Efficient syntheses of four chiral phenylcyclopropanes

Article abstract of DOI:10.1139/V08-033

An efficient four-step synthetic route from each of four crystalline isotopically labeled (1R)-menthyl (1S,2S)-(+)-2-phenylcyclopropanecarboxyates of better than 99% enantiomeric excess has provided multi-gram quantities of the corresponding chiral phenylcyclopropanes. The key transformation involved a bis[1,3-bis(diphenylphosphino)propane]rhodium chloride that catalyzed a highly stereoselective decarbonylation of a trans-2-phenylcyclopropanecarboxaldehyde.

Full text of DOI:10.1139/V08-033

395
Efficient syntheses of four chiral
phenylcyclopropanes
John E. Baldwin and Stephanie R. Singer
Abstract: An efficient four-step synthetic route from each of four crystalline isotopically labeled (1R)-menthyl (1S,2S)-
(+)-2-phenylcyclopropanecarboxyates of better than 99% enantiomeric excess has provided multi-gram quantities of the
corresponding chiral phenylcyclopropanes. The key transformation involved a bis[1,3-bis(diphenylphosphino)propane]rho-
dium chloride that catalyzed a highly stereoselective decarbonylation of a trans-2-phenylcyclopropanecarboxaldehyde.
Key words: cyclopropanes chiral thanks to isotopic substituents, catalytic decarbonylations of
cyclopropanecarboxaldehydes.
Résumé : Une méthode de synthèse efficace et en quatre étapes pour chacun des quatre (1S,2S)-(+)-2-
phénylcyclopropanecarboxylates de (1R)-menthyle isotopiquement marqués avec un excès énantiomérique de plus de 99 %
a permis de préparer des quantités supérieures au gramme des phénylcyclopropanes chiraux correspondants. La transfor-
mation clé implique une réaction très stéréosélective de décarbonylation d’un trans-2-phénylcyclopropanecarboxaldéhyde
catalysée par un chlorure de bis[1,3-bis(diphénylphosphino)propane]rhodium.
Mots-clés : cyclopropanes chiraux en raison de substituants isotopiques, décarbonylation de cyclopropanecarboxaldéhydes.
[Traduit par la Rédaction] Baldwin and Singer 400
Introduction
phenylcyclopropanes, (1S,2R)-1-d1, (1R,2R)-1-d1 and
(1R,2S,3R)-1-d3 (2). The present work extends, and greatly
improves upon these earlier synthetic endeavors.
This article reports the preparation of four chiral isotopi-
cally labeled phenylcyclopropanes (1-di). In the representa-
tions of structures 1-di, and elsewhere, the asterisk symbol
(*) denotes a carbon-13 atom.
To make multi-gram quantities of each of the 1-di com-
′
pounds (1-d1, 1-d2, 1-d3, and 1-d3 ) with reasonable effi-
ciency would require thrifty preparations of chiral
intermediates of high enantiomeric purity, high
stereochemical integrity, and high incorporations of the req-
uisite labels. Once these demands were satisfied, a func-
tional group temporarily used for securing a specific
enantiomer of a given diastereomer of high stereochemical
purity would need to be replaced by a hydrogen, or possibly
a deuterium atom.
The first of these two desiderata has been achieved: each of
A long-standing interest in mechanistic questions posed
by the thermal chemistry exhibited by relatively simple hy-
drocarbons, including cyclopropanes (1), led years ago to the
preparation and utilization in mechanistic studies of 1-d3
and three other isotopically labeled chiral
the
four
crystalline
(1R)-menthyl
(1S,2S)-(+)-2-
phenylcyclopropanecarboxyates (2-di) has been prepared effi-
ciently on a 20 g scale (3). No trace of a (1R)-menthyl
(1R,2R)-(+)-2-phenylcyclopropanecarboxyate was detected by
capillary GC analyses in once recrystallized 2-di compounds.
′
Received 14 November 2007. Accepted 12 February 2008. Published on the NRC Research Press Web site at canjchem.nrc.ca on
1 April 2008.
In Memoriam, Professor Keith Yates (1928–2006).
J.E. Baldwin1 and S.R. Singer. Department of Chemistry, Syracuse University, Syracuse, NY 13244, USA.
Can. J. Chem. 86: 395–400 (2008)
doi:10.1139/V08-033
© 2008 NRC Canada

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Can. J. Chem. Vol. 86, 2008
Scheme 1. Reaction sequence to convert (1R)-menthyl (1S,2S)-
(+)-2-phenylcyclopropanecarboxyate to phenylcyclopropane (R =
(1R)-menthyl).
The first three steps outlined in Scheme 1, starting with
enantiomerically pure 2-di esters, were implemented without
difficulty and in high yields. The two-step hydrolysis-then-
reduction sequence converting a 2-di ester to the correspond-
ing colorless distilled 4-di alcohol was completed with an
average overall yield of 92% 3%.
To satisfy the second requirement, a conventional reaction
sequence was projected: hydroylysis of a menthyl ester to
the corresponding carboxylic acid (4), reduction of the acid
to a trans-2-phenylcyclopropanemethanol (5), oxidation of
the alcohol to an aldehyde with PCC (pyridinium
chlorochromate), and finally
a
Rh-catalyst-promoted
decarbonylation. Not surprisingly, the first three steps were
easily accomplished, while the decarbonylation proved chal-
lenging.
Results and discussion
The reaction sequence used to prepare chiral
′
phenylcyclopropanes (1-d1, 1-d2, 1-d3, and 1-d3 ) is summa-
rized without full detail in Scheme 1 for unlabeled struc-
tures. Earlier preparations of chiral isotopically labeled
phenylcyclopropanes, such as 1-d3′ and stereoisomers
(1S,2R)-1-d1, (1R,2R)-1-d1, and (1R,2S,3R)-1-d3, depended
on this scheme, though two limitations led to far from opti-
mal overall yields (2, 5).
The first limitation was inherent in the presupposition of
Scheme 1 that the starting material 2 or at least intermedi-
ates 3, 4, and 5 would be enantiomerically pure. One enan-
tiomer of acid 3 may be secured through a chiral copper
catalyst promoted diastereoselective and enantioselective
condensation of racemic menthyl α-deuteriodiazoacetate and
styrene or an isotopically labeled styrene, followed by a sep-
aration of cis and trans diastereomers, and then fractional
crystallization of quinine salts of the trans acids (2, 5). This
sequence of steps works, but it is both tedious and ineffi-
cient. This first limitation has been overcome by adopting
and developing an efficient route to crystalline,
The oxidation of a 4-di alcohol with PCC in CH2Cl2 af-
forded the related aldehyde, essentially quantitatively; it was
typically stored briefly under argon and decarbonylated as
promptly as possible. The all-important decarbonylation step
was not so easily dispatched.
Decarbonylations of aldehydes using RhCl(Ph3P)3 in
stoichiometric proportions are generally successful for aryl,
alkenyl, and primary alkyl aldehydes at modest temperatures
(8). They involve a reversible dissociation of the reagent to
RhCl(PPh3)2 and PPh3 as a key step, and usually take place
with excellent stereochemical selectivity, with the retention
of configuration (9). Other reactions may sometimes inter-
vene, but are not of significant concern in the present con-
text. The co-product of the decarbonylation reaction,
RhCl(CO)(PPh3)2, is relatively stable thermally at normal re-
action temperatures; the thermal dissociation of
RhCl(CO)(PPh3)2 to give CO and RhCl(PPh3)2 is typically
too slow to facilitate a catalytic decarbonylation process.
Primary alkyl aldehydes may be decarbonylated catalyti-
cally in high yields when combined with only 5% of
′
enantiomerically pure esters (2-d1, 2-d2, 2-d3, and 2-d3 ) (3).
The second limitation in earlier work was associated with
the decarbonylation step: decarbonylations of trans-2-
phenylcyclopropanecarboxaldehydes using stoichiometric
amounts
of
chlorotris(triphenylphosphine)rhodium(I)
(Wilkinson’s catalyst) (6–8) were realized (in 8 reactions)
with yields ranging from 42% to 72% (55% 12% on aver-
age) on scales affording from 124 to 630 mg of a labeled
phenylcyclopropane product (330
164 mg on average).
Given the investments required to secure 20 g amounts of in-
′
termediates 2-d1, 2-d2, 2-d3, and 2-d3 , and the cost of com-
Wilkinson’s catalyst and a stoichiometric portion of
mercial RhCl(PPh3)3, a catalytic and more efficacious
decarbonylation reaction option seemed essential.
diphenylphosphoryl azide, but more sterically crowded alde-
hydes are relatively inert (10).
© 2008 NRC Canada

Baldwin and Singer
397
Scheme 2. Catalytic decarbonylation of an aldehyde (R-C(O)H)
When a labeled aldehyde, 5-di, was combined with a catalyst
prepared from 3.6 mol% of Cl(CO)Rh(PPh3)2 and 7.2 mol%
of dppp in anhydrous, degassed toluene, and heated at reflux
for about a day, analysis by capillary GC revealed that the
decarbonylation reaction was progressing slowly. An addi-
tional increment of catalyst was prepared and added, and the
reaction mixture was heated at reflux for another day. A
third increment of catalyst was prepared and added. After
another day at reflux, all of the aldehyde had reacted, ac-
cording to GC analyses. Following workup, the labeled
phenylcyclopropane was secured in good yields, with very
little contamination by the side-product α-methylstyrene. In
three of the eight decarboxylation reactions, two from each
of the labeled phenylcyclopropanecarboxaldehydes, four or
five increments of catalyst and additional days of heating the
reaction mixture at reflux were invested, prompted by indi-
cations of incomplete reactions gained from GC analyses.
The annoyingly inconsistent rates of decarbonylation may be
associated with various intrusions of adventitious oxygen,
which binds reversibly but tightly to [Rh(dppp)2]+, and stalls
but does not shut down the catalytic cycle (17).
by [Rh(dppp)2]+.
Proton NMR spectra of 1-d1, 1-d2, 1-d3, and 1-d3′ were
completely consistent with expectations. For 1-d2, for exam-
ple, the three equal-intensity cyclopropyl H multiplet ab-
sorptions were observed at δ 1.97, 1.02, and 0.78 (Fig. 1). In
1-d3, the upfield absorption of the H cis to phenyl at δ 0.78
is nearly absent, and the two cyclopropyl H doublets for cis
related hydrogens at C1 and C3 are separated by 0.95 ppm.
In 1-d3′, the spectrum has the expected pair of doublets for
trans disposed hydrogens at C2 and C3 and no significant
absorption intensity at δ 1.97. For 1-d1, the spectrum is com-
plicated just as one might anticipate from 13C-H spin–spin
couplings.
Bisdiphosphine complexes of Rh(I), such as [bis(1,3-
bisdiphenylphosphino)propane]rhodium
chloride
or
tetrafluoroborate, [Rh(dppp)2]Cl or [Rh(dppp)2]BF4 (11, 12),
will decarbonylate an aldehyde following a mechanistic path
much like one postulated for Wilkinson’s catalyst mediated
decarbonylations. The reductive elimination step gives a
[Rh(dppp)2]+CO complex that dissociates relatively easily to
[Rh(dppp)2]+, and the process can continue, as schematized
in the putative mechanistic model outlined in Scheme 2.
The [Rh(dppp)2]+ structure is probably not square planar
but distorted so as to position the two P-Rh-P planes at a di-
The eight decarbonylation reactions (Scheme 3), two from
each 5-di isotopomer, gave phenylcyclopropanes 1-di in 79%
yields, on average, from the precursor alcohols 4-di. The cat-
alytic decarbonylation reactions were slow but effective. The
four 1-di chiral phenylcyclopropanes prepared, secured in
quantities ranging from 5.4 to 5.9 g, should prove sufficient
for projected stereochemical and mechanistic studies.
The synthetic project reported in this work realized its ob-
jective, but surely did not validate an optimal experimental
protocol for the catalyzed decarbonylation reactions. The
goal was to make useful amounts of the required chiral iso-
topically labeled phenylcyclopropanes efficiently, not to per-
fect the decarbonylation step. Work in that direction might
well find that employing [Rh(dppp)2]Cl or [Rh(dppp)2]BF4
directly, rather than making a catalyst in situ, would bring
better yields or shorter reaction times; perhaps another sol-
vent at another reaction temperature rather than toluene at
reflux would be preferred; perhaps different and more rigor-
ous measures to keep oxygen from the catalyst at every stage
would prove beneficial. A systematic optimization effort
could be most useful, but even now an efficient (though
+
slow)
catalytic
decarbonylation
procedure
for
cyclopropanecarboxaldehydes, one that should work for
other secondary aldehydes, has been exemplified. Especially
for reactions on a multi-gram scale; it should prove a practi-
cal and valuable synthetic method, and in certain situations
make possible mechanistic studies that otherwise would re-
main imagined but not undertaken.
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Can. J. Chem. Vol. 86, 2008
1
Fig 1. H NMR spectra of chiral phenylcyclopropanes 1-d1, 1-d2,
1-d3, and 1-d3′ acquired on a 300 MHz spectrometer; × absorp-
tions = H2O.
Scheme 3. Stereoselective Rh-catalyzed decarbonylation reactions.
mixture was stirred for an additional 30 min, dried over
K2CO3, and filtered through Celite. The solid collected over
Celite was washed with THF (~75 mL) and the filtrates were
combined and concentrated by distillation. The colorless liq-
uid remaining was purified by Kugelrohr distillation at 75–
95 °C under reduced pressure. The colorless oil collected
Experimental section
trans-2-Phenylcyclopropanemethanol
1
(4.45 g, 93% yield) had a H NMR spectrum matching the
The following general procedures for the hydrolysis of
(1R)-menthyl (1S,2S)-(+)-2-phenylcyclopropanecarboxylate
was used to prepare acids 3-d1, 3-d2, 3-d3, and 3-d3′ from es-
ters 2-d1, 2-d2, 2-d3, and 2-d3′, and to reduce these acids to
4-d1, 4-d2, 4-d3, and 4-d3′.
reference data for trans-2-phenylcyclopropanemethanol (5).
In 4-d2 and 4-d3′ the C3-H cis to phenyl was seen at δ 0.95
(1H); in 4-d1, 4-d2, and 4-d3 the benzylic H was recorded at
δ 1.81–1.85 (1H).
(1R)-Menthyl (1S,2S)-(+)-2-phenylcyclopropanecarboxylate
(9.65 g, 31.9 mmol) suspended in 300 mL of CH3OH was
stirred for 30 min. Aqueous 25% NaOH (56 g) was added,
and the reaction mixture was heated at reflux for 21 h. It was
cooled to RT and concentrated by rotary evaporation. The
white residue was dissolved in 225 mL of H2O and the basic
solution was extracted with ether (3 × 100 mL). The aque-
ous layer was separated, acidified with concd HCl (~30 mL),
and extracted with ether (4 × 100 mL). The ether extracts
were combined, dried (MgSO4), and filtered. The colorless
filtrate was concentrated by distillation to remove all but
about 30 mL of the ether.
This concentrate was added dropwise by addition funnel
along with anhydrous THF (~30 mL, used to rinse out the
flask) to a suspension of LiAlH4 (3.03 g, 79.8 mmol) in an-
hydrous THF (250 mL) under argon. The resulting gray sus-
pension was heated at reflux for 42 h, cooled in an ice bath,
and quenched with H2O (3.0 g), 15% aqueous NaOH
(3.03 g), and H2O (9.09 g), with 5 min of stirring the
quenched reaction mixture after each addition. The resulting
trans-2-Phenylcyclopropanecarboxaldehydes
trans-2-Phenylcyclopropanecarboxaldehydes (5-di) were
prepared as described in this specific example, starting from
4-d2. Alcohol 4-d2 (4.87 g, 32.4 mmol) in anhydrous CH2Cl2
(25 mL) was added dropwise by addition funnel to an orange
suspension of PCC (11.18 g, 51.9 mmol) in 100 mL of anhy-
drous CH2Cl2. The flask containing 4-d2 and the addition fun-
nel were rinsed with dry CH2Cl2 (3 × 5 mL). A dark brown
suspension formed immediately, and the reaction mixture was
stirred for 4 h at RT under argon. Ether (150 mL) was added
and a light brown precipitate was formed. The mixture was
stirred for 5 min and then filtered through a short column of
Florisil. The reaction flask residue and the Florisil column
were rinsed with additional ether (3 × 50 mL). All ethereal
material was combined and concentrated by distillation under
an aspirator vacuum, and then using a vacuum pump. The res-
idue, a green oil of crude aldehyde 5-d2 (4.80 g, ≈100%
yield), was stored briefly under argon and subjected to cata-
lytic decarbonation without delay.
© 2008 NRC Canada

Baldwin and Singer
399
Other labeled 1-di phenylcyclopropanes were prepared
from 4-di alcohols in like fashion. For the eight preparations,
times at reflux ranged from 66 to 146 h. Five reactions took
three portions of catalyst (10.5% in all); three took four por-
tions (12.5 mol% in total); the 146 h reaction required five
portions of catalyst (4 × 3.5% + 2%).
Pure 1-di samples could be secured easily by preparative
GC using an Apiezon L column at 125 °C. Under the condi-
tions employed, the retention times for toluene, α-
methylstyrene, and a phenylcyclopropane were 2.66, 9.16,
and 11.83 min, respectively.
Labeled chiral 2-phenylcyclopropanes by catalytic
decarbonylation of labeled chiral trans-2-
phenylcyclopropanecarboxaldehydes
Labeled chiral 2-phenylcyclopropanes (1-di) by catalytic
decarbonylation
of
labeled
chiral
trans-2-
phenylcyclopropanecarboxaldehydes (5-di) were synthesized
as described in this specific example, starting from the sam-
ple of crude 5-d2 prepared immediately above.
An argon atmosphere was established for an oven-dried
500 mL three-necked round-bottomed flask equipped with
an addition funnel, condenser, and a specialized adapter to
connect a septum to the flask through a stopcock to protect
the septum from toluene at reflux. Then RhCl(CO)(PPh3)2
(0.78 g, 1.5 mmol) and 1,3-bis(diphenylphosphino)propane
(dddp; 1.3 g, 2.5 mmol) were added quickly, and 160 mL of
anhydrous, degassed toluene was added by syringe. The re-
sulting mixture was heated to 70–90 °C for 1 h. To the or-
ange solution in the reaction flask was added a yellow
solution of 5-d2 (4.80 g, 32.4 mmol) in anhydrous, degassed
toluene (20 mL) through an addition funnel. The small flask
that had contained the aldehyde solution was rinsed with to-
luene (3 × 5 mL) and the washings were added to the reac-
tion mixture, which was then heated to reflux for 21 h.
A mixture of RhCl(CO)(PPh3)2 (0.78 g, 1.5 mmol), dddp
(1.03 g, 2.5 mmol), and toluene (40 mL) in a separate flask
was heated for 1 h at 65–80 °C and then added to the reac-
tion mixture. The small flask was rinsed with toluene (2 ×
5 mL) and the washings were added to the reaction mixture,
which was then heated to reflux for an additional 24 h.
Analysis by GC showed that the aldehyde (5-
d2)/phenylcyclopropane (1-d2) ratio was about 1:1. An addi-
tional 1.5 mmol of catalyst was prepared and added to the
reaction mixture, which was heated to reflux for another
24 h. The (5-d2)/(1-d2) ratio was 0.46:1. Additional toluene
(50 mL) was added, and the mixture was heated to reflux for
26 h. Another increment of catalyst (0.49 mmol) was pre-
pared and added, and the reaction mixture was heated to re-
flux for an additional 44 h, by which time the reaction had
gone to completion. Pentane (200 mL) was added to the
cooled reaction mixture; the mixture was passed through a
silica gel column, followed by 275 mL of pentane. The col-
orless, clear liquid collected was concentrated by distillation
using an efficient teflon spinning band column (a B/R In-
strument Corporation 36T 100 system rated at up to 200 the-
oretical plates with microprocessor controller). The dark
yellow pot residue (9.69 g) was characterized by capillary
GC: it contained the phenylcyclopropane 1-d2 (31.6
weight%, 3.06 g, 79% yield for the two steps from alcohol
4-d2), toluene (56.1%, 5.44 g) and α-methylstyrene (1.4%,
0.14 g). Further concentration using a micro spinning band
distillation system (B/R 800), followed by a Kugelrohr dis-
Acknowledgments
We thank the National Science Foundation for support of
this work through CHE-0211120 and CHE-0514376.
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4.70 g of 4-d2 used three 3.5 mol% additions of catalyst and
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