Copper-Catalyzed Decarboxylative Radical Silylation of Redox-Active Aliphatic Carboxylic Acid Derivatives

Article abstract of DOI:10.1002/anie.201706611

A decarboxylative silylation of aliphatic N-hydroxyphthalimide (NHPI) esters using Si?B reagents as silicon pronucleophiles is reported. This C(sp³)?Si cross-coupling is catalyzed by copper(I) and follows a radical mechanism, even with exclusion of light. Both primary and secondary alkyl groups couple effectively, whereas tertiary alkyl groups are probably too sterically hindered. The functional-group tolerance is generally excellent, and α-heteroatom-substituted substrates also participate well. This enables, for example, the synthesis of α-silylated amines starting from NHPI esters derived from α-amino acids. The new method extends the still limited number of C(sp³)?Si cross-couplings of unactivated alkyl electrophiles.

Full text of DOI:10.1002/anie.201706611

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Accepted Article
Title: Copper-Catalyzed Decarboxylative Radical Silylation of Redox-
Active Aliphatic Carboxylic Acid Derivatives
Authors: Weichao Xue and Martin Oestreich
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Copper-Catalyzed Decarboxylative Radical Silylation of Redox-
Active Aliphatic Carboxylic Acid Derivatives
Weichao Xue and Martin Oestreich*
Abstract:
A
decarboxylative
silylation
of
aliphatic
N-
hydroxyphthalimide (NHPI) esters using Si–B reagents as silicon
3
pronucleophiles is reported. This C(sp )–Si cross-coupling is
catalyzed by copper(I) and follows a radical mechanism, even with
exclusion of light. Both primary and secondary alkyl groups couple
effectively whereas tertiary groups are probably too sterically
hindered. The functional-group tolerance is generally excellent, and
α-heteroatom-substituted substrates also participate well. This
enables, for example, the synthesis of α-silylated amines starting
from NHPI esters derived from α-amino acids. The new method
3
complements the still limited number of C(sp )–Si cross-couplings of
unactivated alkyl electrophiles.
Scheme 1. Existing and planned methods for the cross-coupling of
1 2 3
unactivated alkyl electrophiles and silicon (pro)nucleophiles. R , R , and R
=
Alkyl and H as well as R = Alkyl and/or Aryl. dtbpy = 4,4’-di-tert-butyl-2,2’-
[
1]
The cross-coupling of unactivated alkyl electrophiles and
silicon (pro)nucleophiles is a deceivingly simple-looking yet
almost unprecedented reaction. Solutions to this challenge have
bipyridine, diglyme
dimethylacetamide, DMF
hydroxyphthalimide, pin = pinacolato.
=
1-methoxy-2-(2-methoxyethoxy)ethane, DMA
=
=
N,N-
N-
=
N,N-dimethylformamide, NHPI
[2]
only been disclosed recently by Fu and co-workers as well as
[3]
[4]
us (Scheme 1, left). The successful catalytic systems differ in
catalyst and silicon source but share the features of a radical
process. Our contribution included an in-depth analysis of the
reaction mechanism, and its radical nature prompted us to find
redox-active alkyl electrophiles other than iodides and bromides
that would engage in a redox reaction with the silicon-based
copper intermediate. Readily accessible aliphatic esters derived
from N-hydroxyphthalimide (NHPI) fall into this category. These
were discovered by Okada and co-workers and are a light-stable
After substantial optimization (see the Supporting
Information), excellent yield was obtained with copper(I)
thiophene-2-carboxylate (CuTc) as (pre)catalyst together with
3
dtbpy and Cy P as ligands in THF/NMP (9/1) at room
temperature (2a→7aa with 1a, Table 1, entry 1). Control
experiments showed that the copper salt, the ligands, and the
base are necessary (entries 2‒5). Notably, the combination of
the bipyridine and the phosphine is crucial to secure high yield:
Neither dtbpy nor Cy P alone promoted the decarboxylative
3
silylation in satisfactory yield (entries 3 and 4). An extensive
screening of alkoxides did not give better results except for
[
5]
alternative to Barton esters. Single-electron transfer reduction
[
6]
initiates decarboxylation to generate an alkyl radical.
[
7,8]
Application of this reaction mode in photo-
and transition-
led to the development of numerous
LiOtBu (entries 6‒9). Using heteroatom-substituted Si‒B reagent
[9,10]
metal catalysis
[13c]
2
Me
2
PhSiB(NiPr
2
)
2
instead of Me PhSiBpin (1a) was
procedures for carbon–carbon bond formation. We report here
the use of NHPI esters for the formation of carbon–silicon
unsuccessful (entry 10), and zinc-based silicon nucleophile
[15]
(Me
2
PhSi)
2
Zn
afforded mediocre yield (entry 11). We also
[
11]
bonds
catalysis with Si–B reagents
Scheme 1, gray box). The new method allows for C(sp )–Si
cross-coupling by decarboxylative silylation of aliphatic
on the basis of our previously established copper
tested an array of other potentially redox-active esters 3a‒6a but
none led to the desired coupling in more than traces (entries 12–
[3]
[12,13]
as silicon pronucleophiles
3
(
15). Importantly, the reaction proceeded equally well in the dark
(entry 16).
[14]
carboxylic acid derivatives (Scheme 1, right).
[a]
Table 1: Selected examples of the optimization of the reaction conditions.
CuTc (10 mol%)
dtbpy/Cy3P (10 mol%)
O
Me PhSiBpin (1a, 2.5 equiv)
2
NaOEt (1.0 equiv)
SiMe Ph
2
THF/NMP (9/1)
RT
2
a–6a
7aa
F5
O
O
O
S
[
*]
W. Xue, Prof. Dr. M. Oestreich
Cl4
N
N
N
N
Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
O
O
O
O
O
O
O
3a
O
2a
4a
5a
6a
Homepage: http://www.organometallics.tu-berlin.de
[b]
Entry
Ester
Variation
Yield [%]
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
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COMMUNICATION
[
c]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
3a
4a
5a
6a
2a
none
91 (86)
w/o CuTc
w/o dtbpy
trace
25
w/o Cy
3
P
46
w/o NaOEt
6
LiOtBu instead of NaOEt
NaOtBu instead of NaOEt
KOtBu instead of NaOEt
KOEt instead of NaOEt
86
42
8
48
0
1
2
3
4
5
6
2
Me PhSiB(NiPr
)
2 2
instead of 1a
trace
30
2
(Me PhSi) Zn instead of 1a
2
none
none
none
none
in dark
10
trace
trace
trace
90
Scheme 2. Scope I: Copper-catalyzed decarboxylative silylation of aliphatic
NHPI esters. [a] Formed along with minor amounts of styrene. Boc = tert-
butoxycarbonyl, Bn = benzyl.
[
a] All reactions performed on a 0.20 mmol scale. [b] Determined by GLC
Considering the relevance of the motif of α-silylated amines
analysis with tetracosane as an internal standard. [c] Isolated yield after
purification by flash chromatography on silica gel. NMP
pyrrolidone, Tc = thiophene-2-carboxylate.
[
16,17]
=
N-methyl-2-
for peptide isosteres,
we set out to extend our
decarboxylative silylation to their synthesis starting from
ubiquitous α-amino acids (Scheme 3). We began with the Boc-
protected (S)-proline derivative 2n employing the different Si‒B
[
13a]
reagents 1a‒c. With phenyl-substituted 1a and 1b,
7na and
With the catalytic system in hand, we explored the substrate
scope of the decarboxylative silylation (Schemes 2 and 3). As
summarized in Scheme 2, the new protocol displayed splendid
functional-group tolerance. In addition to model substrate 2a,
other unfunctionalized cyclic (as 2b), acyclic (as 2c), and
benzylic (as 2d) secondary coupling partners also participated.
An internal alkene (as in 2e) and a Boc-protected amine (as in
7
nb formed in 70% and 43% yield, respectively, while less
[
13b]
reactive, purely alkyl-substituted 1c
failed to give 7nc.
Likewise, non-canonical 2o, (S)-phenylalanine-derived 2p, and
S)-tryptophan-derived 2q yielded the α-silylated amines 7oa–qa
(
in moderate isolated yields. No substrate control was seen in the
decarboxylative silylation of isoleucine derivative 2r, affording
7
ra in 46% yield with 50:50 diastereomeric ratio. When phthalyl
2
f) were accepted. LiOtBu was generally superior to NaOEt as
and Cbz instead of Boc were chosen as the electron-
withdrawing protecting group on racemic alanine (as in 2s) and
(S)-proline (as in 2t), similar outcomes were obtained. Also,
diethyl-protected racemic valine derivative 2u transformed into
7ua in good yield, thereby demonstrating the possibility of
accessing a series of dialkyl-protected α-silylated amines with
our method.
the base in the coupling of primary NHPI esters. A wide range of
functional groups was compatible, including a methyl phenyl
ether (as in 2g), an ethyl ester (as in 2h), and carbonyl groups
(
as in dehydrocholic acid skeleton 2i). To our delight, the NHPI
esters of 3-pyridinepropionic acid (2j) and phenylpropionic acid
2k), which could undergo styrene formation by β-elimination,
(
converted into the corresponding silanes in moderate yields.
Moreover, the silyl group was successfully installed α to an
oxygen atom (2l→7la). Likely for steric reasons, tertiary NHPI
esters reacted only sluggishly (see the Supporting Information
for details) but adamantyl-substituted 2m furnished 7ma in
acceptable yield.
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COMMUNICATION
standard setup
R3SiBpin (1a–c, 2.5 equiv)
NHPI NaOEt or LiOtBu (1.0 equiv)
radical-probe experiments
O
R1
R1
standard setup
NaOEt (1.0 equiv)
SiR3
R3
O
SiMe Ph
2
+
N
THF/NMP (9/1)
RT
N
SiMe2Ph
R2
R3
n–u
R2
THF/NMP (9/1)
RT
NHPI
2
7na–ua and
nb and 7nc
2v
8va: 45%
7va: not formed
7
O
standard setup
SiMe2Ph
+
NaOEt (1.0 equiv)
SiMe2Ph
NBoc
SiMePh2
NBoc
SiEt3
NBoc
SiMe2Ph
NHPI
2
SiMe Ph
THF/NMP (9/1)
RT
NBoc
2w
9wa: 21%
7wa: 30%
7
(
na: 70%
LiOtBu)
7nb: 43%
7nc: trace
7oa: 55%
(LiOtBu)
(NaOEt)
(NaOEt or LiOtBu)
racemization experiment
SiMe2Ph
Me
O
SiMe2Ph
Me
NHBoc
standard setup
NHBoc
pa: 43%
SiMe2Ph
SiMe2Ph
NBoc
NaOEt (1.0 equiv)
NHPI
NBoc
7
N
Boc
THF/NMP (9/1)
RT
NHBoc
(
NaOEt)
(
S)-2n (99% ee)
radical-trapping experiments
standard setup
rac-7na: 70%
7
qa: 51%
7ra: 46% (d.r. = 50:50)
O
(
NaOEt)
(NaOEt)
SiMe Ph
2
N
SiMe2Ph
iPr
SiMe2Ph
NEt2
7ua: 66%
(LiOtBu)
Me
Me
Me
Me
Me
O
N
O
NaOEt (1.0 equiv)
O
NCbz
2
SiMe Ph
NHPI
7
sa: 58% (X-ray)
NaOEt)
7ta: 69%
(LiOtBu)
THF/NMP (9/1)
RT
+
(
2
a
TEMPO
none
7aa:
91%
58%
22%
10a:
0%
11%
9%
Scheme 3. Scope II: Copper-catalyzed decarboxylative silylation of NHPI
esters derived from α-amino acids. Cbz = benzyloxycarbonyl.
1
2
.0 equiv
.0 equiv
O
standard setup
NaOEt (1.0 equiv)
Me
+
NHPI
In analogy to our copper-catalyzed dehalogenative silylation
THF/NMP (9/1)
RT
[
3a]
(
Scheme 1, left),
we assumed that the present
2
a
11 (2.5 equiv)
decarboxylative silylation would also proceed through radical
intermediates. To test for this, precursor 2v was prepared and
subjected to the standard protocol (Scheme 4, top); ring-opened
Me
SiMe2Ph
+
8
va was isolated in moderate yield without the formation of
SiMe2Ph
cyclopropyl-containing 7va. Similarly, 2w as possible
a
7
aa: 65%
12a: 17%
precursor for hex-5-enyl radical cyclization underwent the
competing 5-exo-trig ring closure to 9wa in 21% yield along with
linear 7wa (Scheme 4, top). A racemization experiment with
enantioenriched (S)-2n proceeded with complete loss of the
stereochemical information at the α carbon atom (Scheme 4,
middle); as to be expected, the same result was obtained with
Scheme 4. Control experiments to verify the radical mechanism.
[
3a]
On the basis of the above results and our previous work,
we also suggest a radical catalytic cycle for this copper-
3
(R)-2n. Moreover, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)
catalyzed C(sp )‒Si cross-coupling (Scheme 5). The cationic
+
inhibited our model reaction, and the cyclohexyl/TEMPO adduct
was detected in small quantities (2a→7aa along with 10a,
Scheme 4, bottom). When 4-methylstyrene (11) was used to
copper(I) complex 13 captures the silicon nucleophile (see
quantum-chemical calculations in Ref. [3a] for its base-mediated
release from the Si‒B pronucleophile) to form the silicon-based
copper(I) complex 14a. After single-electron transfer (SET) from
14a to the electron-accepting NHPI ester 2a, the radical cation
3
3
trap the cyclohexyl radical, C(sp )‒C(sp ) bond formation
3
occurred in 17% yield at the expense of C(sp )‒Si bond
•+
•‒
formation (2a→7aa along with 12a, Scheme 4, bottom). These
findings, together with convincing literature precedence that
NHPI esters function as alkyl-radical surrogates in
15a is formed along with the radical anion 16a . The latter
•
further decarboxylates to afford the cyclohexyl radical (Cy ).
•+
•
Radical recombination of 15a and Cy will ultimately deliver the
[7,8]
[9]
+
photocatalysis
and nickel chemistry, strongly support the
cross-coupled 7aa and regenerate copper(I) catalyst 13 .
involvement of radicals in this copper-catalyzed decarboxylative
silylation.
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COMMUNICATION
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Scheme 5. Proposed catalytic cycle of the decarboxylative silylation.
To recap, we have disclosed here
a copper-catalyzed
decarboxylative cross-coupling of NHPI esters and Si‒B
3
reagents to build C(sp )‒Si bonds. It is the first example of NHPI
esters used as coupling partners in copper catalysis. Application
of this method to α-amino acid-derived NHPI esters is
particularly noteworthy. The substrate scope generally compares
well with existing methodologies, e.g., decarboxylative borylation
[
[
10] Iron catalysis: F. Toriyama, J. Cornella, L. Wimmer, T.-G. Chen, D. D.
Dixon, G. Creech, P. S. Baran, J. Am. Chem. Soc. 2016, 138, 11132–
11135.
2
11] For examples of C(sp )–Si bond formation involving decarbonylation,
see: a) L. Guo, A. Chatupheeraphat, M. Rueping, Angew. Chem. Int.
Ed. 2016, 55, 11810–11813; Angew. Chem. 2016, 128, 11989–11992;
b) X. Pu, J. Hu, Y. Zhao, Z. Shi, ACS Catal. 2016, 6, 6692–6698; for an
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Liu, Angew. Chem. Int. Ed. 2016, 55, 236–239; Angew. Chem. 2016,
3
[14]
3
for C(sp )‒B bond formation.
This new C(sp )‒Si cross-
coupling is an addition to the recently reported dehalogenative
[
2,3a]
processes.
1
28, 244–247.
Acknowledgements
[
12] For recent reviews of Si–B chemistry, see: a) L. B. Delvos, M. Oestreich
in Science of Synthesis Knowledge Updates 2017/1 (Ed.: M. Oestreich),
Thieme, Stuttgart, 2017, pp. 65–176; b) M. Oestreich, E. Hartmann, M.
Mewald, Chem. Rev. 2013, 113, 402‒441.
W.X. thanks the China Scholarship Council (CSC) for
a
predoctoral fellowship (2015–2019). M.O. is indebted to the
Einstein Foundation (Berlin) for an endowed professorship. We
thank Dr. Elisabeth Irran (TU Berlin) for the X-ray analyses.
[13] For the preparation of Si–B reagents, see: a) M. Suginome, T. Matsuda,
Y. Ito, Organometallics 2000, 19, 4647–4649 [Me PhSiBpin (1a) and
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Nakamura, Y. Ito, Organometallics 2000, 19, 719–721
[(Me PhSiB(NiPr ) ].
2
2
2
3
Keywords: copper • cross-coupling • decarboxylation • radical
reactions • silicon
2
2 2
[
14] For recent examples of decarboxylative borylation using NHPI esters,
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Kumar, A. W. Yu, K. A. Johnson, A. K. Chatterjee, M. Yan, P. S. Baran,
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Suggestion for the Entry for the Table of Contents
COMMUNICATION
W. Xue, M. Oestreich*
Page No. – Page No.
Copper-Catalyzed Decarboxylative
Radical Silylation of Redox-Active
Aliphatic Carboxylic Acid Derivatives
3
CO
2
neutral. Although seemingly trivial, C(sp )‒Si cross-coupling between
unactivated alkyl electrophiles and silicon (pro)nucleophiles had been elusive until
recently. The present decarboxylative silylation of N-hydroxyphthalimide (NHPI)
esters makes use of Si‒B reagents as the source of silicon and is catalyzed by
copper(I). This broadly applicable reaction is shown to follow a radical mechanism
and proceeds in the dark as well (see Scheme).
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