First example of a total axial to centered chirality transfer in the [2+2+2] cycloadditions of allenediynes

Article abstract of DOI:10.1055/s-2000-6300

The cobalt-mediated [2+2+2] cycloaddition of allenediynes bearing a phosphine oxide group at one alkyne terminus is a completely regio-, chemo- and diastereoselective high yielding process. Moreover, a total transfer of chiral information from axial to centered chirality is evident, which makes this reaction a new and powerful tool for the preparation of enantiomerically enriched tricyclic derivatives.

Full text of DOI:10.1055/s-2000-6300

PAPER
985
First Example of a Total Axial to Centered Chirality Transfer in the [2+2+2]
Cycloadditions of Allenediynes
Olivier Buisine, Corinne Aubert, Max Malacria*
Université P. et M. Curie, Laboratoire de Chimie Organique de Synthèse, associé au CNRS, Tour 44-54, B. 229, 4, Place Jussieu 75252
Paris Cedex, France
Fax (33)0144277360; E-mail: malacria@ccr.jussieu.fr
Received 8 February 2000
R
X
Y
X
Abstract: The cobalt-mediated [2+2+2] cycloaddition of allene-
diynes bearing a phosphine oxide group at one alkyne terminus is a
completely regio-, chemo- and diastereoselective high yielding pro-
cess. Moreover, a total transfer of chiral information from axial to
centered chirality is evident, which makes this reaction a new and
powerful tool for the preparation of enantiomerically enriched tricy-
clic derivatives.
Y
•
Y
X
a
R
R
+
CpCo
CpCo
X, Y :
X, Y :
H
Key words: alkynes, allenes, chirality, cycloadditions, cobalt
72%
R = H
1
2
0%
O
R = Ph
0%
87%
O
Chiral allenes are versatile precursors in asymmetric syn-
thesis that efficiently transfer the axial to centered chiral-
ity and their reactivity has recently attracted much
attention.1 In addition, transition metal-promoted carbon-
carbon bond forming reactions based on allenes have been
extensively investigated.2 These reactions include, for in-
stance, co-oligomerizations,3 intramolecular cycliza-
tions,4 [4+2], [5+1] and [5+2] cyclizations,5 inter- and
intramolecular Pauson-Khand reactions,6 formation and
electrocyclizations of p-allyl metal complexes7 and for-
mal Alder ene reaction.8
Reagents and conditions: a, CpCo(CO)2 (1 equiv), xylenes, D, hn
Scheme 1
the allenediyne 3 was obtained from the dodeca-5,11-
diynal 413 (Scheme 2).
H
O
a
Ph
Although allenes are good ligands in organometallic
chemistry, only few examples involving this moiety have
been reported in nickel(II)/(0)9 and palladium(0)10 medi-
ated [2+2+2] cycloadditions or homo Diels-Alder reac-
tions.11
3
4
H
(b)
5a R = Ph
5b R = Me
1 or 3
O
R
Recently, we disclosed that the cobalt(I) complex, Cp-
Co(CO)2, was able to mediate the intramolecular [2+2+2]
cyclizations of allenediynes with pronounced regio- and
diastereoselectivity.12 Depending on the substitution of
the allene, the regioselectivity of the cycloaddition was
quite different leading either to the h4-complexed tricyclic
[6.6.6] or [6.6.5] compounds (Scheme 1). At this point of
our studies, we were attracted to determine whether this
cobalt-mediated [2+2+2] cyclization can be performed
with a high degree of chirality transfer.
PPh2
Reagents and conditions : a, one-pot i. PhC∫CLi, THF, -78 °C,
ii. CH3SO2Cl, iii. 2 LiCl, MeCuCNMgBr, 74%; b, i. n-BuLi, THF,
-78 °C, 30 min, ii. Ph2P(O)Cl, -78°to 50 °C, 10 min, 5a:90%;
5b:87%
Scheme 2
Indeed, aldehyde 4 was treated in THF, at -78 °C, in a
one-pot operation, subsequently with phenyl lithiu-
macetylide, methanesulfonylchloride and methylcyano-
cuprate (generated from methyl Grignard, copper(I)
cyanide and lithium chloride) affording the allenediyne 3
in 74% yield. The latter was exposed to one equivalent of
(h5-cylopentadienyl)cobalt dicarbonyl in refluxing xy-
lenes under irradiation and was consumed after 30 min-
utes. Surprisingly, despite many attempts, we were unable
to isolate h4-cobalt complexes in pure form, even when
oxygen-free conditions were used during the purification.
In this paper, we report that optically active allenediynes
undergo intramolecular cobalt(I)- mediated [2+2+2] cy-
clization with a total axial to centered chirality transfer.
In order to determine the level of the stereoselectivity of
the cyclization of chiral allenediynes, we first checked the
behavior of racemic precursor 3. The latter has been judi-
ciously selected considering that the cyclization of 1 led to
[6.6.5] complex as a sole diastereomer and in addition,
that the double bond bearing a phenyl group in 2 was not
reactive. By improving the procedure already described,
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986
O. Buisine et al.
PAPER
On the basis of the 1H NMR spectrum of the crude mix- tropic cone of one of the two phenyls on phosphorus. The
ture, we could safely assume that the cyclization had oc- latter effect is particularly helpful for the stereochemical
1
curred but, probably, the existence of strong allylic strains assignment of the double bond. As observed in the H
in the resulting complexes associated with their high sen- NMR spectrum, the vinylic methyl in 6a is strongly
sitivity caused their degradation.
shielded (d 0.83 ppm) relative to the classical chemical
shift. This anisotropic effect was also found with the cy-
cloadduct 6b, obtained by the cyclization of the com-
pound 5b, which bears two vinylic methyl groups: one at
d 0.62 ppm and the other at d 1.32 ppm. Finally, the cy-
clization is totally chemo- and diastereoselective.
Recently, we disclosed that the substitution of the triple
bond of an enediyne with a phosphine oxide group greatly
enhances the stability of the corresponding complex and
thus, the yield of the cyclization.13a, 14 Furthermore, we
have decided to exploit the property of such substituents
in the cyclization of allenediynes.
These results could be explained by the most probable
mechanism of the [2+2+2] cyclization which may involve
a cobaltacyclopentadiene I. The latter could react with the
internal double bond of the allene moiety following two
pathways in the intramolecular [4+2] cycloaddition pro-
cess which will deliver the tricyclic cobalt complex
(Scheme 4).
Thus, deprotonation of 3 or 1 with n-butyllithium fol-
lowed by the alkylation of the corresponding lithium-
acetylide with diphenylphosphinic chloride afforded the
allenediyne 5a and 5b in 90% and 87% yields, respective-
ly. Gratifyingly, addition of one equivalent of CpCo(CO)2
to 5a in boiling THF under irradiation yielded complex 6a
quantitatively as a single diastereomer (Scheme 3). Com-
pound 6a crystallizes spontaneously from a 1:3 mixture of
dichloromethane/hexane and the assigned structure was
unambiguously secured by a single crystal X-ray analysis
(Figure).
H
CpCo(CO)2
CoCp
O
Ph
Ph
PPh2
O
PPh2
I
CpCo
H
E
Ph
H
(a)
H
H
H
approach
anti to R
R
O
P
R
Ph
PPh2
Ph
P
Ph
[4+2]
Co
Cp
Co
O
P
P
Ia
Ph
CpCo
Cp
5a R = Ph
5b R = Me
6a R = Ph
6b R = Me
100%, 6aE
Reagents and conditions: a, CpCo(CO)2 (1 equiv), THF, D, hn,
6a:quantitative; 6b:66%
CpCo
H
H
approach
syn to R
Scheme 3
Ph
P
Co
Cp
H
Co
P
Ph
Ph
[4+2]
P
Ph
Is
Cp
Z
0%, 6aZ
O
=
P
P
Ph
Scheme 4
The most favored approach of the polyunsaturated part-
ners in which the non-bonded interactions and A1,3 strains
were minimized is the exo approach in which the tether is
in a chair-like conformation. However, the cobaltacyclo-
pentadiene I could approach one or the other face of the
allene. The approach anti to R (Ia) would lead to an inter-
mediary-bridged polycyclic cobaltacyclopentene com-
plex in which the relative configuration of all stereogenic
centers is set. After reductive elimination, the final [6.6.5]
tricyclic h4-cobalt complex would have an E exocyclic
double bond (compound 6aE). An approach syn to R (Is)
would also lead to a [6.6.5] tricyclic compound. However,
this compound would possess the Z configured exocyclic
Figure ORTEP Representation of 6a
The ORTEP representation showed: (i) the [6.6.5] frame-
work, confirming that the internal double bond of the al-
lene had reacted in the process, (ii) the anti relationship
between the angular H and the cobalt moiety, (iii) the E
configuration of the exocyclic double bond, (iv) the vinyl-
ic aromatic ring is not conjugated with the exocyclic dou-
ble bond (the strong allylic strain, of the styrene moiety,
evidently forces an out of plane rotation of the phenyl
group), and (v) the vinylic methyl is placed in the aniso-
Synthesis 2000, No. 7, 985–989 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
First Example of a Total Axial to Centered Chirality Transfer in the [2+2+2] Cycloadditions
987
double bond (6aZ). Such an approach is strongly disfa- tried without any success and the 1H NMR chiral resolu-
vored due to the non-bonding interactions between R, the tion reagents were totally ineffective. In our laboratory,
metallacyclopentadiene, and the phosphine oxide group we disclosed that the titration of the enantiomeric cobalt
and in fact, the sole product 6aE was obtained in the reac- complexes bearing a phosphine oxide substituent is possi-
tion of 5a.
ble with 31P NMR in presence of (+) zinc taddolate,19 ob-
tained by the reaction of dimethylzinc with the (+)-taddol.
Thus, in the presence of one equivalent of (+) zinc tad-
dolate, the enantiomers of complex 6a are totally discrim-
inated by 31P NMR. In fact, the cyclization of the
allenediyne 5a (70% ee) provided the adduct 6a in 70% ee
and the enantiopure 5a afforded the enantiomerically pure
cycloadduct 6a meaning a total retention of the optical ac-
tivity during the cyclization.
Then, we turned our attention to the cyclization of the op-
tically active allenediyne 5a. Its straightforward prepara-
tion is outlined in Scheme 5.
O
OH
a
Ph
Ph
Ph
7
8
OH
(S)
b
(S)
c, d
O
PPh2
Ph
7, > 95%ee
5a, > 95% ee
Reagents and conditions: a, PCC (1.4 equiv), Al2O3, CH2Cl2, r.t.,
60%; b, [Ru]* (5 mol%) = [(p-cymene)RuCl2]2, (S,S)-TsDPEN
(2 equiv/Ru), i-PrOH, 80 °C, 20 min, then KOH (20 equiv/Ru), 8,
i-PrOH, r.t., 30 min, 75%; c, i. n-BuLi, THF, -78 °C, ii. MsCl,
iii. MeCuCNMgBr, 71%; d, i. n-BuLi, THF, -78 °C, ii. Ph2P(O)Cl,
-78°to 50 °C, 78%
Scheme 5
The propargylic alcohol 7, obtained as previously de-
scribed by addition of phenyl lithiumacetylide to aldehyde
4, was oxidized with PCC on alumina in dichloromethane
to afford the ketone 8. Its enantioselective reduction was
of 8 carried out by following two methods. The first one15
was run in ether in the presence of LiAlH4 and Chirald to
afford the chiral alcohol 7 in 73% yield and with an enan-
Scheme 6
In summary, the [2+2+2] cyclization of allenediynes was
shown to be completely regio-, chemo- and diastereose-
lective, the facial selectivity occurring on the less hin-
dered face of the allene. Considering the rapid access to
enantiopure allenediynes and the total transfer of the axial
into the centered chirality, the cobalt-mediated [2+2+2]
cylizations of allenediynes may be regarded as a powerful
tool for the construction of enantiomerically pure polycy-
clic frameworks. Indeed, free ligands could be very inter-
esting as constituting the BCD moiety of steroids bearing
an oxidized C-11 position. Approaches to such com-
pounds are under investigations in our laboratories.
1
tiomeric excess of 70% (the ee was determined by H
NMR by using chiral europium salts). The second
method16 involved a transfer hydrogenation from 2-pro-
panol to the ketone under the action of a chiral ruthenium
(II) catalyst. The chiral catalyst was generated in situ by
adding KOH into a mixture of [p-cymeneRuCl2]2 and
(S,S)-N-(p-toluenesulfonyl)-1,2-diphenylethylenedi-
amine [(S,S)-TsDPEN]. Thus, in presence of 5 mol% of
catalyst in 2-propanol, 7 was obtained from 8 in 75% yield
and with 95% ee.17 Each step, particularly the addition of
the methylcyanocuprate being stereoselective,18 the opti-
cally active allenediyne 5a was obtained similarly as the
racemic one in 55% overall yield. However, the titration
of the enantiomers was impossible by using classical
14-Phenylpentadeca-12,13-dien-1,7-diyne (3)
methods (GC, HPLC), nevertheless the optically active al- In a one-pot operation, a solution of n-BuLi in hexane (2.2 M,
1.1 mL, 2.5 mmol) was added dropwise to a THF (10 mL) solution
lenediyne 5a was submitted to the cyclization protocol
and furnished the complex 6a in 95% yield. But, at this
point, the major problem remained the determination of
the enantiomeric excess. Indeed, to our knowledge no
methods have been reported for the titration of the enan-
tiomeric cobalt complexes. The classical ones have been
of phenylacetylene (0.255 g, 2.5 mmol) at -78 °C. After stirring for
5 min, a solution of dodeca-5,11-diynal 413 (0.445g, 2.5 mmol) in
THF (5 mL) was added. The mixture was stirred for 5 min and
methanesulfonyl chloride (0.19 mL, 2.5 mmol) was added at once.
The resulting mixture was transferred via a cannula to a solution
of the methylcyanocuprate previously generated. To a cooled
(-78 °C) THF (100 mL) solution of CuCN (1.34 g, 15 mmol) and
Synthesis 2000, No. 7, 985–989 ISSN 0039-7881 © Thieme Stuttgart · New York

988
O. Buisine et al.
PAPER
LiCl (1.27 g, 30 mmol) (dried overnight at 100 °C under vacuum 1
mm Hg) was added dropwise a solution of methylmagnesium bro-
mide in Et2O (2.4 M, 3.1 mL, 7.5 mmol), the mixture was stirred at
-78 °C for 15 min. After being stirred at –40 °C for 1 h, the reaction
mixture was hydrolyzed at -78 °C with a solution of NH4Cl/
NH4OH (3:1), warmed to r.t. and diluted with Et2O (150 mL). The
organic layer was separated, washed with brine, dried (MgSO4), and
concentrated. Flash chromatography (petroleum ether/Et2O, 95:5)
of the crude mixture afforded 3 (0.51 g, 74%).
Hz), 50.0 (d, J = 143 Hz), 34.1, 32.9, 30.9, 27.7, 24.2, 23.1, 22.1,
13.6.
31P NMR (162 MHz, CDCl3): d = 40.56.
(S)-1-Phenyltetradeca-1,7,13-triyn-3-ol [(S)-7]
In a flame-dried flask, under Ar, [p-(cymene)RuCl2]2 (0.196 g,
0.32 mmol) and (S,S)-TsDPEN (0.234 g, 0.64 mmol) were charged
and dissolved in i-PrOH (10 mL), and degassed by three freeze-
pump-thaw cycles. The mixture was heated at 80 °C for 20 min, the
reaction mixture turned from orange to red. KOH (flame-dried un-
der vacuum) (0.09 g, 1.6 mmol) was then added and the mixture
was held under fast stirring; the color turned quickly to purple. A
degassed solution of the ketone 8 (1.76 g, 6.4 mmol) in i-PrOH
(50 mL) was added slowly via a cannula at r.t.. After being stirred
for 30 min, the reaction mixture was concentrated and subjected
to flash chromatography (petroleum ether/EtOAc, 85:15) to yield
(S)-7 (1.33 g, 75%, ≥ 90% ee).
(S)-3: [a]D25 +146.6 (c 1.3, CHCl3).
IR (neat): n = 3290, 2100, 1940, 1590, 1570, 1490, 1450, 1060,
1020 cm-1.
1H NMR (400 MHz, CDCl3): d = 7.46 (d, 2H, J = 7.7 Hz), 7.37 (t,
2H, J = 7.7 Hz), 7.24 (t, 1H, J = 7.6 Hz), 5.52-5.48 (tq, 1H, J = 6.4,
3.0 Hz), 2.32-2.20 (m, 8H), 2.15 (d, 3H, J = 3.0 Hz), 2.00 (t, 1H,
J = 2.5 Hz), 1.72 (quint, 2H, J = 7.1 Hz), 1.69-1.59 (m, 4H).
13C NMR (100 MHz, CDCl3): d = 204.4, 128.4, 137.7, 126.5 (2C),
125.7 (2C), 100.8, 92.5, 84.4, 80.2, 80.1, 68.6, 28.7, 28.2, 28.1,
27.7, 18.4, 18.1, 17.3.
[a]D25 +5.36 (c 1.35, CHCl3).
IR (neat): n = 3300, 3290, 2200, 2100, 1950, 1880, 1800, 1440,
1330 cm-1.
Anal. Calcd for C21H24: C, 91.25; H, 8.75. Found: C, 91.10; H, 8.86.
1H NMR (400 MHz, CDCl3): d = 7.58-7.40 (m, 2H), 7.36-7.20 (m,
3H), 4.63 (t, 1H, J = 6.1 Hz), 2.30-2.13 (m, 6H), 1.97 (t, 1H,
J = 2.5Hz), 1.95-1.86 (m, 2H), 1.79-1.68 (m, 2H), 1.68-1.54 (m,
4H).
13C NMR (100 MHz, CDCl3): d = 131.8 (2C), 128.5, 128.4 (2C),
122.7, 90.1, 85.0, 84.4, 80.4, 80.1, 68.6, 52.7, 37.0, 28.1, 27.6, 24,9,
18.6, 18.5, 18.4.
Compound (5a)
To a cooled (-78 °C) THF (20 mL) solution of the allenediyne 3
(0.918 g, 3.3 mmol) was added dropwise a solution of n-BuLi in
hexane (2.0 M, 1.65 mL, 3.3 mmol). After stirring the solution for
30 min, diphenylphosphinic chloride (0.7 mL, 3.3 mmol) was add-
ed. After the addition, the reaction mixture was warmed up quickly
with a steam bath (50 °C) and was stirred for 10 min. Then, the mix-
ture was hydrolyzed with sat. NH4Cl and diluted with CH2Cl2 (40
mL). The organic layer was separated, washed with brine, dried
(MgSO4), and concentrated. Flash chromatography (petroleum
ether/EtOAc, 20:80) of the crude mixture yielded 5a (1.4 g, 90%).
Anal. Calcd for C20H22O: C, 86.29; H, 7.97. Found: C, 86.28; H,
8.08.
Acknowledgement
(S)-5a [a]D25 +34.7 (c 0.13, CHCl3).
Financial support was provided by the CNRS and MRES. O.B.
thanks Glaxo-Wellcome for his fellowship. The authors thank Dr.
J. Vaissermann, Université P. et M. Curie for carrying out the X-ray
structure determination of 6a.
IR (neat): n = 3040, 2180, 1940, 1890, 1810, 1770, 1570, 1480,
1430, 1200, 1140, 1020, 840, 750, 720, 690 cm-1.
1H NMR (400 MHz, CDCl3): d = 7.87-7.81 (m, 4H), 7.55-7.40 (m,
6H), 7.33-7.28 (m, 4H), 7.19 (t, 1H, J = 7.2 Hz), 5.45 (tq, 1H,
J = 6.2, 2.8 Hz), 2.46 (dt, 2H, J = 6.9, 2.9 Hz), 2.25-2.13 (m, 6H),
2.09 (d, 3H, J = 2.8 Hz), 1.77-1.46 (m, 6H).
References
13C NMR (100 MHz, CDCl3): d = 200.4, 137.6, 133.5 (d, 2C,
J = 121 Hz), 132.2 (2C), 131.5 (d, 4C, J = 11 Hz), 128.6 (d, 4C,
J = 14 Hz), 128.4 (2C), 126.5, 125.7 (2C), 109.3 (d, J = 30 Hz),
100.7, 92.4, 80.7, 79.6, 75.2 (d, J = 173 Hz), 28.5, 28.2, 28.1, 26.7,
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First Example of a Total Axial to Centered Chirality Transfer in the [2+2+2] Cycloadditions
989
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Llerena, D.; Buisine, O.; Aubert, C.; Malacria, M.
Tetrahedron 1998, 54, 9373.
Synthesis 2000, No. 7, 985–989 ISSN 0039-7881 © Thieme Stuttgart · New York