Organophotoredox/palladium dual catalytic decarboxylative Csp3-Csp3coupling of carboxylic acids and π-electrophiles

Article abstract of DOI:10.1039/d0sc02609c

A dual catalytic decarboxylative allylation and benzylation method for the construction of new C(sp3)-C(sp3) bonds between readily available carboxylic acids and functionally diverse carbonate electrophiles has been developed. The new process is mild, operationally simple, and has greatly improved upon the efficiency and generality of previous methodology. In addition, new insights into the reaction mechanism have been realized and provide further understanding of the harnessed reactivity.

Full text of DOI:10.1039/d0sc02609c

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ARTICLE
Organophotoredox/Palladium Dual Catalytic Decarboxylative
Csp3-Csp3 Coupling of Carboxylic Acids and π-Electrophiles
Kaitie C. Cartwrighta and Jon A. Tunge*a
Received 00th January 20xx,
Accepted 00th January 20xx
A dual catalytic decarboxylative allylation and benzylation method for the construction of new C(sp3)–C(sp3) bonds between
readily available carboxylic acids and functionally diverse carbonate electrophiles has been developed. The new process is
mild, operationally simple, and has greatly improved upon the efficiency and generality of previous methodology. In
addition, new insights into the reaction mechanism have been realized and provide further understanding of the harnessed
reactivity.
DOI: 10.1039/x0xx00000x
A. Ir/Pd-Catalyzed DcA of Amino Alkanoic Acids and Esters5
Introduction
Photoredox catalysis has emerged as a powerful strategy in the
functionalization of feed-stock carboxylic acids.1 Using the carboxylic
acid as a traceless activating group has allowed for the site-specific
generation of radical species under mild conditions, while producing
carbon dioxide as the only stoichiometric byproduct. Moreover, as
compared to traditional anionic decarboxylation, radical
B. Ir/Pd-Catalyzed DcA of N-Protected Amino Acids6
decarboxylation often provides
a facile and more generally
applicable pathway for activation.2
One reaction that has benefited from the employment of
photoredox-promoted decarboxylation is decarboxylative allylation
(DcA). This reaction is a subset of the Tsuji-Trost allylation that
distinguishes itself by the use of carboxylic acids as latent carbanions,
bypassing the need for pre-formed organometallics or strong alkaline
conditions.3 Despite being an attractive and powerful
transformation, anionic decarboxylation is limited to carboxylic acid
substrates which provide sufficient stabilization of the resulting
carbanion following the decarboxylation event. A pKa of less than 25
is often required for facile decarboxylation, and substrates with pKa
values between 25 and 30 demand elevated temperatures.3a,4
C. 4CzIPN/Pd-Catalyzed Decarboxylative Allylation & Benzylation of
Carboxylic Acids (This Work)
Recently, our lab has developed methodology that overcomes this
limitation in pKa through the development of a photoredox/Pd dual
catalytic coupling strategy (Scheme 1A & 1B).5,6 In this system, an
oxidative radical decarboxylation is facilitated by an iridium
photosensitizer, which allows for a facile radical decarboxylation.
Inverting the electronic demand for the decarboxylation event allows
access to carboxylic acid nucleophiles that fall outside the pKa
requirement for thermal decarboxylation. Once the carbon radical
species is generated, combination with an electrophilic Pd-π-allyl
intermediate provides a new C(sp3)–C(sp3) bond.7
Scheme 1: Tunge Dual Catalytic Decarboxylative Couplings
on activated carboxylic acids that contain α-amino5,6 and/or α-
benzylic5 functionalities which provide significant stabilization to the
radical intermediate. Within these substrate classes, the α-amino
precursors tend to provide higher yields than benzylic radical
precursors due to an undesired homocoupling observed between
benzylic radicals (Scheme 1A).5,6 Allylation of carboxylic acids without
these radical-stabilizing functionalities has remained elusive.
Additionally, the desire to utilize this reactivity with more complex
electrophilic coupling partners has not been realized. In fact, the
original method was only operable with a narrow range of 2-
substituted allyl electrophiles, while the more challenging 3-
substituted allyl electrophiles were not effectively coupled.5,6,8
Finally, the methods reported thus far utilized iridium-based
photosensitizers which are ultimately expensive and unsustainable.9
Although a great advancement in DcA, this chemistry remains
underdeveloped. Specifically, the reaction was only demonstrated
a. Department of Chemistry, The University of Kansas, 1567 Irving Rd., Lawrence,
KS, 66045.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Herein, a new organophotoredox/palladium dual catalytic process is influence on the success of the reaction. Change to the ligand
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described that has greatly improved the utility of this technology resulted in either 1) low conversion, 2) high conversion, but little
DOI: 10.1039/D0SC02609C
through 1) achieving higher yields and greater generality in decarboxylation, 3) the alkene/alkane side products were formed in
carboxylic acid nucleophiles, and 2) allowing access to a variety of large quantity, or 4) the allylated product was the major product.
structurally diverse allylic electrophiles for the installation of various
The ligands that allowed the reaction to proceed with full conversion,
complete decarboxylation, and produced the allylated product as the
major product consisted of bidentate phosphine ligands with similar
molecular scaffolds (Table 1, Entries 1-6). Of these catalysts, the
BINAP and SegPhos ligands provided a higher percentage of allylated
product 2a (Table 1, Entries 1 & 3) as compared to their more
sterically demanding counterparts, Tol-BINAP and DTBM-SegPhos
(Table 1, Entries 5 & 6). Gratifyingly, BINAP proved to be the superior
ligand and is also one of the least expensive and most readily
accessible of the ligand set (Table 1, Entry 1).
alkenyl, styrenyl, dienyl, and aryl functionalities (Scheme 1C). In
addition, evaluating this methodology has provided new insights into
the advantages and limitations of radical DcA, as well as the
dominant catalytic pathway.
Results and Discussion
With the goal of developing a broadly applicable catalyst system for
decarboxylative allylation of alkanes, a complete re-evaluation of the
catalyst system was undertaken. In 2016, Zhang reported a set of
carbazole-based donor-acceptor fluorophores that are inexpensive
and easily accessible organic photoredox catalysts.10 Further, this
group of catalysts possesses a range of redox potentials resembling
Table 1: Ligand evaluation
that of the iridium photosensitizers originally employed (Figure 1,
-
[Ir(ppy)2(bpy)]+ BF4 : P*/P-
= +0.95 V & = -1.05 V;
P/P-
[Ir[dF(CF3)ppy]2(dtbbpy)]+ PF6 : P*/P- = +1.21 V & P/P- = -1.37 V).
Employing the carbazole-based photosensitizers (Figure 1) in the
photoredox-facilitated DcA revealed the 4CzIPN catalyst to be the
most compatible with the transformation (Table S1).10a,11 The only
other catalyst to provide notable conversion in the DcA was 4CzPN,
but produced less allylated product than 4CzIPN (Table S1).
-
a All reactions were performed on 0.2 mmol scale and product ratios
were determined by GC/MS.‡ b 2a formed as a 60:40 trans:cis mixture
in Entry 1.
To improve upon the reaction economy and operational simplicity,
an intermolecular process that directly utilizes a carboxylic acid for
reaction with various allylic carbonates was devised (Table S5). To
our delight, the intermolecular reaction proceeded well. Allyl methyl
carbonate was identified as the ideal allylic electrophile and sodium
carbonate proved to be the ideal base (Scheme 3). Lastly, the
reaction was found to produce comparable results in acetonitrile
(73% yield 2a, 60:40 d.r.), which was preferred over the use of
dimethyl sulfoxide due to the more straightforward isolation of the
allylated product.13
Figure 1: Dicyanobenzene carbazole-based fluorophores
The change from Ir to 4CzIPN was first adopted into reaction
conditions that were found to be successful in our previous dual
catalytic DcA (Scheme 2).5,6,12 This reaction produced the allylated
product, but also produced an alkane and alkene side product.
Additionally, this reaction did not proceed to high conversion.
Scheme 3: Intermolecular DcA of 1a‡
With superior decarboxylative allylation conditions established,
attention was turned to the allylation of a variety of carboxylic acids
(Table 2). Initially, the allylation was performed with N-protected
amino acids (2b-q). No alkene or alkane side products were observed
with these substrates. Gratifyingly, the amino acid substrates that
underwent allylation with our first-generation method were able to
be allylated in similar to increased yields under our new conditions.
Scheme 2: Previous DcA conditions with 4CzIPN photosensitizer‡
In order to maximize production of the allylated product, the
influence of changing the Pd ligands was investigated (Table 1, See
Table S2 for full ligand screening and Table S3 for Pd pre-catalyst
evaluation). The change in ligand proved to have a significant
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Table 2: Products from DcA of carboxylic acids
a Reactions were performed on 0.2 mmol scale. b Isolated yields are shown. c (Yields) were determined by q1H NMR with pyridine as the
internal standard for products that were found to be volatile. d Yields are an average of two reactions. e 95% pure, see Supporting
Information for details. f Product was isolated with alkane side product and mass adjusted; product previously used in thermal DcA.17
Interestingly, under these conditions, carbamate-protected amino that produced monosubstituted and unsubstituted α-oxy radicals
acids produced higher yields than their acyl-protected counterparts, were generally allylated in moderate yields (2r-v). The alkene and
in contrast to first-generation conditions that resulted in comparable alkane side products do arise in the DcA of these substrates. Benzylic
yields for both groups (2b-c, 2l-m).6 The N-Boc- and N-Cbz-protected carboxylic acid substrates that provide 3o, 2o, and 1o benzylic radicals
amino acid substrates generally performed well with a variety of upon decarboxylation could be allylated in moderate to good yields
different side chain functionalities. Substrates containing N- (2w-2aa). Of these, the substrates 1w and 1x that produce 2o
heterocycle side chains proceeded smoothly, allowing for high yields benzylic radicals provided yields exceeding 60% (2w & 2x). This is
of the allylated alkaloids 2d and 2g (93% & 79%, respectively).14 notable since the diphenylmethane nucleophile has previously been
Increased steric demand around the carboxylic acid did not interfere employed in anionic allylation under thermal conditions, but the
with the efficiency of DcA (2h & 2l). Cyclic scaffolds also generally yield of allylated product is much less (10% yield) than what can be
underwent DcA well (2n-q). Interestingly, the cyclopropane amino obtained in the 4CzIPN/Pd DcA conditions.16 The homodimeric
acid substrate 2n was allylated in low yield (21%) but larger ring sizes products frequently observed under the Ir/Pd conditions with
such as a cyclobutane (2o), cyclopentane (2p), and cyclohexane (2q) benzylic carboxylic acids were not observed with the disubstituted
were allylated in high yields.15 The DcA of 1b was successfully run in benzylic acids allylated under the new conditions. Instead, alkene
batch on the 1 mmol scale, which provided 2b in 93% yield (see and/or alkane side products were observed. Conversely, benzylic
Supporting Information for details).
acid substrate (1aa) was able to be allylated in 31% yield (2aa) but
In addition to the successful decarboxylative allylation of amino did produce the homocoupled product in 48% yield.
acids, other carboxylic acids that produce more reactive radical
intermediates were allylated with this method (2r-2ff). Substrates
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The dual catalytic DcA was also applicable to carboxylic acids that substituent producing higher yield (4c, 82%) comparedVwiewithAr2tic-mle Oenthlinyel
DOI: 10.1039/D0SC02609C
possess weak radical-stabilizing functionalities (2a, 2bb-2ff). The allyl carbonate (4b, 64%). Having Cl in the 2-position unfortunately
highest yields of allylated products, 70-80%, were observed with resulted in a poor yield (4d, 14%) and formation of a plethora of side
quaternary carboxylic acids such as gemfibrozil (1ee) and 18-β- products. This may be due to poor chemoselectivity between allyl
glycyrrhetinic acid (1ff). The tertiary carboxylic acids of this group carbonate and vinyl halide electrophiles.
such as β-amino acid (1cc) and γ-amino acid (1dd), were allylated in
moderate yields (65% 2cc & 50% 2dd). Unfortunately, unsubstituted 1- and 3-substituted allylic carbonates having a variety of carbon
acids that do not possess functionalities to provide moderate chains also operate well in the radical DcA (4e-4h). These carbonates
stabilization of the radical intermediate underwent poor conversion functioned comparably regardless of the geometric configuration of
and only produced trace amounts of allylated product. To the alkene or the structure of the allylic carbonate (branched/linear).
demonstrate the difference in reactivity, a disubstituted acid (1w) They also produced the E-configured olefin and the linear product
was allylated selectively in the presence of an unsubstituted acid despite the variation in the carbonate starting material. Cyclic allylic
under standard reaction conditions (Scheme 4).18 Substrate 1gg carbonates also provided allylated products, however the yields
remained intact during this reaction.19
were about half what is realized with their acyclic counterparts (4i-
4j).
Utilizing branched acyclic allylic carbonates was quite advantageous
(e.g. 3e’ vs. 3e).23 For one, they can be synthesized from aldehydes,
providing a plethora of inexpensive options for the electrophilic
coupling partner.24 Also, utilizing branched carbonates provided
cleaner reactions than their 3-substituted allyl carbonate
counterparts in some cases. For instance, using carbonates derived
from cinnamyl alcohols produced a complex mixture of products
under these reaction conditions and provided poor yields of allylated
product. In contrast, switching to the branched carbonates allowed
for the formation of styrenyl products in good yields (4k-4o, 70-95%
yield). Potentially, this reaction benefits from a more facile oxidative
addition that is achieved with the branched carbonates as opposed
to the cinnamyl carbonates. The conjugated olefin products are
produced as a mixture of geometric isomers which is likely the result
of a background photoisomerization.24 Here, the styrenyl products
were obtained in higher yields and with greater E-selectivity when
the aryl moiety contained electron-withdrawing functionalities (4m
& 4o, 86% & 81% yield, respectively, >90:10 E:Z) as opposed to
electron-donating functionalities (4l & 4n, 79% & 72% yield,
respectively, ~70:30 E:Z). The branched carbonates also proved
highly useful for the installation of other conjugated functionalities
such as dienes (4p & 4q, 72% & 45% yield, respectively), but these
too provided a mix of geometric isomers.
Scheme 4: Preferential DcA of disubstituted carboxylic acid
While substrates that contain strong radical-stabilizing groups such
as the α-amino acids only produced the allylated product, more
reactive radicals resulted in the formation of the alkene and alkane
side products as was observed in the optimization of DcA of 1a’ and
1a. Potentially, these products form as a result of off-cycle pathways
such as disproportionation or β-hydride elimination.20
In addition to the alkane- and alkene-forming side reactions, a recent
report by KÖnig brought to our attention the possibility of
competitive radical addition to the 4CzIPN photocatalyst.21 Sure
enough, the "Allyl-4CzIPN" was isolated along with intact 4CzIPN
after the catalytic DcA of 1w (Scheme 5A).22 When the "Allyl-4CzIPN"
was utilized as the photocatalyst in the DcA of 1b, a greatly
diminished yield results (Scheme 5B). Thus, the side reaction with
4CzIPN ultimately degrades the photocatalyst activity.
Apart from DcA, decarboxylative benzylation (DcB) with Pd-π-benzyl
electrophiles would be an attractive application of dual catalytic
couplings.25 Through the use of benzylic carbonates, toxic benzylic
halides that are traditionally employed in cross-coupling could be
avoided.26 However, Pd-catalyzed DcB is challenging since aromatic
moieties receive additional stability from conjugation, making the
barrier for oxidative addition higher.27 Therefore, benzylic
carbonates with extended conjugation are generally utilized because
their lower resonance energies lead to lower activation barriers.27,28
Benzylic carbonates with extended conjugation underwent high
conversion under our mild reaction conditions. The highest yielding
benzylations are those that install naphthalene (4r, 71% yield) and
phenanthrene tethers (4t, 78% yield). The benzylic carbonates react
Scheme 5: Allylation of 4CzIPN
Apart from the improved accessibility of carboxylic acid nucleophiles, to full conversion to produce the cross-coupled product and the
another compelling aspect of this methodology is its efficient mass-balance is typically made up of the benzyl dimer.
coupling of carboxylic acids with more functionally diverse π-
electrophiles (Table 3). Allylic carbonates with substituents in the 2-
position proceeded well, with the electron-withdrawing phenyl
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Table 3: Products from DcA/DcB of 1f
a Reactions were performed on 0.2 mmol scale. b Isolated yields are shown. c Isomer ratios were determined by 1H NMR. d Mix of E/Z-isomers.
The regiochemistry of the alkylcarbonate group does influence the systems, the heteroaromatic benzylic carbonates produced a variety
reaction yield. With the naphthalene system, the higher yield is of oligomer side products. Nevertheless, these carbonates have been
obtained when the naphthalene is substituted in the 2-position (3r) utilized to provide the cross-coupled product between 1f and furyl
vs. the 1-position (3s) (4r & 4s isolated in 71% & 35% yield, carbonates as well as with pyrrolyl carbonate, with yields between
respectively). The reaction with carbonate (3s) resulted in 46% yield 20-40% (4v-x). Like the naphthalene system, the regiochemistry of
of naphthalene dimer. Similarly, the 9-anthracenyl carbonate (3u) the carbonate leaving group influences the yield of the reaction. With
provides the cross-coupled product (4u) in just 21% yield due to the furyl carbonate, the 2-alkyl carbonate (3v) provided higher yields
favorable formation of the homocoupled bianthracenyl dimer.
than the analogous 3-substitution (3w). An N-Boc pyrrole group was
also able to be installed through this coupling, albeit with a low
Heteroaromatic systems also underwent successful cross-coupling, isolated yield that is reminiscent of what was observed with the
but with lower yields.14 Compared to the aromatic hydrocarbon related furyl carbonates (4x).
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After exploration of scope and reactivity, mechanistic probes were
utilized to shed light on the dominant catalytic cycle. First, it is
important to note that the reaction does not proceed (<5% yield)
without the Pd, photocatalyst, or light, indicating that this process
relies on a light-promoted dual catalytic system (Table S6).
Pd(OAc) as opposed to π-allyl-PdCl points to a carboxylate oxidation
pathway reminiscent of other metal-carboxylate species.
Furthermore, the higher quenching constant for the π-allyl-PdOAc
complex, as compared to tetrabutylammonium carboxylate,
indicates the key role of the Pd complex in facilitating quenching.
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34
DOI: 10.1039/D0SC02609C
When the radical trap, TEMPO, is incorporated into the reaction, the
allylated product is not produced (Scheme 6). Instead, conversion is
interrupted, and low yields of TEMPO trapped products result. This
result supports the formation of radicals along the reaction pathway.
After the Pd-facilitated quenching and decarboxylation events, there
are several possibilities for the C–C bond formation. First, the
possibility of
a
radical-polar cross-over event must be
considered.21,35 But, due to the large negative reduction potentials of
most alkyl radicals utilized herein (< -1.3 V)36, this pathway is not
favorable and thus determined to be irrelevant in the dominant
catalytic cycle. An alternate route is the radical addition to a π-allyl-
Pd intermediate. This addition can occur at the allyl ligand resulting
in the new C–C bond or through coordination to the Pd centre
followed by inner-sphere reductive elimination to form the new C–C
bond.6,37 In either event, the cross-coupled product would be
liberated along with a Pd(I) species. This species is expected to have
a reduction potential around -1.26 V31 which would be well-matched
with the 4CzIPN radical anion (-1.21 V).10 The decreased efficiency
seen with 4CzPN may be due to the less negative oxidation potential
(-1.16 V)10 that is not as well-matched for Pd(I) reduction as that of
4CzIPN, further supporting the relevancy of a Pd(I) reduction to the
catalytic cycle (Table S1).
Scheme 6: DcA in the presence of TEMPO
A Stern-Volmer analysis was completed to assess the photocatalytic
processes taking place. The potential fluorescence emission
quenching of 4CzIPN with diphenylcarboxylate tetrabutylammonium
salt, as well as with a variety of Pd species, was investigated (see
Supporting Information for experimental details). Quenching of
4CzIPN with diphenylcarboxylate tetrabutylammonium salt was
observed, which was not surprising since 4CzIPN (+1.35 V)10 is
sufficiently reactive to oxidize the carboxylate (+1.0 to +1.3 V)18.
However, the observed 4CzIPN quenching with the carboxylate salt
was significantly less efficient (Figure S2) than the observed
quenching with relevant Pd species investigated.
Taken together, we believe the dominant catalytic pathway to
proceed via a reductive quenching pathway in which the excited
4CzIPN is quenched by a π-allyl-Pd carboxylate species (Scheme 7).38
Single electron transfer from carboxylate to Pd then facilitates
decarboxylation and the formation of a carbon radical. Rebound of
the radical with the π-allyl-Pd results in the formation of the allylated
product and a Pd(I). The Pd(I) can be reduced by one electron via the
4CzIPN radical anion to complete both catalytic cycles.39
The first Pd species investigated was the Pd(OAc)2/BINAP pre-
catalyst. When these reactions were assembled, an initial stirring of
the Pd(OAc)2 pre-catalyst and BINAP was performed to allow for
reduction of the Pd(II) to Pd(0).29 However, this process was not
anticipated to fully reduce the Pd(II) species and thus higher
oxidation states of Pd are expected to be present at the onset of the
reaction.29 The mixture of Pd(OAc)2/BINAP was found to be an
efficient quencher of 4CzIPN (~10-fold more than carboxylate, Figure
S3). Similar Pd species are reported to have reduction potentials
between -1.1 and -1.3 V, which places the potentials for the Pd
catalyst reduction close to the excited state oxidation potential of
4CzIPN (-1.04 V).10,30,31,32 If such a reduction initiates the cycle, the
resulting 4CzIPN radical cation (+1.52 V)10 should easily oxidize
carboxylates (+1.0 to +1.3 V)17 in the reaction, thus turning over the
catalyst.
Alternatively, it is possible that the dominant catalytic pathway
proceeds via oxidative addition of Pd(0) and allyl carbonate to
provide the cationic π-allyl-Pd. This species is anticipated to be highly
relevant to the mechanistic pathway, thus its ability to quench
4CzIPN was also considered. After an evaluation of reported redox
potentials, the cationic π-allyl-Pd is not expected to be reduced by
4CzIPN as the potential reported for this species is -1.35 V, which is
not a favorable match for oxidative quenching of 4CzIPN (-1.04
V).10,33 In agreement with this redox potential mismatch, a π-allyl-
PdCl dimer/BINAP mixture did not quench 4CzIPN (Figure S4).
However, π-allyl-Pd(OAc)/BINAP was found to be an efficient
quencher of 4CzIPN (~10-fold more efficient than carboxylate, Figure
S4). The high quenching ability of the BINAP complex of π-allyl-
Scheme 7: Proposed dominant catalytic pathway38,39
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3
4
Review on decarboxylative allylation and benzylation: a) J. D.
Conclusions
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Weaver, A. Recio, A. J. Grenning, J. A. TDuOngI:e1,0C.1h03e9m/D. 0RSeCv0.22600191C,
111, 1846–1913. Tsuji-Trost reaction: b) J. Tsuji, H. Takahashi,
A catalytic, mild, operationally simple, and minimal waste-producing
decarboxylative method for the site-specific installation of allylic,
dienyl, styrenyl, and benzylic functionalities on carboxylic acids has
been realized. This method has drastically improved upon our
previously reported methodology through the change to a more
sustainable and less expensive organophotocatalyst which also led to
higher yields across a much broader substrate scope. In addition, the
cross-coupling methodology described herein provides a general way
to incorporate diverse electrophiles as well as utilize carboxylic acid
nucleophiles that do not undergo decarboxylation under thermal
control. Thus, the methodology herein may provide a simple and
general approach towards building molecular complexity from easily
accessible and inexpensive starting materials that would be
otherwise difficult to achieve.
M. Morikawa, Tetrahedron Lett. 1965, 6, 4387. c) B. M. Trost,
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2020, 60, 1-10. Select representative examples of various
nucleophiles in thermal decarboxylative allylation: b) J. Tsuji,
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Conflicts of interest
There are no conflicts to declare
16914−16919. Method utilizing Ni/Ir in single electron Tsuji-
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Acknowledgements
This work was supported by the National Science Foundation
(CHE-1800147) and the Kansas Bioscience Authority Rising Star
program. Support for the NMR instrumentation was provided
by NSF Academic Research Infrastructure Grant No. 9512331,
NIH Shared Instrumentation Grant No. S10RR024664, and NSF
Major Research Instrumentation Grant No. 0320648.
10 a) J. Luo, J. Zhang, ACS Catal. 2016,
11 4CzIPN was also found to be a successful alternative to Ir by
the Zhang group in
a decarboxylative cross-coupling:
Reference 9a & Z. Zuo, D. T. Ahneman, L. Chu, J. A. Terrett, A.
G. Doyle, D. W. C. MacMillian, Science 2014, 345, 437-440.
12 Ir[dF(CF3)ppy]2(dtbbpy)PF6 can be purchased from Sigma for
$834/gram. 4CzIPN cost can be synthesized from carbazole
and tetrafluoroisophthalonitrile for $4.01/gram.7a
13 2a q1H NMR yield 73%, ~60:40 d.r. under optimal
intermolecular reaction conditions in acetonitrile (0.1 M).
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favorable radical decarboxylation that results from the
formation of a carbon radical in an orbital with increased s-
character: E. V. Anslyn, D. A. Dougherty, Modern Physical
Organic Chemistry, 1st ed.; University Science Books: 2006; pp.
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Notes and References
‡ All potentials listed are listed as V vs. SCE and were obtained in
MeCN.
‡ GC/MS ratios list represent area percent out of 100% of all
products and starting materials present in the final reaction
mixture.
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23 Allylic carbonates could be made from commercially available
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8 | J. Name., 2012, 00, 1-3
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