Palladium-catalyzed asymmetric [3 + 2] trimethylenemethane cycloaddition reactions

Article abstract of DOI:10.1021/ja0640750

Transition-metal-catalyzed trimethylenemethane (TMM) [3 + 2] cycloadditions provide direct routes to functionalized cyclopentanes. This reaction has been shown to be a highly chemo-, regio-, and diastereoselective process. We report a palladium-catalyzed

Full text of DOI:10.1021/ja0640750

Published on Web 09/22/2006
Palladium-Catalyzed Asymmetric [3 + 2] Trimethylenemethane Cycloaddition
Reactions
Barry M. Trost,* James P. Stambuli, Steven M. Silverman, and Ulrike Schwo¨rer
Department of Chemistry, Stanford UniVersity, Stanford, California 94305-5080
Received June 9, 2006; E-mail: bmtrost@stanford.edu
Scheme 1
Natural products containing five-membered rings are found
throughout the chemical literature. Transition metal-catalyzed [3
+ 2] trimethylenemethane (TMM) cycloadditions provide direct
routes to substituted cyclopentanes.1 In 1979, we first reported that
Pd-TMM complexes generated from 3-acetoxy-2-trimethylsilyl-
methyl-1-propene (1) and catalytic palladium react with electron-
deficient olefins to produce exo-methylenecyclopentanes (eq 1).2
We have shown that this palladium-catalyzed [3 + 2] TMM
cycloaddition reaction is a highly chemo-, regio-, and diastereo-
selective process.3,4 This methodology has also been extended to
[4 + 3] and [6 + 3] cycloadditions5,6 and has been employed in a
number of total syntheses.3,7-9 Unfortunately, the formation of
stereogenic centers in the cycloadducts produced in this chemistry
has mainly been limited to the use of chiral auxiliaries.10 The sole
example of a palladium-catalyzed asymmetric [3 + 2] sulfone-
substituted TMM cycloaddition was reported from the Hayashi lab
with enantiomeric excesses ranging from 4 to 78% ee.11
Scheme 2
The difficulty in designing a catalyst for the asymmetric Pd-
TMM cycloaddition reaction is illustrated in our proposed stepwise
mechanism (Scheme 1).12,13 The formation of the zwitterionic
intermediate is generated by insertion of LnPd into the C-O bond
followed by attack of the displaced acetate anion onto the
trimethylsilyl group, which deposits its electrons onto the TMM
fragment.14 The enantiodetermining step is most likely the initial
nucleophilic attack. This step occurs distal to the set of coordinated
chiral ligands. Our early work on the beneficial effect of phosphite
and phosphorustriamine ligands on the efficiency of the cycload-
dition induced us to examine palladium complexes of chiral
phosphoramidites15a as asymmetric catalysts.
Initially, we examined the reaction of 1 with methyl cinnamate
in the presence of 5 mol % of Pd(dba)2 and 10 mol % ligand (L1-
L6) in toluene at 23 °C (Scheme 2). Interestingly, catalysts formed
from L1, L4, or L5 gave 0% yield; however, catalysts formed from
L2 gave 78% yield and 29% ee. We thought an increase in the
rigidity of L2 may lead to an increase in % ee. Indeed, the synthesis
of L615b and its application in the asymmetric TMM [3 + 2]
cycloaddition gave 80% yield and 58% ee of the desired cyclo-
pentane product.
Although toluene has typically been used in previous TMM [3
+ 2] reactions, we decided to screen a number of solvents to
examine the effect on the yield and selectivity of this process (Table
1). Reactions conducted in THF, ether, and DME gave good yields
(68-80%) with modest (48-55% ee, entries 2-4) selectivities,
while reactions conducted in DCM, CH3CN, DMF, or dioxane gave
low yields of the desired product (entries 5-8). Thus, toluene
appears optimal.
Using the optimized catalytic conditions, we examined the initial
scope of the TMM accepting olefin (Table 2). All cyclopentane
products were formed in >19:1 trans:cis selectivity. The reaction
of methyl cinnamate with 1 at 0 °C increased the enantioselectivity
from 58 (at 23 °C) to 62%. Cooling the reaction to -25 °C failed
to produce >5% product as determined by GC analysis. Employing
the more reactive (E)-benzalacetone allowed the reaction to be
performed at -25 °C, which increased selectivity to 82% ee (entry
2). Lowering the reaction temperature to -50 °C failed to produce
any noticeable product by GC analysis. The selectivity of this
reaction does not rely on the presence of the phenyl group as trans-
nonenone reacts with yields and enantiomeric excesses that are
comparable to those of benzalacetone (entry 3). Switching the
methyl group to an ethyl (entry 4) or phenyl (entry 5) gives yields
and enantiomeric excesses that are comparable to those of benza-
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10.1021/ja0640750 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Solvent Effect on Asymmetric TMM [3 + 2]
Cycloadditions
Table 3. Palladium-Catalyzed [3 + 2] TMM Cycloaddition
Reactions of Aryl- and Alkylidene Tetralones
entry
solvent
% yielda
% ee
1
2
3
4
5
6
7
8
toluene
THF
DME
80
68
80
69
10
0
58
48
53
55
32
0
ether
DCM
CH3CN
DMF
0
35
0
39
dioxane
a Determined by GC analysis with an internal standard.
Table 2. Initial Scope of Palladium-Catalyzed [3 + 2] TMM
Cycloaddition Reactions
a Isolated yields.
(entries 2 and 3). Cycloalkyl-substituted tetralones were also good
substrates (entries 4 and 5); however, cyclohexylidene tetralone
failed to react under our conditions at room temperature.
In summary, we have developed conditions to effect an asym-
metric palladium-catalyzed [3 + 2] cycloaddition reaction between
in situ generated Pd-TMM intermediates and various olefins,
despite the large distance between the chiral ligands and the
asymmetric bond-forming event. Future work will focus on
extending this methodology to include [4 + 3] and [6 + 3]
cycloadditions, as well as developing catalysts that promote
asymmetric [3 + 2n] cycloaddition reactions with substituted TMM
precursors.
Acknowledgment. We thank the NIH (GM13598) and the NSF
for their generous support of our programs. Palladium salts were a
generous gift from Johnson-Matthey. Mass spectra were provided
by the Mass Spectrometry Regional Center at the University of
CaliforniasSan Francisco, supported by the NIH Division of
Research Resources. J.P.S. thanks the NIH (GM069269-02) for a
postdoctoral fellowship.
Supporting Information Available: Experimental details for
procedures and spectral data for all unknown compounds (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
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a Isolated yields.
lacetone. Use of electron-rich or electron-poor aromatic enones
slightly lessened the enantioselectivity with respect to the parent
chalcone. Nitriles were compatible functionalities and gave enan-
tioselectivities comparable to those of esters (entry 7).
The reaction of 1 with various tetralone derivatives in the
presence of Pd(dba)2 and L6 gave high yields and enantiomeric
excesses of the corresponding exo-methylenecyclopentanes. The
reaction of benzylidene tetralone at 23 °C gave 99% yield and 85%
ee (entry 1). Reducing the temperature to -25 °C gave 94% yield
and 92% ee. Exchanging the phenyl group for six- and five-
membered aromatics did not have a large effect on the reaction
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