An Efficient and Reliable Catalyst System Using Hemilabile Aphos for B-Alkyl Suzuki-Miyaura Cross-Coupling Reaction with Alkenyl Halides

Article abstract of DOI:10.1002/ejoc.201201602

The 9-methoxy-9-borabicyclo[3.3.1]nonane-based B-alkyl Suzuki-Miyaura cross-coupling reaction (the 9-MeO-9-BBN variant) has been efficiently performed by using the catalyst consisting of Pd(OAc)₂ and a hemilabile P,O-ligand, Aphos-Y, under mild reaction conditions (K₃PO ₄·3H₂O, THF/H₂O, room. temp.). For applications in the total synthesis of structurally complex natural products, the Johnson protocol commonly uses two ligands (dppf and Ph₃As) and two organic solvents (THF and DMF). In contrast, the new version reported here employs one ligand (Aphos-Y) and one organic solvent (THF). Moreover, the broad substrate scope and the excellent functional group tolerance of the Pd(OAc) ₂-Aphos-Y catalyst have been demonstrated, providing a reliable and simple synthetic tool for fragment coupling in total synthesis through formation of a C(sp³)-C(sp²) bond. Copyright

Full text of DOI:10.1002/ejoc.201201602

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201602
An Efficient and Reliable Catalyst System Using Hemilabile Aphos for B-
Alkyl Suzuki–Miyaura Cross-Coupling Reaction with Alkenyl Halides
Ning Ye[a] and Wei-Min Dai*[a]
Keywords: Alkenes / Amides / Cross-coupling / Palladium / Phosphanes
The 9-methoxy-9-borabicyclo[3.3.1]nonane-based B-alkyl
Suzuki–Miyaura cross-coupling reaction (the 9-MeO-9-BBN
variant) has been efficiently performed by using the catalyst
consisting of Pd(OAc)2 and a hemilabile P,O-ligand, Aphos-
Y, under mild reaction conditions (K3PO4·3H2O, THF/H2O,
room. temp.). For applications in the total synthesis of struc-
turally complex natural products, the Johnson protocol com-
monly uses two ligands (dppf and Ph3As) and two organic
solvents (THF and DMF). In contrast, the new version re-
ported here employs one ligand (Aphos-Y) and one organic
solvent (THF). Moreover, the broad substrate scope and the
excellent functional group tolerance of the Pd(OAc)2–Aphos-
Y catalyst have been demonstrated, providing a reliable and
simple synthetic tool for fragment coupling in total synthesis
through formation of a C(sp3)–C(sp2) bond.
phosphanes (Aphos) such as 1 and 2 can be modified by
thearylgroupappendedattheC4(orC5)positionofthebenz-
amide core (Scheme 1). These Aphos ligands promoted
Suzuki–Miyaura reactions of inactivated aryl chlorides with
arylboronic acids at room temperature,[4a] reactions of
benzyl halides with arylboronic acids at 60 °C,[4b] and B-
alkyl Suzuki–Miyaura reactions of aryl and alkenyl halides
under mild conditions.[4a,4c] In the latter case, addition of
Ph3As was not necessary, which implies that one hemilabile
ligand 2[5] can substitute for dppf and Ph3As.[4c] We report
here a detailed study on a simpler catalyst system consisting
of Pd(OAc)2 and Aphos-Y (2) in the presence of
K3PO4·3H2O and H2O in THF at room temperature for
catalyzing the “9-MeO-9-BBN variant” of the Suzuki–
Miyaura cross-coupling reaction. Our emphasis focuses on
the substrate scope and the practical applicability in the
total synthesis of natural products.
Previously, we established by NMR spectroscopy that
Aphos formed a P,O-chelating palladium(0) complex Ia (Ar
= H) with [Pd2(dba)3] in [D8]THF as the dominant species
along with some detectable monoligated and P-mono-
dentate complex Ib (Scheme 1).[4a] Both P,O-chelating and
P-monodentate palladium(II) complexes and oxidative ad-
ducts IIb (Ar = H; X = Br, Cl, OAc; R1 = 4-tBuC6H4) were
characterized by X-ray crystallographic analysis.[6] It was
suggested that minor palladium complex Ib is the most re-
active catalyst for oxidative addition[4a,7] and the initially
formed adduct, IIa, should be equilibrated into IIb. More-
over, the above three complexes IIb exerted similar reactiv-
ity in the coupling reaction with PhB(OH)2 [THF/H2O
(10:1), K3PO4, room temp., 1 h] to give the biaryl in yields
of 90–95%,[6] indicating that IIb might be converted into
hydroxido complex III prior to transmetallation.[8] We envi-
sioned two scenarios for the formation of intermediate IV
Introduction
The B-alkyl Suzuki–Miyaura cross-coupling reaction is a
powerful tool for the formation of C(sp2)–C(sp3) bonds and
has widespread applications in the total synthesis of natural
products.[1] Two main protocols have been established
by using 9-alkyl-9-borabicyclo[3.3.1]nonane (9-alkyl-9-
BBN)[1b] and the alkyl borinate derived from 9-methoxy-9-
BBN and an alkyl iodide.[1f] In contrast to 9-alkyl-9-BBN
formed by hydroboration of 1-alkenes with 9-BBN-H, the
“9-MeO-9-BBN variant” tolerates a broader spectrum of
functionalities, some of which might not survive under the
hydroboration conditions. This distinct advantage renders
the “9-MeO-9-BBN variant” an attractive protocol for cou-
pling of densely functionalized fragments in the total syn-
thesis.[1b,1e,1f,2] In general, a catalytic amount of [Pd(dppf)-
Cl2] and a mild base such as NaOH, K2CO3, or K3PO4
were used in aqueous DMF for promoting the B-alkyl Su-
zuki–Miyaura cross-coupling reaction.[1a,1b] In 1993, John-
son and Braun first reported the use of an additional li-
gand, Ph3As, for achieving a higher turnover rate and a
cleaner reaction.[3a] Since then, the Johnson protocol is
popularly employed in total synthesis, and omission of
Ph3As has been reported to decrease the product yield sig-
nificantly in some cases.[3b] We have recently demonstrated
that the catalytic activity of aromatic amide-derived
[a] Laboratory of Advanced Catalysis and Synthesis, Department
of Chemistry
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong SAR, P.R. China
Fax: +852-2358-1594
E-mail: chdai@ust.hk
Homepage: http://ihome.ust.hk/~chdai/index.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201602.
Eur. J. Org. Chem. 2013, 831–835
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
831

N. Ye, W.-M. Dai
SHORT COMMUNICATION
took place once the base was omitted (entry 5 vs. entry
4, Table 1). These results demonstrate that the Pd(OAc)2–
Aphos-Y catalyst is as efficient as the Johnson protocol.
Table 1. Effects of ligand(s), base, and water on the coupling of
alkyl iodide 3 with 2-bromoprop-1-ene.[a]
Entry Pdb]
Ligand (mol-%) Base
Additive[c] t [h] Yield [%][e]
1
PdCl2
PdCl2
PdCl2
Pd(OAc)2 2 (7.5)
Pd(OAc)2 2 (7.5)
dppf (5)
dppf (5)
dppf (5)
K3PO4
Cs2CO3 Ph3As
K3PO4 Ph3As
–
12
12
12
20
16
77
46
80
0
2[d]
3
4
5
–
–
–
K3PO4
86
[a] Reaction conditions: 0.2 mmol alkyl iodide 3, 4 equiv. tBuLi,
4 equiv. 9-MeO-9-BBN, 2 mL each of Et2O and THF, –78 °C to
room temp., 1.5 h; then Pd, ligand, additive, 3 equiv. base,
5.5 equiv. 2-bromoprop-1-ene, 9 equiv. H2O, 2 mL DMF (entries
1–3) or THF (entries 4 and 5), room temp. The long reaction times
and large excess of 2-bromoprop-1-ene were purposely used here
for maximizing the conversion. [b] 5 mol-%. [c] 10 mol-% Ph3As
was added. [d] H2O not added. [e] Isolated yield of 4 based on alkyl
iodide 3. PMB = para-methoxybenzyl, TES = triethylsilyl.
Scheme 1. Structures of Aphos 1 and 2 and the Pd–Aphos-pro-
moted catalytic cycle for the B-alkyl Suzuki–Miyaura cross-cou-
pling reaction.
responsible for the transmetallation.[8a–8g] One involves the
reaction of halide complex IIb with hydroxy borinate VIb[8g]
derived from the hydrolysis of MeO borinate species VIa.
As the effect of H2O is profound, a detailed investigation
Alternatively, reaction of hydroxido complex III[8h–8k] with was followed (Table 2). Under the Johnson protocol, the re-
9-alkyl-9-BBN VIc, which is formed by loss of MeO– from action of alkyl iodide 5 with 1.5 equiv. of 2-bromoprop-1-
VIa, should give IV. Both pathways were consistent in that ene in the presence of 19 equiv. of H2O proceeded smoothly
K3PO4 and H2O were required for the coupling reaction as in a mixture of THF/DMF to afford 6a in 91% yield (entry
the source of OH– anion[8k] (vide infra). Transmetallation 1, Table 2). Similarly, the same coupling catalyzed by
within IV was proposed to occur through a four-centered Pd(OAc)2–Aphos-Y in the presence of 9 equiv. of H2O in
hydroxido μ2-bridged transition state (TS)[8g] or via a three- THF furnished 6a in 97% yield (entry 2, Table 2). When
centered TS as in the reaction of arylboronic acids.[8e,8i–8k] K3PO4·3H2O was used, the coupling did not occur without
One remarkable feature of the hemilabile Aphos in complex extra added H2O. It was interesting to note that 6a could
IV is the higher trans effect of the phosphorus atom, which be formed once extra H2O was added (entry 3 vs. entry 4,
weakens the PdII–O+(H)B–(R3) bond and renders the trans- Table 2). In the presence of H2O, similar results were ob-
metallation easier. Finally, reductive elimination from cis- tained regardless of whether K3PO4·3H2O or K3PO4 was
R1PdR2(Aphos) (V) without assistance of OH– anion[8i–8k] used (entry 5 vs. entry 2, Table 2). Moreover, the palladium
should furnish the R1–R2 product with regeneration of Ia,b. loading could be reduced to 1 mol-% without deteriorating
It should be emphasized that the details of the above-pro- the yield, albeit at the expense of prolonged reaction time
posed mechanism, in particular the transmetallation pro- (entry 6, Table 2). Therefore, the Pd(OAc)2–Aphos-Y cata-
cesses, need further investigation. Our current work focuses lyst could efficiently catalyze the coupling reaction with in-
on the practical application of the Pd(OAc)2–Aphos-Y cat- expensive K3PO4·3H2O as the base in THF and without
alyst.
using Ph3As and DMF.
We first compared the Pd(OAc)2–Aphos-Y catalyst with
With the coupling protocol established above, making
the Johnson protocol[9] in the coupling reaction of 2-bro- use of Pd(OAc)2–Aphos-Y, the scope of substituted alkenyl
moprop-1-ene with the alkyl iodide 3 (Table 1). With 5 mol- bromides in the reactions with alkyl iodide 5 was examined
% each of PdCl2 and dppf, product 4[10] was formed in (Table 3). Di-, tri-, and tetrasubstituted alkenes 6a–d were
yields of 77–80% in the presence of 9 equiv. of H2O and prepared in good to excellent yields (entries 1–4, Table 3).
with or without 10 mol-% Ph3As (entries 1 and 3, Table 1). A correlation of the reaction time and product yield with
In the absence of H2O, the coupling did occur in DMF, but the steric bulkiness of the alkenyl bromide is noted. Steric
a much lower yield of 4 was observed (entry 2, Table 1). By hindrance should be the main reason for the lower yield of
using Pd(OAc)2–Aphos-Y, 4 was formed in THF in 86% 6e in which the phenyl group (R3 = Ph) and the alkyl group
yield in the presence of K3PO4 and H2O, but no reaction carried over from 5 are cis to each other (entry 5, Table 3).
832
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B-Alkyl Suzuki–Miyaura Cross-Coupling with Alkenyl Halides
Table 2. Effect of water on the coupling of 5 with 2-bromoprop-1-
Table 4. Coupling reactions of various alkyl iodides with 2-bro-
moprop-1-ene.[a]
ene.[a]
Entry Base[b]
H2O [equiv.] Solvent(s)
t [h]
Yield [%][c]
1
2
3
4
Cs2CO3
K3PO4
19
9
–
THF/DMF 1.5
91
97
0
THF
THF
THF
THF
THF
1.5
16
K3PO4·3H2O
K3PO4·3H2O 9[d]
12 + 3 45
2
10
5
K3PO4·3H2O
K3PO4·3H2O
9
9
93
90
6[e]
[a] Reaction conditions are found in the Experimental Section. The
Johnson protocol [5 mol-% Pd(dppf)Cl2, 10 mol-% Ph3As] was
used in entry 1. [b] 3 equiv. [c] Isolated yield of 6a based on alkyl
iodide 5. [d] H2O was added after 12 h of reaction, and the reaction
was continued for another 3 h. [e] 1 mol-% Pd(OAc)2 and 1.5 mol-
% Aphos-Y were used. TBDPS = tert-butyldiphenylsilyl.
[a] Reaction conditions are found in the Experimental Section. [b]
Isolated yield of 7 based on the alkyl iodides.
Nevertheless, the Pd(OAc)2–Aphos-Y catalyst system is ap-
plicable for the preparation of all types of substituted alk-
enes in synthetically useful yields.
Trisubstituted alkenes with multiple functionalities are
useful fragments for total synthesis of natural products, and
yet their synthesis is often challenging.[10–13] To demon-
strate the practical applicability of the Pd(OAc)2–Aphos-Y
catalyst, Table 5 highlights the highly functionalized trisub-
stituted alkenes synthesized. Compounds 8a,b, obtained in
85% yield each, feature an additional double bond and a
free hydroxy group,[3b] respectively (entries 1 and 2,
Table 5). The cyclic silyl protecting group for chiral 1,3-di-
ols has been proven to be beneficial in highly stereoselective
ketone aldol reactions and has been utilized in the total
synthesis of the plecomacrolides.[14] This protecting group
was also found to be critically important in Altmann’s syn-
thesis of the mycolactone core via the “9-MeO-9-BBN vari-
ant” under the Johnson protocol, presumably due to re-
Table 3. Coupling reactions of alkyl iodide 5 with various alkenyl
bromides.[a]
Entry R1
R2
R3
t [h]
Yield [%][b]
1[c]
2
3
4
5
Me
Me
Me
Ph
H
H
H
Me
H
Ph
1.5
3
6
2
15
6a: 97
6b: 85
6c: 81
6d: 92
6e: 64
Me
Me
H
CHO
H
[a] Reaction conditions are found in the Experimental Section. [b] duced steric hindrance as compared to the bis(TBS) ether
analogue.[9,10b] We used the same cyclic silyl-protected alk-
Isolated yield based on 6. [c] Data taken from entry 2 of Table 2.
enyl iodide in entries 3–6 of Table 5. Although the alkyl
In the total synthesis of polyketide natural products, the iodide with the TES-protected anti-aldol unit underwent
requisite fragments often possess syn- and anti-aldol sub- the coupling reaction with 2-bromoprop-1-ene (entry 1,
units as well as different protecting groups such as the para- Table 4), it nearly failed to form product 9a (entry 3,
methoxybenzyl (PMB) and various bulky silyl groups. Syn- Table 5).[11b] Fortunately, the less sterically demanding
thesis of alkene 4 with the TES-protected syn-aldol moiety PMB ether greatly enhanced the coupling efficiency, fur-
is given in Table 1. The results of reactions of other repre- nishing 9b in 90% yield (entry 4, Table 5). In addition, anal-
sentative chiral alkyl iodides are summarized in Table 4. Al- ogous products 9c,d were synthesized under the same cata-
kene 7a with a TES-protected anti-aldol moiety could be lytic conditions in 83 and 80% overall yields, respectively,
synthesized in 90% yield (entry 1, Table 4). It was found after cleavage of the PMB group (entries 5 and 6, Table 5).
that the PMB group, if close to the coupling reaction site
Finally, we prepared the C13–C23 fragment 12 designed
(i.e. in 1,3-relationship to the iodine atom), might interfere for the synthesis of iriomoteolide-1a diastereomers accord-
with the reaction. While the yield of 7b (85%) is slightly ing to our previously established strategy (Scheme 2).[2] The
lower than that of the TES-protected analogue 7a (90%) same type of reaction has been used by others in the total
(entry 2 vs. entry 1, Table 4), a significantly diminished yield synthesis of the proposed structure of iriomoteolide-1a.[15]
of 65% was obtained for 7c, and extension of the reaction Under the Johnson protocol (Condition A) 12 was isolated
time could not improve the yield (entry 3, Table 4). Chela- in 85% yield. It was pleasing to find that the Pd(OAc)2–
tion of the PMBO group appended at the primary carbon Aphos-Y catalyst (Condition B) performed equally well, af-
with boron would retard the reaction.
fording product 12 in 86% yield.
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833

N. Ye, W.-M. Dai
SHORT COMMUNICATION
Table 5. Synthesis of densely functionalized trisubstituted alkenes
by Pd(OAc)2–Aphos-Y-catalyzed C(sp3)–C(sp2) coupling reac-
tions.[a]
Conclusions
In summary, we have developed a general and efficient
catalyst system consisting of Pd(OAc)2–Aphos-Y[4a,4c] for
the B-alkyl Suzuki–Miyaura cross-coupling reaction using
9-alkyl-9-BBN[1b] and the “9-MeO-9-BBN variant”.[1f] The
catalyst system functions under mild conditions in THF
with K3PO4 or K3PO4·3H2O as the base and without the
use of an additional ligand (such as Ph3As) or solvent (such
as DMF). As exemplified by the analogous fragments used
in the total synthesis of mycolactone[10b] and iriomoteolide-
1a,[2,15] di- and trisubstituted alkenes 8, 9, and 12 possessing
dense functionalities, such as double bonds, free hydroxy
groups, syn- and anti-aldol subunits, or PMB and silyl pro-
tecting groups, can be synthesized at room temperature in
good to excellent yields. The hemilability of Aphos-Y[4a] en-
ables its efficient role in the catalytic cycle as dppf and
Ph3As do under the Johnson protocol, resulting in an ef-
ficient and reliable tool for the construction of the C(sp3)–
C(sp2) bond. Application of the Pd(OAc)2–Aphos-Y cata-
lyst system in the total synthesis of natural products is in
progress in our laboratories.
Experimental Section
Formation of the Alkyl Borinate: A flame-dried two-neck flask of
10 mL capacity was charged with the alkyl iodide (0.20 mmol) and
was then evacuated and backfilled with argon (5 times). 9-MeO-9-
BBN (1 m in hexanes, 0.60 mL, 0.60 mmol) and freshly distilled dry
Et2O (2 mL) were added with a syringe at room temperature. The
colorless solution was cooled to –78 °C in a dry ice/acetone bath.
After stirring for 5 min, tBuLi (1.6 m in heptane, 0.35 mL,
0.56 mmol) was rapidly added with a syringe in one portion at
–78 °C. The resulting milky suspension was stirred for 10 min at
the same temperature, and freshly distilled dry THF (2 mL) was
added. The mixture turned clear, and the cooling bath was removed
after stirring for 10 min. The mixture was stirred at room tempera-
ture for another 1.5 h to form a homogeneous pale yellow solution
of the alkyl borinate.
[a] Reaction conditions are found in the Experimental Section. [b]
Isolated yield based on the alkenyl iodides. [c] With 18 equiv. H2O.
[d] 75% of the alkenyl iodide was recovered. [e] Overall yield for
the coupling and cleavage of the PMB group.
Cross-Coupling Reaction: A 10 mL process vial was charged with
Pd(OAc)2 (2.2 mg, 0.01 mmol), Aphos-Y (7.6 mg, 0.015 mmol),
and K3PO4 (127.4 mg, 0.60 mmol) or K3PO4·3H2O (159.8 mg,
0.60 mmol). The loaded vial was sealed with a cap containing a
silicon septum and was evacuated and backfilled with argon (5
times) through a needle. A solution of the alkenyl halide (5.5 equiv.
for Table 1 and 1.5 equiv. for Tables 2, 3, and 4) in degassed THF
(2 mL) was added with a syringe, followed by the addition of H2O
(33 μL, 1.8 mmol or 66 μL, 3.6 mmol). The mixture was stirred at
room temperature for 5 min, and then the alkyl borinate prepared
as described above was transferred with a syringe. After being stir-
ring at room temperature for the indicated time, the reaction mix-
ture was filtered off through a plug of Celite and rinsed with
EtOAc. The combined organic layer was concentrated under re-
duced pressure, and the residue was purified by flash column
chromatography on silica gel (eluting with EtOAc/hexane) to give
the coupling product. For Table 5, 1.1–1.5 equiv. of the alkyl borin-
ate were used per equiv. of alkenyl iodide.
Scheme 2. Synthesis of the C13–C23 fragment of iriomoteolide-1a.
Condition A: 1.6 equiv. 11, 4 equiv. tBuLi, 6 equiv. 9-MeO-9-BBN,
Et2O/THF, –78 °C to room temp., 1.5 h; then 1 equiv. 10, 6 mol-%
Pd(dppf)Cl2·CH2Cl2, 18 mol-% Ph3As, 4 equiv. Cs2CO3, 25 equiv.
H2O, DMF, room temp., 1 h (85%). Condition B: 1.2 equiv. 11,
2.8 equiv. tBuLi, 3 equiv. 9-MeO-9-BBN, Et2O/THF, –78 °C to
room temp., 1.5 h; then 1 equiv. 10, 5 mol-% Pd(OAc)2, 7.5 mol-%
Aphos-Y, 3 equiv. K3PO4, 18 equiv. H2O, THF, room temp., 1 h
(86%).
Supporting Information (see footnote on the first page of this arti-
cle): Representative procedures reported in the literature, product
characterization data, images of the reaction mixture at various
stages, and copies of NMR spectra.
834
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B-Alkyl Suzuki–Miyaura Cross-Coupling with Alkenyl Halides
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Acknowledgments
This work is supported in part by a General Research Fund grant
(No. 600709) from the Research Grant Council, The Hong Kong
Special Administrative Region, P. R. China and by the Department
of Chemistry, The Hong Kong University of Science and Technol-
ogy (HKUST). The authors thank Jian Wang for synthesizing the
intermediates 10 and 11.
[9] See Supporting Information for details.
[10] a) F. Feyen, A. Jantsch, K.-H. Altmann, Synlett 2007, 415–418;
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Received: November 28, 2012
Published Online: January 2, 2013
Eur. J. Org. Chem. 2013, 831–835
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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