Regioselective Single-Electron Tsuji-Trost Reaction of Allylic Alcohols: A Photoredox/Nickel Dual Catalytic Approach

Article abstract of DOI:10.1021/acs.orglett.9b02473

A radical-mediated functionalization of allyl alcohol derived partners with a variety of alkyl 1,4-dihydropyridines via photoredox/nickel dual catalysis is described. This transformation transpires with high linear and E-selectivity, avoiding the requirement of harsh conditions (e.g., strong base, elevated temperature). Additionally, using aryl sulfinate salts as radical precursors, allyl sulfones can also be obtained. Kinetic isotope effect experiments implicated oxidative addition of the nickel catalyst to the allylic electrophile as the turnover-limiting step, supporting previous computational studies.

Full text of DOI:10.1021/acs.orglett.9b02473

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Regioselective Single-Electron Tsuji−Trost Reaction of Allylic
Alcohols: A Photoredox/Nickel Dual Catalytic Approach
†
†
Zheng-Jun Wang,†,‡ Shuai Zheng,† Eugenie Romero, Jennifer K. Matsui, and Gary A. Molander†,*
́
†Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia,
Pennsylvania 19104-6323, United States
‡State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan
University, Changsha 41000, China
S
* Supporting Information
ABSTRACT: A radical-mediated functionalization of allyl
alcohol derived partners with a variety of alkyl 1,4-
dihydropyridines via photoredox/nickel dual catalysis is
described. This transformation transpires with high linear
and E-selectivity, avoiding the requirement of harsh conditions
(e.g., strong base, elevated temperature). Additionally, using
aryl sulfinate salts as radical precursors, allyl sulfones can also
be obtained. Kinetic isotope effect experiments implicated
oxidative addition of the nickel catalyst to the allylic
electrophile as the turnover-limiting step, supporting previous computational studies.
istorically, palladium catalysis has proven to be a
In an effort to expand the nucleophile scope of the Tsuji−
Trost reaction, reductive allylation strategies have been
explored by numerous groups, enabling the employment of
“hard” nucleophiles.5 Notably, Tunge and co-workers
pioneered the concept of a radical-based approach.5a In this
paradigm, utilizing photoredox catalysis, the excited state
photocatalyst undergoes a single-electron oxidation of a
carboxylate anion substrate. Upon decarboxylation, a carbon-
centered alkyl radical is generated and captured by a Pd(II) π-
allyl complex, forming a Pd(III) intermediate that reductively
eliminates to afford the cross-coupling product.
H
powerful means by which C−C bonds can be forged
in a regioselective manner.1 To date, there have been
numerous reports utilizing palladium catalysis for coupling
allyl alcohol substrates with “soft” (pKa < 25) enolate
nucleophiles (Scheme 1).2 Other “soft” nucleophiles (e.g.,
Scheme 1. Radical-Based Addition to π-Allylnickel
Intermediates
Although numerous coupling methods using various
palladium catalysts have been reported during the past
decades, significantly fewer methods have been disclosed
utilizing the group 10 base metal, nickel.6 In addition to the
advantage of cost-effectiveness, recent studies have shed light
on the complementary reactivity that can arise between nickel
and palladium. For example, Fu et al. successfully carried out
nickel-catalyzed, enantioselective cross-coupling between allyl
chlorides and alkylzinc reagents.7 Nickel/photoredox dual-
catalyzed alkylation of vinyl epoxides has also been developed,
with strong evidence of an inner-sphere mechanism.8
Inspired by previous work, a Ni-based radical approach to
functionalize allylic electrophiles was sought. Herein, alkylation
of allyl alcohol derived motifs has been demonstrated by using
4-alkyl 1,4-dihydropyridines (DHPs)9 as latent radical
precursors. Both allyl carbonates and in situ activated allyl
alcohols have proven effective in the reaction. To highlight the
new chemical space, the disclosed reaction was employed for
nitrogen- and oxygen-based nucleophiles) have also been
employed,3 and significant advances have been accomplished
in the field with the development of stereoselective trans-
formations. By comparison, although known, transformations
using “hard” nucleophiles, which coordinate to metal catalysts
before reductive elimination, are less studied and often require
harsh conditions and/or functional group intolerant reagents.4
Received: July 16, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02473
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the synthesis of allylated monosaccharides. Heteroatomic
radical sources, such as aryl sulfinates, were also used as
partners in this reaction. Finally, from a mechanistic stand-
point, various factors influencing regioselectivity (e.g., the
ligand on the nickel center) were demonstrated, in addition to
kinetic isotope effect studies that support oxidative addition as
the turnover-limiting step.
With suitable conditions in hand, an investigation of the
stereoelectronic effects of the aryl moiety on the reaction was
begun. Similar to reactivity trends reported for Tsuji−Trost
reactions,11 electron-withdrawing groups (2d) did not
significantly decrease overall reaction efficiency (Figure 1).
At the outset of the synthetic studies, optimization was
conducted using isopropyl DHP as the radical precursor, and
the best results were observed with allyl methyl carbonate as
the electrophile. This result provided an opportunity to use
dimethyl dicarbonate (DMDC) as an activator, which had
proven to be compatible in previous studies (Table 1, entry
a
Table 1. Control Studies and Ligand Optimization
entry
deviation from standard conditions
yield (%) E/Z ratio (A:B)
1
2
3
4
5
6
7
8
R1 = CO2Me, R2 = i-Pr, No DMDC
none, R2 = i-Pr
83
66
trace
0
>20:1
>20:1
−
−
−
no light, R2 = i-Pr
Figure 1. Exploring alkylation scope with carbonates. Reaction
conditions: Allyl methyl carbonate (1.0 equiv, 0.30 mmol), DHP (1.5
equiv, 0.45 mmol), 4CzIPN (3 mol %), Ni(phen)Cl2 (5 mol %), and
DMF (3 mL, 0.1 M) thoroughly degassed followed by stirring near
no photocatalyst, R2 = i-Pr
no Ni catalyst, R2 = i-Pr
none, R2 = Cyclohexyl
Ni 2, R2 = Cyclohexyl
Ni 3, R2 = Cyclohexyl
Ni 4, R2 = Cyclohexyl
Ni 5, R2 = Cyclohexyl
Ni 6, R2 = Cyclohexyl
Ni 7, R2 = Cyclohexyl
0
87
26
66
8
10
73
21
>20:1
7:1
1
blue LEDs for 16 h. E/Z ratios were determined by H NMR of the
isolated product. a 1 mmol scale. b dr > 20:1.
>20:1
E only
1.3:1
1:1.2
2:1
9
Conversely, electron-donating moieties (2e) led to slightly
diminished yields (63%). Furthermore, exploration of alkyl-
substituted electrophilic partners was of interest. Notably, 2f
was successfully isolated from the corresponding dienol
derivative, albeit in lower yield. As a general note, alkyl-
substituted allyl carbonates suffered from diminished reactivity
and, in some cases, no reactivity. This occurrence may result
from a decrease in electrophilicity, hampering π-allyl nickel
complex formation.
10
11
12
a
Reaction conditions: Allyl alcohol or carbonate (1.0 equiv, 0.1
mmol), DHP (1.5 equiv, 0.15 mmol), 4CzIPN (3 mol %), Ni catalyst
(5 mol %), DMDC (3.0 equiv), and DMF (0.1 M) thoroughly
degassed followed by stirring near blue LEDs for 16 h. Overall yields
and regioselectivity were determined by GC, and E/Z isomer ratios
were determined by NMR.
Next, attention was focused on incorporating a wider range
of DHPs, beginning with cyclic carbon-centered radicals (2a,
2h, and 2j). Functional groups such as alkenes (2g) and
activated hydrogens (2i) are also compatible, providing good
yields of the desired products. A 1 mmol scale reaction was
also attempted for 1a, and a comparable 85% yield was
obtained.
Although allyl carbonates are both commercially available
and easily accessed, conditions were sought to improve step
economy by alkylating allylic alcohols in a single-step, one-pot
reaction. With DMDC identified as the most efficient activator,
the scope of this approach was examined with various cinnamyl
alcohol derivatives and functionally diverse DHPs (Figure 2).
Comparing previous yields with allyl carbonates and the newly
developed allyl alcohol conditions, comparable results were
achieved (2a and 2b versus 3a and 3b). Therefore,
investigation of the scope of the functional group breadth for
the allyl alcohol and DHP pieces was continued. Similar
compatibility for substitutions about the aryl motif was
observed (3c−3g). Nitrogen- (3k) and oxygen-containing
(3i) heterocyclic DHPs were successfully incorporated into the
reaction manifold, with moderate to good yield. Notably, a
hydroxymethyl radical was successfully applied in this trans-
formation, with a 57% yield of β-hydroxyl product 3l isolated.
2).10 To confirm the necessity of photocatalyst 4CzIPN
(2,4,5,6-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene), nickel
catalyst, and light, control experiments were performed
(Table 1, entries 3−5). For each control experiment, no
desired product was observed. Using high-throughput
experimentation, further optimizations were carried out to
identify a set of suitable reaction conditions (e.g., photo-
catalyst, Ni source, solvent, and ligand; see Supporting
Information for further details). To investigate the regiose-
lectivity (linear/branched) and stereoselectivity (E/Z) of the
reaction further, a series of bipyridine ligands were screened
(entries 6−12). Most of the ligands led to highly selective
linear product formation, although further investigations
showed a substrate dependence on the linear/branched
selectivity (see Supporting Information for further details).
1,10-Phenanthroline was identified as a suitable ligand,
rendering almost exclusively E-selective product with a high
yield (entry 6). Further reactions were therefore carried out
using the preformed complex, Ni(phen)Cl2, as the precatalyst.
B
DOI: 10.1021/acs.orglett.9b02473
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
motifs albeit with less functional group tolerance anticipated in
the aryl subunit.17
To extend the functional group repertoire of allyl alcohol
derived electrophiles further, other carbon- or heteroatom-
centered radical sources were tested. Although toolboxes such
as alkyltrifluoroborates,18lkylsilicates,19 and N-centered radi-
cals20 were incompatible, aryl sulfinate salts were coupled with
allyl carbonates to generate allyl aryl sulfones (Figure 3).21
Figure 2. Coupling allyl alcohols with DHPs. Reaction conditions:
Allyl alcohol (1.0 equiv, 0.30 mmol), DHP (1.5 equiv, 0.45 mmol),
4CzIPN (3 mol %), Ni(phen)Cl2 (5 mol %), DMDC (3.0 equiv, 0.90
mmol), and DMF (3 mL, 0.1 M) thoroughly degassed followed by
stirring near blue LEDs for 16 h. E/Z ratios were determined by H
NMR of the isolated product.
Figure 3. Demonstrating latent radical breadth. Reaction conditions:
Allyl methyl carbonate (1.0 equiv, 0.30 mmol), DHP (1.5 equiv, 0.45
mmol), 4CzIPN (3 mol %), Ni(phen)Cl2 (5 mol %), and DMF (3
mL, 0.1 M) thoroughly degassed followed by stirring near blue LEDs
1
1
for 16 h. E/Z ratios were determined by H NMR of the isolated
To broaden the scope and demonstrate the utility of this
alkylation of allyl alcohol pro-electrophiles, attention was
turned toward more challenging scaffolds, specifically mono-
saccharides.12 Functionalization of carbohydrates has proven
to be pivotal in many fields, including labeling technologies,13
cyclodextrin-mediated catalysis,14 and carbohydrate drug
development.15 Notably, few C-allyl glycoside syntheses have
been reported, most of which are restricted to C-1 alkylation of
the carbohydrate and further limited to unsubstituted propenyl
electrophiles.16 Upon treating both furanose (4a) and
pyranose (4b) DHPs under the standard reaction conditions,
the nontraditional C-allyl glycosides were isolated in acceptable
yields and excellent regio- and diastereoselectivities (Scheme
2). Notably, structurally similar organometallic saccharide
partners cannot be accessed owing to rapid β-alkoxy
elimination. During the course of these studies, Mazet and
co-workers published a complementary approach, using a one-
pot isolation/C−O arylation strategy with a glycoside-derived
allyl electrophile and Grignard reagents, rendering similar
product. a Sample was contaminated with hexane, EtOAc, and H2O.
Both electron-rich (5e) and electron-deficient (5c) allyl
carbonates performed well, and an aliphatic allyl carbonate
was applicable with good regioselectivity (5f), providing the
desired product in 56% yield with a 9:1 E/Z ratio.
Finally, to shed some light on the mechanism, several
preliminary experiments were carried out, including Stern−
Volmer quenching experiments. Although significant fluores-
cent quenching with alkyl DHPs has been previously
reported,22 neither Ni(phen)Cl2 nor allyl methyl carbonate
quenched the fluorescence of 4CzIPN. This finding suggests
that the reductive generation of the allylic radical is not
occurring by interaction with the photocatalyst or the low-
valent Ni species. To determine the turnover-limiting step, we
investigated the kinetic isotope effect by comparing reaction
rates of α-deuterated cinnamyl methyl carbonate vs its
nondeuterated form. A significant secondary isotope effect
(kH/kD = 1.15 0.07) was observed, indicating a change in
hybridization of the allyl carbonate substrate in the turnover-
determining step, suggesting oxidative addition of allyl
carbonate to the nickel complex as the turnover-limiting
step. The regioselectivity would then most likely be dictated by
energy differences between the corresponding transition
structures involved in the reductive elimination step, which is
consistent with previous computational studies.8,23 Based on
these results and previous mechanistic studies, a plausible
mechanism is proposed (Scheme 3): upon excitation, the
photocatalyst oxidizes the radical precursors to generate alkyl
or sulfonyl radicals, which are subsequently captured by Ni(0).
Allyl methyl carbonate then oxidatively adds to the Ni(I)
intermediate to generate the active Ni(III) species, followed by
a
Scheme 2. Direct Allylation of Monosaccharides
a
Reaction conditions: Allyl alcohol (1.0 equiv, 0.30 mmol), DHP (1.5
equiv, 0.45 mmol), 4CzIPN (3 mol %), Ni(phen)Cl2 (5 mol %),
DMDC (3.0 equiv, 0.90 mmol), and DMF (3 mL, 0.1 M) thoroughly
degassed, followed by stirring near blue LEDs for 16 h. E/Z ratios
3
3
3
reductive elimination to form Csp −Csp or Csp −S bonds. The
resulting Ni(I) species is reduced by the radical anion of
4CzIPN, closing both catalytic cycles.
1
were determined by H NMR of the isolated product.
C
DOI: 10.1021/acs.orglett.9b02473
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Organic Letters
Letter
J.W. acknowledges financial support from the Chinese
Scholarship Council. We thank Dr. Charles W. Ross, III
(UPenn) for help in HRMS data acquisition.
Scheme 3. Putative Radical-Based Mechanism
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02473.
Detailed experimental procedures, mechanistic inves-
tigations, characterization data, and NMR spectra for
new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: gmolandr@sas.upenn.edu.
ORCID
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Shuai Zheng: 0000-0001-9085-4410
Jennifer K. Matsui: 0000-0003-0693-0404
Gary A. Molander: 0000-0002-9114-5584
Notes
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The authors are grateful for the financial support provided by
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DOI: 10.1021/acs.orglett.9b02473
Org. Lett. XXXX, XXX, XXX−XXX