Thermal C2-C6 cyclization of enyne-allenes. Experimental evidence for a stepwise mechanism and for an unusual thermal silyl shift

Article abstract of DOI:10.1021/jo062448+

Enyne-allenes 4a-c bearing various cyclopropyl systems as radical clock reporter groups at the allene terminus have been synthesized and subjected to thermal C²-C⁶ cyclization. The ratio of ene versus formal Diels-Alder products could be rationalized on the basis of steric effects. Only the thermolysis of 4c, equipped with the fast diphenylcyclopropylcarbinyl radical clock, afforded a 1,3-butadienyl benzofulvene clearly formed via cyclopropyl ring opening. This finding provides unambiguous evidence for a stepwise mechanism of the C²-C⁶ cyclization making it possible to suggest a lifetime for the intermediate diradical of > 1 × 10^-10 s (at 170 °C). An interesting corollary was the isolation of an unexpected silyl shift product in the thermolysis of all three enyne-allenes that allows explanation of the loss of the TIPS group in some of the Diels-Alder products. For a full understanding of the mechanism, silyl and hydrogen shift processes were interrogated using DFT.

Full text of DOI:10.1021/jo062448+

Thermal C2-C6 Cyclization of Enyne-Allenes. Experimental
Evidence for a Stepwise Mechanism and for an Unusual Thermal
Silyl Shift
Michael Schmittel,*,† Atul A. Mahajan,† Go¨tz Bucher,*,‡ and Jan W. Bats§
Organische Chemie I, FB 8 (Chemie-Biologie), UniVersita¨t Siegen, Adolf-Reichwein-Str., D-57068 Siegen,
Germany, Lehrstuhl fu¨r Organische Chemie II, Ruhr-UniVersita¨t Bochum, UniVersita¨tsstr. 150,
D-44801 Bochum, Germany, and Institut fu¨r Organische Chemie und Chemische Biologie, Johann
Wolfgang Goethe-UniVersita¨t, Marie-Curie-Strasse 11, D 60439 Frankfurt am Main, Germany
schmittel@chemie.uni-siegen.de
ReceiVed NoVember 29, 2006
Enyne-allenes 4a-c bearing various cyclopropyl systems as radical clock reporter groups at the allene
terminus have been synthesized and subjected to thermal C2-C6 cyclization. The ratio of ene versus
formal Diels-Alder products could be rationalized on the basis of steric effects. Only the thermolysis of
4c, equipped with the fast diphenylcyclopropylcarbinyl radical clock, afforded a 1,3-butadienyl
benzofulvene clearly formed via cyclopropyl ring opening. This finding provides unambiguous evidence
for a stepwise mechanism of the C2-C6 cyclization making it possible to suggest a lifetime for the
intermediate diradical of >1 × 10-10 s (at 170 °C). An interesting corollary was the isolation of an
unexpected silyl shift product in the thermolysis of all three enyne-allenes that allows explanation of
the loss of the TIPS group in some of the Diels-Alder products. For a full understanding of the mechanism,
silyl and hydrogen shift processes were interrogated using DFT.
Introduction
allenes, the C2-C6 cyclization (Scheme 1), as disclosed in 1995.4
In the ensuing years, the thermal C2-C6 cyclization has been
extensively studied mechanistically5 and theoretically,6 exhibit-
ing an attractive potential for the synthesis of complex car-
bocycles7 and for DNA cleavage.8
The cyclopropylcarbinyl ring opening has been widely used
in mechanistic investigations as a “radical clock” to report about
the intermediacy of short-lived radicals and diradicals.9 More-
The thermal cyclization of enediynes (Bergman)1 and enyne-
allenes (Myers-Saito, C2-C7)2 has aroused great interest over
the past two decades, because their diradical intermediates play
a key role in the action of natural enediyne antitumor antibiot-
ics.3 Interestingly, the C2-C7 (Myers-Saito) cyclization has to
compete against another thermal reaction mode of enyne-
(5) (a) Schmittel, M.; Strittmatter, M.; Kiau, S. Angew. Chem., Int. Ed.
Engl. 1996, 35, 1843-1845. (b) Schmittel, M.; Strittmatter, M.; Vollmann,
K.; Kiau, S. Tetrahedron Lett. 1996, 37, 999-1002. (c) Schmittel, M.;
Steffen, J. P.; Auer, D.; Maywald, M. Tetrahedron Lett. 1997, 38, 6177-
6180. (d) Schmittel, M.; Keller, M.; Kiau, S.; Strittmatter, M. Chem. Eur.
J. 1997, 3, 807-816. (e) Engels, B.; Lennartz, C.; Hanrath, M.; Schmittel,
M.; Strittmatter, M. Angew. Chem., Int. Ed. 1998, 37, 1960-1963. (f)
Schmittel, M.; Strittmatter, M. Tetrahedron 1998, 54, 13751-13760. (g)
Li, H. B.; Zhang, H. R.; Petersen, J. L.; Wang, K. K. J. Org. Chem. 2001,
66, 6662-6668. (h) Schmittel, M.; Steffen, J. P.; Maywald, M.; Engels,
B.; Helten, H.; Musch, P. J. Chem. Soc., Perkin Trans. 2001, 2, 1331-
1339. (i) Yang, Y. H.; Petersen, J. L.; Wang, K. K. J. Org. Chem. 2003,
68, 5832-5837. (j) Yang, Y. H.; Petersen, J. L.; Wang, K. K. J. Org. Chem.
2003, 68, 8545-8549.
† Universita¨t Siegen.
‡ Ruhr-Universita¨t Bochum.
§ Johann Wolfgang Goethe-Universita¨t.
(1) (a) Bergman, R. G.; Jones, R. R. J. Am. Chem. Soc. 1972, 94, 660-
661. (b) Bergman, R. G. Acc. Chem. Res. 1973, 6, 25-31.
(2) (a) Myers, A. G.; Kuo, E. Y.; Finney, N. S. J. Am. Chem. Soc. 1989,
111, 8057-8059. (b) Nagata, R.; Yamanaka, H.; Okazaki, E.; Saito, I.
Tetrahedron Lett. 1989, 30, 4995-4998.
(3) (a) Nicolaou, K. C.; Dai, W.-M. Angew. Chem., Int. Ed. Engl. 1991,
30, 1387-1416. (b) Borders, D. B.; Doyle, T. W. Enediyne Antibiotics as
Antitumor Agents; Marcel Dekker: New York, 1995.
(4) Schmittel, M.; Strittmatter, M.; Kiau, S. Tetrahedron Lett. 1995, 36,
4975-4978.
10.1021/jo062448+ CCC: $37.00 © 2007 American Chemical Society
Published on Web 02/20/2007
2166
J. Org. Chem. 2007, 72, 2166-2173

Thermal C2-C6 Cyclization of Enyne-Allenes
SCHEME 1. Thermal Myers-Saito (C2-C7) and C2-C6 Cyclizations of Enyne-Allenes
SCHEME 2. Radical Clock Reporter Groups in Thermal
Cyclizations of Enyne-Allenes as Employed by (a) Finn et
al.11 and (b) Schmittel et al.5a,d
First investigations on the cyclopropylcarbinyl ring opening
in the thermal C2-C6 cyclization of enyne-allenes (Scheme
2b) demonstrated that the substituted and unsubstituted cyclo-
propylcarbinyl radical clock remained intact, leading to the
assumption that the rate constant for hydrogen transfer on the
stage of the postulated diradical has to be much faster than 106
s-1 5a,d An equally intact cyclopropyl ring in the C2-C6 products
.
was found by Lipton et al. after an oxyanion-accelerated
cyclization of cyclopropyl-substituted enyne-allenes.12 Both
studies seem at first to argue against diradical intermediates in
the C2-C6 cyclization, but a stringent interpretation of the data
suggests that a short-lived diradical intermediate cannot be
discounted. Recently, kinetic isotope effects have revealed
further insight into the mechanism of the C2-C6 cyclization.
Results by Singleton et al.13 suggested that for a selected
example there is one common transition state for the stepwise
and concerted C2-C6 cyclization. In contrast, the results of
Schmittel et al.14 could rather be reconciled with a stepwise
diradical mechanism, leading either to ene or formal Diels-
Alder products.
The rising dispute about the mechanism of the C2-C6
cyclization asked for further experimental interrogation of the
putative diradical intermediate. As a consequence, we have
designed enyne-allenes 4a-c carrying various cyclopropyl
substrates at the allene terminus, including the ultrafast diphe-
nylcyclopropylcarbinyl radical clock. Herein, we report on the
thermal C2-C6 cyclization of these enyne-allenes and their
ring-closed and ring-opened products. Moreover, the formation
of an unexpected silyl shift product was identified by X-ray,
and its formation was investigated computationally.
over, radical clocks have been exploited to differentiate between
stepwise versus concerted mechanisms in various reaction
scenarios, such as for example in the ene reaction.10 In the
context of the Myers-Saito cyclization (Scheme 2a), Dopico and
Finn studied the thermolysis of enyne-allenes carrying either
a cyclopropyl or a 2-phenylcyclopropyl group at the allene
terminus. Their results clearly suggested that the cyclopropyl-
carbinyl ring opening strongly depends on the reaction condition;
in the presence of 1,4-cyclohexadiene (1,4-CHD) the reaction
proceeded via a diradical intermediate, whereas in the presence
of methanol the reaction advanced via
a zwitterionic
intermediate.11
(6) (a) Engels, B.; Hanrath, M. J. Am. Chem. Soc. 1998, 120, 6356-
6361. (b) Schreiner, P. R.; Prall, M. J. Am. Chem. Soc. 1999, 121, 8615-
8627. (c) Cramer, C. J.; Kormos, B. L.; Seierstad, M.; Sherer, E. C.; Winget,
P. Org. Lett. 2001, 3, 1881-1884. (d) de Visser, S. P.; Filatov, M.; Shaik,
S. Phys. Chem. Chem. Phys. 2001, 3, 1242-1245. (e) Stahl, F.; Moran,
D.; Schleyer, P. V.; Prall, M.; Schreiner, P. R. J. Org. Chem. 2002, 67,
1453-1461. (f) Schreiner, P. R.; Navarro-Vazquez, A.; Prall, M. Acc. Chem.
Res. 2005, 38, 29-37.
(7) (a) Zhang, H.-R.; Wang, K. K. J. Org. Chem. 1999, 64, 7996-7999.
(b) Wang, K. K.; Zhang, H.-R.; Petersen, J. L. J. Org. Chem. 1999, 64,
1650-1656. (c) Li, H. B.; Zhang, H. R.; Petersen, J. L.; Wang, K. K.
J. Org. Chem. 2001, 66, 6662-6668. (d) Li, H. B.; Petersen, J. L.; Wang,
K. K. J. Org. Chem. 2001, 66, 7804-7810. (e) Yang, Y. H.; Petersen,
J. L.; Wang, K. K. J. Org. Chem. 2003, 68, 8545-8549. (f) Dai, W. X.;
Petersen, J. L.; Wang, K. K. J. Org. Chem. 2005, 70, 6647-6652.
(8) (a) Schmittel, M.; Kiau, S.; Siebert, T.; Strittmatter, M. Tetrahedron
Lett. 1996, 37, 7691-7694. (b) Schmittel, M.; Maywald, M.; Strittmatter,
M. Synlett 1997, 165-166.
Results
Synthesis. The required enyne-allenes 4a-c were prepared
according to our established protocol (Scheme 3), starting with
the Sonogashira cross coupling of triisopropylsilyl acetylene
with 2-iodobenzaldehyde furnishing compound 1 in quantitative
yield.15 Addition of BrMgCtC-R (R ) cyclopropyl, phenyl-
cyclopropyl,16 diphenylcyclopropyl17) afforded propargyl alco-
hols 2a-c in good yield18 (the 1H NMR of compound 2c showed
two isomers, which were used in the next step without any
further separation). Compounds 2a-c were treated with Ac2O
and 4-(N,N-dimethylamino)-pyridine (DMAP) in triethylamine/
DCM at room temperature, furnishing propargyl acetates 3a-c
(9) (a) Ingold, K. U.; Griller, D. Acc. Chem. Res. 1980, 13, 317-323.
(b) Engel, P. S.; Keys, D. E. J. Am. Chem. Soc. 1982, 104, 6860-6861. (c)
Campredon, M.; Kanabus-Kaminska, J. M.; Griller, D. J. Org. Chem. 1988,
53, 5393-5396. (d) Castellino, A. J.; Bruice, T. C. J. Am. Chem. Soc. 1988,
110, 1313-1315. (e) Newcomb, M.; Manek, M. B.; Glenn, A. G. J. Am.
Chem. Soc. 1991, 113, 949-958. (f) Adam, W.; Heil, M. J. Am. Chem.
Soc. 1991, 113, 1730-1736. (g) Adam, W.; Finzel, R. J. Am. Chem. Soc.
1992, 114, 4563-4568. (h) Newcomb, M. Tetrahedron 1993, 49, 1151-
1176. (i) Engel, P. S.; Wu, A. J. Org. Chem. 1994, 59, 3969-3974. (j)
Caldwell, R. A.; Zhou, L. J. Am. Chem. Soc. 1994, 116, 2271-2275. (k)
Engel, P. S.; Lowe, K. L. Tetrahedron Lett. 1994, 35, 2267-2270. (l) Tanko,
J. M.; Blackert, J. F. J. Chem. Soc., Perkin Trans. 2 1996, 1775-1779.
(m) Cooksy, A. L.; King, H. K.; Richardson, W. H. J. Org. Chem. 2003,
68, 9441-9492.
(12) Brunette, S. R.; Lipton, M. A. J. Org. Chem. 2000, 65, 5114-5119.
(13) Bekele, T.; Christian, C. F.; Lipton, M. A.; Singleton, D. A. J. Am.
Chem. Soc. 2005, 127, 9216-9223.
(14) Schmittel, M.; Vavilala, C. J. Org. Chem. 2005, 70, 4865-4868.
(15) Kevin, R. R.; Larock, R. C. J. Org. Chem. 2002, 67, 86-94.
(16) The synthesis of trans-(2-ethynylcyclopropyl)benzene is described
in Supporting Information.
(17) Bettinetti, G. F.; Desimoni, G.; Gruenanger, P. Gazz. Chim. Ital.
1964, 94, 91-108. For the modified synthesis of (2-ethynyl-1-phenylcy-
clopropyl)benzene and its molecular structure, as derived from X-ray
diffraction analysis, please see Supporting Information.
(18) Schmittel, M.; Strittmatter, M.; Schenk, W. A.; Hagel, M. Z.
Naturforsch., B: Chem. Sci. 1998, 53, 1015-1020.
(10) Roubelakis, M. M.; Vougioukalakis, G. C.; Angelis, Y. S.; Orfa-
nopoulos, M. Org. Lett. 2006, 8, 39-42.
(11) Dopico, P. G.; Finn, M. G. Tetrahedron 1999, 55, 29-62.
J. Org. Chem, Vol. 72, No. 6, 2007 2167

Schmittel et al.
SCHEME 3. Synthesis of Enyne-Allenes 4a-c
in high yield.19 Enyne-allenes 4a-c were finally received
through a Pd-promoted reaction of R3-ZnCl with 3a-c at
-60 °C in moderate yield.20
SCHEME 4. Thermal Cyclization of Enyne-Allenes 4a-c
Thermal Cyclization. Upon thermolysis of enyne-allene 4a
in toluene at 110 °C (reflux temperature) in the presence of an
excess of 1,4-CHD, the ene product 5a and the formal Diels-
Alder products 6a, 6a′, and 6a′′ (with TIPS and without TIPS
group) were isolated (Scheme 4, Table 1). Most notable was
the formation of the silyl migration product 6a in 13% yield.
The structures of the ene and the formal Diels-Alder products
were assigned on the basis of 1H, 13C, 1H,1H-COSY, and HRMS.
The 1H NMR of the silyl migration product 6a showed a
characteristic singlet for proton Hb at 4.76 and another one for
Ha at 7.91 ppm. Moreover, the structure of compound 6a was
1
assigned on the basis of H,1H-COSY NMR.
In contrast, thermolysis of enyne-allene 4b in toluene/1,4-
CHD (100 equiv) at 110 °C afforded the silyl migration product
6b (two isomers) and the formal Diels-Alder product 6b′′
(without TIPS group). No formal ene product was observed.
The two isomers of compound 6b showed characteristic singlets
in the 1H NMR for proton Hb at 4.69 and 4.78 ppm and for Ha
at 7.92 and 7.94 ppm, respectively. The desilylated product 6b′′
exhibited a characteristic signal at 4.16 ppm for the -CH2- unit
of the fulvene ring.
When enyne-allene 4c was heated at 170 °C in a sealed tube
in the presence of an excess of 1,4-CHD (100-fold excess) in
toluene, the cyclopropyl ring-opened product 5c′ (32%) and silyl
shift compound 6c (25%) yield were isolated. A control
experiment in toluene-d8 did not exhibit deuterium incorporation
into 5c′, suggesting an intramolecular hydrogen transfer.
The structure of ene product 5c′ was assigned on the basis
of 1H, 13C, 1H,1H-COSY, HSQC, and HRMS. The 1H NMR of
compound 5c′ showed a characteristic doublet with a coupling
constant of 11.5 Hz for the two olefinic protons Ha and Hb,
suggesting that both protons are anti to each other. Product 6c
(a formal Diels-Alder) showed a characteristic singlet at δ )
4.87 for proton Hb and δ ) 7.79 for proton Ha. Moreover, the
structure of 6c was established by X-ray structure analysis,
providing unambiguous proof for the silyl group migration (for
the X-ray crystal structure see Supporting Information).
Discussion
(19) Borzilleri, R. M.; Weinreb, S. M.; Parvez, M. J. Am. Chem. Soc.
1994, 116, 9789-9790.
(20) Elsevier, C. J.; Stehouwer, P. M.; Westmijze, H.; Vermeer, P.
J. Org. Chem. 1983, 48, 1103-1105.
Radical Clock Experiments. The only reported example of
a radical clock experiment in the thermal C2-C6 cyclization of
enyne-allenes (Scheme 2b) did not exhibit cyclopropyl ring
2168 J. Org. Chem., Vol. 72, No. 6, 2007

Thermal C2-C6 Cyclization of Enyne-Allenes
TABLE 1. Yields of the Thermal C2-C6 Cyclization of 4a-ca
% yield
reaction
ene
DA
DA
silyl migration
entry
compound
condition
product 5/5′
(with TIPS) 6′
(without TIPS) 6′′
product 6 (%)
1
2
3
4a
4b
4c
110 °C; 14 h
110 °C; 14 h
170 °C; 7 h
25 (5a)
7
20
15
13
32
25
32 (5c′)
a In the presence of excess of 1,4-CHD (100 equiv). DA: formal Diels-Alder product.
cyclopropylcarbinyl system (k ) 1.1 × 103 s-1 at 20 °C, ca.
2.2 × 105 s-1 at 110 °C).21-23
SCHEME 5. Visualization of the Two Competing Pathways
in the Diradical Cyclization of Enyne-Allenes
In the thermolysis of enyne-allene 4b, only Diels-Alder
products 6b, 6b′′ and no ene product were isolated. As the
remote attachment of a phenyl group at the cyclopropyl ring
should not influence the electronic situation in the diradical 7b,
the failure to observe any ene product suggests that s-trans 7b
is sterically not available, most likely due a repulsive interaction
between the phenylcyclopropane and the bulky TIPS group at
the alkyne. Exclusive formation of formal Diels-Alder products
thus proposes that the diradical intermediate s-cis 7b undergoes
radical combination to 6b,6b′′ faster than cyclopropyl ring
opening. As ring opening in s-cis 7b is best mimicked by the
opening of the R,R-diphenyl-(2-phenylcyclopropyl)carbinyl
radical with an activation energy Ea ) 8.5 kcal mol,-1,24 one
has to postulate a barrier for the radical combination in 7b of
less than 8 kcal mol-1
.
The diphenylcyclopropylcarbinyl group with a rate constant
for ring opening of k ) 5.0 × 1011 s-1 at 25 °C25a has been
widely used as a ultrafast “radical clock” to trap short-lived
radical intermediates.25 As formation of 6b,6b′′ in the ther-
molysis of 4b has shown that steric bulk caused by the phenyl
group at the cyclopropyl ring will lead preferentially to the s-cis
diradical, we expected the diphenylcyclopropyl unit in 4c to
entail an even higher population of the s-cis diradical 7c. As a
consequence, its ring opening cannot be impeded by the
sterically demanding TIPS group. Indeed, with 4c we were able
to see, aside of the Diels-Alder product 6c, the first radical
clock ring opening as indicated by the formation of butadienyl
benzofulvene 5c′. A control experiment in toluene-d8 verified
that 5c′ was exclusively formed by an intramolecular hydrogen
transfer, rejecting any mechanistic hypothesis in which the
regular ene product 5c could have served as precursor to 5c′.
Hence, transformation from 4c to 5c′ should involve ring
opening.5a,d At that time, in 1996, a stepwise mechanism was
derived from the finding that the two diastereomeric enyne-
allenes did not convert stereospecifically to the ene product.
As the 1996 experiment used an enyne-allene with a cyclo-
propylcarbinyl reporter group and a phosphine oxide at the allene
terminus, we now decided to interrogate the thermal cyclization
of enyne-allenes carrying an aryl group at the terminal allene
center, as this should emphasize the radical character in the
diradical. Triplet and open shell singlet diradicals should be
perfectly poised for cyclopropyl ring opening at rates similar
to those of monoradicals. Fortunately, rate constants for radical
clock reactions in monoradicals are well gauged or can be rather
reliably approximated. Computations by Engels,6a Schreiner,6b
and Shaik6d et al. advise that the C2-C6 diradical has a triplet
ground state. Whether this state is involved in the thermal
reaction remains unclear, since the thermal reaction conditions
should first lead to an open shell singlet diradical from which
follow-up ene and formal Diels-Alder products should originate
with a low barrier in competition to the radical clock opening.
As described above, thermolysis of enyne-allene 4a afforded
ene and formal Diels-Alder products without ring opening. A
look at the proposed stepwise mechanism (Scheme 5) suggests
that some appreciable part of diradical 7a is available in the
s-cis conformation as the formal Diels-Alder products can only
be derived therefrom. In contrast, for the intramolecular
hydrogen transfer the benzofulvene diradical 7a has to adopt
the s-trans conformation. The ratio of ene versus Diels-Alder
products is 5:8. Since interconversion of s-cis and s-trans 7a
must involve rotation about a C-C bond with partial double
bond character, it is not clear whether rotation can compete with
the fast and irreversible follow-up reactions leading to the
observed products. Clearly, as both reactions prevail in the
competition with the radical clock process, they must be much
faster than the cyclopropyl ring opening for the R,R-diphenyl-
(21) Beckwith, A. L. J.; Bowry, V. W. J. Am. Chem. Soc. 1994, 116,
2710-2716.
(22) Hollis, R.; Hughes, L.; Bowry, V. W.; Ingold, K. U. J. Org. Chem
1992, 57, 4284-4287.
(23) For calculations of cyclopropyl ring-opening rate constants at other
temperatures, we use the pre-exponential factor 1012.85 s-1 as in ref 22.
(24) We assume a rate constant of k (20 °C) ) 3.1 × 106 s-1 for the
ring opening of the R,R-diphenyl-(2-phenylcyclopropyl)carbinyl radical. At
first, the accelerating effect of a 2-phenyl group on the ring opening (2788
x) was evaluated from the comparison of the R-phenyl(cyclopropyl)carbinyl
and the cyclopropylcarbinyl radical opening (see refs 22 and 25). This effect
was translated onto the ring opening of R,R-diphenyl-(cyclopropyl)carbinyl
radical (k ) 1.1 × 103 s-1 at 20 °C).
(25) (a) Newcomb, M.; Johnson, C. C.; Manek, M. B.; Varik, T. R.
J. Am. Chem. Soc. 1992, 114, 10915-10921. (b) Newcomb, M.; Tanaka,
N.; Bouvier, A.; Tronche, C.; Horner, J. H.; Musa, O. M.; Martinez, F. N.
J. Am. Chem. Soc. 1996, 118, 8505-8506. (c) Newcomb, M.; Choi, S.-Y.;
Horner, J. H. J. Org. Chem. 1999, 64, 1225-1231. (d) Newcomb, M.;
Daublain, P.; Horner, J. H. J. Org. Chem. 2002, 67, 8669-8671.
J. Org. Chem, Vol. 72, No. 6, 2007 2169

Schmittel et al.
SCHEME 6. Thermal Cyclization of Enyne-Allene 4c
ring opening of s-cis 7c to s-cis 8c may be estimated to 6 kcal
mol-1,22 providing us with a rate constant of k170°C ) 7 × 109
s-1
.
In concluding this part of the discussion, it seems reasonable
to state that with the rising number of phenyl groups (from 4a
to 4c) there is an concomitant increase in the steric repulsion
between the cyclopropyl unit and the TIPS group. Hence, in
the thermolysis of 4a-c an augmenting amount of the s-cis
diradical 7 is formed as an intermediate, which does not allow
any more for the stepwise ene reaction to occur with 4b,c. On
the other side, the s-cis conformation allows for both the formal
Diels-Alder reaction and for the sterically unimpeded ring
opening of the cyclopropylcarbinyl reporter group. Only in 4c
the ring opening, however, can compete. This allows us to
roughly predict a barrier for the radical combination reaction
in 4c.
Silyl Shift. A further matter of the present work was to
understand the multistep process leading to the formal Diels-
Alder products. As mentioned above, the s-cis diradical 7 suffers
radical combination to the unstable Diels-Alder product 9. In
order to account for the isolated products 6, 6′, and 6′′, at some
stage a partial desilylation process must have occurred after
formation of 9. A clear hint to explain the formation of the
desilylated product 6′′ came from the isolation and identification
of 6, which was observed in the thermal cyclization of all
enyne-allenes 4. Compound 6 is a good candidate to lose a
TIPS group in a very mild hydrolysis reaction.
SCHEME 7
Similar to 6, the benzylic silane 10 should also be apt to lose
its silyl substituent. However, due its thermodynamic instability,
10 is expected to undergo a variety of hydrogen or silyl shifts,
and hence may only be a short-lived intermediate. Thus in
summary, 6a-c seem more to be more promising candidates
for TIPS loss. To investigate this matter, we have performed a
detailed density functional theory study of the potential reaction
pathways leading to the observed products. To reduce the costs
in CPU time, a simplified model system was investigated, in
which any (phenyl)cyclopropyl substituent was replaced by a
methyl group and bromine atoms at the diphenylamine moiety
were omitted. In the calculated reaction sequence the starting
molecule 9 is thus represented by X1 and 10 is represented by
the two conformers X5 h Y1.
We selected the standard B3LYP/6-31G(d) method for the
investigation of the hydrogen and silyl shifts. In order to test
its adequacy, the 1,5-H shift in s-trans-cis-1,3-pentadiene was
initially studied as a benchmark test. The best estimate for the
activation enthalpy of this reaction had been reported as ∆Hq
) 38.4 kcal mol-1 by Lynch and Truhlar.27 Employing B3LYP/
6-31G(d) we obtained ∆Hq ) 36.6 kcal mol-1, which is in
reasonable agreement with the above value. We conclude that
the B3LYP hybrid density functional28 is likely to be suitable
for the system investigated. In constructing the PES connecting
X1 with X9 and Y7, we have limited our investigations to H
and TIPS shifts occurring around the perimeters of the inner
two rings of the benzofluorene ring system.29
opening26 of the intermediate s-cis 7c to s-cis 8c (Scheme 6),
followed by rotation to s-trans 8c and an intramolecular
hydrogen transfer via a seven-membered transition state. The
rate constant of the 2,2-diphenylcyclopropyl methyl radical ring
opening in diradical 7c has become competitive to the rate
constant for radical combination in s-cis 7c to 6c. Unfortunately,
on the basis of known data an estimate of the rate constant for
opening of the R,R-diphenyl-(2,2-diphenylcyclopropyl)carbinyl
radical is imprecise, but it may be approximated with the rate
constant of the R-phenyl-(2-phenylcyclopropyl)carbinyl radical
(k40°C ) 3.6 × 108 s-1).22 Thus, the activation energy for the
Figure 1 gives a summary of the results of our calculations.
Initially, X1 faces a choice between two orbital-symmetry
(27) Lynch, B. J.; Truhlar, D. G. J. Phys. Chem. A 2001, 105, 2936-
2941.
(26) (a) Shimizu, N.; Nishida, S. J. Chem. Soc., Chem. Commun. 1972,
389-390. (b) Rudolph, A.; Weedon, A. Can. J. Chem. 1990, 68, 1590-
1596. (c) Adam, W.; Curci, R.; D’Accolti, L.; Dinoi, A.; Fusco, C.;
Gasparrini, F.; Kluge, R.; Paredes, R.; Schulz, M.; Smerz, A. K.; Veloza,
L. A.; Weinkotz, S.; Winde, R. Chem. Eur. J. 1997, 3, 105-109.
(28) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(29) This limitation appears justified, as a 1,9-H-shift to position 4 in
the benzofluorene framework (a H-shift to the right hand side from X1 as
depicted in Figure 4) results in a thermodynamically very unfavorable
quinoidal product.
2170 J. Org. Chem., Vol. 72, No. 6, 2007

Thermal C2-C6 Cyclization of Enyne-Allenes
FIGURE 1. Structures of X5 (left), the transition state connecting X5 and Y1 (middle), and Y1 (right), as calculated at the B3LYP/6-31G(d) level
of theory. The diphenylamino group present in the molecules was replaced by a simple amino group for the sake of clarity.
FIGURE 2. Calculated reaction pathways leading from X1 to X9 and Y7. Color encoding has been used to facilitate visualization of individual
reaction pathways. “Warm” colors (red, magenta, dark violet) indicate reaction pathways involving hydrogen shift, whereas “cold” colors (dark
blue, light blue, green) indicate a reaction pathway involving a triisopropylsilyl shift. The color black is used for the starting point X1, the end
points X9 and Y7, and for Y1. The energies given are in kcal mol-1 relative to X1 ) 0.
allowed 1,9-hydrogen shifts yielding X2 and X5. Of the two
reactions, the one yielding X5 is predicted to be kinetically
favored by a wide margin. From X2, on the other hand, the
thermodynamically very favorable product X3 may become
accessible with a small barrier. X5 represents 3-diphenylamino-
11-methyl-5-triisopropylsilyl-5H-benzofluorene in the higher
energy conformation, with the TIPS group being located
equatorially and the hydrogen atom 5-H located in axial position,
as required for further hydrogen shifts. In the second, much
more favorable conformer Y1 the bulky TIPS substituent has
moved to the axial position. The ∆Hq ) -17.0 kcal mol-1 for
X5 f Y1 is most likely due to a very unfavorable peri-
interaction with the hydrogen atom located in position 6 leading
to a destabilization of X5. The barrier for conversion of X5 to
Y1 is predicted to be small (4.0 kcal mol-1), suggesting that
X5 will be rapidly converted to Y1 (Figure 2). Obviously, the
axial TIPS substituent is ideally positioned for further TIPS-
shifts that may ultimately lead to the formation to Y7,
corresponding to product 6 in the experiment. Conversion of
X5 into Y1 is analogous to the movement of a hinge connecting
the reaction pathways involving hydrogen transfer and those
involving silyl transfer. In the transition state linking X5 and
Y1, the vibrational mode with the imaginary force constant
corresponds to a rotation around the C5-Si bond coupled to
an out-of-plane movement of the C6-C7-C8-C9 ring frag-
ment. This movement can be compared to the interaction of a
balance spring (C6-C7-C8-C9 ring fragment) and a tooth-
wheel (TIPS group) in a clockwork.
From Y1, the reaction may again bifurcate. The kinetically
preferred pathway (dark blue) runs via the TIPS-benzofluorene
J. Org. Chem, Vol. 72, No. 6, 2007 2171

Schmittel et al.
added dropwise. After stirring for 16 h at room temperature, the
reaction mixture was quenched with aqueous saturated ammonium
chloride solution. The aqueous layer was washed with diethyl ether
(2 × 50 mL). The combined organic layers were dried over sodium
sulfate and evaporated under reduced pressure. After purification
by column chromatography (neutral alumina, n-pentane) compound
4a was isolated as light yellow oil. Rf ) 0.5. Yield: 236 mg, 42%.
IR (film) ν˜: 3068 (w), 2942 (s, C-H), 2864 (s), 2151 (s, CtC),
1928 (m, CdCdC), 1580 (s), 1505 (s), 1482 (m), 1313 (s), 1285
(s), 1178 (m), 1118 (m), 1072 (m), 1008 (m), 821 (s), 665 (m)
Y3 and via the TIPS-benzofluorene Y4 to the final product Y7.
We note that 3-diphenylamino-11-methyl-9b-triisopropylsilyl-
9bH-benzofluorene (Y6, see Supporting Information), which is
expected to be found along the reaction coordinate connecting
Y4 and Y7, in our hands was a minimum structure only in
preoptimizations performed at the RHF/STO-3G level of theory.
It could no longer be located if density functional theory
(B3LYP/6-31G(d) or BHandHLYP/6-31G(d)) was used. Simi-
larly, 3-diphenylamino-11-methyl-10b-triisopropylsilyl-10bH-
benzofluorene (Y5, see Supporting Information) between Y3
and Y7 (reaction pathway indicated by light blue color) and
3-diphenylamino-11-methyl-11b-triisopropylsilyl-11bH-benzof-
luorene (Y9, see Supporting Information) between Y2 and Y8
(reaction pathway indicated by green color) could not be located
at the B3LYP/6-31G(d) level of theory. This finding does not
rule out that these structures could be true minima. However,
it appears highly unlikely that these molecules, even if they
existed, would be protected by significant barriers against decay
by TIPS shift. The barriers for the TIPS shifts are generally
predicted to be smaller than the barriers for the hydrogen shift
reactions. This observation is consistent with our failure to
isolate any of the Y compounds except the final product Y7.
Formation of X5 from X1 (“red” pathway) is predicted to be
kinetically favored over formation of X2/X3 (“magenta”
1
cm-1. H NMR (400 MHz, acetone-d6): δ 0.51-0.55 (m, 1H),
0.59-0.63 (m, 1H), 0.92-0.97 (m, 2H), 1.18 (s, 21H), 1.73-1.80
(m, 1H), 7.01 (d, J ) 8.9 Hz, 4H), 7.08 (d, J ) 8.7 Hz, 2H), 7.21
(d, J ) 2.5 Hz, 1H), 7.24 (td, J ) 7.5, 1.3 Hz, 1H), 7.33 (td, J )
7.6, 1.2 Hz, 1H), 7.39 (d, J ) 7.2 Hz, 1H), 7.44 (d, J ) 8.8 Hz,
4H), 7.52 (dd, J ) 7.6, 0.9 Hz, 1H), 7.57 (d, J ) 8.5 Hz, 2H). 13
C
NMR (100 MHz, acetone-d6): 7.2, 7.9, 11.6, 12.0, 19.0, 96.3, 97.6,
106.0, 116.0, 122.1, 125.4, 126.5, 126.7, 128.0, 128.4, 129.9, 132.1,
133.3, 133.8, 134.3, 134.5, 147.0, 147.4, 207.4 ppm. HRMS: calcd
for C41H4379Br2NSi 735.153, found 735.152.
Bis(4-bromophenyl)-(4-{1-(2-(trans-phenylcyclopropyl)-3-[(2-
triisopropylsilanylethynyl)-phenyl]-propa-1,2-dienyl}-phenyl)-
amine (4b). Procedure as for 4a. Yield: 165 mg, 29%. IR (film)
ν˜: 3065 (w), 2942 (s, C-H), 2864 (s), 2153 (s, C≡C), 1929 (m,
CdCdC), 1747 (s), 1581 (s), 1505 (s), 1486 (s), 1370 (m), 1313
(s), 1285 (s), 1235 (s), 1179 (m), 1118 (m), 1073 (m), 1009 (m),
822 (s), 665 (m) cm-1 1H NMR (400 MHz, acetone-d6) two
.
pathway) by such a wide margin (∆∆Hq ) 10.1 kcal mol-1
)
isomers: δ 1.17-1.20 (m, 21H), 1.26-1.34 (m, 1H), 1.36-1.42
(m, 1H), 2.01-2.06 (m, 1H), 2.16-2.21 (m, 1H), 6.97 (d, J ) 8.8
Hz, 4H), 7.01 (d, J ) 8.7 Hz, 2H), 7.16-7.22 (m, 3H, Ar-H), 7.25-
7.38 (m, 5H), 7.42 (d, J ) 8.8 Hz, 4H), 7.46 (d, J ) 8.4 Hz, 2H),
7.45-7.56 (m, 2H, Ar-H). 13C NMR (100 MHz, acetone-d6) two
isomers : 12.0, 16.6, 17.1, 19.1, 23.6, 24.0, 26.9, 27.5, 96.4, 96.5,
98.1 (2C), 106.0 (2C), 113.3, 113.4, 116.1, 122.2, 124.6, 124.8,
124.9, 125.2, 125.3, 125.5, 125.6, 125.9, 126.4, 126.5 (3C), 126.6,
126.7, 127.0, 128.1, 128.3, 128.4, 129.3, 130.0 (2C), 130.4, 131.6,
131.8, 133.0, 133.2, 133.8, 136.5, 136.8, 142.9, 147.1 (2C), 147.3,
207.4, 207.5 ppm. HRMS: calcd for C47H4779Br2NSi 811.184, found
811.183.
that essentially only the red pathway should be followed. As a
result of the low barrier for conversion of X5 into Y1, all X5
formed in the course of the reaction is likely to convert to Y1,
which will eventually rearrange to final product Y7.
Our calculations on the facile thermal silyl shifts in the model
system thus suggest that only products should be formed that
have undergone TIPS shifts. This is in agreement with the
experimental results obtained with 4b and 4c but contrasts with
the behavior of 4a, where small amounts of the Diels-Alder
product with the TIPS substituent still in its original place were
also formed. The reason for this discrepancy is unclear at
present.
Bis(4-bromophenyl)-(4-{1-(2,2-diphenylcyclopropyl)-3-[(2-tri-
isopropylsilanylethynyl)-phenyl]-propa-1,2-dienyl}-phenyl)-
amine (4c). Procedure as for 4a. Yield: 219 mg, 44%. IR (film)
ν˜: 3031 (w), 2942 (s, C-H), 2864 (s), 2151 (m, CtC), 1929 (w,
CdCdC), 1581 (m), 1505 (s), 1486 (s), 1313 (s), 1285 (s), 1235
(s), 1179 (m), 1073 (m), 1008 (m), 882 (m), 822 (m), 757 (m),
Conclusions
In the present investigation we describe the first successful
trapping of a thermally generated C2-C6 diradical intermediate
using an ultrafast radical clock. Therefore, the present results
furnish direct evidence for a stepwise mechanism in the C2-
C6 cyclization of enyne-allenes. In several cases, the C2-C6
cyclization of enyne-allenes to formal Diels-Alder products
is accompanied by a loss of the TIPS group. Through a
combination of experimental and computational studies it is
suggested that a facile silyl shift followed by a desilylation is
operative.
701 (m), 677 (m) cm-1 1H NMR (400 MHz, acetone-d6) two
.
isomers: δ 1.16 (s, 42H), 1.62-1.67 (m, 2H), 1.71-1.74 (m, 1H),
1.82 (t, J ) 5.5 Hz, 1H), 2.58-2.64 (m, 1H), 2.69 (t, J ) 6.7 Hz,
1H), 5.66 (s, 1H), 5.68 (s, 1H), 6.83 (t, J ) 7.6 Hz, 1H), 6.94-
7.08 (m, 15H), 7.18-7.33 (m, 22H), 7.38-7.49 (m, 14H). 13C NMR
(100 MHz, acetone-d6) two isomers: 11.1, 18.2 (2C), 19.8, 20.0,
26.2, 28.0, 38.7, 39.3, 95.2, 95.3, 96.6, 96.8, 105.0, 105.1, 109.5,
109.6, 115.2 (2C), 120.8. 120.9, 123.6 (2C), 123.7 (2C), 124.1 (2C),
124.6 (2C), 125.1, 125.2, 125.6 (2C), 125.7, 125.8, 126.1, 126.2,
126.4, 126.7, 126.8, 127.0, 127.2, 127.3, 127.4, 127.7, 128.3, 128.4,
128.9, 129.0, 129.5, 130.4, 131.1, 131.5, 131.6, 132.1 (2C), 132.3
(2C), 132.4, 132.9, 135.0, 135.8, 140.3, 141.2, 145.9 (2C), 146.0
(2C), 146.3, 207.6, 208.1 ppm. HRMS: calcd for C53H5181Br79-
BrNSi 889.215, found 889.215.
Experimental Section
Bis(4-bromophenyl)-(4-{1-cyclopropyl-3-[(2-triisopropylsila-
nylethynyl)phenyl]-propa-1,2-dienyl}phenyl)amine (4a). In 50
mL of dry diethyl ether tris(4-bromophenyl)-amine (1.46 g, 3.02
mmol) was cooled to 0 °C in an ice bath. n-BuLi (2.5 M) (1.21
mL, 3.02 mmol) was added dropwise and stirred. After 4 h this
reaction mixture was added dropwise to a 1 M ZnCl2 solution (412
mg in 3.02 mL of diethyl ether) and stirred for 60 min at room
temperature. Now the reaction mixture was cooled to -60 °C, and
Pd(PPh3)4 (87.0 mg, 750 µmol) in dry THF (5 mL) was added
dropwise. After stirring for 30 min at the same temperature,
propargyl acetate 3a (300 mg, 730 µmol) in dry THF (10 mL) was
Standard Protocol for Thermolysis. Enyne-allenes 4a-c were
dissolved in dry toluene in the presence of 1,4-CHD (100 equiv)
and heated at 110 (reflux temperature) or 170 °C (sealed tube) for
7-14 h. After removal of toluene under reduced pressure and
purification by chromatography (aluminum sheet, silica gel 60F254
,
n-hexane) products were isolated as a yellow oil/solid.
Bis(4-bromophenyl)-[4-(cyclopropylidene-{1-[(triisopropylsi-
lanyl)-methylene]-1H-inden-2-yl}-methyl)-phenyl]-amine (5a).
Yield: 5.0 mg, 25%. 1H NMR (400 MHz, CDCl3): δ 1.00 (d, J )
7.4 Hz, 18H), 1.21-1.25 (m, 2H), 1.32 (septet, J ) 7.5 Hz, 3H),
2172 J. Org. Chem., Vol. 72, No. 6, 2007

Thermal C2-C6 Cyclization of Enyne-Allenes
1.56-1.61 (m, 2H), 6.20 (s, 1H), 6.81 (s, 1H), 6.92 (d, J ) 8.8
Hz, 4H), 6.95 (d, J ) 8.6 Hz, 2H), 7.12-7.18 (m, 1H), 7.22-7.25
(m, 2H), 7.31 (d, J ) 8.8 Hz, 4H), 7.43 (d, J ) 8.5 Hz, 2H), 7.65
(d, J ) 7.5 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 2.2, 5.5,
12.9, 19.1, 115.3, 120.5, 122.1, 124.1, 124.5, 124.7, 125.2, 125.3,
125.6, 128.1, 130.6, 132.2, 132.5, 136.2, 136.8, 144.0, 144.3, 145.2,
146.4, 155.1 ppm. HRMS: calcd for C41H4381Br79BrNSi 737.152,
found 737.151.
140.4, 140.5, 142.8, 143.0, 143.2, 143.9, 144.9, 146.1, 146.6 (2C),
147.0, 147.3 ppm. HRMS: calcd for C47H4781Br79BrN 813.183,
found 813.182.
Bis(4-bromophenyl)-[10-(2-phenylcyclopropyl)-11H-benzo[b]-
1
fluoren-7-yl]-amine (6b′′). Yield: 3.3 mg, 15%. H NMR (400
MHz, CDCl3): δ 1.59-1.62 (m, 1H), 1.66-1.70 (m, 1H), 2.25-
2.32 (m, 1H), 2.50-2.57 (m, 1H), 4.16 (s, 2H), 7.01 (d, J ) 8.8
Hz, 4H), 7.20 (dd, J ) 9.1, 2.3 Hz, 1H), 7.30-7.41 (m, 11H),
7.52 (d, J ) 2.4 Hz, 1H), 7.58 (d, J ) 7.2 Hz, 1H), 7.84 (d, J )
6.9 Hz, 1H), 7.92 (s, 1H), 8.26 (d, J ) 9.1 Hz, 1H). HRMS: calcd
for C38H2779Br2N 655.051, found 655.050.
Bis(4-bromophenyl)-(10-cyclopropyl-11-triisopropylsilanyl-
1
11H-benzo[b]fluoren-7-yl)-amine (6a). Yield: 2.6 mg, 13%. H
NMR (400 MHz, CDCl3): δ 0.38-0.44 (m, 1H), 0.74-0.79 (m,
1H), 0.83 (d, J ) 7.5 Hz, 9H), 0.85 (d, J ) 7.5 Hz, 9H), 1.18-
1.23 (m, 5H), 2.26-2.38 (m, 1H), 4.76 (s, 1H), 7.03 (d, J ) 8.8
Hz, 4H), 7.23 (dd, J ) 9.1, 2.4 Hz, 1H), 7.28-7.32 (m, 2H), 7.37
(d, J ) 8.8 Hz, 4H), 7.48 (d, J ) 2.3 Hz, 1H), 7.54 (d, J ) 7.2 Hz,
1H), 7.82 (d, J ) 6.7 Hz, 1H), 7.91 (s, 1H), 8.40 (d, J ) 9.1 Hz,
1H). 13C NMR (100 MHz, CDCl3): δ 8.9, 10.3, 12.1, 12.9, 18.5,
18.7, 29.7, 38.5, 115.4, 116.1, 120.5, 121.7, 122.9, 124.1, 125.3,
125.5, 126.6, 126.8, 132.3 (2C), 132.7, 133.4, 139.9, 140.4, 143.0,
145.4, 146.6, 147.2 ppm. HRMS: calcd for C41H4381Br79BrNSi
737.152, found 737.152
Bis(4-bromophenyl)-[4-((E)-4,4-diphenyl-1-{1-[1-trimethylsi-
lanyl-meth-(E)-ylidene]-1H-inden-2-yl}-buta-1,3-dienyl)-phen-
yl]-amine (5c′). Yield: 12 mg, 32%. IR (film) ν˜: 3051 (m), 2942
(s, C-H), 2865 (s), 1582 (m), 1486 (s), 1312 (s), 1265 (s), 1178
(w), 1072 (m), 1008 (m), 882 (m), 823 (s), 740 (s), 702 (s) cm-1
.
1H NMR (400 MHz, C6D6): δ 1.03 (d, J ) 7.4 Hz, 18H), 1.32
(septet, J ) 7.5 Hz, 3H), 6.52 (d, J ) 8.8 Hz, 4H), 6.60 (d, J )
8.7 Hz, 2H), 6.69 (s, 1H), 6.83 (s, 1H), 6.90-6.93 (m, 3H), 7.10
(d, J ) 8.8 Hz, 4H), 7.12-7.16 (m, 3H), 7.16-7.24 (m, 8H), 7.30
(d, J ) 11.5 Hz, 1H), 7.37 (d, J ) 6.9 Hz, 2H), 7.94 (dd, J ) 5.4,
2.7 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 12.9, 19.1, 115.6,
120.9, 122.2, 123.7, 125.0, 125.5, 126.4, 127.3, 127.4, 127.6, 127.7,
127.9, 128.1, 128.2, 130.7, 132.3, 132.7, 133.5, 136.2, 137.2, 137.7,
139.9, 142.3, 142.7, 143.3, 143.9, 145.9, 146.2, 155.4 ppm.
HRMS: calcd for C33H2781Br79BrN 889.215, found 889.215.
Bis(4-bromophenyl)-[10-(2,2-diphenylcyclopropyl)-11-triiso-
propylsilanyl-11H-benzo[b]fluoren-7-yl]-amine (6c). Yield: 9.3
mg, 25%. IR (film) ν˜: 3053 (m), 2978 (s, C-H), 2872 (s), 2306
(w), 1488 (m), 1446 (m), 1384 (m), 1266 (s), 1152 (m), 1114 (m),
1076 (m), 896 (m), 740 (s) cm-1. 1H NMR (400 MHz, CDCl3): δ
0.82 (d, J ) 7.4 Hz, 9H), 0.86 (d, J ) 7.5 Hz, 9H), 1.25 (septet,
J ) 7.5 Hz, 3H), 1.90 (dd, J ) 8.9, 4.9 Hz, 1H), 2.19 (dd, J ) 6.2,
5.2 Hz, 1H), 3.63 (t, J ) 7.7 Hz, 1H), 4.87 (s, 1H), 6.28 (d, J )
7.4 Hz, 2H), 6.58 (t, J ) 7.7 Hz, 2H), 6.71 (t, J ) 7.2 Hz, 1H),
6.83 (d, J ) 8.9 Hz, 4H), 6.93 (dd, J ) 9.0, 2.3 Hz, 1H), 7.23 (d,
J ) 2.2 Hz, 1H), 7.29-7.35 (m, 2H), 7.32 (d, J ) 8.8 Hz, 4H),
7.43 (t, J ) 7.6 Hz, 2H), 7.61 (d, J ) 7.9 Hz, 3H), 7.79 (s, 1H),
7.84 (dd, J ) 6.8, 1.4 Hz, 1H), 8.18 (d, J ) 9.0 Hz, 1H). 13C NMR
(100 MHz, CDCl3): δ 12.1, 18.6 (CH3 (CH) Si), 18.7 (CH3 (CH)
Si), 24.5, 29.0, 38.0, 38.9, 115.0, 116.3, 120.5, 121.9, 122.9, 124.1,
124.6, 125.0, 125.4, 126.2, 126.5, 126.6, 127.4, 127.8, 128.7, 129.3,
130.6, 131.9, 132.1, 132.7, 139.9, 140.2, 140.7, 142.6, 146.1, 146.6,
146.7, 146.8 ppm. HRMS: calcd for C33H2781Br79BrN 889.215,
found 889.215.
Bis(4-bromophenyl)-(10-cyclopropyl-5-triisopropylsilanyl-
1
11H-benzo[b]fluoren-7-yl)-amine (6a′). Yield: 1.5 mg, 7%. H
NMR (400 MHz, CDCl3): δ 0.83-0.88 (m, 2H), 1.11 (d, J ) 7.4
Hz, 18H), 1.20-1.23 (m, 2H), 1.46 (septet, J ) 7.4 Hz, 3H), 2.15-
2.21 (m, 1H), 4.16 (s, 2H), 7.02 (d, J ) 8.8 Hz, 4H), 7.23 (dd,
J ) 9.1, 2.3 Hz, 1H), 7.37 (d, J ) 8.8 Hz, 4H), 7.47 (d, J ) 7.4
Hz, 1H), 7.51 (d, J ) 2.3 Hz, 1H), 7.58 (d, J ) 7.5 Hz, 1H), 7.92
(s, 1H), 7.96 (s, 1H), 8.46 (d, J ) 9.1 Hz, 1H). 13C NMR (100
MHz, CDCl3): δ 7.2, 10.9, 11.3, 18.6, 29.7, 36.2, 115.5, 116.0,
119.5, 121.9, 123.0, 124.3, 125.6, 126.7, 127.1, 130.1, 132.3, 133.0,
133.4, 134.6, 140.1, 140.6, 141.0, 143.7, 144.4, 146.6 ppm.
HRMS: calcd for C41H4379Br2NSi 735.153, found 735.152.
Bis(4-bromophenyl)-(10-cyclopropyl-11H-benzo[b]fluoren-7-
yl)-amine (6a′′). Yield: 3.1 mg, 20%. 1H NMR (400 MHz,
CDCl3): δ 0.86-0.90 (m, 2H), 1.19-1.23 (m, 2H), 2.16-2.23 (m,
1H), 4.16 (s, 2H), 7.02 (d, J ) 8.8 Hz, 4H), 7.23 (dd, J ) 9.1, 2.3
Hz, 1H), 7.34 (m, 2H, Ar-H), 7.37 (d, J ) 8.8 Hz, 4H, Ar-H), 7.51
(d, J ) 2.1 Hz, 1H), 7.58 (d, J ) 7.1 Hz, 1H), 7.83 (d, J ) 6.8 Hz,
1H), 7.90 (s, 1H), 8.47 (d, J ) 9.1 Hz, 1H). 13C NMR (100 MHz,
CDCl3): δ 7.2, 11.3, 29.7, 36.2, 115.5, 116.3, 120.5, 122.1, 123.3,
125.1, 125.5, 126.7, 126.8, 127.6, 130.2, 132.3, 133.4, 134.6, 140.7,
79
140.8, 140.9, 143.8, 143.9, 146.6 ppm. HRMS: calcd for C32H23
Br2N 579.020, found 579.019.
-
Bis(4-bromophenyl)-[10-(2-phenylcyclopropyl)-11-triisopro-
pylsilanyl-11H-benzo[b]fluoren-7-yl]-amine (6b). Yield: 8.6 mg,
32%. IR (film) ν˜: 2865 (s, C-H), 1580 (m, CdC), 1485 (s), 1282
(m), 1072 (m), 883 (m), 821 (m), 755 (m), 698 (m) cm-1. 1H NMR
(400 MHz, CDCl3) two isomers: δ 0.54 (d, J ) 7.5 Hz, 9H), 0.66
(d, J ) 7.5 Hz, 9H), 0.89 (2 × d, J ) 7.6 Hz, 18H + 3H), 0.98-
1.07 (m, 3H), 1.46-1.50 (m, 2H), 1.66-1.73 (m, 2H), 1.86-1.91
(m, 1H), 2.03-2.07 (m, 1H), 2.57-2.63 (m, 1H), 2.65-2.71 (m,
1H), 4.69 (s, 1H), 4.78 (s, 1H), 6.77 (d, J ) 8.9 Hz, 1H), 7.02 (d,
J ) 8.8 Hz, 4H), 7.05 (d, J ) 8.8 Hz, 4H), 7.15 (dd, J ) 9.1, 2.4
Hz, 1H), 7.19-7.24 (m, 5H), 7.33-7.41 (m, 17H), 7.48-7.57 (m,
4H), 7.85 (d, J ) 7.2 Hz, 2H), 7.92 (s, 1H), 7.94 (s, 1H), 8.04 (d,
J ) 9.1 Hz, 1H), 8.40 (d, J ) 9.1 Hz, 1H). 13C NMR (100 MHz,
CDCl3) two isomers: δ 11.5, 12.1, 18.3, 18.5, 18.6, 18.7, 24.9,
26.6, 26.7, 28.1, 38.4, 40.3, 115.5 (2C), 116.2, 116.3, 120.6, 120.7,
121.5, 121.8, 123.0, 123.1, 124.0, 124.1, 124.9, 125.2, 125.4, 125.6
(2C), 125.8, 125.9, 126.6, 126.7, 126.8, 127.1, 128.5, 128.7, 129.7,
130.4, 131.9, 132.1, 132.4 (2C), 133.5, 133.6, 139.7, 139.8, 140.0,
Acknowledgment. We are very much indebted to the
Deutsche Forschungsgemeinschaft and the Fonds der Chemis-
chen Industrie for continued support of our research. Moreover,
we thank Dr. Demuth for a careful check of the experimental
data. This work is dedicated to Prof. Dr. A. Maercker on the
occasion of his 75th birthday.
Supporting Information Available: Experimental procedures
for compound 2a,c-3a,c, 1H, 13C spectra for all compounds,
ORTEP drawings of compound (2-ethynyl-1-phenylcyclopropyl)-
benzene (17), and crystallographic data for the X-ray diffraction
analyses of 6c and 17 in CIF format. Details of the computation.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO062448+
J. Org. Chem, Vol. 72, No. 6, 2007 2173