Preparation, structure, and reaction of a sterically encumbered 1-phosphaallene containing a cyclopropylidene moiety

Article abstract of DOI:10.1021/ol0341750

(Matrix presented) Sterically protected (Z)-1-(2,4,6-tri-tert-butylphenyl)-2,5-dibromo-1-phosphapent-1-ene was allowed to react with potassium tert-butoxide to afford a cyclopropylidenephosphaethene, which was characterized spectroscopically and by X-ray

Full text of DOI:10.1021/ol0341750

ORGANIC
LETTERS
2003
Vol. 5, No. 7
1111-1114
Preparation, Structure, and Reaction of
a Sterically Encumbered
1-Phosphaallene Containing a
Cyclopropylidene Moiety
Shigekazu Ito, Shigeo Kimura, and Masaaki Yoshifuji*
Department of Chemistry, Graduate School of Science, Tohoku UniVersity,
Aoba, Sendai 980-8578, Japan
yoshifj@mail.cc.tohoku.ac.jp
Received January 31, 2003
ABSTRACT
Sterically protected (Z)-1-(2,4,6-tri-tert-butylphenyl)-2,5-dibromo-1-phosphapent-1-ene was allowed to react with potassium tert-butoxide to afford
a cyclopropylidenephosphaethene, which was characterized spectroscopically and by X-ray crystallography. Construction of the cycloalkyl
groups and isomerization of 1-phosphapenta-1,2,4-trienes to cyclopropylidenephosphaethenes are also described.
Kinetic stabilization with bulky substituents has been widely
utilized for the preparation of various unstable chemical
species such as the unsaturated bonds of heavier main-group
elements and the chemistry of multiple-bonded phosphorus
compounds.1 Since we reported the first example of a stable
phosphorus-phosphorus double-bonded species (diphos-
phene),2 we have now synthesized a number of low-
coordinated phosphorus compounds bearing the 2,4,6-tri-tert-
butylphenyl (abbreviated to Mes*) group.3 In the course of
research on low-coordinated phosphorus compounds, several
1-phosphaallene derivatives4 have been prepared as “phos-
phacumulenes”. Since allene derivatives have been widely
used in organic synthesis due to their high reactivities,5
1-phosphaallenes might be of interest as novel synthons for
unusual organophosphorus compounds.1,3a However, studies
on 1-phosphaallenes have not been performed so extensively
and only a few 1-phosphaallenes have been prepared.1 On
the other hand, cyclopropane derivatives as a group are one
of the most attractive organic counterparts due to their ring
distortion and unique electronic properties.6 Additionally,
cyclopropanes bearing unsaturated bonds as in methylene-
cyclopropane have shown extensive utility and versatility in
reactions.6,7 We now report the preparation, structure, and a
(4) (a) Kato, T.; Gornitzka, H.; Baceiredo, A.; Bertrand, G. Angew.
Chem., Int. Ed. 2000, 39, 3319. (b) Escudie´, J.; Ranaivonjatovo, H.; Rigon,
L. Chem. ReV. 2000, 100, 3639.
(1) (a) Regitz, M.; Scherer, O. J. Multiple Bonds and Low Coordination
in Phosphorus Chemistry; Thieme: Stuttgart, Germany, 1990. (b) Dillon,
K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The Carbon Copy; Wiley:
Chichester, UK, 1998.
(2) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J.
Am. Chem. Soc. 1981, 103, 4587. Yoshifuji, M.; Shima, I.; Inamoto, N.;
Hirotsu, K.; Higuchi, T. J. Am. Chem. Soc. 1982, 104, 6167.
(3) (a) Yoshifuji, M. J. Chem. Soc., Dalton Trans. 1998, 3343. (b)
Yoshifuji, M. J. Organomet. Chem. 2000, 611, 210. (c) Tokitoh, N. J.
Organomet. Chem. 2000, 611, 217.
(5) (a) Patai, S. The Chemistry of Ketenes, Allenes and Related
Compounds; Wiley: New York, 1980. (b) Landor, S. R. The Chemistry of
the Allenes; Academic Press: London, UK, 1982. (c) Schuster, H. F.;
Coppola, G. M. Allenes in Organic Synthesis; Wiley: New York, 1984.
(6) (a) de Meijere, A. Methoden der Organischen Chemie (Houben-Weyl)
Vol. E17: Carbocyclic Three- and Four-Membered Ring Compounds;
Thieme: Stuttgart, Germany, 1996. (b) de Meijere, A. Angew. Chem., Int.
Ed. Engl. 1979, 18, 809.
10.1021/ol0341750 CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/04/2003

selection of reactions of a bulky cyclopropylidenephospha-
ethene stabilized by the Mes* group, as exemplified by the
construction of the cyclopropyl moiety. Moreover, the
preparation of a cyclobutylidenephosphaethene is also de-
scribed.
Scheme 1a
The sterically encumbered 2,2-dibromo-1-phosphaethene
(1)8 bearing the Mes* group was allowed to react with
butyllithium3a,9 and then with 1,3-dibromopropane to afford
(Z)-2,5-dibromo-1-phosphapent-1-ene Z-2 in excellent yield
(>90%).10 Phosphapentene Z-2 was treated with potassium
tert-butoxide to afford a novel cyclopropylidenephospha-
ethene derivative 3, which was first characterized by
spectroscopic methods.11 In the 31P NMR spectrum, the signal
of 3 was observed at a lower field than that reported for
3-methyl-1-(2,4,6-tri-tert-butylphenyl)-1-phosphabuta-1,2-
diene (Mes*PdCdCMe2; δP 60), whereas in the 13C NMR
spectrum the signals due to the two sp2 carbon atoms of 3
appeared at a higher field than those reported for Mes*Pd
CdCMe2 (δP)C 235.0; δC)C 117.0).12 The UV spectrum of
3 displayed a bathochromic shift compared with that of
1-(2,4,6-tri-tert-butylphenyl)-1-phosphaallene [λmax 275 nm
(sh, log ꢀ 3.18)],13 probably due to a hyperconjugation effect
enhanced by the cyclopropyl group.6,14 On the other hand,
the (Z)-5-bromo-1-phosphapent-1-ene Z-5, prepared from
2-bromo-1-phosphaethene 4,8a was allowed to react with
potassium tert-butoxide to afford the (Z)-2-cyclopropyl-1-
phosphaethene Z-6.10 The formation of Z-6 under these
a Reagents and conditions: (a) (i) n-BuLi, THF, -78 °C; (ii)
1,3-dibromopropane, -78 °C to room temperature. (b) t-BuOK,
THF, 0 °C.
conditions indicated that, in the reaction of Z-2 with
potassium tert-butoxide, the cyclopropyl ring was first
formed by γ-elimination before the 1-phosphaallene skeleton
was constructed. No â-elimination took place to afford either
1-phosphapenta-1,2,4-triene or 1-phosphapenta-1,4-diene
derivative from Z-2 or Z-5 with potassium tert-butoxide,
probably indicating that the acidity of the protons at the
3-position is sufficiently high to generate the requisite
anion.15
Next, the cyclopropylidenephosphaethene 3 was allowed
to react with W(CO)5(thf) to afford the corresponding
complex 7 in 70% yield (Scheme 2).16 The structure of 7
(7) (a) Nakamura, I.; Yamamoto, Y. AdV. Synth. Catal. 2002, 344, 111.
(b) Nakamura, E.; Yamago, S. Acc. Chem. Res. 2002, 35, 867.
(8) (a) Appel, R.; Casser, C.; Immenkeppel, M. Tetrahedron Lett. 1985,
26, 3551. (b) Goede, S. J.; Bickelhaupt, F. Chem. Ber. 1991, 124, 2677.
(c) Goede, S. J.; Dam, M. A.; Bickelhaupt, F. Recl. TraV. Chim. Pays-Bas
1994, 113, 278.
Scheme 2a
(9) Yoshifuji, M.; Ito, S. Top. Curr. Chem. 2003, 223, 67.
(10) Z-2: Colorless crystals, mp 108-109 °C dec; 31P{1H} NMR (162
MHz, CDCl3) δ 250; 1H NMR (400 MHz, CDCl3) δ 3.50 (t, 2H, 3JHH ) 7
3
3
Hz, CH2Br), 3.05 (dt, 2H, JPH ) 21 Hz, JHH ) 7 Hz, PdCCH2), 2.24
(quin, 2H, JHH ) 7 Hz, CH2). Z-5: 31P{1H} NMR (162 MHz, CDCl3) δ
3
1
2
3
250; H NMR (400 MHz, CDCl3) δ 7.12 (dd, 1H, JPH ) 39 Hz, JHH
)
8 Hz, PdCH), 3.21 (t, 2H, 3JHH ) 7 Hz, CH2Br), 1.76 (m, 2H, CH2), 1.66
(m, 2H, PdCCH2). Z-6: 31P{1H} NMR (162 MHz, CDCl3) δ 232; 1H NMR
(400 MHz, CDCl3) δ 6.57 (dd, 1H, 2JPH ) 38 Hz, 3JHH ) 11 Hz, PdCH),
0.85 (m, 1H, CH), 0.69 (m, 2H, CHH), 0.48 (m, 2H, CHH); 13C{1H} NMR
1
2
(101 MHz, CDCl3) δ 177.7 (d, JPC ) 43 Hz, PdC), 18.3 (d, JPC ) 21
Hz, CH), 10.6 (d, JPC ) 7 Hz, CH2). Z-10: 31P{1H} NMR (162 MHz,
3
1
3
CDCl3) δ 247; H NMR (400 MHz, CDCl3) δ 3.48 (t, 2H, JHH ) 7 Hz,
3
3
a Reagents and conditions: (a) W(CO)5(thf), rt. (b) LiAlH4, THF,
0 °C.
CH2Br), 2.91 (dt, 2H, JPH ) 21 Hz, JHH ) 7 Hz, PdCCH2), 1.99 (quin,
2H, JHH ) 7 Hz, CH2), 1.85 (quin, 2H, JHH ) 7 Hz, CH2). 11: 31P{1H}
3
3
1
NMR (162 MHz, CDCl3) δ 76; H NMR (400 MHz, CDCl3) δ 3.03 (m,
2H, CH2), 2.96 (m, 2H, CH2), 1.92 (m, 2H, CH2); 13C{1H} NMR (101
1
2
MHz, CDCl3) δ 229.4 (d, JPC ) 24 Hz, PdC), 122.1 (d, JPC ) 14 Hz,
PdCdC), 31.2 (d, 3JPC ) 15 Hz, CH2), 17.4 (s, CH2). 12: 31P{1H} NMR
(162 MHz, CDCl3) δ 66; 1H NMR (400 MHz, CDCl3) δ 5.68 (dt, 1H, 2JPH
) 27 Hz, 3JHH ) 8 Hz, dCH), 3.35 (t, 2H, 3JHH ) 7 Hz, CH2Br), 1.99 (m,
was confirmed by X-ray crystallographic analysis as shown
in Figure 1.17 The C1-C2 distance is shorter whereas the
P-C1 distance is slightly longer than the corresponding data
for [Mes*PdCdCPh2][W(CO)5] [CdC 1.311(10), PdC
1.632(7) Å].18 On the other hand, the C2-C3 and C3-C4
distances in 7 are elongated compared to the proximal bonds
2H, PdCCH2), 1.96 (quin, 2H, JHH ) 7 Hz, CH2); 13C{1H} NMR (101
3
1
2
MHz, CDCl3) δ 239.3 (d, JPC ) 27 Hz, PdC), 109.2 (d, JPC ) 13 Hz,
PdCdC), 33.1 (s, CH2Br), 32.7 (d, 4JPC ) 2 Hz, CH2), 27.8 (d, 3JPC ) 13
Hz, PdCCH2).
(11) 3: Colorless crystals, mp 85-86 °C dec; 31P{1H} NMR (162 MHz,
CDCl3) δ 70; 1H NMR (400 MHz, CDCl3) δ 1.74 (m, 2H, CHH), 1.69 (m,
2H, CHH); 13C{1H} NMR (101 MHz, CDCl3) δ 221.9 (d, JPC ) 26 Hz,
1
2
3
PdCdC), 94.5 (d, JPC ) 14 Hz, PdCdC), 11.0 (d, JPC ) 7 Hz, CH2);
UV (hexanes) λmax (log ꢀ) 210 (4.71), 224 (4.63), 263 (3.95), 308 (sh, 3.34).
(12) Etemad-Moghadam, G.; Tachon, C.; Gougyou, M.; Koenig, M.
Tetrahedron Lett. 1991, 32, 3687.
(15) (a) Ito, S.; Kimura, S.; Yoshifuji, M. Chem. Lett. 2002, 708. (b)
Ito, S.; Kimura, S.; Yoshifuji, M. Bull. Chem. Soc. Jpn. 2003, 76, 405.
(16) 7: Orange crystals, mp 122-124 °C.; 31P{1H} NMR (162 MHz,
CDCl3) δ 40 (1JPW ) 265 Hz); 1H NMR (400 MHz, CDCl3) δ 1.96 (m,
2H, CHH), 1.85 (m, 2H, CHH); 13C{1H} NMR (101 MHz, CDCl3) δ 223.2
(d, 1JPC ) 94 Hz, PdCdC), 200.5 (d, 2JPC ) 34 Hz, COax), 197.5 (d, 2JPC
) 10 Hz, COeq), 95.2 (d, 2JPC ) 11 Hz, PdCdC), 12.0 (d, 3JPC ) 15 Hz,
CH2); IR (KBr) ν 2071, 1955, 1930 cm-1. Anal. Calcd for C27H33O5PW:
C, 49.71; H, 5.10. Found: C, 49.73; H, 5.04.
(13) Ma¨rkl, G.; Reitinger, S. Tetrahedron Lett. 1988, 29, 463.
(14) (a) de Meijere, A. Chem. Ber. 1974, 107, 1684. (b) de Meijere, A.;
Jaekel, F.; Simon, A.; Borrmann, H.; Ko¨hler, J.; Johnels, D.; Scott, L. T. J.
Am. Chem. Soc. 1991, 113, 3935. (c) Bader, R. F.; Slee, T. S.; Cremer, D.;
Kraka, E. J. Am. Chem. Soc. 1983, 105, 5061.
1112
Org. Lett., Vol. 5, No. 7, 2003

several isomerizations of ethenylidenephosphiranes giving
the corresponding phospha[3]radialenes have been reported.26
Since the γ-elimination by potassium tert-butoxide was
established, we then examined the reaction of Z-2 with
another base and an alkoxide was selected. Compound Z-2
was thus allowed to react with sodium ethoxide to afford
1-phosphapenta-1,2,4-triene 9 in 11% isolated yield together
with 3 (Scheme 3).27 It is suggested that the weaker basicity
Scheme 3a
Figure 1. An ORTEP drawing of the molecular structure for 7
with 50% probability ellipsoids. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): P-W
2.5311(8), P-C1 1.637(4), P1-CMes* 1.847(4), C1-C2 1.269(5),
C2-C3 1.481(6), C2-C4 1.480(7), C3-C4 1.53(1), W-P-C1
115.4(1), W-P-CMes* 141.8(1), C1-P1-CMes* 102.6(2), P1-C1-
C2 171.7(3), C1-C2-C3 148.8(6), C1-C2-C4 149.0(5), C3-
C2-C4 62.2(5), C2-C3-C4 58.9(4), C2-C4-C3 58.9(4).
a Reagents and conditions: (a) NaOEt, THF, reflux. (b) t-BuOK,
THF, rt.
of the ethoxide might facilitate the formation of the 1-phos-
phaallene skeleton rather than the formation of the cyclo-
propylidene group. Interestingly, compound 9 isomerized to
in methylenecyclopropane [1.457(14) Å] and the C3-C4
distance is close to the distal bond [1.5415(3) Å].19 It is
suggested that the high-energy HOMO of the cyclopropyl
group can interact with the PdCdC skeleton, especially, to
raise the bond order of the CdC part.6,14
(20) To a solution of 3 (123 mg, 0.38 mmol) in THF was added a THF
solution of LiAlH4 (0.75 mmol) at 0 °C. The reaction mixture was warmed
to room temperature and then refluxed for 1 h. After cooling to room
temperature, the mixture was treated with ethyl acetate at 0 °C. The solvent
was removed in vacuo and the residue was extracted with hexane. In the
31P NMR spectrum E/Z-6 (E/Z 4:1) and 8 were observed in a 2:1 ratio
together with trace amounts of unidentified products. E-6: 31P{1H} NMR
(162 MHz, CDCl3) δ 234; 1H NMR (400 MHz, CDCl3) δ 7.00 (dd, 1H,
Taking into account the existence of the phosphaallene
group and the cyclopropyl ring in the same molecule, the
reactivity of 3, especially the transformation of the cyclo-
propylidenephosphaethene skeleton, is of great interest and
the reaction of 3 with a hydride reagent was carried out.
Compound 3 was thus reacted with lithium aluminum
hydride to mainly afford a geometric mixture of phospha-
ethenes E/Z-6 (E/Z ) 4:1) together with the phosphino-
methylenecyclopropane 8.20,21 Interestingly 8 isomerized to
E/Z-6 (E/Z ) 5:1) upon heating in the presence of a base
such as triethylamine.22 Although it is not obvious whether
the PdC or CdC moiety displays higher reactivity,23 it
should be noted that the cyclopropyl ring remained un-
changed under similar reaction conditions that were em-
ployed for vinylidenecyclopropane (Scheme 2).24 Neither
thermolysis (80 °C in toluene) nor photolysis (λ >300 nm
in benzene-d6) of 3 afforded any skeletal isomerization
product probably due to the bulky Mes* group,25 even though
3
2JPH ) 25 Hz, JHH ) 11 Hz, PdCH), 2.10 (m, 1H, CH), 0.96 (m, 2H,
CHH), 0.57 (m, 2H, CHH). 8: 31P NMR (162 MHz, CDCl3) δ -70 (dd,
1JPH ) 230 Hz, 2JPH ) 22 Hz); 1H NMR (400 MHz, CDCl3) δ 5.72 (d, 1H,
1JPH ) 230 Hz, PH), 6.03 (m, 1H, CH). Phosphaethene E-6 was alternatively
obtained in a similar manner for Z-6.
(21) The reaction of 1-(2,4,6-tri-tert-butylphenyl)-1-phosphaallene (Mes*Pd
CdCH2)13 with lithium aluminum hydride gave Mes*P(H)CHdCH2 (δP
-66) and (E)-Mes*PdCHCH3 (δP 250) in a 1:4 ratio. As for the preparation
of (E)-Mes*PdCHCH3: (a) Ma¨rkl, G.; Bauer, W. Angew. Chem., Int. Ed.
Engl. 1989, 28, 1695. (b) Ito, S.; Toyota, K.; Yoshifuji, M. Chem. Commun.
1997, 1637.
(22) (a) Mercier, F.; Hugel-Le Goff, C.; Mathey, F. Tetrahedron Lett.
1989, 30, 2397. (b) Nguyen, M. T.; Landuyt, L.; Vanquickenborne, L. G.
Chem. Phys. Lett. 1993, 212, 543.
(23) A theoretical study concluded that the HOMO and the LUMO of
HPdCdCH2 are dominated by the PdC moiety. Nguyen, M. T.; Hegarty,
A. F. J. Chem. Soc., Perkin Trans. 2 1985, 1999.
(24) Maercker, A.; Tatai, A.; Grebe, B.; Girreser, U. J. Organomet. Chem.
2002, 642, 1.
(25) No isomerization of an ethenylidenephosphirane bearing the Mes*
group has been reported. Breen, T. L.; Stephan, D. W. J. Am. Chem. Soc.
1995, 117, 11914.
(26) (a) Miyahara, I.; Hayashi, A.; Hirotsu, K.; Yoshifuji, M.; Yoshimura,
H.; Toyota, K. Polyhedron 1992, 11, 385. (b) Toyota, K.; Yoshimura, H.;
Uesugi, T.; Yoshifuji, M. Tetrahedron Lett. 1991, 32, 6879. (c) Maercker,
A.; Brieden, W. Chem. Ber. 1991, 124, 933. (d) Komen, C. M. D.; Horan,
C. J.; Krill, S.; Gray, G. M.; Lutz, M.; Spek, A. L.; Ehlers, A. W.;
Lammertsma, K. J. Am. Chem. Soc. 2000, 122, 12507. (e) Bloch, R.; Le
Perchec, P.; Conia, J.-M. Angew. Chem., Int. Ed. Engl. 1970, 9, 798.
(27) 9: Colorless crystals, mp 82-84 °C dec; 31P{1H} NMR (162 MHz,
CDCl3) δ 68; 1H NMR (400 MHz, CDCl3) δ 6.42 (m, 1H, dCH), 6.37,
(m, 1H, dCH), 5.25 (m, 1H, dCH), 5.06 (m, 1H, dCH); 13C{1H} NMR
(17) Crystal data for 7: C27H33O5PW: M 652.38, red prisms crystallized
from dichloromethane at 0 °C, crystal dimensions 0.30 × 0.30 × 0.25 mm3,
monoclinic, space group P21/c (no. 14), a ) 13.2761(4) Å, b ) 10.1614(3)
Å, c ) 20.8686(8) Å, â ) 101.7659(9)°, V ) 2756.1(2) Å3, Z ) 4, Fcalcd
) 1.572 g cm-3, F(000) ) 1296.00, µ ) 4.287 mm-1, T ) 150 K, 21968
reflections measured (2θmax ) 55.0°), 6225 were observed (Rint ) 0.044),
R1 ) 0.031 [I > 2.0σ(I)], Rw ) 0.042 (all data), S ) 1.23 (439 parameters).
CCDC-200284.
1
2
(18) Yoshifuji, M.; Toyota, K.; Sato, T.; Inamoto, N.; Hirotsu, K.
Heteroat. Chem. 1990, 1, 339.
(19) Laurie, V. W.; Stigliani, W. M. J. Am. Chem. Soc. 1970, 92, 1485.
(101 MHz, CDCl3) δ 242.5 (d, JPC ) 25 Hz, PdCdC), 131.7 (d, JPC )
5
3
13 Hz, PdCdC), 118.1 (d, JPC ) 3 Hz, CH2), 112.9 (d, JPC ) 11 Hz,
CH).
Org. Lett., Vol. 5, No. 7, 2003
1113

3 in the presence of potassium tert-butoxide probably through
cyclization involving the [1,2]-migration of the allenic
proton.28
plex 7. Reaction of 3 with a hydride reagent afforded 6 and
8 without cleavage of the cyclopropyl rings. The cyclopro-
pylidenephosphaethene skeleton remained unchanged by heat
and by light. Potassium tert-butoxide promoted not only
γ-elimination but also isomerization of 9 to 3. The phos-
phaethenes carrying the cycloalkyl group are expected to be
utilized as a synthon for a variety of organophosphorus
compounds to reveal several unique properties that are
enhanced by the electronic effects of the cyclopropane ring.
Second, we applied the above procedure to a cyclobutyl-
idene derivative. The 6-bromo-1-phosphahex-1-ene Z-10,
prepared by a similar method for Z-2 with 1,4-dibromo-
butane, was allowed to react with potassium tert-butoxide
to afford the cyclobutylidenephosphaethene 11 in only 15%
isolated yield.10,29 On the other hand, the reaction of Z-10
with sodium ethoxide afforded 6-bromo-1-phosphahexa-1,2-
diene 12,10 which was converted to 11 in the presence of
potassium tert-butoxide (Scheme 3).
Acknowledgment. This work was supported in part by
a Grant-in-Aid for Scientific Research (No. 90011676) from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
In conclusion, we have demonstrated that it is possible to
prepare a novel 1-phosphaallene derivative 3 containing the
cyclopropylidene moiety and the carbonyltungsten(0) com-
Supporting Information Available: Full spectroscopic
data for Z-2, 3, Z-5, Z/E-6, 7, 9, Z-10, 11, and 12,
experimental details for the preparation of 3, 7, and 9, and
X-ray crystallographic data (CIF) for 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
(28) (a) Griffin, G. W.; Covell, J.; Petterson, R. C.; Dodson, R. M.; Klose,
G. J. Am. Chem. Soc. 1965, 87, 1410. (b) Griffin, G. W.; Marcantonio, A.
F.; Kristinsson, H.; Petterson, R. C.; Irving, C. S. Tetrahedron Lett. 1965,
2951.
(29) We obtained 1,7-bis(2,4,6-tri-tert-butylphenyl)-1,7-diphosphacyclo-
dodeca-2,8-diyne [δP ) -50; νC≡C 2189 cm-1; m/z 684 (M+)] as a
byproduct in the reaction of Z-10 under condition b in Scheme 3.
OL0341750
1114
Org. Lett., Vol. 5, No. 7, 2003