Enantioselective construction of all-carbon quaternary centers by branch-selective Pd-catalyzed allyl-allyl cross-coupling

Article abstract of DOI:10.1021/ja2039248

The Pd-catalyzed cross-coupling of racemic tertiary allylic carbonates and allylboronates is described. This reaction generates all-carbon quaternary centers in a highly regioselective and enantioselective fashion. The outcome of these reactions is consistent with a process that proceeds by way of 3,3′-reductive elimination of bis(η¹-allyl)palladium intermediates. Strategies for distinguishing the product alkenes and application to the synthesis of (+)-α-cuparenone are also described.

Full text of DOI:10.1021/ja2039248

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pubs.acs.org/JACS
Enantioselective Construction of All-Carbon Quaternary Centers by
Branch-Selective Pd-Catalyzed AllylꢀAllyl Cross-Coupling
Ping Zhang, Hai Le, Robert E. Kyne, and James P. Morken*
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States
S Supporting Information
b
ABSTRACT: The Pd-catalyzed cross-coupling of racemic
tertiary allylic carbonates and allylboronates is described.
This reaction generates all-carbon quaternary centers in a
highly regioselective and enantioselective fashion. The out-
come of these reactions is consistent with a process that
proceeds by way of 3,30-reductive elimination of bis-
(η1-allyl)palladium intermediates. Strategies for distinguish-
ing the product alkenes and application to the synthesis of
(þ)-R-cuparenone are also described.
Figure 1. Substrate classes for allylic substitution.
Scheme 1
he catalytic enantioselective construction of all-carbon qua-
T
ternary centers remains a challenging task in organic syn-
thesis.1 Relative to reactions that establish tertiary centers, efficient
reactions that form quaternary centers are often much more
difficult to develop because of barriers imposed as a result of
establishing highly congested carbon centers; selectivity is also
more challenging as a result of the diminished steric bias between
the enantiotopic faces of substrates and intermediates. In terms
of catalytic CꢀC bond-forming constructions of quaternary
centers, enantioselective cycloadditions, Heck reactions,2 enolate
R-arylations,3 and enolate R-allylations4 have proven to be
valuable strategies. Conjugate addition5 and allylic substitution6
reactions have also proven to be of significant value. Under the
purview of Cu catalysis, SN20 allylic substitutions can offer high
levels of enantiocontrol in the construction of quaternary
centers.7 However, these reactions generally require construc-
tion of isomerically pure trisubstituted alkene substrates (i.e., A
or B in Figure 1). While Ru, W, and Ir complexes undergo
branch-selective allylic substitution with either internal or term-
inal allylic electrophiles, πꢀσꢀπ isomerization with these com-
plexes is generally slower than nucleophilic addition, and thus, as
in the case of copper complexes, the use of isomerically pure
substrates is required.8 In contrast, Pd and Mo allyl complexes
undergo rapid πꢀσꢀπ isomerization and under appropriate
conditions can favor the branched addition product. This feature
allows these complexes to process mixtures of stereoisomeric and
regioisomeric allylic electrophiles (i.e., AꢀD in Figure 1).9 With
these catalysts, remarkable progress in the enantioselective
substitution of allylic electrophiles to generate tertiary centers
has been made; however, only three examples offering a protocol
for the asymmetric construction of all-carbon quaternary centers
by branch-selective substitution reactions have appeared. Trost
described Pd-catalyzed enantioselective substitutions of isoprene
monoepoxide and more recently developed a substrate-specific
linalylation reaction.10 Additionally, Hou described the Pd-
catalyzed addition of malonates to tertiary allylic acetates, wherein
a key requirement for high selectivity is the use of hindered
substrates (e.g., 1-napthyl derivatives).11 In this manuscript, we
detail an effective protocol for Pd-catalyzed enantioselective
construction of quaternary centers by allylic substitution. Nota-
bly, this reaction employs readily available racemic tertiary allylic
carbonates and provides high levels of regio- and enantioselec-
tivity across a range of substrates.
We recently described a Pd-catalyzed regio- and enantioselec-
tive allylꢀallyl cross-coupling that enables the asymmetric assembly
of tertiary stereocenters (Scheme 1; R1 = aryl, alkyl; R2 = H).12
Mechanistic experiments suggested that this transformation likely
proceeds by way of π-allyl complexes (1a and 1b) and that a critical
feature of the mechanism is the likely intermediacy of bis-
(η1-allyl)palladium intermediates. These compounds undergo
inner-sphere 3,30-reductive elimination13 (2a/2b in Scheme 1),
thereby delivering the branched allylation products selectively. In
light of the discussion above, it was of interest to determine
whether this reaction could be extended to the much more
demanding case of quaternary center assembly. A primary
concern in developing such a process arises from the fact that
with tertiary allylic carbonates, mixtures of syn and anti π-allyl
complexes (1a and 1b) would likely be generated, and their
interconversion, which would be necessary for a stereocon-
vergent reaction, would require access to hindered tertiary
η1-allylpalladium intermediates.
Received: April 28, 2011
Published: June 07, 2011
r
2011 American Chemical Society
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Table 1. Optimization of the AllylꢀAllyl Cross-Coupling of 3
Table 2. Substrate Scope for the AllylꢀAllyl Cross-Coupling
entry
variation of conditions
10% catalyst
4:5a
1:1
yieldb
er of 4c
1
2
3
4
5
6
7
38
90
79
82
77
88
90
96:4
96:4
96:4
95:5
95:5
96:4
96:4
10% catalyst, Cs2CO3 (1.2 equiv)
10% catalyst, CsF (1.2 equiv)
CsF (3 equiv)
2:1
5:1
9:1
CsF (10 equiv)
20:1
14:1
>20:1
THF/H2O (10:1)
CsF (3 equiv), THF/H2O (10:1)
a Determined by 1H NMR analysis. b Isolated yield of purified product.
Compounds 4 and 5 were inseparable by chromatography, and the yield
refers to the mixture. c Determined by chiral GC chromatography.
To initiate these studies, racemic allyl carbonate 3 was
subjected to cross-coupling with allylB(pin), 5 mol % Pd2(dba)3,
and 10 mol % MeO-furyl-biphep.14 As depicted in entry 1 of
Table 1, this reaction indeed delivered the allylꢀallyl coupling
product 4 with very high levels of enantiomeric purity; however, a
significant amount of 1,3-diene 5 was generated, and the reaction
proceeded with very low efficiency. While 5 might be produced
from an intermediate bis(η1-allyl)Pd complex by intramolecular
H-atom abstraction (to generate propene),15 it might also be
produced prior to transmetalation by elimination from the allyl
intermediate (i.e., 1 or related; Scheme 1).16 That the latter pro-
cess may operate was verified by treating rac-3 with the catalyst in
the absence of allylB(pin); this experiment provided complete
conversion to 5 in 12 h. To minimize this side reaction during the
coupling process, additives thought to accelerate transmetala-
tion were examined. Both Cs2CO317 and CsF18 proved beneficial
(entries 2 and 3) and enhanced the 4:5 ratio in a concentration-
dependent manner. Since addition of water has also been shown
to facilitate transmetalation,19 aqueous solvent systems were
examined and also found to minimize production of the 1,
3-diene (entry 6). Optimal conditions were found to involve CsF
(3 equiv) and employ 10:1 THF/H2O as the solvent system. In
this case, the allylꢀallyl coupling product was obtained in
excellent yield with high enantioselectivity as a single (>20:1)
regioisomer (entry 7).
The scope of the catalytic allylꢀallyl coupling was examined
using a panel of substrates. As depicted in Table 2, a range of
aromatic tertiary allylic carbonates participated in the reaction,
and a good level of substitution was tolerated. Notably, oxygen
and halogen-substituted substrates were processed in good yield
with good chemo- and enantioselectivity. Importantly, ortho sub-
stitution in the substrate was also tolerated, as the example in
entry 8 indicates. In addition to the methyl ketone-derived elec-
trophiles in entries 1ꢀ10, longer alkyl chains and those bearing
protected oxygenation were also converted to the corresponding
chiral 1,5-dienes with excellent selectivity. Lastly, the results for
the aliphatic substrates in entries 14 and 15 suggested that
excellent enantiodiscrimination does not require an aromatic
substituent. As long as the two enantiotopic groups on the
a Isolated yield of purified product. The product:1,3-diene (elimination
product) ratio (pdt:elim ratio) was determined by 1H NMR analysis. In
general, the product and the 1,3-diene were inseparable by chromatog-
raphy, and the yield refers to the mixture. b Determined by chiral GC,
SFC, or HPLC analysis and the average of two or more experiments.
c Substrate was a mixture of branched and linear allylic carbonates.
d Reaction at 80 °C. e Reaction for 36 h. f Reaction employed 10 equiv of
CsF and 3 equiv of allylB(pin) in 5:1 THF/H2O. g Reaction in anhydrous
THF. h Reaction employed a mixture of linear and branched allylic
chloride substrates.
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Scheme 2a
Scheme 3
a Conditions: (a) PhI, 10 mol % Pd(OAc)2, Bu4NCl, DMF, 80 °C, 16 h.
(b) 5 mol % HG2 catalyst, ethyl acrylate, CH2Cl2, 40 °C, 20 h. (c)
B2(pin)2, 3 mol % Pt(dba)3, 3.6 mol % 3,5-Ph2-taddol-PPh, THF; then
NaOH, H2O2.
As the preceding data suggest, allylB(pin) can be employed
with a broad range of substituted allyl electrophiles. As depicted
in Scheme 3, substitution on the allylboronate was also tolerated.
Coupling of carbonate 9 and methallylB(pin) occurred with
excellent levels of asymmetric induction. Importantly, reaction
product 10 is well-suited for construction of cyclopentenones. As
depicted, ozonolysis delivered ketoaldehyde 11, which was
converted to cyclopentenone 12 by intramolecular aldol con-
densation. In analogy to a study by Meyers,23 12 was converted
to R-cuparenone.24 This five-step route from 9 represents the
shortest catalytic asymmetric synthesis of this target structure.25
In conclusion, the 3,30-reductive elimination reaction that
operates in the course of allylꢀallyl cross-couplings allows for
contrasteric CꢀC bond constructions between allylic electro-
philes and allylboronates. Importantly, these reactions can be
used for the asymmetric construction of hindered quaternary
carbon centers. Further studies of the utility of these processes
are in progress.
substrate bear a significant difference in size, high levels of
selectivity can be observed. For example, cyclohexyl and methyl
(entry 14) were well-distinguished, and the product was obtained
with 92:8 er, whereas the similar size of the alkyl chain and the
methyl group in entry 15 resulted in diminished stereoselection.
The example in entry 14 also highlights the fact that allylic halides
may serve as coupling partners in this process.
An important feature of the allylꢀallyl cross-coupling reaction
is that it can employ racemic tertiary allylic alcohol derivatives,
which are readily prepared by addition of vinylmagnesium
bromide to the corresponding ketone. In some cases, however,
the regioisomeric primary allylic electrophile might be more
readily available, so it was important to determine whether these
substrates could be employed. As depicted in entries 5ꢀ7, both
the E and Z terminal allylic carbonates were converted to the
same quaternary-center-containing coupling product with simi-
lar levels of efficiency and selectivity. One noteworthy difference
between the E and Z substrates is that the Z substrate reacted at a
substantially lower rate (36 vs 12 h). It is tenable that in the Z
configuration, the aryl group is oriented orthogonal to the alkene
σ-bond framework to avoid an A(1,3) interaction; in this
orientation, the aryl group may shield the alkene from attack,
thereby slowing the oxidative addition step.
’ ASSOCIATED CONTENT
S
Supporting Information. Characterization data and pro-
b
cedures. This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
To examine features associated with practicality, the experi-
ments in Scheme 2 were undertaken. First, it was shown that a
more economical Pd source, PdCl2, can be employed and that
the reaction can be conducted without the aid of a glovebox.
Second, it was found that the alkenes in the cross-coupling
products could be effectively differentiated, an important pre-
requisite for target-directed synthesis. As depicted in Scheme 2,
this objective may be accomplished in a number of ways.
Reaction a shows that a regioselective Heck reaction20 where-
in the less-hindered alkene is selectively transformed can be
accomplished. Ostensibly, the high regioselection in this
reaction results from incipient torsional strain should the
other olefin undergo migratory insertion with an arylpalla-
dium complex. Likely for similar reasons, olefin cross-
metathesis21 converts 4 to unsaturated ester 7 in a highly regio-
and stereoselective fashion (reaction b). Lastly, regio- and
diastereoselective dihydroxylation can be accomplished by
way of Pt-catalyzed diboration in the presence of a chiral
phosphonite catalyst (reaction c).22 In this case, diastereocon-
trol results from the enantiofacial selectivity that occurs in the
diboration step.
Corresponding Author
morken@bc.edu
’ ACKNOWLEDGMENT
Support from the NIGMS (GM-64451) and the NSF (DBI-
0619576, BC Mass Spectrometry Center) is gratefully acknowl-
edged. P.Z. is grateful for LaMattina and AstraZeneca Fellow-
ships. We thank Frontier Scientific for a generous donation of
allylB(pin) and AllyChem for B2(pin)2.
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