Enabling the Cross-Coupling of Tertiary Organoboron Nucleophiles through Radical-Mediated Alkyl Transfer

Article abstract of DOI:10.1021/jacs.7b06288

The construction of quaternary centers is a common challenge in the synthesis of complex materials and natural products. Current cross-coupling strategies that can be generalized for setting these centers are sparse and, when known, are typically predicated on the use of reactive organometallic reagents. To address this shortcoming a new, photoredox-Ni dual catalytic strategy for the cross-coupling of tertiary organoboron reagents with aryl halides is reported. In addition to details on the cross-coupling scope and limitations, full screening efforts and mechanistic experiments are communicated.

Full text of DOI:10.1021/jacs.7b06288

Communication
pubs.acs.org/JACS
Enabling the Cross-Coupling of Tertiary Organoboron Nucleophiles
through Radical-Mediated Alkyl Transfer
David N. Primer and Gary A. Molander*
Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323,
United States
S
* Supporting Information
electrophilic and protic functional groups and are often
ABSTRACT: The construction of quaternary centers is a
common challenge in the synthesis of complex materials
and natural products. Current cross-coupling strategies
that can be generalized for setting these centers are sparse
and, when known, are typically predicated on the use of
reactive organometallic reagents. To address this short-
coming a new, photoredox-Ni dual catalytic strategy for
the cross-coupling of tertiary organoboron reagents with
aryl halides is reported. In addition to details on the cross-
coupling scope and limitations, full screening efforts and
mechanistic experiments are communicated.
accompanied by isomerization, generating byproducts when
the temperature of these reactions is not carefully controlled.
Beyond these seminal reports, there are additional isolated
examples using Kumada6 or Negishi7 reagents, but they do not
represent a unified strategy for tertiary cross-couplings.
Particularly from a diversification perspective, approaches using
these sensitive, pyrophoric reagents are wholly impractical when
applied to a wide array of tertiary nucleophiles paired with an
equally broad palette of aryl electrophiles. The clear absence of
methods predicated on the use of bench-stable partners inspired
the laboratory to explore tertiary organoboron nucleophiles
more closelyespecially given the recent surge in methods for
their preparation.8
he installation of quaternary centers has been a long-
Tstanding challenge, going back to the earliest days in the art
of steroid synthesis. Woodward et al. succeeded in setting some
of the first all-carbon stereocenters through cycloaddition
chemistry over 60 years ago.1 Although methods have evolved
dramatically, there are still relatively few ways to install these
centers reliably across a broad array of functionalized partners.2
Among the most commonly employed methods, three
predominate: (1) alkylation of 1,1-disubstituted allylic leaving
groups,3a (2) conjugate addition to α,β-unsaturated carbonyls,3b
and (3) alkylation of substituted enolate equivalents.3c In many
cases, each of these subclasses can cleanly access the requisite
quaternary center, but they are constrained by the requirement
and natural reactivity pattern for a particular structural element
(e.g., α,β-unsaturated carbonyls, allyl, or carbonyl systems). Most
notably, none of these methods adequately addresses the
synthesis of arylated quaternary centers.
In addition to these common approaches, a variety of niche
metal-catalyzed reactions have also been developed for installing
such centers between various nucleophile and electrophile
combinations. Key contributions include Tsuji−Trost allyl-
ations,4a Heck couplings,4b Pauson−Khand cyclizations,4c redox
relay arylations,4d and carbene C−H insertion reactions.4e These
examples are by no means comprehensive, but they represent
landmark catalytic approaches to the installation of challenging
quaternary centers.
Studies were initiated using tert-butyltrifluoroborate 13 as the
chosen nucleophile with 4′-bromoacetophenone as the aryl
electrophile. Conditions (Figure 1A) successful in previous
studies9 proved completely ineffective for achieving the desired
cross-coupling. For all bipyridyl ligands, only starting material or
proto-dehalogenation products were observed. Furthermore, the
alkyltrifluoroborate was often minimally consumed after 24 h.
Given the lack of conversion under our initially developed
conditions, we sought to understand in a more comprehensive
manner the divergent reactivity of these tertiary radicals.
First, oxidation of the reagent was considered (Figure 1B).
Notably, in previous work static quenching of alkyltrifluoro-
borates in the presence of a cationic photocatalysts was
observed,10 indicating the formation of a salt pre-complex that
precedes a single-electron-transfer event. Because of the
enhanced steric bulk of these tertiary precursors, it seemed
prudent to confirm these reagents were indeed generating radical
intermediates. Gratifyingly, both cyclic voltammetry analysis of
the alkyltrifluoroborate (E1re/d2 = +1.26 V vs SCE) and Giese-type
trapping experiments11 confirmed the viability of this oxidation,
the latter affording the desired alkylated product 14 in 74% yield.
Next, attention was turned to the key bond-forming reductive
elimination step in these couplings (Figure 1C). Here, the
desired NiIII intermediate was deemed to be accessible through
the use of a preformed NiII(dtbbpy) oxidative addition complex
15 in the presence of alkyltrifluoroborate 13 and an appropriate
oxidant.12 Surprisingly, stoichiometric experiments exposing this
complex to a variety of oxidant combinations in the presence of
13 failed to afford any of the desired coupling product, even
under prolonged reaction times. The only products observed
Although other metal-mediated catalytic reactions have
enabled access to quaternary centers, there are, by contrast,
vanishingly few examples of tertiary nucleophiles in classical
cross-coupling chemistry. Seminal contributions from Biscoe5a
and Glorius5b et al. highlighted the success of Grignard reagents
with N-heterocyclic carbene-ligated nickel complexes. Unfortu-
nately, these strongly basic nucleophiles are poorly tolerant of
Received: June 16, 2017
© XXXX American Chemical Society
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Figure 1. Plausible catalytic cycle and key mechanistic experiments. (A) Previously developed conditions for alkyltrifluoroborate coupling. (B)
Oxidation experiment and cyclic voltammetry analysis of 13. (C) Stoichiometric experiments with Ni(II) oxidative addition complex 15.
details). Across all ligand classes, only diketonate-type species
showed significant product formation. Interestingly, these
ligands have seen use in mechanistically analogous reductive
coupling chemistry reported by Gong et al.21
Further optimization of this ligand architecture identified a
bulkier tetramethylheptanedione (TMHD) scaffold as suitable
for promoting the desired coupling. Additional evaluation of
bases showed that adding inorganic bases reduced proto-
dehalogenation byproducts and increased yield. A solvent screen
showed that DMA was essential for coupling success. Use of
other polar, aprotic solvents such as DMF, DMSO, or NMP
resulted in poorer conversions. ZnBr2 and other Lewis acids were
tested as additives to facilitate the coupling. Although these
additives were deemed non-essential, rate studies show that they
reduce the initial induction period observed for the coupling,
particularly at larger reaction scales. Table 1 summarizes the
effects of deviations from the optimized conditions. It should be
Figure 2. Constrained cyclic alkyltrifluoroborates in cross-coupling.
Table 1. Optimized Conditions for tert-Butyltrifluoroborate
Cross-Coupling and Effects of Deviation from Standard
Conditions
were THF adduct 18 (known to be generated under the reaction
conditions through C−H abstraction13 or halide abstraction
pathways14) and protodehalogenation. Given these results, it
appeared unlikely that the established bipyridyl ligand systems
would be broadly applicable to their tertiary counterparts.
Although daunted by the failures here, we did achieve targeted
success with adamantyltrifluoroborate and other constrained ring
systems because of their unique structural properties. As a tied
back (tetrahedral) radical possessing no sites for possible β-
hydride elimination, slight modification of the previously
developed conditions allowed access to an array of compounds
(Figure 2). Electron-poor (19−23, 25), electron-rich (24), and
heteroaryl halides (20) all proved viable. Unfortunately, this
rather niche reactivity was not unprecedented7a,15 and by no
means represented a general solution to tertiary radical coupling.
To identify a set of more robust general conditions, a screening
effort was initiated to evaluate a wide variety of ligand classes that
might be amenable to tertiary radical progenitors. Representative
ligands from the Ni cross-coupling literature spanning
phosphines,16 pyridines,17 terpyridines,18 amino alcohols,19 N-
heterocyclic carbenes,4 olefins,20 and 1,3-diketones21 were all
introduced to both a Ni(0) and Ni(II) source in parallel. The
reactions were then followed by UPLC to observe the desired
coupling product (see Supporting Information (SI) for full
a
HPLC yield with 4,4′-di-tert-butylbiphenyl as internal standard.
b
CzlPN = 2,4,5,6-tetra-9H-carbazol-9-yl-1,3-benzenedicarbonitrile.
B
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Figure 4. Further evaluation of aryl bromide scope with enone-derived
alkyltrifluoroborate 38.
Figure 3. Aryl bromide scope in the cross-coupling of potassium tert-
butyltrifluoroborate under the developed conditions. ⧧Reactions
irradiated with four H150 blue LED Kessil lamps.
noted that although reactions run in front of the low wattage
LEDs were sluggish (48−72 h), increasing the light intensity
through the use of blue aquarium lamps as in Figure 3, entries 27
and 35, substantially reduced the reaction times (24 h).
With suitable conditions in hand, we evaluated this coupling
across an array of aryl bromides (Figure 3). The use of numerous
electron-poor aryl bromides resulted in moderate to excellent
yields (26−34). Of note, a number of electrophilic functional
groups that are intolerant of more reactive Kumada conditions
could be employed to afford nitrile 27, aldehyde 28, sulfonyl
fluoride 30, Weinreb amide 34, and ketones 26 and 36. A bromo
sulfonamide could also be engaged in the coupling to give 31,
demonstrating tolerance of some protic functional groups.
Product 30 demonstrates electronic tolerance of ortho-
substitution; attempts to use a more sterically demanding o-
methyl-substituted bromide, however, resulted in trace con-
version. A heteroaryl thiophene and benzofuran were acceptable
partners to give 35 and 36. Unfortunately, the more desirable N-
heteroaryl bromides were ineffective under the coupling
conditions (see 37). At this time, we surmise that pyridine
serves to ligate the metal center competitively, inhibiting the
active catalyst. This is supported by doping experiments where
adding 10 mol% of pyridine to the active catalyst mixture arrests
conversion to desired product (see SI).
To evaluate the scope further, enone-derived alkyltrifluoro-
borate 38 was used to ease separation of reactions with
incomplete conversion (Figure 4). Here, electron-poor systems
worked well to give 39−44. Meta-substitution was tolerated,
giving 43 and 46 in moderate yields. Overall, the use of electron-
neutral bromides resulted in poorer yields (45, 46), and truly
electron-rich systems such as 4-bromoanisole afforded trace
product (47).
With this survey of electrophiles established, we next turned
our attention to more complex tertiary alkyltrifluoroborate
precursors, using electron-poor bromides as model electrophiles
(Figure 5A). Cyclohexanone- and cyclopentanone-based ring
systems could be coupled with an array of substituted aryl
bromides (aldehydes, sulfones, ketones) to afford 48−51.
Figure 5. (A) Tertiary alkyltrifluoroborate scope with electron poor aryl
bromides. (B) Comparison of enone- and enoate-derived trifluoro-
borate cross-coupling to metal-catalyzed 1,4-addition chemistry.
Notably, the products obtained from the coupling of these
enone- and enoate-derived trifluoroborates are analogous to
those accessed through 1,4-addition of aryl nucleophiles to α,β-
unsaturated carbonyl systems.22 As such, the coupling here
represents an umpolung approach to these methods, where the
carbon β to the carbonyl is rendered nucleophilic (Figure 5B).
Considering the far greater number of commercially available
aryl electrophiles relative to the corresponding aryl nucleophiles,
this alternative approach has clear advantages for diversity-
oriented synthesis. Other functionalized trifluoroborates could
be employed to give quaternary centers in 52−56. Here, ester-,
ether-, and ketone-containing trifluoroborates all work well in the
established protocol. Incorporating an amide in a nearby position
shut down the reaction, returning only β-hydride elimination and
reduction products (see 57), suggesting that changes in the
ligation around the nickel have drastic effects on the reaction
profile for this tertiary cross-coupling. Importantly, the 1-
adamantyl- and 1-phenylcyclopropyltrifluoroborates worked
under these conditions as well.
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In summary, the first general method for the cross-coupling of
tertiary organotrifluoroborates is described. The development of
this protocol hinged on the use of photoredox high-throughput
screening technology to identify a new ligand architecture
amenable to bulkier tertiary radical nucleophiles. Using the
conditions identified therein, an array of functionalized,
unactivated tertiary organoboron reagents were employed in
transition metal-catalyzed cross-coupling for the first time.23
These reactions generate quaternary centers, notoriously
challenging to access, that are typically forged in cross-coupling
through the use of highly reactive Kumada and Negishi partners.
In contrast, these mild conditions tolerate a number of
electrophilically sensitive functional groups such as aldehydes,
ketones, esters, and amides.
At present, a few key limitations of this protocol remain, as the
aryl bromide scope for this cross-coupling is currently limited to
electron-poor and electron-neutral systems. From the standpoint
of organoboron coupling, this limitation actually complements
existing metal-free tertiary coupling work by Aggarwal et al.,
wherein tertiary alkyl pinacol boronates can be best coupled with
electron-rich arene systems.24 The current method also exhibits a
notable absence of N-containing heteroaryl partners. In some
ways, this failure of heteroaryl systems is mitigated by the
successful implementation of tertiary alkyltrifluoroborates in
Minisci processes,25 but extension to other electrophilic sites on
heteroaryl compounds would add significant value. Further
refinement to the ligand scaffold and conditions will likely
address these two limitations, as well as the ortho-substitution
challenge; efforts in the laboratory toward this end are ongoing.
Given the growing suite of methods for the synthesis of tertiary
organoboron compounds,8 this photoredox cross-coupling is a
timely addition, enabling tertiary organoboron reagents as
partners in the installation of arylated quaternary centers.
Although limitations remain, the ligand screening data and
mechanistic interrogation will allow practitioners seeking to
employ other tertiary radical feedstocks (carboxylic acids,
halides, aldehydes, silicates, etc.) to channel these lessons into
developing new tertiary alkyl cross-couplings of broad interest.
Taken together, these findings introduce tertiary alkyl radicals to
the photoredox cross-coupling portfolio.
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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/jacs.7b06288.
Experimental details and spectral data (PDF)
AUTHOR INFORMATION
Corresponding Author
*gmolandr@sas.upenn.edu
■
ORCID
́
(19) Gonzalez-Bobes, F.; Fu, G. C. J. Am. Chem. Soc. 2006, 128, 5360.
Gary A. Molander: 0000-0002-9114-5584
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank NIGMS (R01 GM113878) for support of this
research. We thank the NIH (S10 OD011980) for supporting
the University of Pennsylvania (UPenn) Merck Center for High-
Throughput Experimentation, which funded the equipment used
in screening efforts.
D
DOI: 10.1021/jacs.7b06288
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX