(CDC)-Rhodium-catalyzed hydroallylation of vinylarenes and 1,3-dienes with allyltrifluoroborates

Article abstract of DOI:10.1021/acscatal.9b01579

Catalytic site-selective hydroallylation of vinyl arenes and 1,3-dienes is reported. Transformations are promoted by a readily accessible bidentate carbodicarbene-rhodium complex and involve commercially available allyltrifluoroborates and an alcohol. The reaction is applicable to vinyl arenes and aryl-or alkyl-substituted 1,3-dienes (30 examples). Allyl addition products are generated in 40-78% yield and in up to >98:2 site selectivity. Reaction outcomes are consistent with the intermediacy of a Rh(III)-hydride generated by protonation of Rh(I) by an acid. A number of key mechanistic details of the reaction are presented: (1) Deuterium scrambling into the product and starting alkene indicates reversible Rh(III)-H migratory insertion. (2) A large primary kinetic isotope effect is observed. (3) With substituted allyltrifluoroborates (e.g., crotyl-BF₃K) mixtures of site isomers are generated as a result of transmetalation followed by Rh-(allyl) complex equilibration, consequently disproving outer-sphere addition of the allyl nucleophile to Rh(III)-(η³-allyl). (4) Stereochemical analysis of a cyclohexadiene allyl addition product supports a syn Rh(III)-hydride addition. (5) A Hammett plot shows a negative slope. Finally, utility is highlighted by a iodocyclization and cross metathesis.

Full text of DOI:10.1021/acscatal.9b01579

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Article
(CDC)-Rhodium-Catalyzed Hydroallylation of
Vinylarenes and 1,3-Dienes with Allyl Trifluoroborates.
Justin S. Marcum, Tia N. Cervarich, Rajith S Manan, Courtney C. Roberts, and Simon J. Meek
ACS Catal., Just Accepted Manuscript • Publication Date (Web): 22 May 2019
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(CDC)-Rhodium-Catalyzed Hydroallylation of Vinylarenes and 1,3-Dienes with
AllylTrifluoroborates.
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Justin S. Marcum, Tia N. Cervarich, Rajith S. Manan, Courtney C. Roberts, and Simon J. Meek*
Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290.
Supporting Information Placeholder
ABSTRACT: Catalytic site-selective hydroallylation of vinyl arenes and 1,3-dienes is reported. Transformations are promoted by a
readily accessible bidentate carbodicarbene-rhodium complex and involve commercially available allyltrifluoroborates and an
alcohol. The reaction is applicable to vinyl arenes, and aryl or alkyl-substituted 1,3-dienes (30 examples). Allyl addition products
are generated in 40–78% yield and in up to >98:2 site-selectivity. Reaction outcomes are consistent with the intermediacy of a
Rh(III)-hydride generated by protonation of Rh(I) by an acid. A number of key mechanistic details of the reaction are presented: (1)
Deuterium scrambling into the product and starting alkene indicates reversible Rh(III)–H migratory insertion. (2) A large primary
kinetic isotope effect is observed. (3) With substituted allyltrifluoroborates (e.g., crotyl-BF3K) mixtures of site isomers are generated
as a result of transmetalation followed by Rh-(allyl) complex equilibration; consequently, disproving outer-sphere addition of the
allyl nucleophile to Rh(III)-(3-allyl). (4) The stereochemical analysis of a cyclohexadiene allyl addition product supports a syn
Rh(III)–hydride addition. (5) A Hammett plot shows a negative slope. Finally, utility is highlighted by a iodocyclization and cross
metathesis.
KEYWORDS: alkene, hydroallylation, carbodicarbene ligands, rhodium catalysis, allyltrifluoroborates
proceed through a nucleophilic (L)nCu(I)-alkyl intermediate
that engages with an electrophilic allyl component.13 In
1. INTRODUCTION
Catalytic allyl additions to polarized C=O, and C=N -bonds
Scheme 1. Catalytic Hydroallylation of Olefins.
represent important C–C bond forming methods central to
chemical synthesis.1 In contrast, the closely related catalytic
hydroallylations of unsaturated hydrocarbon compounds
remain less well developed.2 Such reactions are valuable as they
employ simple hydrocarbon feedstocks, form a new C–C bond,
and install a versatile alkene functional group. While
hydroallylation represents an attractive synthetic strategy for
C–C bond formation,3 such processes have generally focused
on activated C–C -bonds (e.g., enones, dialkylidene ketones,
and alkylidene malonates).4 Accordingly, a number of efficient
Ni-,5 Pd-,6 Cu-catalyzed7 allyl conjugate addition methods with
activated -unsaturated carbonyls have been reported.
Furthermore, through the appropriate choice of chiral ligands
nickel-5b-c and copper-catalyzed7a,d-e reactions have been
rendered highly enantioselective. In spite of these efforts, the
direct hydroallylation of less activated carbon-carbon  bonds
(alkenes, styrenes, dienes, allenes, alkynes) remain limited.2 A
challenge in achieving such a reaction arises from the absence
of significant C–C bond polarization from an activating group.
In conjugate allyl addition reactions, generally, carbon–carbon
bond formation precedes C–H bond formation.
comparison, only
a single report of catalytic olefin
Despite these challenges, recent advances in (L)nCu–H
catalyzed multicomponent coupling reactions have led to the
development of efficient methods for the site- and/or
enantioselective hydroallylations of styrenes,8 alkenylborons,9
alkynes,10 allenes,11 and cyclopropenes12 (Scheme 1a). In such
instances reactions employ a silane as the hydride source, and
hydroallylation that proceeds with nucleophilic allyl reagents
has been disclosed. In 2015, Zhao and co-workers reported on
a cobalt-catalyzed hydroallylation of heterobicyclic alkenes
with allyltrifluoroborates and ethanol (Scheme 1b).14 Under
phosphine free conditions, cobalt(II)bromide catalyzes the
diastereoselective hydroallylations of a variety of oxabicyclic
alkenes in good yields. Notably, the authors observed
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interesting divergent chemical reactivity when phosphine comparison, reaction with allyl reagents 2b or 2c, under
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2
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ligands are employed, and phosphine-cobalt species result in
allylative ring-opening reactions instead.15 Related allyl–olefin
couplings via allyl C–H activation have also been reported.16
identical conditions, yielded no desired product, likely due to
the inability of 2b-c to easily generate H–X through reaction
with a methanol. Furthermore, control reactions with 5 or 10
mol %[H(OEt2)2][BArF ] and either 2b and
4
As part of an initiative to explore olefin
hydrofunctionalization reactions catalyzed by carbodicarbene
(CDC) transition metal complexes,17 we previously discovered
that the in situ metalation of CDC–bisphospine ligands by Rh(I)
results in a transient Rh(III)-hydride that is able to initiate the
site- and enantioselective hydroarylation of 1,3-dienes.18
Reactions proceed via the formation of electrophilic Rh(III)-
(3-allyl). In this regard, we hypothesized that (CDC)-Rh(III)–
H species formed through oxidative protonation could be
employed to effect other C–C bond forming reactions, such as
catalytic hydroallylation of olefins. A key requirement for
achieving olefin hydroallylation, however, is the need to for a
proton source and an allyl nucleophile. Allyltrifluoroborate
reagents provide an effective solution to these criteria as they
are both a nucleophile, and by alcoholysis of organo
trifluoroborates with an alcohol provide a controlled release of
H–F.19 A proposed mechanism for olefin hydroallylation is
depicted in Scheme 1c. Transmetalation to form Rh(I)-allyl
followed by oxidative protonation by H–X leads to a Rh(III)–
hydride A that can insert across a bound olefin to form B. The
resulting Rh(III)-alkyl complex could then undergo reductive
elimination to furnish the product and regenerate the catalyst.
Several pitfalls can be envisioned, most notably the
protodemetalation to generate propene. The generation of
metal hydrides through protonation at the metal has become an
effective strategy for accessing metal hydrides for
hydrofunctioanlization of 1,3-dienes.20 For example,
nucleophile additions to electrophilic metal-(3-allyl) species
generated by M–H additions to 1,3-dienes include Ni-, Pd- and
Table
1.
Optimization
of
(CDC)-Rh-Catalyzed
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Hydroallylation of Vinyl Arenes.a
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Rh-catalyzed
hydroarylation,18 23
hydrophosphinylation.25
hydroalkylation,21
hydroamination,22
and
,
hydrothiolation,24
Herein, we describe an efficient Rh-catalyzed intermolecular
hydroallylation of vinyl arenes and 1,3-dienes that proceeds
with high site-selectivity to afford the branched products in
good yields. Reactions employ common olefins, commercially
available allyltrifluorborates, an alcohol, and are promoted by
an in situ generated (CDC)-Rh catalyst.
2. RESULTS AND DISCUSSION
aReactions performed under N2 atmosphere. Conversion to 3a
and 4 (E + Z isomers) determined by analysis of 400 or 600
MHz 1H NMR spectra of crude reactions with DMF as internal
b
Scheme 2. Initial Result
standard. cProducts formed in <60:40 er. 1:1 mixture of E:Z
d
isomers.
Butylphosphinoyl]-2-[(R)-1-
(diphenylphosphino)ethyl]ferrocene.
e(SP,R)-JoSPOphos
=
(SP)-1-[(R)-tert-
2c resulted in <2% conversion to product and olefin
polymerization.
To optimize the selectivity and efficiency of the catalytic
hydroallylation reaction, a series of CDC ligands (L1–L8) were
evaluated in combination with [Rh(coe)2Cl]2 (Table 1). We
have previously shown that (CDC)-Rh complexes are
efficiently formed in situ via rhodium C–H metalation of
pyrazolium salts.18 In this regard, initial reactions involving in
situ pincer catalyst derived from tridenate L2, afforded <2%
conversion with methanol as alcohol additive (Table 1, entry 2),
2.1. The Method and Its Scope. 2.1.1. Hydroallylation of Aryl-Substituted Alkenes.
We began our investigations by studying the reaction between
2-vinyl naphthylene 1, and allyl reagents 2a-c (Scheme 2).
Treatment of 1 and 2a with 5 mol % in situ generated (L1)-Rh
in the presence of methanol in DCE at 60 °C afforded 3a in 54%
NMR yield as a 1:1 mixture of allyl and alkenyl isomers. In
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which indicated a bidentate ligand scaffold was crucial for
were employed in reactions, 3a was formed in <60:40 er in all
cases.
With optimal conditions in hand we next set out to study the
scope of vinyl arenes with allyltrifluoroborate 2. The results of
these studies are illustrated in Scheme 3. A wide array of vinyl
arenes undergo allyl additions with 5 mol % in situ generated
(L7)-Rh and two equivalents of allyltrifluoroborate 2 within 18
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reactivity. However, when alkene 1 and allylboron 2a (2 equiv)
are subjected to 5 mol % in situ generated (L1)-Rh (Table 1,
entry 2), 2 equivalents of MeOH in DCE at 35 °C, 57%
conversion to homoallyl naphthalene 3a is observed in 18 h as
a 4:1
Scheme 3. Scope of Vinyl Arene Hydroallylationa–c
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Scheme 4. Synthesis of 1,5-Dienes by Hydroallylation of 1,3-
Dienesa–c
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aReactions were carried out under N2 atmosphere. Conversion >98%
in all cases, determined by analysis of 1H spectra of unpurified product
mixtures (±2%). Yields are reported as a combination of isomers
(±5%); allyl/alkenyl ratio given in parentheses. bExperiments were run
in duplicate or more. See the SI for details. cRatio in parentheses
corresponds to crude allyl:alkenyl isomer ratio.
aReactions were carried out under N2 atmosphere. Conversion
>98% in all cases, determined by analysis of 1H spectra of unpurified
product mixtures (±2%). Yields are reported as a combination of
mixture of allyl and alkenyl isomers. Varying the alcohol
additive to i-PrOH and H2O (Table 1, entries 3–4) led to an
increase in reaction efficiency; in the presence of H2O 3a is
formed in 77% conversion and decreased in isomerization (3a:4
6:1). With isoindoline (L3) or aryloxy (L4) substituted
pyrazoliums (Table 1, entries 5–6) there is an increase in
conversion to desired product 74% (1:1, 3a:4) and 85% (8:1;
3a:4), respectively. Placement of electron-donating and
electron-withdrawing substituents on the diarylphosphine, L5–
L8 (Table 1, entries 7–10), identified (p-F-C6H4)2P L7 as the
optimal CDC ligand furnishing 3a in 80% NMR yield and 14:1
(3a:4). The less electron-donating phosphine likely minimizes
alkene isomerization by facilitating substitution of the product
from the catalyst by another alkene prior to formation of the
(CDC)-Rh-hydride. Reaction with related bidentate (NHC)–
phosphine L9 (Table 1, entry 11), or chiral bidenate phosphines
(Sp,R)-JoSPOphos (L10), and (R)-binap (L11) (Table 1, entries
12–13) resulted in minimal transformation (<25% conv).
Lastly, achiral DPEphos (L12) (Table 1, entry 14) resulted in
the selective formation to isomerized 4 in 12% conversion. It
should be noted that, while chiral enantiopure CDC ligands
b
isomers (±5%); allyl/alkenyl ratio given in parentheses. Experiments
were run in duplicate or more. See the SI for details. cRatio in
parentheses corresponds to crude allyl:alkenyl isomer ratio.
hours at 35 °C (Scheme 3). Additions to naphthalene- and
phenanthrene-derived vinyl substrates are efficient, and
products 3a-c are formed in 53–78% yield. Notably, while the
naphthyl products were generated in 10:1 selectivity,
phenanthrene-derived 3c was formed as a 3:1 mixture of
allyl:alkenyl isomers. Similarly, a variety of non-polycyclic
arene olefins react efficiently. Reaction of one equivalent of
alkene furnishes the allyl addition products 3d–3h in 55–78%
yield. The reaction protocol also extends to heteroaryl-
substituted alkenes, which proceed with efficient allyl group
transfer. For example, catalytic hydroallylation allows for the
preparation of allyl additions products containing pyrrole (3i),
indole (3j), benzofuran (3k), and benzothiophene (3l) rings in
61–78% yield, and >7:1 allyl:alkenyl isomers. Under the
current protocol 1,1-disubstituted styrenes and alkyl-substituted
terminal alkenes do not undergo hydroallylation.
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run in duplicate or more. See the SI for details. cRatio in parentheses
corresponds to crude allyl:alkenyl isomer ratio.
2.1.2. Hydroallylation of 1,3- Dienes. After having demonstrated the
protocol worked well for a broad selection of alkenylarenes, we
investigated the applicability of the method towards reactions
involving 1,3-dienes (Scheme 4). A significant feature of the
Rh-catalyzed method is the modularity of the CDC ligand
scaffold. Consequently, a short survey of CDC ligands and
alcohol additives identified pyrazolium L3 in combination with
2.0 equivalents of MeOH afforded more optimal results
compared to L7 and H2O. In the presence of 2.5 mol %
[Rh(coe)2Cl]2 and 6.0 mol % L3 in DCE at 35 °C, a variety of
aryl-substituted 1,3-dienes bearing electron-withdrawing and
electron-donating groups undergo efficient hydroallylation in
24 h to afford 1,5-diene products (5a–5j) in 46–65% isolated
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4
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2.1.4. Utility. The homoallyl arenes synthesized through the
current hydroallylation method can be functionalized in a
number of ways. Two brief examples are illustrated in Scheme
6: (1) Iodocyclization of 3j by reaction with I2 in MeCN at 22
°C affords tricycle 8 in 40% isolated yield, and 3:1 dr. (2) Cross
metathesis of 3k by treatment with 5 mol % Hoveyda-Grubbs-
II and 2 equivalents of phenylvinylketone furnishes 9 in 63%
yield and >20:1 E/Z selectivity).
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2.3. Reaction Mechanism. 2.3.1. Deuterium Labeling Experiments. With an
effective method for hydroallylation in hand, we undertook a
number of mechanistic experiments to gain insight into the
(CDC)-Rh-catalyzed reaction mechanism. First, we conducted
a variety of deuterium labeling experiments
yields, and >5:1 allyl:alkenyl selectivity.
Similar to
vinylarenes, the reaction is also tolerant of heteroaromatic
substrates to afford heteroaryl-containing 1,5-dienes; the
formation of thiophenene 5k in 73% yield, and carbazole 5l in
62% yield, are representative. Finally, under the current
catalytic protocol, hydroallylation of alkyl-substituted 1,3-
dienes also proceeds to high conversion, however, the products
are generated as an inseparable mixture of 1,2- and 1,4-
hydroallyl addition products. For example, regioisomers 5m’
and 5m” are formed in 76% yield as 1.5:1 mixture. Phosphine–
CDC ligand development is currently in progress to address
challenges associated with site-selectivity.
Scheme 6. Functionalization of Homoallyl Arenes
2.1.3. Hydroallylation with 2-Substituted Allyltrifluoroborates. To increase the
reaction scope, 2-allyltrifluoroborates were examined (Scheme
5).
Both 2-methyl (6a) and 2-phenyl-substituted (6b)
allyltrifluoroborates were prepared and employed in the
hydroallylation reaction. Under the optimal reaction conditions
with pyrazolium L7 (Scheme 3, vinyl arenes) and L3 (Scheme
4, 1,3-dienes) as ligands both organoborons reacted smoothly to
provide allyl addition products 7a–7d in 54–72% yield and
excellent selectivity (>20:1). Notably, the formation of
isomerized product is prevented owing to the increased sterics
of the 1,1-disubstituted alkene. Additionally, product 7b
arising from the reaction of cyclohexadiene represents an
example of an internal alkyl-substituted 1,3-diene.
with both vinyl arene and 1,3-diene substrates. As depicted in
Scheme 7a, the hydroallylation reactions of vinyl naphthalene
1, and phenylbutadiene 10 were carried out in the presence of
CD3OD to examine deuterium scrambling into the product and
starting olefins. For vinyl naphthalene, 0.97 deuterium is
incorporated into the methyl group of 3a, 0.44 deuterium in the
alkene of 3a (0.11 D internal, and 0.33 D terminal), and 0.28
deuterium (0.16 D and 0.12 D) into the terminal alkene C–H
bonds of unreacted 1. The corresponding reaction with 2-
methylallyl trifluoroborate 6a and CD3OD furnishes 6c in 67%
yield (>20:1 selectivity), and 0.89 deuterium in the methyl
group and <0.02 deuterium is exchange into terminal alkene.
Notably, the unreacted alkene 1 only contains <0.05 deuterium.
These data indicate the Rh-catalyst rapidly and reversibly
exchanges in hydrogen/deuterium prior to C–C bond formation.
While 1,3-dienes are less stable under the reaction conditions,
catalytic hydroallylation of phenylbutadiene 10 with 2a in the
presence of two equivalents of CD3OD leads to similar
deuterium distributions (Scheme 7b). Reaction with 5 mol %
in situ generated (L3)-Rh and a reaction time of 18 h, afforded
5a in 45% NMR yield (>98% conv of 5a) in >20:1 allyl/alkenyl
selectivity with 0.88 deuterium incorporated into the methyl
group. As with vinyl naphthalene 3a, 0.16 D is deuterium is
incorporated into the terminal alkene of 5a, and <0.1 D into
unreacted 10. The slightly lower deuterium incorporation into
unreacted 1,3-diene 10 (vs 3a) is likely a result of the slower
substitution of a diene from rhodium. Control reactions run
with CD3OD and without [Rh(coe)2Cl]2 do not result in
deuterium incorporation into either 1 or 10 by Brønsted acid
pathways.
Scheme 5. Catalytic Hydroallylation with 2-Substituted
Allyltrifluoroboratesa–c
aReactions were carried out under N2 atmosphere. Conversion >98%
in all cases, determined by analysis of 1H spectra of unpurified product
mixtures (±2%). Yields are reported as a combination of isomers
b
(±5%); allyl/alkenyl ratio given in parentheses. Experiments were
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aReactions were carried out under N2 atmosphere. Conversion
In search of additional evidence, to further probe the hydride
determined by analysis of 1H spectra of unpurified product
mixtures (±2%). Yields are reported as a combination of isomers
(±5%); crude allyl/alkenyl ratio given in parentheses. bExperiments
were run in duplicate or more. See the SI for details.
incorporation step, a KIE study was performed. The absence of
competing alkene isomerization in the formation of 6c,
permitted the relative reaction rates between CH3OH:CD3OD to
be established (Scheme 7c). In the catalytic reaction between 1
and 6a and CH3OH:CD3OD (1:1), a kH/kD of 6.13 was observed
indicating a large primary KIE. This likely arises from the
Rh(I) protonation, however, the competition reaction does not
provide information about the rate determining step.26 The
magnitude of the KIE is similar to those observed for
protonation at the metal in other systems.26 Unfortunately,
attempts to obtain a KIE based on initial rates was impeded by
the heterogeneity of the reactions caused by the
allyltrifluoroborate.
analysis of the product by 1H NMR indicates a syn relationship
between the allyl moiety and deuterium at the homoallylic
position. This reaction, however, proved not conclusive due to
competitive incorporation of deuterium into cyclohexadiene
starting material prior to C–C bond formation (See Supporting
Information for details). This data further supports that Rh(III)–
H migratory insertion is fast and reversible prior to C–C bond
formation.
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2.3.2. Reactions with Crotyl-BF3K. In addition to reversible hydride
incorporation, the isomerization of the organoboron fragment in
the reaction was studied (Scheme 9). The
In an effort to distinguish between the either Rh(III)–H syn
migratory insertion vs outer-sphere protonation of a rhodium
bound alkene mechanism,27 the synthesis of cyclohexadiene 7b
was carried out with 2.0 equivalents of CD3OD, and the
stereochemical position of the deuterium determined (Scheme
8). Through detailed 1H NMR analysis of 7b, assignment and
Scheme 8.
Syn vs Anti Hydroallylation: Rh(III)–H
Insertion vs Diene Protonationa
Scheme 7. Deuterium Labeling Experimentsa-b
aSee SI for reaction details and stereochemical assignment.
Scheme 9. Reactions with (E)-Crotyl-BF3K and Reverse
Crotyl-BF3K
application of either (E)-crotyltrifluoroborate E-13 or reverse
crotyl reagent 14, in the (CDC)-Rh-catalyzed hydroallylation of
12, both result in similar ratio of 15:15’ (3:1 vs 2.5:1)
irrespective of which reagent is used. This result indicates, (i)
the allyl fragment arising from the trifluoroborate transfers to
rhodium and rapidly isomerizes via a Rh-allyl in the reaction
prior to C–C bond formation, and (ii) the allyl nucleophile
addition is not outer-sphere addition to a Rh-3-allyl since both
reagents afford the same ratio of product regioisomers. Use of
the more sterically demanding prenyl-BF3K resulted in <2%
conversion, with the resulting mass balance being unreacted
starting material.
2.3.3. Hammett Plot. We then investigated the electronic nature of
the transition state in the product-determining step by
performing a Hammett analysis of the catalytic hydroallylation
of alkenylarenes (Figure 1). A plot kH/kX vs + results in a  =
-0.48 (R2 = 0.9422) indicating a buildup of positive charge on
the olefin in the transition state of the product-determining step.
A reasonable explanation for the negative slope that is in
agreement with the deuterium scrambling data and large KIE,
is that a more electron-rich olefin would help facilitate oxidative
protonation to form Rh(III)–H. In addition, the negative slope
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is inconsistent with rhodium-hydride insertion onto the alkene. BF3K. Transmetalation with allyl-BF3K then affords Rh(I)–
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While the negative slope could also possibly be explained by
reductive elimination being the product determining step, which
is supported by studies by Hartwig where a similar trend was
observed in reductive elimination in phosphine-Pt-diaryl
complexes,28 such a notion would be inconsistent with the large
KIE.
allyl B, which can undergo irreversible oxidative protonation by
H–X (generated by the reaction of BF3 and MeOH) to afford
Rh(III)–hydride C. Subsequent, rapid reversible rhodium-
hydride migratory insertion across the olefin generates D,
which can proceed via C–C reductive elimination to afford the
product and regenerate complex A. It should be noted that Rh–
H formation prior to allyl group transmetalation is also
consistent with the data. The formation of deuterated alkene
recovered at the end of the reaction can be rationalized by an
equilibrium between alkene complex C and unbound alkene
complex C’, with the amount of deuterium incorporation
related to the relative rate of dissociation of the alkene
compared to reductive elimination. Low D incorporation would
indicate alkene dissociation from C is slower than reductive
elimination. Consequently, the much lower D incorporation
observed in reaction of sterically more demanding 2-Me-allyl
6a (Scheme 7a), is likely a result of increased sterics more
significantly effecting
2.3.4. Isolated (CDC)-Rh Complex. To obtain support for the
intermediacy of a bidentate (CDC)-Rh complex we synthesized
9
[(L1)Rh(cod)]BF4 16 (Scheme 10).
The complex was
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synthesized by deprotonation of pyrazolium salt L1 with n-
BuLi at -78 °C in THF, followed by reaction with [Rh(cod)Cl]2
to afford a orange-brown solid in 98% yield. While (L1)-Rh 16
was spectroscopically characterized (31P{1H}= 25.04 ppm (d,
JRh–P = 164.85 Hz)), unfortunately the X-ray structure of which
0.15
0.1
Scheme 11. Proposed Catalytic Cycle
p-Ph
p-Me
y = -0.4787x - 0.0235
0.05
R² = 0.9422
H
0
-0.05
-0.1
m-OMe
-0.15
-0.2
m-Cl
-0.25
-0.3
p-CO2Me
-0.4
-0.2
0
0.2
0.4
0.6
Hammett σ+ Values
Figure 1. Plot of Log(kX/kH) versus Hammett  values derived
from the scope presented in Scheme 3 and see also SI.
could not be obtained. Catalytic reactions with (CDC)-Rh
complex 16 demonstrate that it is a competent hydroallylation
catalyst (Scheme 10). Under standard conditions (see Scheme
2), with 5 mol % 16, 1,5-diene 5a is delivered in 54%
conversion, and 4:1 allyl/alkenyl selectivity. As a control,
reactions with [Rh(cod)Cl]2 established that 1,5-cyclooctadiene
is an ineffective ligand for promoting hydroallylation,
providing support for a bidentate (CDC)-Rh as the active
catalyst.
the rate of reductive elimination versus the rate of alkene
dissociation.
Finally, the observation of product alkene isomerization is
expected to occur due to slow product release off the catalyst
by another molecule of starting olefin. This provides
opportunity for the product bound rhodium species to re-enter
the catalytic cycle and generate the alkenyl isomer H via -
hydride elimination from D to afford G. Overall, these data
point towards Rh(III)–H formation being the product-
determining step.
Scheme 10. Synthesis and Reactivity of [(L1)Rh(cod)]BF4
 CONCLUSION
In summary, we have developed the first catalytic site-
selective hydroallyation of vinyl arenes and 1,3-dienes with
allyltrifuloroborates enabled by Rh catalysts supported by new
bidentate pyrazolium-based carbodicarbene ligands. This
represents the first example of bidentate CDCs as effective
ligands in transition metal catalysis. Catalyst is generated in
situ and the reactions are efficient and proceed in good yield to
afford a broad range of homoallylarenes and 1,5-diene products
with high levels of olefin site-selectivity with only minimal
double bond isomerization. Mechanism experiments are
consistent with the intermediacy of a Rh–allyl that is formed via
reversible Rh–hydride insertion on to the olefin. The highly
On the basis of our mechanistic findings, we propose the
catalytic cycle outlined in Scheme 11 for the catalytic olefin
hydroallylation reaction. Initial in situ formation of cationic
(CDC)-Rh(I) complex A, likely occurs via rhodium oxidative
C–H metalation of the ligand followed by H–Rh–allyl reductive
elimination to form propene, where the allyl arises from allyl-
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modular aspect of the bidentate CDCs disclosed in this study
are expected to be applicable to other sets of transformations.
1
2
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8
9
Finally, studies aimed at expanding olefin scope, organoboron
nucleophile, and developing additional chiral CDCs to render
this process highly enantioselective continue to be investigated
in these laboratories.
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Zeng, X. Recent Advances in Catalytic Sequential Reactions Involving
Hydroelement Addition to Carbon–Carbon Multiple Bonds. Chem.
Rev. 2013, 113, 6864–6900.
ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
*sjmeek@unc.edu.
ORCID
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Simon J. Meek: 0000-0001-7537-9420
Rajith S. Manan: 0000-0001-7081-0138
Courtney C. Roberts: 0000-0001-8177-4013
Notes
Authors declare no competing financial interests.
Supporting Information
(3) Hartwig, J. F. Raising the Bar for the "Perfect Reaction" Science
2002, 297 (5587), 1653
Experimental procedures, analytical data for new compounds, and
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
(4) For examples of stoichiometric allyl conjugate additions, see:
Silicon (a) Hosomi, A.; Sakurai, H. Chemistry of Organosilicon
Compounds. 99. Conjugate Addition of Allylsilanes to .alpha.,.beta.-
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ACKNOWLEDGMENT
Financial support was provided by the NSF (CHE-1665125) and
University of North Carolina at Chapel Hill. JSM is grateful for a
Burroughs-Welcome fellowship from the UNC Chemistry
Department. We thank the University of North Carolina’s
Department of Chemistry Mass Spectrometry Core Laboratory for
their assistance with mass spectrometry analysis. This material is
based upon work supported by the National Science Foundation
under Grant No. (CHE1726291).
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