Palladium-Catalyzed Insertion of α-Diazoesters into Vinyl Halides To Generate α,β-Unsaturated γ-Amino Esters

Full text of DOI:10.1002/anie.200805483

Angewandte
Chemie
DOI: 10.1002/anie.200805483
Synthetic Methods
Palladium-Catalyzed Insertion of a-Diazoesters into Vinyl Halides To
Generate a,b-Unsaturated g-Amino Esters**
Romas Kudirka, Sean K. J. Devine, Christopher S. Adams, and David L. Van Vranken*
Palladium-catalyzed insertion of trimethylsilylcarbenes,
derived from trimethylsilyldiazomethane, into vinyl halides
has recently been shown to be a powerful method for the
formation of vinylsilanes with allylic amino groups.[1] The
analogous process, when carried out with a-diazoesters,
would provide direct access to chiral a,b-unsaturated g-
amino acids; such as those found in various bioactive natural
products, like miraziridine A (Scheme 1),[2] and in synthetic
for trimethylsilyldiazomethane; albeit in lower yields. Two
potential competing pathways make palladium-catalyzed
insertions using a-diazoesters more challenging than the
corresponding reactions of trimethylsilyldiazomethane:
firstly, cross-coupling of the vinyl halide and the diazo
compound can occur,[8] secondly, palladium chloride can
catalyze polymerization of ethyl diazoacetate.[9] In the
unoptimized reaction, there was a significant amount of
unreacted starting material (1a; Table 1, entry 1). We
Table 1: Optimization of the three-component coupling reaction to
generate g-aminoesters.
Entry
Equivalents
of amine
Equivalents
of EtO2CCHN2
t [h]
T [8C]
Yield [%]
1
2
3
4
5
6
7
8
3
3
3
3
3
3
1.5
1.5
1.5
3
10
10
5
20
24
24
24
24
46
46
66
66
66
66
66
ꢀ30
31
34
41
65
Scheme 1. Three-component coupling of amines, vinyl iodides, and
ethyl diazoacetate to generate g-amino acid derivatives.
4/4[a]
2
90
94
inhibitors.[3] Peptides composed solely of g-amino acids can
readily form secondary structures and tend to adopt helical
conformations, even in protic solvents.[4] a,b-Unsaturated g-
amino esters are generally made through olefination of the
corresponding N-protected a-amino aldehydes;[5] the a-
amino aldehydes can be highly sensitive to racemization, so
they are generally prepared through reduction of N-protected
a-amino acids to give a-amino alcohols, which are then
reoxidized to the aldehydes under mild reaction conditions.[5]
A modular method for the direct synthesis of g-amino esters
would be highly valuable, particularly if applicable to non-
natural amino acid side chains (Scheme 1).
[a] The EtO2CCHN2 was added over the first 4 hours, and was followed by
an additional 4 hours of stirring. dba=trans,trans-dibenzylideneacetone,
THF=tetrahydrofuran.
hypothesized that the large excess of nucleophilic amine
could deactivate the catalyst through direct ligation or
formation of chelating by-products. To test this hypothesis
we lowered the stoichiometry of the amine from eight to three
equivalents, and resulted in a slight improvement in yield
(Table 1, entry 2). To improve catalyst turnover the reaction
temperature was increased to 668C, thus resulting in another
slight increase in yield (Table 1, entry 3). To consume more of
this starting material, we added an additional 1.5 equivalents
of diazoester, which led to a further improvement in
conversion of vinyl iodide 1a into product (Table 1,
entry 4). Since additional EDA seemed to exert a beneficial
effect on the yield, the reaction was repeated using 10 equiv-
alents of EDA. Although the yield was further improved to
65% (Table 1, entry 5), none of the remaining vinyl iodide
was recovered. a-Diazoesters are known to undergo
[3+2] cycloadditions with a,b-unsaturated esters.[10] Indeed,
when the enoate 2a was heated with 2 equivalents of ethyl
diazoacetate in THF over 24 hours, none of the enoate could
There is growing interest in palladium-catalyzed carbene
insertion reactions.[6,7] In previous work involving palladium-
catalyzed three-component coupling to generate vinylsilanes,
we noted that ethyl diazoacetate (EDA) could be substituted
[*] R. Kudirka, S. K. J. Devine, C. S. Adams, Prof. D. L. Van Vranken
Department of Chemistry
University of California at Irvine
1102 Natural Sciences 2, Irvine, CA 92697-2025 (USA)
Fax: (+1)949-824-8571
E-mail: david.vv@uci.edu
[**] This work was supported by the ACS PRF 42780-AC1.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805483.
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Table 2: Variation of the amine nucleophile.
be recovered, thus confirming that the enoate product is
sensitive to ethyl diazoacetate.
These results seem to suggest that extended exposure of
the product to EDA under these reaction conditions leads to
destruction of the product. To test this hypothesis, 10 equiv-
alents of EDA were added over 4 hours; as GC-MS analysis
showed the presence of unreacted vinyl iodide, the reaction
was allowed to continue for an additional 4 hours. The result
was a satisfying 90% yield of the desired allylamine (Table 1,
entry 6). Slow addition of the EDA was essential; when all
10 equivalents of EDA were added immediately and the
reaction was allowed to stir for 7 hours, the yield of the
product was only 24% and the remaining starting material
was recovered. Therefore, the palladium catalyst and/or the
ethyl diazoacetate is depleted by bolus addition of the diazo
compound. To optimize the yield of the allylamine it was best
to stop the reaction as soon as the vinyl iodide was consumed.
Typically this point was reached after five to seven equiv-
alents of the diazo compound had been added over about
2.5 hours. Under these reaction conditions, the desired allyl-
amine 2a was generated in 94% yield (Table 1, entry 7).
We hypothesize that the reaction (Scheme 2) starts with
oxidative addition of palladium to the vinyl halide to generate
vinylpalladium complex A, and subsequent formation of a
Entry
Conditions[a]
Product
Yield [%]
1
2
A
B
2a
2a
94
62
3
4
A
B
A
A
B
A
A
B
2b
2b
2b
2c
2c
2c
2d
2d
40
34
68
51
28
71
75
61
5[b]
6
7
8[b]
9
10
11[b]
A
2e
91
12
A
B
B
A
B
2 f
2 f
2 f
2g
2g
55
71
80
42
62
13
14[b]
15
16
[a] Conditions A: R1R2NH (3 equiv); Conditions B: R1R2NH (1 equiv),
Et3N (2 equiv). [b] [Pd2(dba)3·CHCl3] (5 mol%) was used. Bn=benzyl.
groups can be readily removed from amides under acidic
conditions. The cyclic secondary amines piperidine and
pyrrolidine are slightly less effective than morpholine
(Table 2, entries 12 and 15).
The yield of the desired g-amino ester seemed to be
inversely correlated with the basicity (and nucleophilicity) of
the secondary amine: morpholine (pKa = 7.41) < BnNHMe
(pKa = 9.34) < piperidine (pKa = 11.22) < pyrrolidine (pKa =
11.27). To reduce the nucleophilicity of the reaction medium,
we explored an alternative set of reaction conditions that
employed one equivalent of the amine nucleophile along with
two equivalents of triethylamine (pKa = 10.65) to help
neutralize the HI generated in the reaction. These alternative
reaction conditions proved to be better for amines that are
more basic than triethylamine (Conditions B; Table 2,
entries 13 and 16), but not for amines that are less basic
than triethylamine (Table 2, entries 2, 4, 7, and 10). The vinyl
iodide was not consumed, regardless of how much EDA was
added; therefore we attributed catalyst deactivation as the
cause of the low yields. However, the yield was improved by
increasing the catalyst loading (Table 2, entries 5, 8, 11, and
14). To ensure efficiency, all of the subsequent reactions were
carried out using 2.5 mol% of palladium catalyst, unless
otherwise noted.
Scheme 2. Proposed mechanism for the palladium-catalyzed reaction.
L=ligand, Nu=nucleophile, X=halide.
palladium carbene B.[11] Migration of the vinyl ligand to the
empty p orbital of the carbene ligand generates the h1-
allylpalladium complex C. Migratory insertion is well pre-
cedented for CO ligands, but has only recently been
demonstrated for palladium carbenes.[12] This step sets the
absolute configuration in the reaction. Presumably, the h1-
allylpalladium complex generates an h3-allylpalladium inter-
mediate D that is then attacked by the amine nucleophile at
the position distal from the ester group.[13]
With the reaction conditions now optimized for use of
morpholine (Conditions A; Table 2, entry 1) as both nucleo-
phile and base, we set out to explore the generality of the
amine nucleophiles. Benzylamine and 2,4-dimethxoxybenzyl-
amine gave modest yields of the desired coupling product
under the general reaction conditions (Table 2, entries 3 and
6), but N-methylbenzylamine proved to be more effective
(Table 2, entry 9). We note that 2,4-dimethoxybenzyl (DMB)
We next set out to evaluate the effect of substituents on
the vinyl iodide component. Not surprisingly, silyl-protected
ethers perform well in the palladium-catalyzed coupling
reaction (Table 3). The trans substrate (E)-1b fared poorly
relative to the cis substrate (Z)-1b; as observed in previous
studies of palladium-catalyzed carbene insertion reactions.[1]
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Table 3: Variation of the vinyl iodide.
entry 3). tert-Butyl esters are readily deprotected under mild
acidic conditions.
To demonstrate the utility of the g-amino acids, which can
be generated by this new three-component coupling reaction,
we demonstrated that the DMB-protected amine 2c could be
coupled with FmocGlyOH and the DMB group removed to
afford an Fmoc-protected dipeptide (Scheme 3).
Entry
1
Vinyl iodide
Product
Yield [%]
90
(Z)-1b
(E)-1b
3b
3b
2[a]
18
3
1c
1c
3c
3c
33
67
4[b]
Scheme 3. Formation of peptide bonds with DMB-protected amino
acids. Fmoc=9-fluorenylmethyloxycarbonyl, HATU=O-(7-azabenzo-
triazol-1-yl)-N,N,N’,N’-tetramethyluronium hexafluorophosphate,
NEM=N-ethylmorpholine, TFA=trifluoroacetic acid.
5
1d
3d
91
[a] EtO2CCHN2 (10 equiv) was added over 4 hours. [b] [Pd2(dba)3·CHCl3]
(5 mol%) was used. TBS=tert-butyldimethylsilyl.
In conclusion, we have developed a powerful new method
for the synthesis of a,b-unsaturated g-amino esters through a
palladium-catalyzed three-component coupling reaction. The
key step in the catalytic cycle is believed to involve migratory
insertion of a carbene unit into a vinyl-palladium bond to
generate an h3-allylpalladium intermediate. This unique
transformation is important because it complements the
multistep synthesis of a,b-unsaturated g-amino esters from a-
amino acids and it can be used to synthesize variants with
both natural and non-natural side chains.
The internal iodoalkene 1c gave the corresponding g-amino
ester 3c as the Z stereoisomer in only 33% yield. This pattern
of stereoselection has previously been observed in palladium-
catalyzed allylic alkylations.[14] Nitrile substituents appear to
be well-tolerated in the reaction and led to a high yield of g-
amino ester 3d. As triphenylphosphine was used as the ligand
in these reactions, all of the products (except 3c) were
generated as racemic mixtures. However, the wide range of
available chiral phosphine ligands offers the promise of
straightforward development of an enantioselective variant of
this reaction.
Received: November 10, 2008
Revised: February 27, 2009
Published online: April 7, 2009
Next we compared the reaction of related a-diazo esters
(Table 4). The extra methyl group of ethyl a-diazopropio-
nate[15] clearly lowered the efficiency of the reaction, and
afforded the product as a 2:1 mixture of E and Z isomers
(Table 4, entries 1 and 2). tert-Butyl diazoacetate was also
effective in the reaction when a larger excess of the diazo
compound was added over a longer period of time (Table 4,
À
Keywords: amino acids · C C coupling · diazo compounds ·
palladium · peptide mimics
.
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Diazo substrate
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[%]
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[equiv]
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H
Et
ꢁ5
3
2
94
Me Et
1.5
6
53[a]
83
H
tBu
10
[a] 2:1 mixture of E and Z isomers.
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