Thermal Rearrangements of Spiro[2.4]heptatrienes
J . Org. Chem., Vol. 67, No. 13, 2002 4439
Ta ble 1. Com p a r ison of 1H a n d 13C NMR Ch em ica l
Sh ifts for Meth in e Gr ou p s a m on g th er m olysis p r od u cts
of 1a (d ih yd r o), 1b (d im eth yl), a n d 1c (n -p r op yl)a
chromatography (TLC) carried out on precoated silica gel 60
A plates (layer thickness 250 µm) with detection by UV light
(λ ) 254 nm) and iodine exposure. Purification by preparative
TLC was carried out using 2 mm silica gel plates. Diazocyclo-
pentadiene, prepared as described previously,7 was vacuum
distilled through a Vigreaux column, bp 52-53 °C/50 mm, and
could be stored at -25 °C for several months.
NMR chemical shifts (δ)
substituents
HA
HK
CA
CK
Lower TLC Spots
Rea ction of Dia zocyclop en ta d ien e w ith (2-Br om ovi-
n yl)tr im eth ylsila n e. A solution of diazocyclopentadiene (600
mg, 6.52 mmol) dissolved in commercially available (2-bromo-
vinyl)trimethylsilane (10 mL) was purged with argon for 15
min. The solution was then irradiated for 2.5 h with a 450-W
lamp in a quartz immersion apparatus (Pyrex filter). The
temperature of the reaction mixture was kept at 0-5 °C. The
excess (2-bromovinyl)trimethylsilane was removed and the
black residue chromatographed quickly through a Florisil
column using pentane as eluent. The solvent was removed
under reduced pressure, and the crude mixture was separated
by means of preparative gas chromatography (injector tem-
perature, 180 °C; column temperature, 160 °C; detector
temperature, 150 °C; carrier gas pressure, 120 kPa). The
retention time was 4.6 min for 2 and 8.14 min for the coproduct
identified as 3. The desired adduct 2 was isolated as a colorless
liquid (90 mg, 5.7%). Spectral data for trans-2: 1H NMR (250
MHz, CDCl3) δ 6.51 (m, 2H), 6.34 (m, 1H), 6.12 (m, 1H), 3.85
(d, 1H, J ) 8.0 Hz), 1.73 (d, 1H, J ) 8 Hz), 0.09 (s, 9H); 13C
NMR (63 MHz, CDCl3) δ 139.04 (CH), 134.88 (CH), 131.60
(CH), 130.27 (CH), 46.23 (C), 30.12 (CH), 25.38 (CH), -1.19
(Me3Si); HRMS (EI) m/z 244.0104, 242.0124, calcd for
dihydro (6a )
dimethyl (6b)
n-propyl (6c)
3.39
3.32
3.28
3.67
3.39
3.53
50.4
48.6
50.6
59.7
59.3
60.3
Upper TLC Spots
dihydro (7a )
dimethyl (7b)
n-propyl (7c)
2.73
2.65
2.61
3.63
3.55
3.47
53.4
51.5
53.3
58.6
57.6
59.0
a
Definitions of HA, HK, and their corresponding carbon atoms
(CA and CK) are shown in Scheme 3, as are the location of dihydro
(R1 ) R2 ) H), dimethyl (R1 ) R2 ) Me), and n-propyl (R1 ) H, R2
) n-Pr) substituents.
Comparison of the NMR spectra of the thermolysis
products of 1a -c indicated distinctive patterns of chemi-
cal shifts (Table 1).25 For example, in the chromatographi-
cally less mobile dimers, both methines resonated at δ
3.3-3.7, whereas the monoallylic methine was shifted
upfield to ca. δ 2.6-2.7 in the more mobile dimer. These
patterns indicate that the dimeric products from 1a and
1c have structures analogous to those from 1b. Thus, the
products of lower TLC mobility can tentatively be as-
signed as 6a -c and those of higher mobility as 7a -c.
In conclusion, the thermal rearrangements of spiro-
[2.4]hepta-1,4,6-trienes may be explained in terms of
[1,5]-sigmatropic rearrangements to yield bicyclo[3.2.0]-
hepta-1,3,6-trienes, which then dimerize by formal cyclo-
addition across the strained bridgehead double bond. At
the present level of understanding, neither free radical
nor thermally forbidden concerted mechanisms account
persuasively for the observed formation of only two of
the four possible dimers. Finally, we note that at much
higher temperatures, bicyclo[3.2.0]hepta-1,3,6-trienes can
undergo gas-phase reactions other than dimerization.9d,11,13
Studies on the direct observation of bicyclo[3.2.0]hepta-
1,3,6-triene using low-temperature NMR spectroscopy
are contemplated.
C
10H1581BrSi 244.0106, calcd for C10H1579BrSi 242.0126. The
corresponding signals of the cyclopropanyl protons CHSiMe3
and CHBr of the cis isomer appear at δ 1.41 (d, J ) 10 Hz)
and 4.20 (d, J ) 10 Hz). Spectral data for 3: 1H NMR (250
MHz, CDCl3) δ 3.98 (dd, AB system, 1H, J ) 11.1, 4.6 Hz),
3.72 (t, 1H, J ) 11.1 Hz), 3.49 (dd, AB system, 1H, J ) 11.1,
4.6 Hz), 0.21 (s, 9H); 13C NMR (63 MHz, CDCl3) δ 42.88 (CH),
36.48 (CH2), -2.51 (Me3Si); HRMS (CI) m/z 260.9124, 258.9166,
calcd for C5H1279Br81BrSi 260.9133, calcd for C5H1279Br2Si
258.9154. Prolonged photolysis resulted in a higher yield of
this product.
P r ep a r a tion of Sp ir o[2.4]h ep ta -1,4,6-tr ien e (1a ). Dry
CsF (225 mg, 1.48 mmol) was placed in a 50-mL two-neck
round-bottom flask connected to a series of three cold traps
maintained at -45 °C and liquid nitrogen temperature. The
system was then evacuated so that the vacuum reached ∼10
mTorr. Dry dimethyl sulfoxide (5 mL) was added to the flask
via a syringe, followed by a solution of 2 (120 mg, 0.49 mmol)
in DMSO (5 mL). The volatile products were condensed into
the liquid nitrogen trap over a period of 30 min. The triene
Exp er im en ta l Section
1
was condensed as a slightly yellowish solid (30 mg, 68%). H
Compounds are numbered under the assumption that 6a -c
and 7a -c are the structures of the lower and upper TLC spots
of the dimers from thermolysis. 1H and 13C NMR spectra were
obtained in CDCl3 [50-100 mM, 25 °C for 6b and 7b and
referenced to Me4Si, residual CHCl3 (1H, 7.26 ppm), or CDCl3
(13C, 77.0 ppm)]. NOE difference spectroscopy was done on
nondegassed samples on the 500 MHz spectrometer with a 90°
read pulse. PCMODEL (version 7, MMX and MM3 force fields)
and Gaussian 98 were used to determine energies and
NMR (500 MHz, CDCl3) δ 7.88 (s, 2H), 6.57 (m, 2H), 6.09 (m,
2H); 13C NMR (125 MHz, CDCl3) δ 141.51 (CH), 131.06 (CH),
117.95 (CH), 40.07 (C); IR 1718 cm-1 (cyclopropenyl stretch-
ing); UV (MeOH) λmax 254 nm.
Th er m olysis of Sp ir o[2.4]h ep ta -1,4,6-tr ien e (1a ). The
spirene was heated in an NMR tube at 50 °C for 8 h as the
reaction was monitored by 1H NMR spectroscopy. Two isomeric
dimers, corresponding to a 2:3 mixture of faster and slower
moving components (7a and 6a ), were isolated by preparative
1
1
geometries of the dimers. Vicinal H NMR coupling constants
TLC (hexane) in 80% combined yield and characterized by H
and 13C NMR (Table S3 and Figures S9 and S10 of the
Supporting Information). 1H NMR spectra (recorded in CDCl3)
for the dimers showed that they are identical to the dimers of
bicyclo[3.2.0]hepta-1,3,6-triene 5 reported earlier by Bauld et
al.14
were predicted in PCMODEL using a Karplus relationship
applied to MM3 or Gaussian 98 structures. Spin simulation
was done with Bruker Daisy software. Mass spectra (MS) were
acquired at 70 eV on a sector instrument. High-resolution mass
spectra were recorded using perfluorokerosene as the stan-
dard. Chemical reagents were used without further purifica-
tion. Preparative gas chromatography was carried out using
a thermal conductivity detector and a 2 m × 5 mm column
(20% Carbowax 1450 on 80/100 Chromosorb W Acid Washed)
using prepurified (99.999%) helium as the carrier gas. Column
chromatography was performed using reagent-grade silica gel
(230-400 mesh). All reactions were monitored by thin-layer
P r epar ation of 1,2-Dim eth ylspir o[2.4]h epta-1,4,6-tr ien e
(1b).8 A solution of diazocyclopentadiene (1 g, 0.0109 mol) in
2-butyne (30 mL) was cooled to 0 °C in a Pyrex tube and
purged with argon for 15 min. The contents of the tube were
irradiated at 0 °C for 2 h using a 450-W lamp in a quartz
immersion apparatus (Diaza filter, λ ) 366 nm). During this
time, 90% of the theoretical amount of nitrogen was evolved.
The excess 2-butyne was removed in vacuo, and the black
crude product was passed through a silica gel column using
dichloromethane/pentane (30:70) as eluent. Evaporation of the
(25) Table S3 of the Supporting Information shows a full comparison
of 1H and 13C NMR chemical shifts for the six dimeric products.