The thermal C2-C6/[2 + 2] cyclisation of enyne-allenes: Reversible diradical formation

Article abstract of DOI:10.1039/c0ob01275k

New enyne-allenes, structurally designed toward the thermal C ²-C⁶/[2 + 2] cyclisation mode, were prepared and characterised, one of them even by X-ray crystallography. The mechanism of their transformation to formal [2 + 2] cycloadducts was interrogated by trapping experiments and DFT computations. The results support a stepwise mechanism that involves the reversible formation of the C²-C⁶ diradical intermediate.

Full text of DOI:10.1039/c0ob01275k

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PAPER
The thermal C2–C6/[2 + 2] cyclisation of enyne-allenes: Reversible diradical
formation†‡
Mehmet Emin Cinar, Chandrasekhar Vavilala,§ Jian Fan and Michael Schmittel*
Received 30th December 2010, Accepted 7th March 2011
DOI: 10.1039/c0ob01275k
New enyne-allenes, structurally designed toward the thermal C2–C6/[2 + 2] cyclisation mode, were
prepared and characterised, one of them even by X-ray crystallography. The mechanism of their
transformation to formal [2 + 2] cycloadducts was interrogated by trapping experiments and DFT
computations. The results support a stepwise mechanism that involves the reversible formation of the
C2–C6 diradical intermediate.
The (benzo)fulvene diradical 2,1,2 generated in the thermal
C2–C6 cyclisation of enyne-allene 1, is well known to undergo
intramolecular follow-up processes, such as formal ene3,4 and
Diels–Alder5,6 (DA) cycloadditions, depending on the nature of the
substituents at the allene terminus. With R2 and/or R3 being aryl
substituents the reaction furnished benzofluorene 3 along the DA
pathway whereas with R2 and/or R3 being CHRR¢ benzofulvenes
4 were obtained (Scheme 1).7 In contrast, the involvement of 2
in the formal [2 + 2] cycloaddition of 1 affording 5 has not been
investigated.
The first example of a [2 + 2] cycloaddition8 was reported by
Gillmann9 in the thermolysis of an enyne-allene with a TMS group
at the alkyne terminus and two hydrogens at the allene terminal.
Despite excess amounts of 1,4-cyclohexadiene (1,4-CHD), the
C2–C6 diradical was not intercepted by hydrogen abstraction.
Nevertheless, a stepwise mechanism via the C2–C6 diradical was
proposed by Gillmann, a suggestion taken over later by Wang,
who designed a number of ingenious examples along the [2 + 2]
reaction channel.10
Basically all C2–C6/[2 + 2] reactions of enyne-allenes known as
of today command absence of aryl and alkyl groups at the allene
terminus. Such substituent pattern may be seen as a hint to the
intermediacy of 2 also in the [2 + 2] cycloaddition mode, as the
competing DA and ene reactions of 2 exhibit activation barriers
of less than 9 and 3 kcal mol-1, respectively.11
In the present paper we address the intermediacy of the
C2–C6 diradical in the [2 + 2] cycloaddition in more detail. It is
important, however, to note that not all enyne-allenes structurally
designed toward a [2 + 2] route undergo this cycloaddition.
For example, thermolysis of 6a in the presence of 1,4-CHD
furnished 11% of 8a (Scheme 2),1d visibly arising by trapping of
the corresponding diradical intermediate 7a through hydrogen
abstraction. Equally, there is evidence for the intermediacy of
7b in the thermal cyclisation of 6b, by both product study and
DNA strand cleavage.5a Thus, to clarify the intermediacy of the
C2–C6 diradical in the [2 + 2] channel, we reinvestigated 6b and
complemented the series by adding 6c–6d. The results of our
experimental and computational study strongly support a stepwise
diradical mechanism with a reversible C2–C6 cyclisation.
Scheme 1 Various products arising from the thermal C2–C6 cyclisation
of enyne-allenes.
Dept. of Chemistry and Biology, Universita¨t Siegen, Adolf-Reichwein-Str.,
D-57068, Siegen, Germany. E-mail: schmittel@chemie.uni-siegen.de
† This paper is dedicated to the commemoration of the late Athel L. J.
Beckwith for his numerous insightful and valuable contributions to the
field of radical chemistry.
‡ Electronic supplementary information (ESI) available: Experimental
details, 1H and 13C-NMR spectra of all new compounds. Crystal data
and structure refinement details for 6c and cartesian coordinates of all
stationary points of 6d. See DOI: 10.1039/c0ob01275k
§ C. Vavilala deceased on 18 February 2010. We will remember him for his
friendship and many scientific stimuli.
Scheme 2 Trapping of the C2–C6 diradical 7 with 1,4-CHD.
3776 | Org. Biomol. Chem., 2011, 9, 3776–3779
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6b was prepared along a literature procedure,5a while 6c
was obtained in two steps starting with the Grignard addi-
tion of tert-butylacetylene to 2-(2-phenylethynyl)benzaldehyde (9)
(Scheme 3).12 The resultant propargyl alcohol 10 was reacted with
chlorodiphenyl phosphine, affording 6c in 82% yield. Its solid
state structure, the second one of the enyne-allene family,13 was
unambiguously determined by X-ray crystallography.
Table 1 Thermolysis conditions for enyne-allenes 6b–6d (solvent:
toluene) and product yields
Enyne-
Product
allene R1
R2
1,4-CHDa Temp. [◦C] Time [h] (Yield [%])
6b
6c
6c
6d
POPh2
POPh2 tBu
H
—
—
111
150b
160b
111
4
5
42.5
8
15b (24)
15c (72)c
15c (67)
15d (26)
POPh2 tBu 100 eq.
tBu
H
—
a 1,4-CHD = 1,4-cyclohexadiene. b In sealed tube. c At 29% conversion.
in the presence of CuCN providing enyne-allene 14 in moderate
yield (48%). Removal of the TMS group was carried out in 1 M
KOH solution, affording enyne-allene 6d in 79% yield (Scheme 3).
The thermolysis of enyne-allenes 6b–6d was performed in
dry toluene under nitrogen. All compounds underwent C2–C6
cyclisation to [2 + 2] cycloadducts in moderate (15b and 15d)
to good (15c) yields (Scheme 4). The reaction conditions are listed
in Table 1.
Scheme 4 Thermolysis of enyne-allenes 6b–6d (for R1,R2 see Table 1).
Thermolysis of 6b in toluene (111 ◦C, 4 h), but in absence of
1,4-CHD, furnished the corresponding [2 + 2] cycloadduct 15b in
24% yield. It was identified from its doublet at 5.39 ppm with a
coupling constant of 9.9 Hz (for 1-H) and by a singlet at 5.93 ppm
(for 10-H) in the 1H-NMR. The 13C-NMR shows a characteristic
doublet at 51.1 ppm for C-1with a carbon phosphorous coupling
constant JC,P = 70 Hz. As the thermal reaction of 6b in presence
of 1,4-CHD had earlier led to 8b,5a diradical 7b (Scheme 2) is a
likely precursor to both products. Notably, the thermolysis of 6c
in toluene afforded 72% of the [2 + 2] cycloadduct 15c, whose
13C-NMR shows the expected doublet at 72.0 ppm for C-1 with a
coupling constant JC,P = 64 Hz. In contrast to the situation with 6b,
however, thermolysis of 6c in presence of 100 equiv. of 1,4-CHD
did not furnish a single trace of the putative trapping product 8c,
as analysed by 1H and 13C-NMR, excluding any major follow-up
reaction of 7c via hydrogen abstraction. In lieu thereof, the [2 + 2]
cycloadduct 15c was isolated in 67% yield (Scheme 4 and Table 1)
To investigate whether the combined steric effects of the tBu and
POPh2 subunits are responsible for the high [2 + 2] cycloaddition
yield, we turned our attention to 6d that is an analogue of 6b with
the tBu replacing the POPh2 group. The thermal cyclisation of 6d
was performed in refluxing toluene for 8 h affording 15d (26%), the
latter being identified by characteristic singlets at 4.40 ppm (1-H)
Scheme 3 Synthesis of enyne-allenes 6b–6d.
In the solid state, enyne-allene 6c is present in the s-trans
conformation (C2–C1–C15–C16 dihedral angle = 169.3◦) that
unlike the thermally reactive s-cis conformation does not allow
for cyclisation (Fig. 1).14 The lengths of the allene double bonds,
C15–C16 and C16–C17, are dCC = 130.3(4) and 131.5(4) pm and
therefore well in line with those of regular allenes.15
Fig. 1 X-ray structure of enyne-allene 6c.¶
To prepare enyne-allene 6d, propargyl alcohol 11 was first
protected with benzoyl chloride followed by treatment with t-BuLi
1
and 6.08 ppm (10-H) in the H NMR and a distinct 13C NMR
resonance at 61.6 ppm (C-1).
¶ X-ray data collection and structure determinations. X-ray single-crystal
diffraction data for 6c was collected on a STOE IPDS one-circle image
plate diffractometer. The structure was solved using the SHELXL-97
crystallographic software package and refined by the full-matrix least-
squares technique.21 The hydrogen atoms were generated theoretically onto
the specific atoms and refined isotropically with fixed thermal factors.
The non-H atoms were refined with anisotropic thermal parameters. The
crystal parameters, data collection, and refinement results are summarised
in Table S1 (ESI‡). Selected bond lengths and angles are listed in Table S2
(ESI‡).
The low yields of 15b (24%) and 15d (26%) together with their
low total product balances (representing otherwise an untraceable
and complex mixture) as well as the high yield for 15c may
actually be taken as evidence for the involvement of a reactive
intermediate, whose follow-up reaction is heavily controlled by
steric effects. Indeed, intermediates 7b and 7d may readily adopt a
planar conformation (Scheme 5), which is surely precluded for 7c.
Hence, the preferred conformation of 7c is well biased to furnish
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concerted mechanism and presents a particularly clear-cut case of
a stepwise intramolecular [2 + 2] cycloaddition of allene-ynes.19
In summary, the present experimental and computational
results suggest that the thermal C2–C6/[2 + 2] cyclisation of
enyne-allenes proceeds via a stepwise pathway. In contrast to the
ene and DA pathway of enyne-allenes, formation of the C2–C6
diradical is not rate determining in the [2 + 2] cycloaddition.
As a consequence, we would expect that upon mild thermolysis
the chiral enyne-allenes 6a–6d, if prepared in an optical active
form, should undergo partial racemisation via the benzofulvene
diradical. Moreover, the deep shallow minimum about diradical
7d indicates that non-statistical dynamic effects can reliably be
excluded in the [2 + 2] pathway, quite in contrast to the situation
of enyne-allenes following the thermal ene reaction pathway.4,20
Scheme 5 Preferred conformations of diradicals 7b–7d.
15c effortlessly, while that of 7b and 7d allows for a variety of
follow-up radical processes.
The sum of all experimental findings thus presents convincing
facts for the involvement of the reactive diradical 7, as it is
a reasonable intermediate for the [2 + 2] cycloadducts 15, the
hydrogenation products 8 and the unidentified side products.
To shed more light on the mechanism of the thermal C2–C6/[2 +
2] cyclisation of enyne-allenes, we performed DFT calculations16
with an unrestricted broken-spin-symmetry ((BS)-(U)BLYP/6-
31G(d))17 on 6d (Fig. 2). Pure DFT functionals, like BLYP, in com-
bination with a 6-31G(d) basis set were demonstrated by Schreiner
to provide high chemical accuracy and reasonable computational
costs in Bergman and Myers-Saito diradical reactions.18
Experimental Section
The preparation of all precursors for enyne-allenes 6c–6d is
described in the ESI‡. 6b was prepared as described in the
literature.5a
4,4-Dimethyl-3-(diphenylphosphinoyl)-1-[2-(phenylethynyl)-
phenyl]-pent-1,2-diene (6c). To a solution of 10 (740 mg,
2.56 mmol) and NEt3 (311 mg, 3.08 mmol) in THF (20 mL) at
-78 ◦C was added chlorodiphenylphosphine (679 mg, 3.08 mmol)
in THF (5 mL) over 15 min. The reaction mixture was stirred for
1 h at the same temperature and then allowed to slowly warm up
to RT. After stirring for 1 h, it was hydrolysed with water and
extracted with ethyl acetate. The combined organic layers were
dried over magnesium sulfate and concentrated under reduced
pressure. After purification by column chromatography (silica gel,
n-hexane/ethyl acetate = 3 : 2, Rf 0.34) 6c was isolated in 82% yield
(994 mg, 2.10 mmol) as white solid. Mp: 38–40 ◦C. IR (KBr, cm-1)
3057, 2962, 2213, 1936, 1599, 1494, 1438, 1190, 1116, 813, 756.
1H-NMR (400 MHz, C6D6) d 1.50 (s, 9H), 6.80–6.84 (m, 1H),
6.88–7.02 (m, 11H), 7.30 (d, J = 7.8 Hz, 1H), 7.36–7.39 (m, 3H),
7.87–7.96 (m, 4H). 13C-NMR (100 MHz, C6D6) d 31.0 (d, JC,P
=
Fig. 2 Potential energy curve obtained from (BS)-(U)BLYP/6-31G(d)
level calculations for the C2–C6/[2 + 2] cyclisation of 6d. (Electronic
energies including unscaled zpe are relative to the starting material 6d).
2.6 Hz), 38.0 (d, JC,P = 5.1 Hz), 87.9, 94.9, 96.0 (d, JC,P = 14 Hz),
113.0 (d, JC,P = 92 Hz), 121.5 (d, JC,P = 2.6 Hz), 123.5, 126.5 (d,
JC,P = 1.7 Hz), 127.4, 127.9, 128.2, 128.4 (d, JC,P = 1.7 Hz), 128.7 (d,
JC,P = 1.7 Hz), 131.3 (d, JC,P = 2.6 Hz), 131.5 (d, JC,P = 2.6 Hz), 131.7,
The DFT results show an overall downhill process of 12.4 kcal
mol-1 for the two-step transformation 6d → 7d → 15d. 16, the tran-
sition state (TS) to diradical 7d, is characterised by bond formation
between C2 and C6 (dCC = 194.3 pm), whereas transition state 17
still shows the C1–C7 bond formation in the cyclobutene ring at an
initial stage (dCC = 263.7 pm). In 17, the C2–C6 bond has already
shortened to 146.7 pm, which is close to that in 15d (dCC = 145.4
pm). In comparison to the stepwise [2 + 2] cycloaddition of hepta-
1,2-dien-6-yne the barrier of the diradical formation is substan-
tially lowered, while that of the final ring closure is much higher.19
Unlike ene3,4,20 and DA5 reactions of enyne-allenes via the
C2–C6 diradical, the [2 + 2] ring closure transition state 17 has a
significantly higher energy than 16, the transition state of the initial
diradical cyclisation.2b As hydrogen incorporation was not always
seen during thermolysis in the presence of 1,4-CHD, the possibility
of a concerted mechanism was also taken into consideration.
However, 18, the TS for the concerted process 6d → 15d, is located
21 kcal mol-1 above the rate-determining TS 17 of the stepwise
variant. The high activation energy for TS 18 thus excludes any
131.8, 131.9, 132.0, 132.8, 135.0 (d, JC,P = 102 Hz), 135.1 (d, JC,P
=
104 Hz), 135.3 (d, JC,P = 6.8 Hz), 209.4 (d, JC,P = 6.8 Hz); HRMS-EI
(m/z) for C33H29OP [M]+ calcd. 472.196, found 472.196.
4,4-Dimethyl-1-[2-(phenylethynyl)phenyl]-penta-1,2-diene (6d).
To a solution of 14 (100 mg, 290 mmol) in 5 mL of methanol
was added 10 mL of 1 N KOH and 5 mL of THF. The reaction
mixture was stirred for 4 h at room temperature. After extraction
with diethyl ether (3 ¥ 15 mL) the organic phase was washed with
water. The combined organic layers were dried over MgSO4, and
the solvent was removed under reduced pressure. After purification
by column chromatography (silica gel, n-pentane, Rf 0.44) 6d was
isolated in 79% yield (62 mg, 228 mmol) as viscous yellow oil. IR
(KBr, cm-1) 3058, 3031, 2960, 2901, 2864, 2215, 1947, 1599, 1494,
1
1445, 1363, 1248, 882, 756. H-NMR (400 MHz, CDCl3) d 1.05
(s, 9H), 5.52 (d, J = 6.4 Hz, 1H), 6.88 (td, J = 7.6, 1.1 Hz), 6.96–
6.99 (m, 3H), 7.05 (td, J = 7.6, 1.1 Hz, 1H), 7.30 (d, J = 6.4 Hz,
1H), 7.40–7.44 (m, 2H), 7.52 (dd, J = 7.6, 1.1 Hz, 1H), 7.67 (d,
J = 7.6 Hz, 1H). 13C-NMR (100 MHz, CDCl3) d 30.3, 32.8, 88.4,
3778 | Org. Biomol. Chem., 2011, 9, 3776–3779
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94.8, 95.1, 107.2, 121.6, 123.8, 126.3, 126.8, 128.4, 128.6, 128.8,
131.9, 132.8, 137.3, 203.8. HRMS-EI (m/z) for C21H20 [M]+ calcd
272.157, found 272.157.
128.6, 129.3, 129.4, 135.0, 146.2, 148.3, 149.9, 152.8. HRMS-EI
(m/z) for C21H20 [M]+ calcd 272.157, found 272.157.
Acknowledgements
2-Phenyl-1-diphenylphosphinoyl-1H-cyclobuta[a]indene (15b).
6b5a (50.0 mg, 120 mmol) was dissolved in dry toluene (30 mL)
and, after degassing, the resulting solution was refluxed for
4 h. After removal of toluene under reduced pressure and
purification by chromatography (preparative TLC, silica gel 60
F254, n-hexane/ethyl acetate = 1 : 1, Rf 0.34) 15b was isolated
in 24% yield (12.0 mg, 28.8 mmol) as a dark yellow solid. Mp:
>194 ◦C decomposition; IR (KBr, cm-1) 3053, 2953, 2923, 2857,
We are very much indebted to the Deutsche Forschungsgemein-
schaft for continued support of our research (Schm 647/18-1). J.
F. thanks the Alexander von Humboldt foundation for a stipend
and Hans-Jo¨rg Deiseroth as well as Marc Schlosser for technical
help with the X-ray measurements.‡
Notes and references
1621, 1547, 1437, 1385, 1180, 1157, 1119, 1102, 1070, 1029,
1
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755, 726, 693. H-NMR (400 MHz, CDCl3) d 5.39 (d, JH,P
=
9.9 Hz, 1H), 5.93 (s, 1H), 7.08 (td, J = 7.5, 1.7 Hz), 7.18–7.24 (m,
4H), 7.32–7.44 (m, 4H), 7.51–7.62 (m, 6H), 7.87–7.95 (m, 4H).
13C-NMR (100 MHz, CDCl3) d 51.1 (d, JC,P = 70 Hz), 113.9,
122.0, 123.8, 124.2, 128.0 (d, JC,P = 12 Hz, 2 ¥ C), 128.7 (d, JC,P
12 Hz, 2 ¥ C), 128.8 (2 ¥ C), 128.9, 129.1 (2 ¥ C), 129.6, 129.7 (d,
JC,P = 113 Hz, 2 ¥ C), 131.2 (d, JC,P = 9.4 Hz, 2 ¥ C), 131.7 (d, JC,P
=
=
3 M. Schmittel, M. Strittmatter and S. Kiau, Tetrahedron Lett., 1995, 36,
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8.6 Hz, 2 ¥ C), 131.8 (d, JC,P = 2.6 Hz), 132.0 (d, JC,P = 2.6 Hz),
132.8, 133.4, 138.4 (d, JC,P = 6.0 Hz), 144.7 (d, JC,P = 6.8 Hz),
147.2 (d, JC,P = 14 Hz), 151.5. HRMS-EI (m/z) for C29H21OP [M]+
calcd. 416.133, found 416.133.
1-t-Butyl-2-phenyl-1-diphenylphosphinoyl-1H -cyclobuta[a]-
indene (15c). To a degassed solution of 6c (250 mg, 529 mmol) in
dry toluene (40 mL) was added 100 equiv. of 1,4-CHD (4.47 mL,
◦
52.9 mmol). The resulting solution was heated up to 160 C for
7 M. Schmittel, M. Maywald and M. Strittmatter, Synlett, 1997, 165.
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42.5 h in a sealed vessel. After removal of toluene under reduced
pressure and purification by column chromatography (silica gel,
n-hexane/ethyl acetate = 3 : 2, Rf 0.65) 15c was isolated in 67%
yield (168 mg, 356 mmol) as orange solid. Mp: 216–218 ◦C; IR
(KBr, cm-1) 3061, 2973, 2928, 2867, 1592, 1537, 1477, 1438, 1368,
1175, 1153, 1107, 761, 697. 1H-NMR (400 MHz, C6D6) d 1.32 (s,
9H), 6.31 (s, 1H), 6.60–6.62 (m, 3H), 6.91 (td, J = 7.3, 1.1 Hz, 1H),
6.98–7.06 (m, 3H), 7.10–7.20 (m, 4H), 7.30 (d, J = 7.5 Hz, 1H),
7.36 (d, J = 7.3 Hz, 1H), 7.99–8.04 (m, 2H), 8.53–8.59 (m, 4H).
13C-NMR (100 MHz, C6D6) d 30.4 (d, JC,P = 4.3 Hz, 3 ¥ C), 39.7
(d, JC,P = 2.6 Hz), 72.0 (d, JC,P = 64 Hz), 116.2, 122.3, 124.4, 124.8,
127.6, 127.7, 128.6 (d, JC,P = 11 Hz, 2 ¥ C), 128.9 (2 ¥ C), 129.2,
129.6, 130.3, 130.6 (2 ¥ C), 130.9 (d, JC,P = 2.6 Hz), 131.4, 131.5,
132.5 (d, JC,P = 8.6 Hz, 2 ¥ C), 134.4 (d, JC,P = 98 Hz), 134.7, 135.8
(d, JC,P = 87 Hz), 144.1 (d, JC,P = 3.4 Hz), 148.2 (d, JC,P = 13 Hz),
151.6 (2 ¥ C), 152.6 (d, JC,P = 4.3 Hz). Anal. calcd. for C33H29OP
(472.56): C, 83.87; H, 6.19. Found: C, 83.36; H, 6.46.
15 (a) E. V. Banide, C. O’Connor, N. Fortune, Y. Ortin, S. Milosevic, H.
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¨
McGlinchey, Eur. J. Org. Chem., 2007, 2611.
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See ESI for full reference.
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724.
1-t-Butyl-2-phenyl-1H-cyclobuta[a]indene (15d). 6d (50.0 mg,
184 mmol) was dissolved in dry toluene (40 mL) and, after
degassing, the resulting solution was refluxed for 8 h. After
removal of toluene under reduced pressure and purification by
chromatography (preparative TLC, silica gel 60 F254, n-hexane,
Rf 0.62) 15d was isolated in 26% yield (13 mg, 47.7 mmol) as
red waxy solid. IR (KBr, cm-1) 3063, 2958, 2903, 2867, 1711, 1676,
1596, 1547, 1460, 1436, 1364, 1180, 754, 696. 1H-NMR (400 MHz,
CDCl3) d 1.10 (s, 9H), 4.40 (s, 1H), 6.08 (s, 1H), 7.10 (td, J = 7.5,
1.1 Hz, 1H), 7.25 (td, J = 7.5, 1.1 Hz, 1H), 7.29–7.32 (m, 1H), 7.36–
7.40 (m, 1H), 7.45–7.49 (m, 2H), 7.66 (dd, J = 7.5, 1.1 Hz, 1H),
7.71–7.74 (m, 2H). 13C-NMR (100 MHz, CDCl3) d 28.6 (3 ¥ C),
33.9, 61.6, 111.0, 121.4, 123.3, 123.5, 128.3 (2 ¥ C), 128.5 (2 ¥ C),
18 M. Prall, A. Wittkopp and P. R. Schreiner, J. Phys. Chem. A, 2001, 105,
9265.
19 M. R. Siebert, J. M. Osbourn, K. M. Brummond and D. J. Tantillo, J.
Am. Chem. Soc., 2010, 132, 11952.
20 M. Schmittel, C. Vavilala and R. Jaquet, Angew. Chem., Int. Ed., 2007,
46, 6911.
21 (a) G. M. Sheldrick, SHELXS97, Program for Crystal Structure
DeterminationUniversity of Go¨ttingen: Go¨ttingen, Germany, 1997;
(b) G. M. Sheldrick, SHELXL97, Program for Crystal Struc-
tural Refinement, University of Go¨ttingen: Go¨ttingen, Germany,
1997.
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The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 3776–3779 | 3779
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