Unexpected consecutive propargyl-allenyl isomerization in nucleophilic trapping reactions of (arene)Cr(CO)3-substituted propargyl cations

Article abstract of DOI:10.1021/om000921i

Instead of giving rise to the expected propargylated phosphonium salts 4, the addition of triphenylphosphane to (arene)Cr(CO)₃-substituted α-propargyl cations furnishes allenyl phosphonium salts 3 via a prototropic isomerization.

Full text of DOI:10.1021/om000921i

376
Organometallics 2001, 20, 376-378
Un exp ected Con secu tive P r op a r gyl-Allen yl
Isom er iza tion in Nu cleop h ilic Tr a p p in g Rea ction s of
(a r en e)Cr (CO)3-Su bstitu ted P r op a r gyl Ca tion s
Astrid Netz, Kurt Polborn, and Thomas J . J . Mu¨ller*
Department Chemie der Ludwig-Maximilians-Universita¨t Mu¨nchen,
Butenandtstrasse 5-13 (Haus F), D-81377 Mu¨nchen, Germany
Received October 30, 2000
Summary: Instead of giving rise to the expected propar-
gylated phosphonium salts 4, the addition of triphenyl-
phosphane to (arene)Cr(CO)3-substituted R-propargyl
cations furnishes allenyl phosphonium salts 3 via a
prototropic isomerization.
trapped with nucleophiles9 and electrophiles,8 respec-
tively. Here, we wish to communicate an unusual
prototropic propargyl-allene isomerization in Lewis
acidic medium to give solely (arene)Cr(CO)3-substituted
allenyl phosphonium salts as addition products of the
reaction of triphenylphosphane with (arene)Cr(CO)3-
substituted R-propargyl cations.
Among several organometallic substituents, Cr(CO)3-
complexed arenes1 are the most interesting functional
groups, since they allow an efficient stabilization of both
positive2 and negative3 benzylic charges.4 This peculiar
feature results in an electronically “hermaphroditic”2b
or amphoteric behavior and has been exploited in quite
some elegant syntheses of complex molecules.5 In the
past years we have demonstrated that conjugated
substituents on Cr(CO)3-complexed arenes6 can be suc-
cessfully applied for charge stabilization, even if the
charge is generated at a remote position.7 In particular,
stabilized propargyl cations7a,b and anions8 have been
generated, structurally characterized, and selectively
The base-catalyzed propargyl-allene isomerization
(Scheme 1) proceeding via a resonance-stabilized pro-
pargyl anion is a well-documented and synthetically
useful procedure to obtain allenes,10 in particular, since
alkynes are readily available by several approaches.11
However, the alternative isomerization process in acidic
media proceeding via a highly reactive and elusive vinyl
cation intermediate (Scheme 1) is almost unknown.12
In the course of our studies of cationic propargylations
with (arene)Cr(CO)3-substituted R-propargyl cations we
have investigated various trapping nucleophiles.7,9 Al-
though for planar chiral (arene)Cr(CO)3-substituted
R-propargyl cations the trapping reactions with π-, S-,
O-, and N-nucleophiles give exclusively the propargy-
lated products with high diastereoselectivity,9a we have
now observed a deviating behavior for the reaction with
phosphanes such as triphenylphosphane. Upon ioniza-
tion of the propargyl acetates 1 according to our
standard procedure7-9 with a slight excess of trimeth-
ylsilyl triflate (TMSOTf) in dichloromethane at -78 °C
the deeply colored propargyl cations 2 are subsequently
trapped with an excess of triphenylphosphane (Scheme
2). Instantaneously, the deeply colored solutions of 2 are
decolorized, and after neutral aqueous workup, surpris-
ingly, exclusively the allenyl phosphonium salts 3 are
obtained in good yields as yellow crystalline solids (3a
* To whom correspondence should be addressed. E-mail: tom@
cup.uni-muenchen.de.
(1) For excellent recent reviews see, e.g.: (a) Semmelhack, M. F.
Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F.
G. A., Wilkinson, G., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 12, p
979. (b) Davies, S. G.; McCarthy, T. D. Comprehensive Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.;
Pergamon: Oxford, U.K., 1995; Vol. 12, p 1039.
(2) (a) Solladie´-Cavallo, A. Polyhedron 1985, 4, 901. (b) J aouen, G.
Pure Appl. Chem. 1986, 58, 597. (c) Davies, S. G.; Donohoe, T. J . Synlett
1993, 323.
(3) (a) Semmelhack, M. F.; Seufert, W.; Keller, L. J . Am. Chem. Soc.
1980, 102, 6584. (b) Davies, S. G.; Coote, S. J .; Goodfellow, C. L. In
Advances in Metal-Organic Chemistry; Liebeskind, L. S., Ed.; J AI
Press: London, 1991; Vol. 2, p 1.
(4) For recent computational studies on the DFT level of theory see,
e.g.: (a) Pfletschinger, A.; Dargel, T. K.; Bats, J . W.; Schmalz, H.-G.;
Koch, W. Chem. Eur. J . 1999, 5, 537. (b) Merlic, C. A.; Walsh, J . C.;
Tantillo, D. J .; Houk, K. N. J . Am. Chem. Soc. 1999, 121, 3596.
(5) (a) Semmelhack, M. F.; Zask, A. J . Am. Chem. Soc. 1983, 105,
2034. (b) Uemura, M.; Minami, T.; Hayashi, Y. J . Chem. Soc., Chem.
Commun. 1984, 1193. (c) Uemura, M. In Advances in Metal-Organic
Chemistry; Liebeskind, L. S. Ed.; J AI Press: London, 1991; Vol. 2, p
195. (d) Corey, E. J .; Helal, C. J . Tetrahedron Lett. 1996, 37, 4837. (e)
Schellhaas, K.; Schmalz, H.-G.; Bats, J . W. Chem. Eur. J . 1998, 4, 57.
(f) Schmalz, H.-G.; Siegel, S. In Transition Metals for Organic
Synthesis; Beller, M., Bolm, C., Eds.; Wiley-VCH: Weinheim, Germany,
1998; Vol. 1, p 550. (g) Schmalz, H.-G.; de Koning, C. B.; Bernicke, D.;
Siegel, S.; Pfletschinger, A. Angew. Chem., Int. Ed. 1999, 38, 1620.
(6) (a) Mu¨ller, T. J . J .; Lindner, H. J . Chem. Ber. 1996, 129, 607. (b)
Mu¨ller, T. J . J . Tetrahedron Lett. 1997, 38, 1025. (c) Mu¨ller, T. J . J .;
Ansorge, M.; Polborn, K. J . Organomet. Chem. 1999, 578, 252. (d)
Ansorge, M.; Mu¨ller, T. J . J . J . Organomet. Chem. 1999, 585, 174. (e)
Mu¨ller, T. J . J . J . Organomet. Chem. 1999, 578, 95. (f) Mu¨ller, T. J . J .;
Netz, A.; Ansorge, M.; Schma¨lzlin, E.; Bra¨uchle, C.; Meerholz, K.
Organometallics 1999, 18, 5066.
(9) (a) Mu¨ller, T. J . J .; Netz, A. Tetrahedron Lett. 1999, 40, 3145.
(b) Netz, A.; Mu¨ller, T. J . J . Tetrahedron 2000, 56, 4149.
(10) (a) Murray, M. In Methoden zur Herstellung und Umwandlung
von Allenen bzw. Kumulenen; Houben-Weyl, Eds.; Thieme: Stuttgart,
Germany, 1977; Vol. 5/2a, p 991. For comprehensive reviews of allene
chemistry see, e.g.: (b) Taylor, D. R. Chem. Rev. 1967, 67, 317. (c)
Rutledge, T. F. Acetylenes and Allenes; Reinhold: New York, Amster-
dam, London, 1969; Parts 1-3. (d) Brady, W. T. In The Chemistry of
Ketenes, Allenes and Related Compounds; Patai, S., Ed.; Wiley:
Chichester, New York, Brisbane, Toronto, 1980; Part 1, p 298. (e) Hopf,
H. In The Chemistry of Ketenes, Allenes and Related Compounds; Patai,
S., Ed.; Wiley: Chichester, New York, Brisbane, Toronto, 1980; Part
2, p 779. (f) Smadja, W. Chem. Rev. 1983, 83, 263. (g) Pasto, D. J .
Tetrahedron 1984, 40, 2805. (h) Schuster, H. F.; Coppola, G. M. Allenes
in Organic Synthesis; Wiley: Chichester, New York, Brisbane, Toronto,
1984.
(7) (a) Mu¨ller, T. J . J .; Netz, A. Organometallics 1998, 17, 3609. (b)
Mu¨ller, T. J . J .; Ansorge, M.; Polborn, K. Organometallics 1999, 18,
3690. (c) Mu¨ller, T. J . J .; Ansorge, M. Tetrahedron 1998, 54, 1457. (d)
Ansorge, M.; Polborn, K.; Mu¨ller, T. J . J . Eur. J . Inorg. Chem. 1999,
225.
(11) For a comprehensive overview see, e.g.: Modern Acetylene
Chemistry; Stang, P. J ., Diederich, F., Eds.; VCH: Weinheim, Ger-
many, 1995.
(12) Barry, B. J .; Beale, W. J .; Carr, M. D.; Hei, S.-K.; Reid, I. J .
Chem. Soc., Chem. Commun. 1973, 177.
(8) Netz, A.; Mu¨ller, T. J . J . Organometallics 2000, 19, 1452.
10.1021/om000921i CCC: $20.00 © 2001 American Chemical Society
Publication on Web 01/06/2001

Communications
Organometallics, Vol. 20, No. 3, 2001 377
Sch em e 1. Ba se- a n d Acid -Ca ta lyzed P r op a r gyl-Allen yl Isom er iza tion s
Sch em e 2
and 3b) or as a yellow foam (3c).13 Trapping the
configurationally stable racemic planar chiral propargyl
cations 2b and 2c with triphenylphosphane gives rise
to the isolation of a racemic mixture of the diastereo-
mers of 3b (diastereomeric ratio (dr) ) 61:39) and 3c
(dr ) 60:40). The diastereomeric ratio can be readily
reproduced by MM2 calculations14 on both diastereo-
mers of 3b with a difference in steric energy of 0.14 kcal/
mol (T ) 195 K; K ) 59:41), indicating a thermody-
namically controlled formation of the diastereomeric
allenyl phosphonium salts 3.
and 104.3 (3J PC ) 12.0-13.8 Hz).15 In the 31P NMR
spectra the characteristic singlets for the phosphorus
nuclei in phosphonium salts can be found between δ
20.64 and 23.05.16 By the aid of 2D NOESY experiments
of 3b a geminal orientation of the (arene)Cr(CO)3 and
the triphenylphosphonium substituents at the R-posi-
tion is supported by the appearance of strong cross-
peaks for the meta anisyl and allenyl protons. Addition-
ally, cross-peaks are found for the ortho methyl protons
on the complexed arene ring and the ortho phenyl pro-
tons of the triphenylphosphane moiety. Additionally, the
structure of the allenes 3 was unambiguously corrobo-
rated by X-ray crystal structure analyses for both 3a
(Figure 1) and 3b (Figure 2).17,18 Furthermore, the bond
lengths in the allenyl fragments (3a , C(10)-C(11) )
1.304 Å, C(11)-C(12) ) 1.297 Å; 3b, C(11)-C(12) )
1.303 Å, C(12)-C(13) ) 1.307 Å) are in agreement with
those of other allenyl-substituted (arene)chromium car-
bonyl complexes.19 Therefore, the formation of 3 can be
rationalized by a regioselective attack of the phosphane
at the propargylic position followed by a subsequent
prototropic rearrangement20 to give the allenyl deriva-
tive 3. In particular, this propargyl-allenyl rearrange-
ment is rather unusual, since it proceeds in a Lewis
According to the NMR spectra the structure of the
unexpectedly formed allenyl derivatives is supported by
the characteristic appearance of the allenyl proton
1
resonances in the H NMR spectra as doublets between
δ 7.01 and 7.12 (the allenyl proton resonance in the
spectrum of 3c is found between δ 5.89 and 6.01 as a
multiplet due to vicinal coupling) with large phosphorus
proton coupling constants 4J (11.3-11.5 Hz). In the
carbon NMR spectra the characteristic signals for the
R-allenyl carbon atoms are found between δ 86.3 and
93.9 (1J PC ) 83.8-92.6 Hz), for the central â-allenyl
resonances between δ 216.5 and 219.7 (2J PC ) 4.1-7.3
Hz), and for the terminal γ-allenyl nuclei between δ 98.5
(13) Syn th esis of th e a llen yl p h osp h on iu m sa lts 3: To a solution
of 1 equiv (0.5 mmol) of the acetate 1 in 5 mL of dry dichloromethane
was added dropwise 1.3 equiv of TMSOTf at -78 °C. Immediately a
deeply colored solution of the propargyl cation 2 was formed, which
was stirred at that temperature for 40-60 min. To this reaction
mixture was then added a solution of 2.2 equiv of triphenylphosphane
in 3 mL of dichloromethane at -78 °C. The reaction can be followed
by a color change from deep purple to yellow. After 55-65 min
subsequently 20 mL of diethyl ether and 20 mL of water were added
and the external cooling was removed. After extraction of the aqueous
phase with dichloromethane (2 × 25 mL) the combined organic layers
were dried with magnesium sulfate and filtered and the solvents were
evaporated in vacuo. The residue was dried in vacuo and triturated
(15) Kalinowski, H.-O.; Berger, S.; Braun, S. 13C NMR Spektroskopie;
Georg Thieme Verlag: Stuttgart, New York, 1984; p 129.
(16) Tebby, J . C. Phosphorus-31 Nuclear Magnetic Resonance Data;
CRC Press: Boca Raton, FL, 1980.
(17) Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as Supplementary Publica-
tion Nos. CCDC-151549 (3a ) and CCDC-151548 (3b). Copies of the data
can be obtained free of charge on application to the CCDC, 12 Union
Road, Cambridge CB2 1EZ, U.K. (fax, + 44-1223/336-033; e-mail,
deposit@ccdc.cam.ac.uk).
(18) The crystals suitable for the X-ray structure analysis of 3b
showed a modest enrichment of the major diastereomer (dr ) 70.5:
29.5), as determined from the displacement of the ortho methyl group
in positions C5 and C9 with a distribution of 70.5% to 29.5%.
(19) Mu¨ller, T. J . J .; Ansorge, M. Chem. Ber./Recl. 1997, 130, 1135.
(20) For a base-catalyzed isomerization of propargyl phosphonium
salts see, e.g.: Khachatryan, R. A.; Mkrtchyan, G. A.; Kinoyan, F. S.;
Indzhikyan, M. G. J . Gen. Chem. USSR (Engl. Transl.) 1986, 207-
208.
with diethyl ether to give as
a yellow crystalline solid. Further
purification was achieved by recrystallization from dichloromethane/
diethyl ether. 3a : yield 270 mg (69%); yellow crystals; mp >185 °C
dec. 3b: yield 270 mg (73%, dr ) 61:39); yellow crystals (dichlo-
romethane/diethyl ether); mp >183 °C dec. 3c: yield 219 mg (77%, dr
) 60:40); yellow foam.
(14) Quantum CAChe 3.0 Program, Oxford Molecular Group, 1997.

378 Organometallics, Vol. 20, No. 3, 2001
Communications
that the propargylic proton of the initially formed
propargyl phosphonium salt 4 is acidified due to the
enhanced cooperative electron-withdrawing nature of
the chromium carbonyl complexed arene and the cat-
ionic phosphonium substituent. Obviously, even the
weakly basic phosphane present in excess now affords
a deprotonation, at least in equilibrium, to give the
highly resonance stabilized propargyl Wittig ylide21
5
that is irreversibly protonated to furnish the thermo-
dynamically more stable allene isomer 3.
Sch em e 3
F igu r e 1. ORTEP plot of 3a . Selected bond lengths (Å)
and dihedral and torsional angles (deg): C(4)-C(10) )
1.488, C(10)-C(11) ) 1.304, C(11)-C(12) ) 1.297, C(12)-
C(13) ) 1.462, C(10)-P(1) ) 1.812; C(12)-C(11)-C(10) )
176.41, C(5)-C(4)-C(10)-C(11) ) 84.72, C(18)-C(13)-
C(12)-C(11) ) 10.06.
In conclusion, we have observed that the addition of
a phosphane to (arene)Cr(CO)3-substituted R-propargyl
cations results in an unusual propargyl-allenyl isomer-
ization in a Lewis acidic medium. This process takes
advantage of the electronically amphoteric nature of the
(arene)chromium carbonyl substituent, which stabilizes
not only the propargyl cation 2 but also the presumed
propargyl anion intermediate 5. Recently, we have
demonstrated the benefits of (arene)chromium carbonyl
substituted allenyl phosphonic acid ester metal-tem-
plated synthesis of heterocycles,7c and thus, the con-
secutive phosphane addition, propargyl-allenyl isomer-
ization sequence allows an alternative approach to
allenyl phosphorus species. The reactions of this novel
class of organometallic allenes will be the goal of future
investigations.
Ack n ow led gm en t. Financial support of this work
by the Fonds der Chemischen Industrie and the Deut-
sche Forschungsgemeinschaft is gratefully acknowl-
edged. We wish to express our appreciation to Prof. H.
Mayr for his generous support.
F igu r e 2. ORTEP plot of 3b (only the major diastereomer
is depicted). Selected bond lengths (Å) and dihedral and
torsional angles (deg): C(4)-C(11) ) 1.496, C(11)-C(12)
) 1.303, C(12)-C(13) ) 1.307, C(13)-C(14) ) 1.457,
C(11)-P(1) ) 1.817; C(11)-C(12)-C(13) ) 178.20, C(5)-
C(4)-C(11)-C(12) ) 84.37, C(12)-C(13)-C(14)-C(19) )
4.80.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental procedures and characterization data for 3a , 3b, and
3c and tables giving data collection parameters, bond lengths
and angles, positional and thermal parameters, and least-
squares planes for 3a and 3b. This material is available free
of charge via the Internet at http://pubs.acs.org.
acidic medium in a nonprotic solvent. However, the
peculiar isomerization can be rationalized on the basis
of the amphoteric electronic nature of the (arene)-
chromium carbonyl substituent (Scheme 3). This means
OM000921I
(21) For stable propargyl ylides see, e.g.: Pommer, H. Angew. Chem.
1960, 72, 811.