, 2004, 14(6), 299–301
Br
Br
77%
Br
exo-anti-13 / endo-anti-14 = 4.4
MeLi
1.1 equiv.
Ph
Ph
O
Ph
O
Ph
O
Br
Ph
6
4, 77%
Ph
Br
Br
18 h, –30 °C
Ph
10, 70%
13
14
Ph
Ph
3
DPIBF
Furan
Ph
7
8
1
:
Ph
Scheme 1
CH2N2
Ph
N
–20 °C, it gave 3-phenylcyclopropene 1 in 35 % yield, as deter-
mined by 1H NMR.¶ This was trapped on addition of DPIBF as
a single adduct, 10 (70% based on 6).¶
N
17, 62%
1
15, 81%
PhCNO
Ph
Br
Li
E
Ph
Br
MeLi
2.2 equiv.
E–X
Ph
Ph
Ph
N
O
Br
6
9
1 E = H, 35%
2 E = Me, 53%
16, 78%
Scheme 3
3 E = C(OH)Me2, 73%
5 E = Me3Si, 30%
caused by the introduction of the methyl group. In support of
this, calculations for 3-tert-butyl-3-phenylcyclopropene suggest
a preferred geometry in which the plane of the benzene ring is
parallel to the cyclopropene π-bond. The HOMO of the pre-
ferred conformation of 1 is shown in Figure 1, alongside the
calculated minimum energy structure.
Attempts to trap the intermediate 1-lithio-3-phenylcyclo-
propene 9 by the addition of acetone or benzaldehyde to the
carbonyl group gave a good yield of cyclopropen-1-yl carbinol
3 in the former case†† and no adduct in the latter. However, it
could be trapped by reaction with methyl iodide at room tem-
perature for 9 h, giving compound 2 in 53% yield.‡‡ Trapping
with methyl chloroformate or carbon dioxide led only to apparent
polymers. However, reaction with TMSCl did give unstable
cyclopropene 5, which could be trapped in 30% yield by reaction
with DPIBF, 12.§§
O
Ph
Ph
E
E
DPIBF
Ph
Ph
1,4,5
10 E = H, 70%
11 E = Br, 64%
12 E = Me3Si, 30%
Scheme 2
Cyclopropene 1 decomposed completely after a week in a
CDCl3 solution; no clear products could be identified. How-
ever, it could also be trapped efficiently in a range of other
cycloadditions. In the case of the Diels–Alder adducts 10 and
13–15 with diphenylisobenzofuran, furan and isoprene, in each
case the phenyl substituent was exo- on the cyclopropane
ring. Similar selectivities have been observed for reactions of
3-methyl-3-phenylcyclopropene with DPIBF.18 In contrast, this
alkene forms endo-cycloadducts with up to 80 % selectivity,
or exclusively in the case of o-benzoquinone, in reactions with
cyclones.18 With compound 1 the adducts with diazomethane
and benzonitrile oxide also had the exo-stereochemistry.
Ab initio calculations at the B3LYP/6-31G* level suggest
that 3-phenylcyclopropene 1, like the 3-methyl-3-phenyl system,
has a preferred bisected conformation; the rotation barrier for
cyclopropene 1 is calculated to be 13.7 kJ mol–1, whereas that
for the 3-methyl-3-phenyl system is lower at 5.6 kJ mol–1.18 This
probably reflects the lower stability of the bisected geometry
Thus, efficient procedures have been developed for the pre-
paration of 3-phenylcyclopropenes allowing the effect of the
substituent on the reactivity of the cyclopropene π-bond to be
studied.
This work was carried out with the support of a grant from
INTAS.
§
This was a white powder, mp 157–159 °C (MeOH) (found, M+:
464.0757; C29H2179BrO requires: 464.0776). H NMR, d: 2.50 (d, 1H,
1
J 4.4 Hz), 3.85 (d, 1H, J 4.4 Hz), 7.19 (m, 1H), 7.25–7.55 (m, 14H),
7.69 (m, 2H), 7.85 (m, 2H). 13C NMR, d: 37.3 (CH), 38.4 (CH), 50.9
(C), 89.1 (C), 92.4 (C), 119.8 (CH), 122.6 (CH), 126.5 (CH), 127.0
(CH), 127.2 (CH), 127.9 (CH), 128.2 (CH), 128.59 (CH), 128.62 (CH),
128.8 (CH), 128.9 (CH), 129.3 (CH), 129.6 (CH), 133.1 (C), 135.1 (C),
136.8 (C), 147.7 (C), 148.7 (C). IR (CHCl3, nmax/cm–1): 3061 (m), 3029
(m), 1605 (m), 1497 (s), 1449 (s), 1304 (s), 1217 (s), 981 (s), 910 (w),
749 (s), 696 (s).
Figure 1
3-Phenylcyclopropene 1. 1H NMR, d: 2.7 (s, 1H), 7.1–7.3 (m, 7H).
†† This was stable for 3 weeks at –20 °C. The product contained some
4-hydroxy-4-methylpentan-2-one formed in the reaction conditions from
acetone.
¶
13C NMR, d: 19.5 (CH), 108.6 (CH), 125.6 (CH), 128.0 (CH), 128.6
(CH), 147.1 (C). IR (film, nmax/cm–1): 1646 (s). Phenylcyclopropene 1
was trapped on the addition of DPIBF (1.0 equiv.) as single adduct 10
(70%, based on tribromide 6), a white powder, mp 156–158 °C (MeOH)
‡‡ Compound 2 was stable for two weeks at –20 °C when neat and for
several months at that temperature in ethereal solution.
1
(found, M+: 386.1669; C29H22O requires: 386.1671). H NMR, d: 2.20
§§ Compound 5, a colourless oil. 1H NMR (–40 °C) d: 0.28 (s, 9H),
2.61 (s, 1H), 7.17–7.56 (m, 6H). 13C NMR, d: –1.3 (SiMe3), 20.2 (CH),
116.7 (C), 119.6 (CH), 124.7 (CH), 125.0 (CH), 127.7 (CH), 148.4 (C).
IR (film, nmax/cm–1): 3059 (w), 3025 (m), 2955 (s), 2897 (m), 1693 (s),
1602 (m), 1492 (m), 1446 (s), 1407 (w), 1248 (s), 1070 (w), 841 (s), 756
(s), 698 (s). An optimised synthesis of this cyclopropene, as well as its
unusual dimerisation, will be described elsewhere.
(d, 2H, J 3.2 Hz), 3.30 (t, 1H, J 3.2 Hz), 7.11–7.76 (m, 19H). 13C NMR,
d: 33.1 (CH), 35.2 (CH), 89.6 (C), 119.5 (CH), 126.1 (CH), 126.3 (CH),
126.4 (CH), 128.3 (CH), 128.40 (CH), 128.43 (CH), 128.6 (CH), 136.1
(C), 140.1 (C), 150.2 (C). IR (CHCl3, nmax/cm–1): 3061 (m), 3030 (m),
1954 (w), 1810 (w), 1604 (m), 1497 (m), 1454 (s), 1342 (w), 1307 (s),
1248 (w), 1217 (m), 1059 (w), 981 (s), 905 (m), 747 (s), 698 (s).
300 Mendeleev Commun. 2004