH, DMF, rt. (c) t-BuO2H, TBAF, CsOH, DMF, 11, rt. (d)
TMSOTf, CH2CHCH2Si(CH3)3, THF, rt. (e) ICl, CH2Cl2, rt. (f) IBX,
DMSO, 40 °C. (g) i) (MeO)2P(O)CH2CO2Me, KOTMS, THF, rt; ii)
TMSOTf, CH2Cl2, rt.
Scheme 3 Synthetic applications of cyclic silyl acetals and
ortho-formyl arylsilanols.
To gain insight into the reaction mechanism, we performed
two KIE experiments (Scheme 2). The observed minimal
isotopic selectivity (kH/kD=1.3) in the intermolecular KIE
experiment suggests that C–H bond cleavage is not turnover-
limiting and the small KIE suggests that a preceding,
irreversible step (likely substrate binding) is the product-
determining step (Scheme 2a). Assuming that C−H bond
cleavage is irreversible, the significant KIE observed in the
intramolecular experiment (kH/kD=2.1) arises only from the C–
H bond cleavage step being product-determining in this case, as
the proceeding irreversible and turnover-limiting step cannot
select the product (Scheme 2b).26 Together, these studies
indicate that the turnover-determining step is an irreversible
step involving substrate metal coordination that proceeds C–H
bond cleavage.
Conclusions
To summarize, we have developed a single-pot reductive arene
ortho-silanolization of esters 1. Two sequential transition metal
catalytic reactions, followed by a mild hydrolysis step, allow
direct access to ortho-formyl arylsilanols 2 and 3. Our strategy
interconnects Ir-catalyzed ester hydrosilylation with Rh-
catalyzed C–H silylation to facilitate the reductive arene ortho-
silylation in a single vessel. Hydrolysis reveals ortho-silanol
and aldehyde functionalities. Notably, ester hydrosilylation was
achieved with 0.1 mol % of [Ir(coe)2Cl]2 and the labile
hydridosilyl acetal-directed C–H silylation was accomplished
within 10 min, employing 0.4 mol % of [Rh(nbd)Cl]2/P(4-
MeOPh)3. Further efforts toward application of this tactic to
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more complex substrates and studies on the mechanistic details
of reductive C–H bond silylation are currently underway.
This research was supported by start-up funds provided by
the University of Texas Arlington, UTA Research
Enhancement Program, and the American Chemical Society
Petroleum Research Fund (PRF# 54831-DNI1). We also thank
Prof. Frank Foss at UTA for generous donation of the flavin
catalyst 11. The NSF (CHE-0234811 and CHE-0840509) is
acknowledged for partial funding of the purchases of the NMR
spectrometers used in this work.
Notes and references
Department of Chemistry and Biochemistry, University of Texas at
Arlington, Arlington, Texas 76019, USA. E-mail: jjeon@uta.edu
† Electronic Supplementary Information (ESI) available: Experimental
1
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R
yield
i) [Ir(coe)2Cl]2 (0.1 mol %)
O
Et2Si
O
H2SiEt2 (3 equiv), CH2Cl2, rt
98%
85%
61%
4%
6a-Me Me
6a-Et
OR
OR
Et
ii) [Rh(nbd)Cl]2 (0.4 mol %)
P(4-OMePh)3 (2.4 mol %)
nbe (2 equiv)
6a-iPr iPr
6a-tBu tBu
6a-Ph Ph
6a-Bn Bn
1
6
6%
98%
C. L. Frye, R. M. Salinger, F. W. G. Fearon, J. M. Klosowski, T.
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THF, 120 °C, 10 min
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4 | J. Name., 2012, 00, 1-3
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