Lewis Base Activation of Silyl Acetals: Iridium-Catalyzed Reductive Horner-Wadsworth-Emmons Olefination

Article abstract of DOI:10.1021/acs.orglett.5b02901

A Lewis base promoted deprotonative pronucleophile addition to silyl acetals has been developed and applied to the iridium-catalyzed reductive Horner-Wadsworth-Emmons (HWE) olefination of esters and the chemoselective reduction of the resulting enoates. Lewis base activation of silyl acetals generates putative pentacoordinate silicate acetals, which fragment into aldehydes, silanes, and alkoxides in situ. Subsequent deprotonative metalation of phosphonate esters followed by HWE with aldehydes furnishes enoates. This operationally convenient, mechanistically unique protocol converts the traditionally challenging aryl, alkenyl, and alkynyl esters to homologated enoates at room temperature within a single vessel.

Full text of DOI:10.1021/acs.orglett.5b02901

Letter
pubs.acs.org/OrgLett
Lewis Base Activation of Silyl Acetals: Iridium-Catalyzed Reductive
Horner−Wadsworth−Emmons Olefination
Udaya Sree Dakarapu, Apparao Bokka, Parham Asgari, Gabriela Trog, Yuanda Hua, Hiep H. Nguyen,
Nawal Rahman, and Junha Jeon*
Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019, United States
S
* Supporting Information
ABSTRACT: A Lewis base promoted deprotonative pronu-
cleophile addition to silyl acetals has been developed and
applied to the iridium-catalyzed reductive Horner−Wads-
worth−Emmons (HWE) olefination of esters and the
chemoselective reduction of the resulting enoates. Lewis base
activation of silyl acetals generates putative pentacoordinate
silicate acetals, which fragment into aldehydes, silanes, and alkoxides in situ. Subsequent deprotonative metalation of
phosphonate esters followed by HWE with aldehydes furnishes enoates. This operationally convenient, mechanistically unique
protocol converts the traditionally challenging aryl, alkenyl, and alkynyl esters to homologated enoates at room temperature
within a single vessel.
cetals 1, metal acetals 2, and silyl acetals 3 can serve as
aldehyde equivalents in organic synthesis (Figure 1).
reactivity, we envisioned the application of Lewis base promoted
deprotonative pronucleophile addition reactions to 3 (Scheme
1b).10 Specifically, such substrates 3 can be generated through Ir-
catalyzed hydrosilylation of esters 4.11,12 Subsequent nucleo-
philic attack of the Lewis base (LB) on 3 generates putative
pentacoordinate silicate acetals 6,13,14 which are likely to
fragment into silanes 7, aldehydes 8, and alkoxides 9. Subsequent
deprotonative metalation of pronucleophiles, with an anion-
stabilizing group, by 9 yields a metalated pronucleophile anion,
which in turn traps aldehydes 8 to furnish alcohols 10.
We report a Lewis base promoted deprotonative pronucleo-
phile addition to silyl acetals, an application to a single-pot Ir-
catalyzed reductive HWE olefination at rt as an alternative to the
traditional HWE olefination (Scheme 1c). Initial dialkylhydri-
dosilyl acetals 3, produced via Ir-catalyzed ester hydrosilylation,
are available for the HWE reaction upon addition of appropriate
Lewis bases and phosphonate esters (i.e., pronucleophile).
Advantages of this single-pot Ir-catalyzed reductive HWE
olefination approach are 4-fold: (1) This approach can be
carried out under mild reaction conditions (e.g., at rt). (2) The
experimental procedure is substantially simpler than that of
aluminum hydride based reductive homologation method by
virtue of in situ generation of 8 and 9 via a fragmentation of silyl
acetals, thus avoiding the need for premetalation of nucleophiles.
(3) This catalytic reductive method allows direct conversion of
traditionally challenging aryl esters, enoates, and ynoates to
corresponding homologated esters. (4) Due to use of a
substoichiometric amount of Ir catalyst (0.1 mol %), the
reactions are feasible for a range of scales from subgram to gram
quantities of the esters.
A
Figure 1. Various acetals.
Acetals 1 are acid-labile/base-stable, whereas metal acetals 2
are only reasonably stable at low temperature. On the other hand,
silyl acetals 3 are tetrahedral intermediate mimics which are
reasonably stable even at elevated temperatures, yet are acid- and
silaphile-labile. Mukaiyama,1 Tietze,2 and Oshima3 demonstra-
ted mixed O,O-silyl acetals and their synthetic applications; these
were predominantly exploited for Lewis acid catalyzed allylsilane
additions. Nonetheless, silyl acetals have seen limited application
in synthetic chemistry so far.4
Reductive Horner−Wadsworth−Emmons (HWE) strategies
involving the in situ generation of the 2/nucleophilic capture
have been developed by Takacs,5a Burton,5b and Hoye5c
(Scheme 1a).6,7 Although these approaches have been successful,
they are restricted in substrate scope due to the inherent
instability of metal acetals. Consequently, the carbonyl
substituents are primarily alkyl in nature, in addition to a limited
number of aryl and alkenyl substituentsto the best of our
knowledge, no example pertaining to alkynyl esters has been
reported. Trost and Herzon demonstrated that the single-pot
HWE olefination of challenging enals is viable at −90 or −95 °C.8
Alternatively, the An group developed lithium diisobutyl-tert-
butoxyaluminum hydride and other congeners, which improve
the stability of the aluminum acetals (typically below 0 °C).9
To develop useful synthetic transformations utilizing silyl
acetals directly as aldehyde equivalents and understand their
Received: October 6, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02901
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Letter
excellent yield (84−95% NMR yield) and high stereoselectivity
(>20:1 E/Z). KOSiMe3 was identified as the best Lewis base for
the Lewis base promoted reductive HWE reaction (87% isolation
yield, entry 7).16,17
Scheme 1. Design for the Reductive HWE Olefination
Involving Silyl Acetals
Based on these encouraging initial results, we probed the
substrate scope for the single-pot catalytic reductive HWE
olefination at rt (Scheme 2).18 The ortho-, meta-, and para-
Scheme 2. Scope of Aromatic and Acyclic Esters for Single-Pot
Ir-Catalyzed Reductive HWE Olefination
Diethylhydridosilyl acetal 3a was conveniently prepared
through the hydrosilylation of ester 4a with the iridium catalyst
[Ir(coe)2Cl]2 (0.1 mol %) and diethylsilane (2 equiv) (Table
1).11,15 The resulting silyl acetal 3a functions as a stable metal
Table 1. Study of Lewis Base for Single-Pot Ir-Catalyzed
a
Reductive HWE Olefination
a
b
Determined by 1H NMR spectroscopy. Reaction of 1a on 10 mmol
c
scale. Saturated esters (ca. 3:1 of 5/saturated 5) were produced.
d
e
Combined yields of 5q/saturated 5q (6:1). H2Sii-Pr2 (1.2 equiv) was
f
used. Tetraethyl methylenediphosphonate was used as a pronucleo-
phile. Reaction at rt for 2 h.
g
b
c
entry
Lewis base
yield of 5a (%)
E/Z (5a)
1
2
3
4
5
6
7
TBAF
26
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
d
e
e
e
NaOEt
33 (69)
substituted aromatic esters afforded the corresponding enoates
(5b−l) with good to excellent yields and excellent E/Z ratios
(>20:1). For example, the reaction of methyl benzoate on 10
mmol scale afforded 5a in 85% yield. Heterocycles such as furan,
thiophene, pyridine, and indole also tolerated the reaction
conditions to provide 5m−p. Reactions with aliphatic esters
afforded enoates 5q−s with excellent E/Z ratios, albeit with
somewhat diminished yields due to competitive olefin saturation
(ca. 3:1 of 5/saturated 5). Hindered methyl pivalate gave 5t with
good yield and excellent stereoselectivity. This method would be
particularly useful for cases in which the isolation of aldehydes is
impractical, due to the instability or inconvenience of the
necessary operation involved with separation. Reductively
generated phenylsulfonylacetaldehyde from methyl phenyl-
sulfonylacetate was notoriously difficult to isolate and tends to
decompose by methods employing either extraction or
chromatography.11 Our developed reductive HWE reaction of
d
NaOt-Bu
KOt-Bu
28 (76)
d
66 (80)
LiOSiMe3
NaOSiMe3
KOSiMe3
84
85
f
95 (87)
a
Conditions: 4a (0.2 mmol), (i) CH2Cl2 (3.3 M); (ii) THF (0.1 M).
Determined by H NMR spectroscopy utilizing an internal standard
(CH2Br2). Determined by H NMR spectroscopy. No completion
was observed after 48 h. With 3 equiv of Lewis base. Isolated yield.
b
1
c
d
1
e
f
acetal species. We then studied the impact of Lewis base upon the
single-pot reductive HWE reaction. When 3a was subjected to
HWE reaction conditions without purification, Lewis bases such
as TBAF, NaOEt, NaOt-Bu, and KOt-Bu (1.2 equiv) provided
enoate 5a in moderate yields (entries 1−4). Interestingly, metal
trimethylsilanoates (entries 5−7) as Lewis bases afforded 5a in
B
DOI: 10.1021/acs.orglett.5b02901
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Letter
methyl phenylsulfonylacetate afforded the corresponding enoate
5u with 52% yield and excellent stereoselectivity. Bidirectional
addition of nucleophiles to succinaldehyde has been challenging
due to its instability and intramolecular engagement of initially
produced metal alkoxide with the appended aldehyde to form a
cyclic acetal adduct.5c Bidirectional reductive HWE using diethyl
succinate, diethylpyridine-2,6-dicarboxylate, and diethyl tereph-
thalate under the identical conditions allowed access to dienoates
5v, 5w, and 5x, respectively. Chemoselective reductive HWE
olefination of isopropyl methyl terephthalate provided mono-
homologated enoate 5y, which is difficult to achieve through
traditional reductive HWE tactics. Finally, when a bisphospho-
nate as a pronucleophile was employed, vinylphosphonate 5z was
produced in moderate yield.
trienoate 12 require three iterations of a three-step reduction,
oxidation, and HWE sequence (a total of 9 linear steps). A triply
iterated, single-pot catalytic reductive HWE reaction provided
(2E,4E,6E)-trienoates 12 with 43% yield.
Furthermore, single-pot Ir-catalyzed reductive HWE olefina-
tion and in situ chemoselective olefin saturation were achieved to
provide saturated esters 11 (Scheme 6). We conjectured that Ir
Scheme 6. In Situ Olefin Saturation
Next, challenging enoates and ynoate substrates were
examined (Scheme 3). We achieved the catalytic reductive
Scheme 3. Scope of Enoates and Ynoates
catalyst and hydrosilane (not dihydrosilane, which can allow
ester hydrosilylation) can effect chemoselective saturation of
enoates.19 When triethylsilane was added to the reaction mixture
from HWE reaction of esters 4, which was warmed to 80 °C in a
closed vessel, we observed chemoselective olefin reduction to
afford two-carbon-homologated saturated esters 11a−e.
The HWE reaction, using sodium hydride as a base, of TBS-
protected Roche aldehyde led to partial epimerization.20 We
further investigated whether the stereogenic center of the ester
would be epimerized under our reaction conditions coupled with
Ando’s Z-selective HWE reaction.21 When TBS-protected (R)-
(−)-Roche ester 13 was subjected to these reaction conditions,
erosion of enantioselectivity in 14 was not observed (Scheme 7).
HWE reactions of α,β-unsaturated esters at rt, which afforded the
corresponding dienoates 5aa−ac and ynenoates 5ad with good
yields and excellent E,E/E,Z ratios.
We then studied sequential reductive HWE (and cyclo-
etherification) of lactones (Scheme 4). Substrates differing in
Scheme 4. Scope of Lactones for Sequential Reductive HWE
(and Cycloetherification)
Scheme 7. Z-Selective Ir-Catalyzed Reductive HWE of (R)-
(−)-Roche Ester
In summary, an operationally convenient, single-pot, catalytic
reductive HWE olefination was developed. The strategy is based
on Lewis base promoted deprotonative pronucleophile addition
to silyl acetals. This sequential reaction proceeds at rt and does
not require premetalation of phosphonates. Notably, tradition-
ally difficult ester substrates bearing aryl, alkenyl, and alkynyl
moieties converted to the corresponding cinnamates, dienoates,
and ynenoates with excellent stereoselectivities and good to
excellent yields. Finally, we demonstrated single-pot Ir-catalyzed
reductive HWE olefination and in situ chemoselective olefin
saturation to directly provide two-carbon-homologated saturated
esters.
a
b
Saturated esters (<5%) were produced. Ratio of hydroxy enoates/
cycloetherification products.
ring size afforded hydroxy enoates 5ae−ag with excellent E/Z
ratios and good yields, along with minor saturated esters and
cycloetherification products. Particularly, benzolactone sponta-
neously underwent cycloetherification after reductive HWE
reaction, which furnished 5ah.
We also investigated iterative reductive HWE olefination of
silyl acetals (Scheme 5). Traditional methods for the synthesis of
ASSOCIATED CONTENT
■
S
* Supporting Information
Scheme 5. Iterative Reductive HWE Olefination
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b02901.
Spectroscopic characterization data and procedures for
preparation of all new compounds (PDF)
C
DOI: 10.1021/acs.orglett.5b02901
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
2006, 2006, 1391−1401. (d) Denmark, S. E.; Beutner, G. L. Angew.
Chem., Int. Ed. 2008, 47, 1560−1638.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jjeon@uta.edu.
Notes
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Fukumoto, Y.; Chatani, N. Synlett 2006, 2006, 869−872.
The authors declare no competing financial interest.
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(b) Corriu, R. J. P.; Perz, R.; Reye, C. Tetrahedron 1983, 39, 999−1009.
(c) Danheiser, R. L.; Fink, D. M.; Okano, K.; Tsai, Y. M.; Szczepanski, S.
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ACKNOWLEDGMENTS
■
We thank UT Arlington start-up fund for support of our
program, ACS Petroleum Research Fund (PRF# 54831-DNI1),
and NSF (CHE-0234811 and CHE-0840509).
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mmol) were added to a flame-dried, nitrogen-purged septum-capped
vial. The mixture was dissolved with CH2Cl2 (0.30 mL, 3.3 M), and
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