Rhodium-catalyzed enantioselective cyclizations of γ-alkynylaldehydes with acyl phosphonates: ligand- and substituent-controlled C-P or C-H bond cleavage

Article abstract of DOI:10.1021/ja201337x

It has been established that a cationic rhodium(I)/(R)-H₈-BINAP or (R)-Segphos complex catalyzes two modes of enantioselective cyclizations of γ-alkynylaldehydes with acyl phosphonates via C-P or C-H bond cleavage. The ligands of the Rh(I) comp

Full text of DOI:10.1021/ja201337x

COMMUNICATION
pubs.acs.org/JACS
Rhodium-Catalyzed Enantioselective Cyclizations
of γ-Alkynylaldehydes with Acyl Phosphonates: Ligand- and
Substituent-Controlled CꢀP or CꢀH Bond Cleavage
Kengo Masuda,† Norifumi Sakiyama,† Rie Tanaka,† Keiichi Noguchi,‡ and Ken Tanaka*,†
†Department of Applied Chemistry, Graduate School of Engineering, and ‡Instrumentation Analysis Center, Tokyo University of
Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
S Supporting Information
b
Scheme 1
ABSTRACT: It has been established that a cationic
rhodium(I)/(R)-H8-BINAP or (R)-Segphos complex cata-
lyzes two modes of enantioselective cyclizations of γ-alky-
nylaldehydes with acyl phosphonates via CꢀP or CꢀH
bond cleavage. The ligands of the Rh(I) complexes and the
substitutents of both γ-alkynylaldehydes and acyl phospho-
nates control these two different pathways.
Scheme 2
ransition-metal-catalyzed reductive coupling is a valuable
T
strategy for regio- and stereocontrolled formation of CꢀC
bonds.1 A number of transition-metal-catalyzed intramolecular
reductive cyclizations of γ-alkynylaldehydes have been developed
for the stereoselective synthesis of cyclic allylic alcohols with a
trisubstituted alkene component.2ꢀ4 For these cyclizations, the
combinations of nickel catalysts and organosilanes,2c,f and rho-
dium catalysts and dihydrogen,3c have been most frequently
employed. Montgomery proposed that the Ni-catalyzed cycliza-
tion using organosilanes proceeds via σ-bond metathesis of the
organosilane SiꢀH bond and the NiꢀO bond of the oxanick-
elacyclopentene intermediate.2c,f Krische proposed that the Rh-
catalyzed cyclization using dihydrogen proceeds via σ-bond
metathesis of the dihydrogen HꢀH bond and the RhꢀO bond
of the oxarhodacyclopentene intermediate.3c Recently, our re-
search group discovered the cationic Rh(I)/(R)-H8-BINAP
complex-catalyzed enantioselective reductive cyclization of γ-
alkynylaldehydes with heteroatom-substituted acetaldehydes,
leading to cyclic allylic esters with a trisubstituted alkene compo-
nent (Scheme 1).5 Mechanistic studies indicated that the reaction
would proceed through oxarhodacyclopentene intermediate A
with the chelating heteroatom-substituted acetaldehyde.5
cyclization of γ-alkynylaldehydes with acyl phosphonates, which
furnishes cyclic R,β-unsaturated ketones with a tetrasubstituted
alkene component6 through CꢀH bond cleavage.
We first examined the reaction of tosylamide-linked γ-alky-
nylaldehyde 1a and diethyl benzoyl phosphonate (2a) in the
presence of the cationic Rh(I)/(R)-H8-BINAP complex (10 mol %).
Gratifyingly, the expected cyclization of 1a with 2a via CꢀP bond
cleavage7,8 proceeded at 80 °C to give cyclic allylic ester 3aa in
good yield with excellent ee (Table 1, entry 1). The effect of
bisphosphine ligands (Figure 1) was then examined, which
revealed that biaryl bisphosphine ligands are effective for this
reaction and yields of 3aa are linearly correlated with dihedral
angles of ligands [dihedral angles,9 H8-BINAP > BINAP >
Segphos; yields of 3aa, H8-BINAP > BINAP > Segphos]
(entries 1ꢀ3), while non-biaryl bisphosphine ligands are totally
ineffective (entries 4 and 5). Interestingly, the use of (R)-
Segphos as a ligand furnished another cyclization product, cyclic
R,β-unsaturated ketone 4aa, via the aldehyde CꢀH bond
cleavage in 24% yield with 98% ee (entry 3). The yield of 3
was significantly decreased and that of 4 was slightly increased by
employing γ-alkynylaldehyde 1b, possessing the n-butyl group at
the alkyne terminus, instead of 1a (entry 6). Importantly, the
correlation between yields of products and dihedral angles of
We anticipated that reaction of the γ-alkynylaldehyde and a
chelating carbonyl compound, possessing the heteroatomꢀ
carbonyl bond (Z/X/Y or Y0 = heteroatom), would furnish
cyclization product C or D through cleavage of the hetero-
atomꢀcarbonyl bond (CꢀY or CꢀY0 bond) in oxarhodacyclo-
pentene intermediate B (Scheme 2). In this Communication,
this hypothesis is embodied as the cationic Rh(I)/(R)-H8-
BINAP complex-catalyzed enantioselective cyclization of γ-
alkynylaldehydes with acyl phosphonates, leading to cyclic allylic
esters with a tetrasubstituted alkene component6 through CꢀP
bond cleavage. In addition, it was found that a cationic Rh-
(I)/(R)-Segphos complex catalyzes another enantioselective
Received: February 11, 2011
Published: April 18, 2011
r
2011 American Chemical Society
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|

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COMMUNICATION
Table 1. Effect of Ligands and Substituents of γ-Alkynylaldehydes 1 and Benzoyl Phosphonates 2
entry
1 (R1)
2 (R2)
ligand
3/% yielda (% ee)
4/% yielda (% ee)
1
1a (Me)
1a (Me)
1a (Me)
1a (Me)
1a (Me)
1b (n-Bu)
1b (n-Bu)
1b (n-Bu)
1a (Me)
1a (Me)
1a (Me)
1b (n-Bu)
1b (n-Bu)
1b (n-Bu)
2a (Et)
2a (Et)
2a (Et)
2a (Et)
2a (Et)
2a (Et)
2a (Et)
2a (Et)
2b (Me)
2c (i-Pr)
2c (i-Pr)
2b (Me)
2c (i-Pr)
2c (i-Pr)
(R)-H8-BINAP
(R)-BINAP
(þ)-3aa/73 (>99)
(þ)-3aa/67 (>99)
(þ)-3aa/55 (>99)
3aa/0
4aa/0
4aa/7b
2
3
(R)-Segphos
(S,S)-DIOP
(ꢀ)-4aa/24 (98)
4aa/0
4c
5c,d
6
(S,S)-Chiraphos
(R)-H8-BINAP
(R)-BINAP
3aa/0
4aa/0
(þ)-3ba/20 (>99)
(þ)-3ba/13 (>99)
3ba/0
(þ)-4ba/26 (97)
(þ)-4ba/29 (97)
(þ)-4ba/67 (99)
4ab/0
7
8
(R)-Segphos
(R)-H8-BINAP
(R)-H8-BINAP
(R)-Segphos
(R)-H8-BINAP
(R)-H8-BINAP
(R)-Segphos
9
(þ)-3ab/73 (>99)
(þ)-3ac/30 (>99)
(þ)-3ac/17 (>99)
(þ)-3bb/26 (>99)
3bc/0
10
11
12
13
14
4ac/10b
(ꢀ)-4ac/60 (>99)
4bb/11b
(ꢀ)-4bc/36 (98)
(ꢀ)-4bc/77 (>99)
3bc/0
a Isolated yield. b NMR yield. The product could not be isolated in a pure form. c For 16 h. d [Rh(nbd)2]BF4 was used.
ligands in the formation of ketone 4ba is opposite to that in the
formation of ester 3aa [yields of 4ba, H8-BINAP < BINAP <
Segphos] (entries 6ꢀ8). The effect of the alkoxy groups of
benzoyl phosphonates was also examined (entries 9ꢀ14): the
use of sterically less demanding dimethyl benzoyl phosphonate
(2b) furnished the corresponding esters 3 as major products
Figure 1. Structures of chiral bisphosphine ligands.
(entries 9 and 12); in contrast, the use of sterically demanding
diisopropyl benzoyl phosphonate (2c) increased the yields of the
corresponding ketones 4 (entries 10 and 13). The combined use
of sterically demanding acyl phosphonate 2c and (R)-Segphos,
possessing the smallest dihedral angle, furnished ketones 4 in the
highest yields (entries 11 and 14).
Table 2. Rh-Catalyzed Enantioselective Cyclization of 1 with
2 via CꢀP Bond Cleavage
Thus, we explored the scope of the enantioselective cyclization
of γ-alkynylaldehydes with acyl phosphonates via the CꢀP bond
cleavage by using 10 mol % of the cationic Rh(I)/(R)-H8-BINAP
complex at 80 °C, as shown in Table 2. With respect to γ-
alkynylaldehydes, 1c and 1a, possessing hydrogen and the methyl
group, respectively, at the alkyne terminus reacted with 2a to give
the corresponding esters in good yields with excellent ee's (entries
1 and 2). On the other hand, aldehydes 1d, 1b, and 1e, possessing
sterically demanding ethyl, n-butyl, and phenyl groups, respec-
tively, at the alkyne terminus, reacted with 2b to give the
corresponding esters in moderate to low yields (entries 3ꢀ5).
Not only tosylamide-linked γ-alkynylaldehydes 1aꢀe but also
ether-linked γ-alkynylaldehyde 1f could participate in this reaction
by using excess 2a (entry 6). With respect to acyl phosphonates,
sterically and electronically diverse aryl groups could be employed
for the carbonyl substituent to give the corresponding esters in
good yields with excellent ee's (entries 7ꢀ11). Alkyl groups could
also be employed for the carbonyl substituent to give the
corresponding esters in good yields with excellent ee's (entries
12ꢀ14). The absolute configuration of (þ)-3ca was unambigu-
ously determined to be S by the anomalous dispersion method.
Reaction of γ-alkynylaldehyde 1e and ketoester 5 instead of acyl
phosphonate 2 was also examined under the same reaction condi-
tions employed in Table 2, but an unidentified complex mixture of
entry
1 (Z, R1)
2 (R3, R2)
2a (Ph, Et)
3
yield (%)aee (%)
1
2
3
4
5
1c (NTs, H)
1a (NTs, Me)
1d (NTs, Et)
(S)-(þ)-3ca
(þ)-3aa
(þ)-3db
(þ)-3bb
(þ)-3eb
(þ)-3fa
64
73
40
26
39
33
67
71
65
65
53
71
69
72
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
2a (Ph, Et)
2b (Ph, Me)
1b (NTs, n-Bu) 2b (Ph, Me)
1e (NTs, Ph) 2b (Ph, Me)
6b 1f [O, (CH2)3Ph]2a (Ph, Et)
7
8
9
1a (NTs, Me)
1a (NTs, Me)
1a (NTs, Me)
2d (2-MeC6H4, Et) (þ)-3ad
2e (4-ClC6H4, Et) (þ)-3ae
2f (4-FC6H4, Et)
(þ)-3af
10 1c (NTs, H)
11 1a (NTs, Me)
12 1a (NTs, Me)
13 1a (NTs, Me)
14 1a (NTs, Me)
2g (4-MeOC6H4, Et)(þ)-3cg
2g (4-MeOC6H4, Et)(þ)-3ag
2h (Me, Et)
2i (Et, Et)
(þ)-3ah
(þ)-3ai
(þ)-3aj
2j (i-Pr, Et)
a Isolated yield. b 2a, 2.5 equiv.
products was generated. At room temperature, the corresponding
dimer 65 was generated with high yield and ee (Scheme 3).10
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COMMUNICATION
Scheme 3
Scheme 4. Possible Mechanism for Formation of 3a
Table 3. Rh-Catalyzed Enantioselective Cyclization of 1 with
2 via CꢀH Bond Cleavage
a Bisphosphine ligand is omitted for clarity except in intermediate E.
Scheme 5. Possible Mechanism for Formation of 4a
entry
1 (Z, R1)
2 (R3, R2)
2c (Ph, i-Pr)
4
yield (%)aee (%)
1
2
3
4
5
6
7
8
9
1c (NTs, H)
1a (NTs, Me)
1d (NTs, Et)
4cc
0
60
74
77
50
0
ꢀ
>99
>99
>99
94
2c (Ph, i-Pr)
2c (Ph, i-Pr)
(ꢀ)-4ac
(ꢀ)-4dc
(R)-(ꢀ)-4bc
(þ)-4ec
4fc
1b (NTs, n-Bu) 2c (Ph, i-Pr)
1e (NTs, Ph) 2c (Ph, i-Pr)
1f [O, (CH2)3Ph]2c (Ph, i-Pr)
1b (NTs, n-Bu) 2a (Ph, Et)
ꢀ
99
(þ)-4ba
67
39
60
52
0
1b (NTs, n-Bu) 2d (2-MeC6H4, Et) (ꢀ)-4bd
1b (NTs, n-Bu) 2e (4-ClC6H4, Et) (ꢀ)-4be
99
99
10 1b (NTs, n-Bu) 2f (4-FC6H4, Et)
(ꢀ)-4bf
99
11 1b (NTs, n-Bu) 2g (4-MeOC6H4, Et)4bg
ꢀ
12 1b (NTs, n-Bu) 2h (Me, Et)
a Isolated yield.
4bh
0
ꢀ
a Bisphosphine ligand is omitted for clarity except in intermediate L.
elimination affords intermediate H, which undergoes reductive
elimination to afford ester 3.
We subsequently explored the scope of the enantioselective
cyclization of γ-alkynylaldehydes with acyl phosphonates via the
aldehyde CꢀH bond cleavage by using 10 mol % of the cationic
Rh(I)/(R)-Segphos complex at 80 °C as shown in Table 3. With
respect to γ-alkynylaldehydes, the reaction of sterically less
demanding terminal γ-alkynylaldehyde 1c and 2c failed to furnish
the corresponding ketone(entry 1). Onthe otherhand, internal γ-
alkynylaldehydes 1a, 1d, 1b, and 1e reacted with 2c to give the
corresponding ketones in moderate to good yields with high ee's
(entries 2ꢀ5). Unfortunately, the reaction of ether-linked γ-
alkynylaldehyde 1f and 2c led to an unidentified complex mixture
of products (entry 6). With respect to acyl phosphonates, not only
phenyl (2a, entry 7) but also sterically demanding o-tolyl (2d,
entry 8) and electron-deficient aryl-substituted acyl phosphonates
(2e and 2f, entries 9 and 10) reacted with 1b to give the
corresponding ketones in moderate to good yields with high ee's.
However, electron-rich aryl- and alkyl-substituted acyl phospho-
nates (2g and 2h, entries 11 and 12) could not participate in this
reaction. The absolute configuration of (ꢀ)-4bc was unambigu-
ously determined to be R by hydrolysis to the known chiral
secondary alcohol, (R)-(þ)-(hydroxyphenylmethyl)phosphonic
acid diisopropyl ester [(R)-(þ)-7].11
Alternatively, activation of the CꢀP bond of 2 with the Rh(I)
catalyst7,8 affords intermediate I, which reacts with 1 to afford
intermediate K through intermediate J. Subsequent reductive elim-
ination of Rh affords ester 3. No productive reaction was observed
under the reaction conditions when substrate 1 was replaced with a
simple alkyne or a simple aldehyde. This result suggests that the
sequence involving intermediates E and H is most likely, since both
the aldehyde and alkyne must be present for reaction to occur.
Scheme 5 depicts possible mechanisms for the formation of
ketone 4. γ-Alkynylaldehyde 1 reacts with the Rh(I) catalyst,
affording oxarhodacyclopentene L with chelating acyl phospho-
nate 2. We believe that coordination mode of 2 to Rh in this
intermediate L is opposite to that in intermediate E, which would
furnish ester 3. β-Hydride elimination to generate intermediate
M followed by insertion of the carbonyl group of 2 affords
intermediate N, which undergoes reductive elimination to afford
(Z)-4. Subsequent double bond isomerization with the cationic
Rh(I) catalyst14 affords (E)-4. Electron-rich aryl- and alkyl-
substituted acyl phosphonates (2g and 2h) failed to react with
1b, presumably due to low reactivity toward the carbonyl
insertion into the less polar RhꢀH bond of intermediate M
compared with the RhꢀO bond of intermediate E.
Scheme 4 depicts possible mechanisms for the formation of
ester 3. γ-Alkynylaldehyde 1 reacts with the Rh(I) catalyst,
affording oxarhodacyclopentene E with chelating acyl phospho-
nate 2. σ-Bond metathesis of the CꢀP bond and the RhꢀO bond
in intermediate F or carbonyl insertion12 into the RhꢀO bond to
generate dioxarhodacycle G13 followed by β-phosphorus
Alternatively, insertion of the carbonyl group of 2 into the
RhꢀC bond of intermediate L to generate dioxarhodacycle P
followed by β-hydride elimination and reductive elimination
would also afford (Z)-4. In addition, activation of the aldehyde
CꢀH bond of 1 to generate intermediate P followed by
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Journal of the American Chemical Society
COMMUNICATION
carborhodation would also furnish intermediate M. However,
these mechanisms are unlikely because they involve an unusual
attack of the (alkenyl)rhodium carbon at the carbonyl oxygen
and hydrorhodation not carborhodation which is common in the
hydroacylation of alkynylaldehydes.15,16
The ligand- and substituent-controlled formation of ester 3 or
ketone 4 might be explained as follows, although the precise
mechanism cannot be determined at the present stage. Increasing
the steric bulk of R1 of 1 and/or R2 of 2 would favor the
formation of intermediate L over intermediate E due to steric
repulsion between substituents R1 and R2. Use of the biaryl
bisphosphine ligand with the small dihedral angle increases the
steric repulsion between R1 of 1 and the equatorial phenyl group
of the ligand, which would also favor the formation of sterically
less demanding intermediate L over intermediate E.
In conclusion, it has been established that a cationic Rh(I)/(R)-
H8-BINAP or (R)-Segphos complex catalyzes two unprecedented
modes of cyclizations of heteroatom-linked γ-alkynylaldehydes with
acyl phosphonates via CꢀP or CꢀH bond cleavage with out-
standing enantioselectivity. These two different reaction pathways
depend on the dihedral angles of the ligands and the substitutents on
both γ-alkynylaldehydes and acyl phosphonates. Future studies will
focus on elucidating the reaction mechanism and expanding the
reaction scope to include chelating carbonyl compounds possessing a
variety of heteroatomꢀcarbonyl bonds.
G. M.; Montgomery, J. J. Am. Chem. Soc. 2005, 127, 13156.(f) Baxter,
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’ ASSOCIATED CONTENT
S
Supporting Information. Procedures, characterization
b
data, and X-ray crystallographic information. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
tanaka-k@cc.tuat.ac.jp
(8) In the Rh-catalyzed hydrogenation of acyl phosphonates, the
cleavage of the CꢀP bond is predominant over the desired reduction of
the carbonyl group: Goulioukina, N. S.; Bondarenko, G. N.; Bogdanov,
A. V.; Gavrilov, K. N.; Beletskaya, I. P. Eur. J. Org. Chem. 2009, 510.
(9) Shimizu, H.; Nagasaki, I.; Saito, T. Tetrahedron 2005, 61, 5405.
(10) We reported that the cationic Rh(I)/H8-BINAP complex can
catalyze enantio- and diastereoselective [2þ2þ2] cycloaddition of 1,6-
enynes with ketoesters: Tanaka, K.; Otake, Y.; Sagae, H.; Noguchi, K.;
Hirano, M. Angew. Chem., Int. Ed. 2008, 47, 1312.
’ ACKNOWLEDGMENT
This work was supported partly by Grants-in-Aid for Scientific
Research (Nos. 20675002 and 23105512) from MEXT, Japan.
We deeply appreciate Takasago International Corp. for the
gift of H8-BINAP and Segphos, Umicore for generous support
in supplying a rhodium complex, and reviewers for valuable
suggestions regarding the reaction mechanism.
(11) Nakanishi, K.; Kotani, S.; Sugiura, M.; Nakajima, M. Tetrahe-
dron 2008, 64, 6415.
(12) Cationic Rh(I) complex-catalyzed [2þ2þ2] and [4þ2] cy-
cloadditions involving acyl phosphonates: (a) Tanaka, K.; Tanaka, R.;
Nishida, G.; Hirano, M. Synlett 2008, 2017. (b) Tanaka, K.; Tanaka, R.;
Nishida, G.; Noguchi, K.; Hirano, M. Chem. Lett. 2008, 37, 934.
(13) Alternatively, the formation of intermediate G via the coupling
of Rh with 1 and 2 to generate dioxarhodacycle followed by alkyne
insertion cannot be excluded. Ni-catalyzed Tishchenko reactions pre-
sumably through dioxanickelacycles: (a) Ogoshi, S.; Hoshimoto, Y.;
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M.; Ogoshi, S. J. Am. Chem. Soc. 2011, 133, 4668.
(14) Cationic Rh(I) complex-catalyzed E/Z isomerization of R,
β-unsaturated carbonyl compounds: Tanaka, K.; Shoji, T.; Hirano, M.
Eur. J. Org. Chem. 2007, 2687.
(15) Cationic Rh(I)/BINAP complex-catalyzed intramolecular hy-
droacylation (and subsequent olefin isomerization) of methylene-linked
γ-alkynylaldehydes: Takeishi, K.; Sugishima, K.; Sasaki, K.; Tanaka, K.
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