An unusually robust triple bond: Synthesis, structure and reactivity of 3-alkynylcyclopropenes

Article abstract of DOI:10.1016/j.tet.2003.11.074

Several 1,2-diphenyl- and 1,2,3-triphenyl-3-alkynylcyclopropenes have been prepared in moderate to very good yields by the reaction of acetylenic nucleophiles with the appropriate cyclopropenylium salt. Single crystal X-ray structures of four of the cyclopropenes were obtained. Stereoselective reduction of the triple bond failed in all cases, whereas model compounds lacking the cyclopropene moiety were reduced successfully. A rational for this lack of reactivity is proposed. The solution-phase thermochemistry of the 3-alkynyl-1,2,3-triphenylcyclopropenes was explored, affording 3-alkynyl-1H-indenes in moderate to good yields.

Full text of DOI:10.1016/j.tet.2003.11.074

Tetrahedron 60 (2004) 1215–1223
An unusually robust triple bond: synthesis, structure and
reactivity of 3-alkynylcyclopropenes
Robert D. Gilbertson,a He-Ping Wu,a Drew Gorman-Lewis,a Timothy J. R. Weakley,a
Hans-Christoph Weiss,b Roland Boeseb and Michael M. Haleya
,
*
aDepartment of Chemistry, University of Oregon, Eugene, OR 97403-1253, USA
¨ ¨ ¨
Institut fur Anorganische Chemie, Universitat Duisburg-Essen, Universitatstrasse 5-7, 45117 Essen, Germany
b
Received 26 August 2003; revised 18 November 2003; accepted 18 November 2003
Abstract—Several 1,2-diphenyl- and 1,2,3-triphenyl-3-alkynylcyclopropenes have been prepared in moderate to very good yields by the
reaction of acetylenic nucleophiles with the appropriate cyclopropenylium salt. Single crystal X-ray structures of four of the cyclopropenes
were obtained. Stereoselective reduction of the triple bond failed in all cases, whereas model compounds lacking the cyclopropene moiety
were reduced successfully. A rational for this lack of reactivity is proposed. The solution-phase thermochemistry of the 3-alkynyl-1,2,3-
triphenylcyclopropenes was explored, affording 3-alkynyl-1H-indenes in moderate to good yields.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
3-alkynylcyclopropenes, their solid-state structures, and the
surprising inertness of the triple bond towards reduction. We
also describe the thermal reactivity of several derivatives.
Recently we described the syntheses of an iridabenzene1
and an iridabenzene valence isomer2 starting from a
nucleophilic 3-vinylcyclopropene.3 Key to this new tech-
nique was stereospecific preparation of the requisite organic
ligand, a (Z)-3-(2-iodoethenyl)cyclopropene (1). Although
this cyclopropene could be prepared in high yield and
excellent stereoselectivity,1 we sought additional versatile
routes for ligand synthesis. As an alternative to the Wittig
approach, we presumed that selective reduction of a triple
bond (Fig. 1) could be a viable method to access the needed
ligands since a number of stereospecific and high yielding
reagents have been reported in recent years.4 This pathway
was particularly attractive in our studies because of the ease
of preparing the ethynylcyclopropene precursors. Although
reports of 3-alkynylcyclopropenes are limited,5 there are
many preparations of cyclopropenes involving cyclopro-
penylium ions and various nucleophiles.6 Cyclopropenyl-
ium ions are highly electrophilic and their reactions with
nucleophiles ordinarily proceed in high yields.7 We
surmised that reaction of an acetylenic Grignard reagent
with a cyclopropenylium ion, stereoselective reduction of
the triple bond, and halodesilation8 of the terminal
substituent would give the (Z)-3-(2-haloethenyl)cyclopro-
penes needed for metallabenzene and valence isomer
formation. We report herein the synthesis of a family of
2. Results and discussion
2.1. Cyclopropene synthesis
For our studies we focused on the use of 1,2,3-triphenyl-
cyclopropenylium bromide (2a)9 and 1,2-diphenylcyclo-
propenylium perchlorate (2b)10 as the synthesis and
chemistry of these salts is well delineated.7 For example,
treatment of 2a with the appropriate acetylenic nucleophile
in THF at 278 8C afforded cyclopropenes 3a and 4a in very
good yields (Scheme 1). The trimethylsilyl group of 3a was
removed readily by K2CO3 in MeOH and Et2O to give the
Keywords:
Alkynes;
Cyclopropenes;
Strained
compounds;
Thermochemistry.
*
Corresponding author. Tel.: þ1-541-346-0456; fax: þ1-541-346-0487;
e-mail address: haley@oregon.uoregon.edu
Figure 1. Synthetic approaches to (Z)-3-(2-haloethenyl)cyclopropenes.
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.11.074

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R. D. Gilbertson et al. / Tetrahedron 60 (2004) 1215–1223
Scheme 1.
terminal acetylene 5a in quantitative yield. Cyclopropene
5a was then converted into a 1,4-bis(cyclopropenyl)-1,3-
butadiyne system (6a) using a modified Eglinton–Glaser
reaction.11 The corresponding ethynyl-linked biscyclopro-
pene 7a was prepared in modest yield by treating 2a with
0.5 equiv. of ethynyldimagnesium dibromide. The synthesis
of the corresponding diphenylcyclopropenes 3b–7b, which
proceeded in an analogous fashion, has been described
previously.12
Selected bond lengths and angles for all four systems, as
well as comparison with the analogous values in the C5
parent 3-ethynylcyclopropene,5e are given in Table 1.
Typical of cyclopropenes in the solid state,13 the carbon–
carbon double bond lengths in the phenyl substituted
3
˚
derivatives are quite short (1.290–1.302 A) and the sp
bond angles about C3 (50.2–50.88) are highly constrained.
The carbon–carbon triple bond lengths are in the normal
˚
range (1.198–1.208 A). Almost all these values are longer
or greater than in 3-ethynylcyclopropene,5e (CvC 1.255 A,
˚
˚
2.2. Solid-state structures
CuC 1.184 A, C1–C3–C2 49.48), even if libration
corrected values are considered.5f These increases are
clearly due to the phenyl substituents which not only reduce
the libration but also release the electron concentration
and thus relieve some of the tension in the highly strained
parent molecule.
Owing to the highly unsaturated and inherently reactive
nature of cyclopropenes, we obtained single crystal X-ray
structures for several derivatives (3a, 4b, 6b, 7a). The
molecular structures of 3a and 4b are shown in Figure 2.
Interestingly, the packing diagrams for 6b and 7a show
several close intermolecular C–H· · ·p contacts. For 6b, the
˚
hydrogen on C3 sits 2.69 A above/below the p-bond of the
˚
Table 1. Selected bond lengths (A) and bond angles (deg) for
3-alkynylcyclopropenes 3a, 4b, 6b, 7a, and the parent molecule
cyclopropene ring in the next molecule (Fig. 3). The
intermolecular H3–C4 distance in 6b is also quite short
Parenta
3a
4b
6b
7ab
˚
(2.78 A). These contacts result in a packing motif that is
C(1)–C(2)
C(1)–C(3)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–R0
C(1)–C(2)–C(3)
C(1)–C(3)–C(2)
C(1)–C(2)–C(Ph)
C(2)–C(1)–C(Ph)
C(3)–C(4)–C(5)
C(4)–C(5)–R0
1.278(2)c 1.290(2) 1.297(2) 1.297(3) 1.302(3)
1.500(2) 1.515(2) 1.517(2) 1.507(3) 1.527(4)
1.502(2) 1.525(2) 1.514(2) 1.516(3) 1.529(3)
1.448(2) 1.462(2) 1.444(2) 1.447(3) 1.472(3)
1.184(2) 1.198(2) 1.200(3) 1.200(3) 1.208(5)d
reminiscent of one necessary for a topochemical diacetylene
polymerization. However, H3 pushes the neighboring
diynes apart, thus yielding 1,4-polymerization parameters
˚
˚
(d¼5.43 A, g¼538, and S1¼3.81 A) that are somewhat
e
e
1.827(2) 1.432(2) 1.387(4)
outside the range typically observed for such monomers
(d<5 A, g<458).
65.2(1)
49.1(1)
64.5(1) 64.8(1) 64.3(2) 64.7(2)
50.2(1) 50.7(1) 50.8(1) 50.5(2)
154.6(2) 153.1(1) 154.6(2) 151.5(3)
153.8(2) 154.7(1) 155.2(2) 152.2(3)
14
˚
e
e
The crystal packing in 7a is considerably more complex
(Fig. 4). The individual molecules alternate their orientation
and are offset with respect to one another. This arrangement
is such that one of the meta protons of a cyclopropenyl
phenyl group is pointed into the p-bond of a neighboring
179.0(1) 178.1(2) 175.9(1) 178.6(2) 177.0(4)d
e
e
177.2(2) 174.8(1) 179.8(2)
a
Ref. 5e.
b
Averages values for two independent molecules.
Libration corrected value; see Ref. 5f.
C(4)–C(4A) in 7a.
c
d
e
˚
cyclopropenyl unit with a C–H· · ·p contact of 2.72 A
(center of the double bond). In addition, the ortho and para
No comparable value.

R. D. Gilbertson et al. / Tetrahedron 60 (2004) 1215–1223
1217
Figure 2. Molecular structures of cyclopropenes 3a (top) and 4b (bottom).
protons on the same cyclopropenyl phenyl group possess
˚
short C–H· · ·p contacts (3.24 and 3.47 A) with the two
2.3. Attempted reduction
phenyl rings that are attached to the same neighboring
cyclopropenyl unit.
With a number of 3-alkynylcyclopropenes now available,
we attempted the stereospecific reduction of the triple bond.

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R. D. Gilbertson et al. / Tetrahedron 60 (2004) 1215–1223
Figure 3. Crystal packing of cyclopropene 6b.
Figure 4. Crystal packing of cyclopropene 7a.
Reaction of 3a with Schwartz’s reagent (Cp2ZrHCl)15
followed by aqueous work-up resulted in complete recovery
of starting material. Use of ‘Cp2TiH’, generated in situ from
Cp2TiCl2 and i-BuMgBr,16 also resulted in recovery of 3a.
Careful hydrogenation with Lindlar’s catalyst faired no
better as the cyclopropene double bond was reduced
preferentially. DIBAL-H (in a variety of solvents and
temperatures) and LiAlH4, both of which would give the
undesired E-geometry, either failed to work or reduced the
double bond preferentially. Examination of Figure 2(a)
suggested that steric bulk might be the culprit for the
unreactivity17 and that use of 3b, missing the Ph substituent
at C3, would be more suitable. Unfortunately, exposure of
3b to the various reagents and conditions described above
also led to recovery of starting material or over-reduced
products.
Scheme 2.

R. D. Gilbertson et al. / Tetrahedron 60 (2004) 1215–1223
1219
To what then can this surprising inertness of the triple bond
in 3 be ascribed? Reagents and reaction conditions were
ruled out as repetition of the above experiments with both
(1-hexynyl)trimethylsilane and (2-phenylethynyl)trimethyl-
silane gave in every instance the correct product from
cis-reduction (J¼10–11 Hz). This result then suggested
that electronics, and not sterics, were the cause of the
unreactivity of the triple bond. To confirm this
hypothesis, we prepared alkyne 8 (Scheme 2), the
cyclopropane analog of 3b. Exposure of aldehyde 918 to
the Corey–Fuchs procedure19 and quench of the resultant
acetylide ion with Me3SiCl gave 8 in 63% yield. Treatment
of 8 with either Cp2ZrHCl or ‘Cp2TiH’ followed by aqueous
work-up resulted in complete, stereospecific reduction of
the triple bond, furnishing alkene 10 in 66% yield.
Surprisingly, the expected Z-geometry about the double
bond was instead E-, based on an alkene coupling constant
of 14.2 Hz. Isomerization of Z-alkenylsilanes to E- is a
relatively facile process, and we have encountered this
problem before while investigating alternative syntheses of
Z-2-(cyclopropenyl)ethenylsilanes.20
Unlike the related 3-vinylcyclopropenes, which furnish
cyclopentadienes via electrocyclic ring closure (in addition
to indenes),26 the distal alkyne carbon is too remote to
undergo analogous reactivity.5d Brown rearrangement23a
(vinylidene formation and subsequent C–H insertion) could
lead to the cyclopent[cd]indene skeleton (12),27 or with
sufficient energy (pyrolysis or photolysis), to indene/
phenanthrene hybrid 13.28
Our initial attempts focused on the thermolysis of 3a–5a.
DSC analysis showed that these molecules underwent
exothermic reaction around 225 8C. Heating dodecane
solutions of the cyclopropenes to 220 8C for 2 h proceeded
with complete conversion to indenes 11a–11c (Scheme 4).28
These compounds result from insertion of the resultant
vinylcarbene into one of the ortho C–H bonds of the Ph
substituent on C3 in 5. Interestingly, indenes 11 do not
undergo sigmatropic rearrangement under the reaction
conditions to give the corresponding indenes with the
double bond residing between the two Ph substituents,
which would be the expected, thermodynamically preferred
double bond configuration.29 In an attempt to prepare a
larger quantity of 11c, desilylation of 11a with K2CO3 in
MeOH/Et2O unexpectedly furnished methoxyfulvene 14 as
the sole product, isolated in 77% yield as a 14:1 mixture of
regioisomers. This presumably occurs by base-induced
isomerization to form unstable vinylidene 15,30 which
then is attacked by the nucleophilic base and subsequently
protonated. This instability/facile isomerization is possibly
why 11c was produced in low yield. Unfortunately, neither
12 nor 13 were observed in our experiments. Formation of
these compounds will likely require temperatures greater
than 500 8C, attainable only by flash vacuum pyrolysis.
One possible explanation for the lack of reactivity of the
triple bond in the alkynylcyclopropenes is due to hyper-
conjugation. The s-bond to an sp hybridized carbon might
be a reasonably good hyperconjugative electron acceptor for
delocalization of the electron pair in the cyclopropenyl
p-bond, since
a
resonance structure that is
(cyclopropenium)þ (alkyne)2 seems reasonable. This in
turn deactivates the triple bond to attack of the electrophilic
reagents. Such hyperconjugation has been shown to occur in
both difluorocyclopropanes21 and fluorocyclopropenes.22
Unfortunately, high level theoretical calculations on our
molecules have not been illuminating.
2.4. Thermochemistry
Given the current interest in ‘buckybowls’ and other related
fullerene fragments,23 we investigated the thermal chem-
istry of the 3-alkynyl-1,2,3-triphenylcyclopropenes as they
might permit access to novel polycyclic aromatic hydro-
carbons. After initially opening to the vinylcarbene,6a,24 the
alkynylated systems should undergo C–H insertion on the
phenyl at C3 to yield indenes25 such as 11 (Scheme 3).
Scheme 4.
3. Conclusions
In summary, we have prepared in very good yield a family
of 3-ethynylcyclopropenes by the reaction of cycloprope-
nylium salts with acetylenic nucleophiles. The X-ray crystal
structures of four of these compounds were determined,
exhibiting short carbon–carbon double bonds and highly
constrained bond angles at C3, both attributable to the
Scheme 3.

1220
R. D. Gilbertson et al. / Tetrahedron 60 (2004) 1215–1223
strained nature of the three-membered ring. Attempts to
reduce the carbon–carbon triple bond in a stereoselective
fashion failed to give the desired products, likely due to
electronic effects resulting from hyperconjugation of the
cyclopropene with the alkyne moiety. Thermolysis of the
cyclopropenes gave alkynylindenes, the product of ring-
opening with C–H insertion on the Ph substituent on C3.
Treatment of the silyl-protected indene with base afforded
an unstable vinylideneindene which reacted further to
furnish a benzofulvene. Future studies will be directed
toward the high temperature pyrolysis of the alkynylindenes
and stereoselective reduction of alkynylstannanes.
under N2. The solution of alkynylmagnesium bromide was
added to the cold suspension of 2a using a double-ended
needle under N2 pressure over a 5 min period. The flask was
rinsed with THF (10 mL) and the rinse was added to the
suspension of 2a. After stirring at 278 8C for 1 h, the
cooling bath was removed and the mixture was stirred at
ambient temperature for 12 h. Excess Grignard reagent was
quenched with saturated aqueous NH4Cl. The mixture was
extracted with Et2O, the phases were separated, and the
aqueous phase was extracted again with Et2O. The com-
bined organics were washed with water, saturated NaHCO3
solution, and brine. The organic layer was dried (MgSO4),
filtered through celite, and concentrated to give a yellow oil.
Purification by preparative radial thin-layer chromato-
graphy (2 mm rotor, hexanes) furnished 3a (765 mg, 84%)
4. Experimental
4.1. General
1
as a white solid, mp 150.6 8C (DSC). H NMR (CDCl3):
7.70 (dd, J¼7.6, 1.8 Hz, 4H), 7.52–7.34 (m, 8H), 7.26 (t,
J¼7.3 Hz, 2H), 7.18 (t, J¼7.2 Hz, 1H), 0.18 (s, 9H). 13C
NMR (CDCl3): 142.19, 129.92, 129.88, 128.93, 128.03,
126.39, 126.10, 125.72, 111.79, 108.55, 82.84, 23.93, 0.31.
IR (KBr): 2153, 1834. Calcd for C26H24Si: C, 85.66; H,
6.64. Found: C, 85.46; H, 6.67.
1H and 13C NMR spectra were recorded in either CDCl3 or
CD2Cl2 using a Varian Inova 300 NMR (1H: 299.94 MHz,
13C: 75.43 MHz) spectrometer Chemical shifts (d) are
expressed in ppm downfield from SiMe4 using the residual
solvent as internal standard (CDCl3–1H: 7.27 ppm, 13C:
77.0 ppm; CD2Cl2–1H: 5.32 ppm, 13C: 54.0 ppm). Coup-
ling constants are expressed in Hz. IR spectra were recorded
using a Nicolet Magna-FTIR 550 spectrometer. Melting
points were determined on a Meltemp II apparatus and are
uncorrected. DSC analyses were performed using a TA
Instruments DSC 2920. Elemental analyses were performed
by Robertson Microlit Laboratories, Inc.
4.1.2. 3-(Phenylethynyl)-1,2,3-triphenylcyclopropene
(4a). Use of phenylethyne (1.43 g, 14 mmol) in the
procedure described above for 3a gave after purification
compound 4a (728 mg, 79%) as a white solid, mp 155.5 8C
1
(DSC). H NMR (CDCl3): 7.76 (dd, J¼7.9, 1.3 Hz, 4H),
7.60 (d, J¼7.7 Hz, 2H), 7.51–7.37 (m, 8H), 7.33–7.25 (m,
5H), 7.19 (t, J¼7.3 Hz, 1H). 13C NMR (CDCl3): 142.52,
131.81, 131.59, 129.91, 129.84, 128.99, 128.10, 127.45,
126.48, 126.16, 125.80, 124.02, 112.16, 92.35, 78.47, 23.77.
IR (KBr): 2215, 1829. Calcd for C29H20: C, 94.53; H, 5.47.
Found: C, 94.44; H, 5.57.
Triphenylcyclopropenylium bromide (2a)9 was prepared
according to the literature. All other reagents were
purchased from commercial suppliers and used as received.
CH2Cl2 and pyridine were distilled from CaH2 under an N2
atmosphere prior to use. Et2O and THF were distilled from
Na/benzophenone ketyl immediately prior to use. All other
chemicals were of reagent quality and used as obtained from
the manufacturers. Column chromatography was performed
on Whatman reagent grade silica gel (230–400 mesh).
Rotary chromatography was performed on a Chromatotron
using silica gel (60 PF254) plates (1–4 mm). Baker pre-
coated silica gel plates were used for analytical
(200£50£0.25 mm3) thin layer chromatography. Reactions
were carried out in an inert atmosphere (dry N2 or Ar) when
necessary.
4.1.3.
3-Ethynyl-1,2,3-triphenylcyclopropene
(5a).
Cyclopropene 3a (547 mg, 1.5 mmol) was dissolved in
Et2O (8 mL) and MeOH (20 mL). Anhydrous K2CO3
(221 mg, 1.6 mmol) was added and the resulting suspension
was stirred at ambient temperature for 8 h. Et2O and water
were added, the layers separated, and the aqueous phase
extracted with Et2O. The combined organics were washed
with brine and dried (MgSO4). The suspension was filtered
through celite and concentrated to give 5a (436 mg, 99%) as
a pale yellow solid. Recrystallization from hexanes afforded
1
a pure, colorless sample, mp 117.8 8C (DSC). H NMR
(CDCl3): 7.72 (d, J¼8.0 Hz, 4H), 7.56–7.36 (m, 8H), 7.28
(t, J¼7.2 Hz, 2H), 7.18 (t, J¼7.2 Hz, 1H), 2.18 (s, 1H). 13C
NMR (CDCl3): 141.77, 129.86, 129.41, 129.02, 128.12,
126.11, 126.00, 125.88, 111.43, 86.42, 66.16, 22.86. IR
(KBr): 3291, 2152, 1834. Calcd for C23H16: C, 94.48; H,
5.52. Found: C, 94.41; H, 5.59.
4.1.1. 3-(Trimethylsilylethynyl)-1,2,3-triphenylcyclopro-
pene (3a). Ethylmagnesium bromide was prepared from
magnesium (0.30 g, 12 mmol) and dropwise addition of
bromoethane (1.30 g, 12 mmol) in THF (20 mL). Once the
addition of bromoethane was complete, the reaction was
heated at reflux for 20 min. The suspension was then cooled
to 0 8C under an atmosphere of N2 and ethynyltrimethyl-
silane (1.38 g, 14 mmol) was added quickly via syringe.
The resulting gray suspension was stirred at 0 8C for
15 min with the evolution of ethane gas. The mixture was
then warmed to ambient temperature and stirred for an
additional 15 min. THF (10 mL) was added to dissolve the
solids.
4.1.4. 1,4-Bis(1,2,3-triphenylcycloprop-2-enyl)-1,3-buta-
diyne (6a). Cyclopropene 5a (117 mg, 0.4 mmol) and
Cu(OAc)2 (250 mg, 1.2 mmol) were suspended in pyridine
(2 mL), water (1 mL) and dioxane (1 mL). The suspension
was warmed with stirring to 45 8C and all solids dissolved to
give a blue solution. A precipitate started to form after
20 min at 45–50 8C. After a total of 2 h of stirring at 45–
50 8C, during which the blue color gradually changed to
green, the flask was cooled to 0 8C and the precipitate was
collected by filtration. The tan solid was washed with 10%
In a separate flask triphenylcyclopropenyl bromide (2a,
869 mg, 2.5 mmol) in THF (175 mL) was cooled to 278 8C

R. D. Gilbertson et al. / Tetrahedron 60 (2004) 1215–1223
1221
1
HCl solution and water, then dried in vacuo. Recrystalliza-
tion from benzene (,3 mL) gave 6a (84 mg, 72%) as small
white cubes, mp 229 8C (melt with decomp.). H NMR
(CD2Cl2): 7.69 (d, J¼7.7 Hz, 8H), 7.51–7.38 (m, 16H),
7.26 (t, J¼7.5 Hz, 4H), 7.17 (t, J¼7.3 Hz, 2H). 13C NMR
(CD2Cl2): 141.94, 130.39, 130.18, 129.62, 128.86, 126.63,
126.45, 126.32, 111.81, 80.49, 64.11, 24.31. IR (KBr):
2142, 1832. Calcd for C46H30: C, 94.81; H, 5.19. Found: C,
94.68; H, 5.41.
63% from 9) as a white solid, mp 51.6–52.5 8C. H NMR
(CDCl3): 7.46–7.22 (m, 10H), 2.87 (t, J¼6.0 Hz, 1H), 2.63
(dd, J¼8.6, 6.3 Hz, 1H), 2.37 (dd, J¼8.6, 5.5 Hz, 1H), 0.11
(s, 9H). 13C NMR (CDCl3): 139.97, 137.15, 128.50, 128.31,
127.75, 126.52, 126.44, 126.23, 104.74, 86.10, 33.47, 33.43,
20.39, 20.14. IR (KBr): 2164, 1598. Calcd for C20H22Si: C,
82.70; H, 7.63. Found: C, 82.47; H, 7.67.
1
4.1.7. (E)-2-(trans-2,3-Diphenylcyclopropyl)ethenyltri-
methylsilane (10). Cyclopropane 8 (49 mg, 0.17 mmol)
was placed in a flame-dried round bottom flask and
dissolved in dry THF (3 mL). Cp2ZrHCl (53 mg,
0.20 mmol) was added in three portions and the reaction
was stirred at room temperature for 1 h. Wet pentane (3 mL)
was added to the mixture, which was then stirred for an
additional 2 h. Concentration of the mixture and flash
chromatography of the residue over silica (hexanes) gave
alkene 10 (33 mg, 66%) as colorless oil. 1H NMR (CDCl3):
7.42–7.21 (m, 10H), 5.76 (dd, J¼14.2, 10.1 Hz, 1H), 5.52
(d, J¼14.2 Hz, 1H), 2.81 (dd, J¼9.1, 6.1 Hz, 1H), 2.59 (t,
J¼5.6 Hz, 1H), 2.37 (td, J¼9.6, 5.1 Hz, 1H), 0.19 (s, 9H).
13C NMR (CDCl3): 146.38, 141.40, 137.84, 129.22, 129.15,
128.49, 128.19, 126.31, 126.22, 125.99, 34.12, 33.07, 31.45,
0.19. HRMS calcd for C20H24Si: 292.1647. Found:
292.1653.
4.1.5. Bis(1,2,3-triphenylcycloprop-2-enyl)ethyne (7a). A
0.5 M solution of ethynyldimagnesium dibromide in THF
was prepared according to the literature.31 A portion of the
solution (1.3 mL, 0.65 mmol) was diluted with THF
(40 mL) and cooled to 278 8C under N2. Cyclopropenylium
salt 2a (434 mg, 1.25 mmol) was added to the cold Grignard
solution and the reaction was stirred for 2 h. The reaction
was warmed to ambient temperature and stirred for an
additional 12 h. Et2O and saturated NH4Cl solution were
added. The phases were separated and the organic phase was
washed with saturated NaHCO3 solution, water, and brine.
The organic phase was dried (MgSO4), filtered through
celite, and concentrated. The residual material was chro-
matographed (preparative radial thin-layer chromatography,
2 mm rotor, 9:1 petroleum ether/EtOAc) affording a pale
yellow solid. Recrystallization from boiling hexanes
(,10 mL) yielded 7a (97 mg, 27%) as white needles, mp
4.1.8. 1,2-Diphenyl-3-(trimethylsilylethynyl)-1H-indene
solution of cyclopropene 3a (50 mg,
1
190 8C (melt with decomp.). H NMR (CDCl3): 7.71 (d,
(11a).
A
J¼7.6 Hz, 8H), 7.45–7.32 (m, 12H), 7.19 (t, J¼7.3 Hz,
4H), 7.10 (t, J¼7.2 Hz, 2H). 13C NMR (CDCl3): 143.37,
129.86, 129.02, 128.86, 127.91, 126.82, 126.23, 125.46,
112.58, 80.94, 23.54. IR (KBr): 1828. Calcd for C44H30: C,
94.59; H, 5.41. Found: C, 94.61; H, 5.47.
0.137 mmol) in dodecane (2 mL) was heated in a ,220 8C
sand bath for 2 h. The dodecane was removed under vacuum
and the residue purified by column chromatography
(hexanes/CH2Cl2, 3:1), giving 11a (39 mg, 78%) as a
beige powder, mp 127.0–127.5 8C. 1H NMR (CDCl3): 7.95
(br d, J¼7.0 Hz, 2H), 7.57 (d, J¼7.3 Hz, 1H), 7.39–7.09
(m, 11H), 5.13 (s, 1H), 0.36 (s, 9H). 13C NMR (CDCl3):
152.20, 146.74, 143.04, 139.94, 133.53, 128.83, 128.28,
128.04, 127.96, 127.94, 127.18, 126.78, 126.33, 123.54,
121.39, 120.49, 103.83, 100.17, 56.78, 20.06. IR (KBr):
2140, 1596. Calcd for C26H24Si: C, 85.66; H, 6.64. Found:
C, 85.53; H, 6.48.
4.1.6. 2-(trans-2,3-Diphenylcyclopropyl)ethynyltri-
methylsilane (8). CBr4 (2.12 g, 6.4 mmol), PPh3 (3.53 g,
13.4 mmol), and dry CH2Cl2 (25 mL) were combined and
stirredatroomtemperaturefor20 min,formingadeeporange-
red solution. After cooling to 0 8C, aldehyde 9 (1.34 g,
6 mmol) in CH2Cl2 (2 mL) was added dropwise and the
mixture stirred at 0 8C for 30 min, then room temperature for
3 h. The suspension was diluted with Et2O (100 mL), filtered
through celite, and the solids washed with additional Et2O
(30 mL). After concentration, the residue was passed over a
short silica gel column using hexanes (100 mL). Concen-
tration of the solution gave the crude dibromoalkene (1.95 g,
86%) as a viscous yellow oil. 1H NMR (CDCl3): 7.41–7.21
(m, 10H), 5.87 (d, J¼9.0 Hz, 1H), 2.87 (dd, J¼9.0, 6.2 Hz,
1H), 2.63 (t, J¼5.7 Hz, 1H), 2.37 (td, J¼9.0, 5.1 Hz, 1H). 13C
NMR (CDCl3): 140.25, 137.05, 136.66, 128.90, 128.59,
128.54, 126.87, 126.48, 88.07, 33.13, 32.83, 30.43.
4.1.9. 1,2-Diphenyl-3-(phenylethynyl)-1H-indene (11b).
Thermolysis of 4a (60 mg, 0.163 mmol) as described
above gave 11b (49 mg, 82%) as a light yellow powder,
1
mp 148.5–149.1 8C. H NMR (CDCl3): 8.05 (d, J¼8 Hz,
2H), 7.76–7.69 (m, 3H), 7.52–7.36 (m, 6H), 7.32–7.20
(m, 8H), 5.25 (s, 1H). 13C NMR (CDCl3): 151.42, 146.87,
143.19, 139.97, 134.77, 131.73, 128.83, 128.46, 128.28,
128.18, 127.95, 127.88, 127.20, 126.77, 126.36, 123.26,
120.48, 97.40, 84.71, 56.91. IR (KBr): 2203, 1597.
Calcd for C29H20: C, 94.53; H, 5.47. Found: C, 94.26;
H, 5.33.
To a stirred solution of the above dibromoalkene (1.89 g,
5 mmol) and dry Et2O (40 mL) at 0 8C was added
butyllithium (5 mL, 2.5 M, 12.5 mmol) dropwise. After
stirring at 0 8C for 15 additional min, freshly distilled
Me3SiCl (2.17 g, 20 mmol) in Et2O (3 mL) was added. The
mixture was allowed to stir and warm to room temperature
over 1 h. The reaction was quenched by addition of
saturated NH4Cl solution (10 mL). The aqueous layer was
removed and the organics dried (MgSO4), filtered, and
concentrated. Purification by preparative radial thin-layer
chromatography (4 mm rotor, hexanes) afforded 8 (1.07 g,
4.1.10. 1,2-Diphenyl-3-ethynyl-1H-indene (11c). Thermo-
lysis of 5 (50 mg, 0.171 mmol) as described above gave 11c
(13 mg, 26%) as a pale yellow solid, mp 113–114 8C. H
NMR (CDCl3): 7.96 (d, J¼7.2 Hz, 2H), 7.79 (d, J¼7.8 Hz,
1H), 7.40–7.25 (m, 4H), 7.21–7.08 (m, 7H), 5.01 (s, 1H),
2.24 (s, 1H). 13C NMR (CDCl3): 152.23, 146.17, 143.21,
140.79, 134.49, 129.66, 128.01, 128.07, 128.17, 127.70,
127.02, 126.36, 123.37, 121.23, 120.02, 103.03, 101.32,
59.93. IR (KBr): 3297, 2142, 1597. Calcd for C23H16: C,
94.48; H, 5.52. Found: C, 94.27; H, 5.41.
1

1222
R. D. Gilbertson et al. / Tetrahedron 60 (2004) 1215–1223
4.1.11. 1-(a-Methoxyethylidene)-2,3-diphenylindene
(14). Indene 11a (29 mg, 0.08 mmol) was subjected to the
protiodesilylation reaction conditions described for the
preparation of 5a. Purification of the crude material by
column chromatography (hexanes/CH2Cl2, 1:1) furnished
14 (20 mg, 77%) as a yellow solid, mp 170.5–171.0 8C. 1H
NMR (CDCl3): 8.21 (br d, J¼7.0 Hz, 1H), 7.39 (br d,
J¼7.3 Hz, 1H), 7.33–7.16 (m, 12H), 3.97 (s, 3H), 1.91 (s,
3H). 13C NMR (CDCl3): 162.26, 140.17, 138.57, 137.43,
137.27, 135.71, 135.06, 130.60, 129.83, 127.94, 127.72,
126.63, 126.23, 125.29, 124.96, 124.25, 120.85, 119.48,
55.02, 15.97. IR (KBr): 2935, 2840, 1610, 1592. Calcd for
C24H20O: C, 88.85; H, 6.21. Found: C, 88.61; H, 6.07.
observed (I$2s(I)), 397 parameters, R1¼0.0627,
wR2¼0.1482.
Acknowledgements
We thank the National Science Foundation, The Camille
and Henry Dreyfus Foundation (Teacher-Scholar Award
1998–2003 to M. M. H.), and the Fonds der Chemischen
Industrie for support of this research. D. G.-L. acknowl-
edges the McNair Foundation for a summer research
fellowship (2000). We thank Dr Alex Blumenfeld and
Professor Richard Williams (University of Idaho) for
acquisition and interpretation of the COSY, HMBC, and
HMQC NMR spectra for 11a–b.
4.2. X-ray crystal structures
Data for 3a were obtained on an Enraf-Nonius CAD-4
Turbo diffractometer; solution and refinement (C atoms
anisotropic, H atoms riding) were accomplished with
teXsan (v. 1.7 for SGI workstations). Data for 4b, 6b, and
7a were obtained on a Siemens SMART diffractometer;
solution and refinement (C atoms anisotropic, H atoms
riding) were performed on F 2 with SHELXTL program
suite (v. 5.03). Crystallographic data (excluding structure
factors) for the reported structures have been deposited with
the Cambridge Crystallographic Data Center as supplemen-
tary publications no. CCDC-217174 (3a), 216916 (4b),
216917 (6b), and 216918 (7a).
References and notes
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Organometallics 2003, 22, 3279–3289.
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3. See also: (a) Wu, H.-P.; Lanza, S.; Weakley, T. J. R.; Haley,
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Weakley, T. J. R.; Haley, M. M. Angew. Chem. Int. Ed. 2002,
41, 3470–3473. (c) Wu, H.-P.; Weakley, T. J. R.; Haley, M. M.
Organometallics 2002, 21, 4320–4322. (d) Jacob, V.;
Weakley, T. J. R.; Haley, M. M. Organometallics 2002, 21,
5394–5400.
4.2.1. Compound 3a. C26H24Si, Mr¼364.56, colorless
tablet, 0.20£0.05£0.04 mm3, monoclinic, space group
˚
P21/c, a¼3.8508(2), b¼21.5422(3), c¼21.4937(8) A,
3
˚
b¼91.860(2)8, V¼1782.06(9) A , Z¼4, rcalc¼1.298
21
g cm23, Mo Ka radiation (l¼0.71069 A), m¼0.74 cm
,
˚
F(000)¼720, T¼2114 8C, 2umax¼458, 2657 independent
reflections scanned, 548 reflections in refinement (I$3s(I)),
113 parameters, R¼0.085, Rw¼0.089.
4. Larock, R. C. Comprehensive organic transformations; 2nd ed.
Wiley-VCH: New York, 2000.
5. (a) Domnin, I. N.; Ivanov, A. L.; Fovorskaya, I. A. Zh. Org.
Khim. 1986, 22, 1780–1783. (b) Ivanov, A. L.; Domnin, I. N.
Zh. Org. Khim. 1988, 24, 2547–2552. (c) Maier, G.; Rang, H.;
Emrich, R. Liebigs Ann. 1995, 153–160. (d) Haley, M. M.;
Biggs, B.; Looney, W. A.; Gilbertson, R. D. Tetrahedron Lett.
4.2.2. Compound 4b. C23H16, Mr¼292.36, colorless plate,
0.28£0.24£0.13 mm3, monoclinic, space group P21/c,
˚
a¼10.3133(1), b¼19.7581(1), c¼8.1048(1) A, b¼
3
100.070(1)8, V¼1626.08(3) A , Z¼4, rcalc¼1.194 g cm23
,
,
˚
¨
1995, 36, 3457–3460. (e) Baldridge, K. K.; Biggs, B.; Blaser,
Mo Ka radiation (l¼0.71073 A), m¼0.068 mm21
˚
D.; Boese, R.; Haley, M. M.; Maulitz, A. H.; Siegel, J. S.
Chem. Commun. 1998, 1137–1138. (f) Dunitz, J. D. Chem.
Commun. 1999, 2547–2548. (g) Maier, G.; Neudert, J.; Wolf,
O.; Pappusch, D.; Sekiguchi, A.; Tanaka, M.; Matsuo, T. J. Am.
Chem. Soc. 2002, 124, 13819–13826.
F(000)¼616, T¼2115 8C, 2umax¼56.78, 3166 independent
reflections scanned, 2276 reflections observed (I$2s(I)),
208 parameters, R1¼0.0509, wR2¼0.1179.
4.2.3. Compound 6b. C34H22, Mr¼430.52, colorless plate,
6. (a) Halton, B.; Banwell, M. G. Cyclopropenes. In The
chemistry of the cyclopropyl group; Rappaport, Z., Ed.;
Wiley: New York, 1987; pp 1223–1339. (b) Baird, M. S. In
Cyclopropenes—synthesis by construction of the system.
Methods of organic chemistry; de Meijere, A., Ed.; Thieme:
Stuttgart, 1996; Vol. E17d, pp 2695–2744.
0.27£0.22£0.06 mm3, monoclinic, space group P21/c,
˚
a¼15.3093(12),
b¼4.1431(3),
c¼18.9043(15) A,
b¼106.678(2)8, V¼1148.6(2) A , Z¼2, rcalc¼1.245 g cm23
,
,
3
˚
Mo Ka radiation (l¼0.71073 A), m¼0.070 mm21
˚
F(000)¼452, T¼2115 8C, 2umax¼49.48, 1666 independent
reflections scanned, 1223 reflections observed (I$2s(I)),
154 parameters, R1¼0.0523, wR2¼0.1238.
7. (a) Komatsu, K.; Yoshida, Z. In Cyclopropenylium salts.
Methods of organic chemistry (Houben-Weyl); de Meijere, A.,
Ed.; Thieme: Stuttgart, 1996; Vol. E17d, pp 3079–3192.
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1371–1427.
4.2.4. Compound 7a. C44H30, Mr¼558.68, colorless plate,
0.21£0.18£0.05 mm3, triclinic, space group P1, a¼
˚
10.3207(2), b¼12.9619(1), c¼14.4042(3) A, a¼64.213(1),
8. There are a number of possible procedures for dehalosilation
with retention of the Z-geometry; see Ref. 4, pp 432–434 and
749–750.
3
˚
b¼79.148(1), g¼67.475(1)8, V¼1602.14(5) A , Z¼2,
rcalc¼1.158 g cm23
,
Mo Ka radiation (l¼0.71073 A),
˚
m¼0.066 mm21, F(000)¼588, T¼2115 8C, 2umax¼49.98,
9. Xu, R.; Breslow, R. Organic Syntheses; Freeman, J. P., Ed.;
Wiley: New York, 1998; Coll. Vol. IX, pp 730–732.
4720 independent reflections scanned, 2880 reflections

R. D. Gilbertson et al. / Tetrahedron 60 (2004) 1215–1223
1223
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analogy.
17. We have encountered other systems where substituents at C3
have also impeded subsequent reactivity. For example, see:
Gilbertson, R. D.; Weakley, T. J. R.; Haley, M. M.; Weiss,
H.-C.; Boese, R. Organometallics 1998, 17, 3105–3107.
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