The photo-dehydro-diels-alder reaction: An efficient route to naphthalenes

Article abstract of DOI:10.1055/s-2005-865316

The photo-dehydro-Diels-Alder reaction (PDDA), a new photochemical route to naphthalenes is presented. The [4+2] cycloaddition takes place between 3-arylynones and arylacetylenes. in which these moieties may be located in two molecules (intermolecular PDD

Full text of DOI:10.1055/s-2005-865316

PAPER
1445
The Photo-Dehydro-Diels–Alder Reaction: An Efficient Route to
Naphthalenes
Naphthalenes
a
via Photo-D
b
ehydro-Die
l
o
r
Reaction Wessig,* Gunnar Müller, Andreas Kühn, Robert Herre, Haiko Blumenthal, Stefanie Troelenberg
Institut für Organische und Bioorganische Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, 12489 Berlin
Fax +49(30)20937450; E-mail: pablo.wessig@chemie.hu-berlin.de
Received 10 March 2005
acetylenic unit between the phenyl group and the carbonyl
carbon atom. Norrish–Yang products (e.g. 2) from such
acetylenic ketones should be easily degraded by oxida-
tion. Surprisingly, we did not isolate Norrish–Yang prod-
Abstract: The photo-dehydro-Diels–Alder reaction (PDDA), a
new photochemical route to naphthalenes is presented. The [4+2]
cycloaddition takes place between 3-arylynones and arylacetylenes,
in which these moieties may be located in two molecules (intermo-
lecular PDDA, 1) or in the same molecule (intramolecular PDDA, uct 2 but the naphthalene 3 upon irradiation of 1c
5 and 9). Especially the latter approach is attractive from a prepara-
tive point of view and permits a straightforward access to highly
functionalized naphthalenes. The irradiation of unsymmetrical re-
actants 9 provides, in contrast to symmetrical reactants 5, two iso-
meric naphthalenes. We found that the regioselectivity can be easily
influenced by suitably located substituents in the aromatic rings and
by blocking undesired positions. Notably, the PDDA may be used
for the preparation of binaphthyls, as shown by the formation of 14
and 16. The mechanism of the PDDA was elucidated by spectro-
scopic analyses and theoretical (DFT) calculations.
(Scheme 1).
O
a)
1c
hν
OH
hν
O
O
Key words: dehydro Diels–Alder reaction, naphthalenes, binaph-
thyls, photocyclization
Ph
3
Ph
2
The photochemistry of carbonyl compounds belongs to
the well-investigated and most frequently used areas of
organic photochemistry. Most photochemical reactions of
carbonyl compounds are based on the electrophilic oxy-
gen atom formed as a result of an n→p* excitation of the
carbonyl group. Typical processes starting from this excit-
ed state are hydrogen migration, which is the initial step
of the Norrish–Yang reaction,1 or an attack of a carbon–
carbon double bond, which initiates the Paternó–Büchi re-
action.2
b)
Dehydro-
Diels–Alder
reaction
+
Diels–Alder
reaction
+
Scheme 1 a) Dimerization of 1c; b) Diels–Alder vs. dehydro-Diels–
Alder reaction
A careful literature survey revealed that dimerization of
arylacetylenes affording naphthalenes is known since
more than one century. Already in 1895 Michael and
Bucher reported the dimerization of phenylpropiolic acid
upon prolonged boiling with acetic anhydride.5 Twelve
years later Pfeiffer and Möller described a similar reaction
of ethyl phenylpropiolate under drastic conditions
(200 °C, sealed tube).6 In the following period some pre-
parative applications were published proving the scope of
the reaction.7 A cyclization such as 1c → 3 may be con-
sidered as a [4+2] cyclization between an alkyne and an
enyne, which resembles the Diels–Alder reaction in which
the diene moiety is dehydrogenated (cf. Scheme 1b).
Therefore, the reaction was, above all by Whitlock,8 char-
acterized as dehydro-Diels–Alder reaction (DDA). Con-
sequently, we suggest to name the photochemical variant
of this reaction as photo-dehydro-Diels–Alder reaction
(PDDA).
Although both aliphatic and aromatic ketones may be
used in the Norrish–Yang reaction, there are much more
synthetic applications with aromatic ketones. This is
mainly due to the special property of these ketones to form
the triplet excited state exclusively. Selectivity and quan-
tum yields of triplet Norrish–Yang reactions, and conse-
quently their preparative value, are often much higher
than via the corresponding singlet pathway, which is char-
acteristic for aliphatic ketones and 1,2-dicarbonyl com-
pounds.3 The phenyl group, which is responsible for the
very fast intersystem crossing (ISC), is certainly the major
drawback of these applications because of the difficulty to
modify or degrade this group under mild conditions.4
To circumvent these limitations, we prepared 1-phenyl-
hex-1-yn-3-one (1c) as a model compound, bearing an
The classic examples of the DDA proceed under very
drastic conditions. Considerable improvement was
achieved by transition metal catalysis. Thus, palladium,9
SYNTHESIS 2005, No. 9, pp 1445–1454
Advanced online publication: 18.04.2005
DOI: 10.1055/s-2005-865316; Art ID: C01705SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.
x
x
.2
0
0
5

1446
P. Wessig et al.
PAPER
rhodium10 and nickel11 catalysts were used in the DDA. The unsymmetrical diynones 9 were prepared in four
To the best of our knowledge, photochemical excitation steps commencing with various arylacetylenes 4. After
was never used to trigger the DDA between acetylenes lithiation with butyllithium, a C-3 building block was in-
and arylacetylenic ketones. Herein we wish to report on troduced by ring opening of oxetane in the presence of
the first preparative applications of the inter- and intramo- BF3·OEt2. The resulting primary alcohols 6 were oxidized
lecular PDDA.
to the corresponding aldehydes 7, which were used with-
out further purification. Treatment of 7 with another lithi-
um arylacetylide afforded the secondary alcohols 8, which
were finally oxidized to the target ketones 9 (Scheme 3).
Preparation of Ynones 1, Diynediones 5 and
Diynones 9
Ynones 1 and diynediones 5 were prepared from phenyl-
acetylene (4a) by different methods. The first step is al-
Photochemical Behavior of Compounds 1, 5 and 9
ways a deprotonation with butyllithium, followed by The UV spectra of all compounds 1, 5 and 9 show, besides
treatment with an ester in the presence of BF3·OEt2 (Meth- strong absorption between 250 and 280 nm, a very weak
od A12) or transmetalation with cadmium chloride and shoulder between 300 and 350 nm corresponding to the
subsequent addition of an acid chloride (Method B13). In n→p* excitation of the carbonyl group. Upon irradiation
Method C, the lithium acetylide was added to an aldehyde of 1, 5 and 9 using a high pressure mercury arc lamp and
giving a secondary propargylic alcohol, followed by its pyrex vessels (which absorb light with wavelength below
oxidation with Dess–Martin periodinane14 (Scheme 2).
300 nm) we obtained almost always naphthalenes, admit-
tedly in varying yields, which are summarized in
Tables 1– 4.
R
method yield
O
1a Me
1b Et
1c Pr
1d Ph
A
B
C
A
49 %
48 %
41 %
19 %
A, B or C
The PDDA dimerization of ynones 1 gave naphthalenes
10 in moderate yields only (Table 1). Hoping to improve
the yields by switching from inter- to intramolecular
PDDA we prepared compounds 5 and 9. The diynediones
5 met our expectations at least partly. Thus, the diketone
11b, bearing three methylene groups between the ynone
moieties, was formed in 54% yield, while with less or
more methylene groups the corresponding naphthalenes
were formed only in moderate yields or the PDDA did not
proceed (5d). The formation of compound 11e is notewor-
thy, despite the low yield, because its formation proved
the usability of the PDDA for macrocyclizations
(Scheme 4, Table 2).
Ph
R
O
1
Ph
n
method yield
4a
5a
5b
5c
5d
5e
2
3
4
7
8
A
A
A
A
B
39 %
20 %
19 %
15 %
41 %
Ph
Ph
A or B
(CH2)n
O
5
methods: A: 1. BuLi, 2. RCOOEt/BF3.Et2O, B: 1. n-BuLi, 2. CdCl2, 3. RCOCl
C: 1. n-BuLi, 2. RCHO, 3. Dess-Martin periodinane
Scheme 2 Preparation of ynones 1 and diynediones 5
O
1. BuLi
DM
Ar1
Ar1
Ar1
2.
O
O
R
OH
O
hν
4
7
6
Ph
BF3⋅OEt2
R
Li
R
Ar2
1
Ph
O
10
O
DM
Ar1
Ar1
O
Ph
Ph
O
OH
hν
Ar2
Ar2
(CH2)n
O
(CH2)n
O
9
8
Ar1
Ph
yield
98 %
4-MeOC6H4 67 %
4-ClC6H4 40 %
4-CF3C6H4 75 %
Ph
5
11
6a
DM = Dess–Martin periodiane
1-Naph = 1-Naphthyl
6b
6c
6d
6e
6f
Mes = 2,4,6-Trimethylphenyl
1-Naph
Mes
87 %
71 %
R1
R2
hν
O
Ar2
Ar1
Ar1
12
Ar1
Ar2
Ph
Ar1
9g 4-CF3C6H4 Ph
Ar2
+
yield
84 %
25 %
83 %
62 %
62 %
75 %
yield
81 %
77 %
80 %
59 %
79 %
O
9a Ph
9b 4-MeOC6H4 Ph
9c Ph
9d Ph
9e 4-ClC6H4
9f Ph
Ar2
9h 1-Naph
9i Ph
9j Mes
9k Ph
Ph
1-Naph
Ph
4-MeOC6H4
4-ClC6H4
Ph
9a-g
Mes
4-CF3C6H4
O
13
Scheme 3 Preparation of diynones 9
Scheme 4 Photochemical behavior of compounds 1, 5 and 9a–g
Synthesis 2005, No. 9, 1445–1454 © Thieme Stuttgart · New York

PAPER
Naphthalenes via Photo-Dehydro-Diels–Alder Reaction
1447
Table 1 Compounds 10a–d Prepared
Product R
Yield1H NMR (CDCl3/TMS)
(%) d, J (Hz)
13C NMR (CDCl3/TMS)
d
HRMS (EI, 70 eV) IR (KBr)
m/z
(cm–1)
10a
10b
10c
Me 34 8.32 (s, 1 H, CHarom), 7.91 (d, 1 H,
206.1 (C=O), 198.7 (C=O), 139.0, 136.4,
calcd for C20H16O2: 1699, 1678,
288.1150; found: 1422, 1351,
J = 8.2, CHarom), 7.37 (m, 8 H, CHarom), 134.3, 133.2, 131.2, 130.8 (Cq), 129.3,
2.67 (s, 3 H, CH3), 2.03 (s, 3 H, CH3)
128.3, 128.2, 127.4, 126.8 (CHarom), 31.8
(CH3), 27.1 (CH3)
288.1151
1250, 751,
703
Et 38 8.33 (s, 1 H, CHarom), 7.38 (m, 9 H,
209.1 (C=O), 201.5 (C=O), 139.1, 136.4,
calcd for C22H20O2: 1682, 1442,
316.1463; found: 1374, 1225,
CHarom), 3.09 (q, 2 H, J = 7.1, CH2), 2.27 134.1, 133.2, 130.9, 130.2 (Cq), 129.2,
(q, 2 H, J = 7.1, CH2), 1.19 (t, 3 H, 128.2, 128.0, 127.2, 126.7 (CHarom), 37.4
J = 7.1, CH3), 0.82 (t, 3 H, J = 7.1, CH3) (CH2), 32.3 (CH2), 8.3 (CH3), 7.7 (CH3)
316.1463
1167, 1069,
963, 755, 703
a
a
Pr 32 8.29 (s, 1 H, CHarom), 7.90 (d, 1 H, 208.2 (C=O), 201.3 (C=O), 139.1, 136.5,
–
–
J = 7.2, CHarom), 7.35 (m, 8 H, CHarom), 134.0, 133.6, 131.0, 130.1 (Cq), 129.1,
3.00 (t, 2 H, J = 7.1, CH2), 2.24 (t, 2 H, 128.2, 128.0, 127.2, 126.6 (CHarom), 46.2
J = 7.1, CH2), 1.72 (m, 2 H, CH2), 1.34 (CH2), 41.2 (CH2), 17.8 (CH2), 16.6 (CH2),
(m, 2 H, CH2), 0.96 (t, 3 H, J = 7.1,
CH3), 0.57 (t, 3 H, J = 7.1, CH3)
13.8 (CH3), 13.4 (CH3)
10d
Ph 23 7.85 (s, 1 H, CHarom), 7.09–7.55 (m, 19 198.5 (C=O), 196.1 (C=O), 139.0, 138.6,
calcd for C30H20O2: 1653, 1447,
412.1463; found: 1314, 1266,
H, CHarom
)
137.6, 137.2, 136.2, 135.1, 133.5, 132.3
(Cq), 131.5, 131.0, 130.5, 129.1, 128.9,
128.3, 127.8, 127.6, 127.5, 126.9 (CHarom
412.1463
1026, 755,
671
)
a The by-products could not be separated from 10c.
Table 2 Compounds 11a–e Prepared
Product n
Yield 1H NMR (CDCl3/TMS)
(%) d, J (Hz)
13C NMR (CDCl3/TMS)
d
HRMS (EI, 70 eV) IR (KBr)
m/z
(cm–1)
11a
11b
11c
2
3
4
30
54
32
8.62 (s, 1 H, CHarom), 7.97 (d, 1 H,
J = 7.9, CHarom), 7.40 (m, 8 H, CHarom), 134.4, 131.6, 130.1, 129.3 (Cq), 129.1,
3.00 (m, 4 H, 2 CH2)
196.9 (C=O), 196.1 (C=O), 138.5, 135.9,
calcd for C20H14O2: 1694, 1686,
286.0994;
found: 286.0994
1262, 1181,
981, 795,
128.8, 128.7, 128.2, 127.4 (CHarom), 39.8
(CH2), 37.9 (CH2)
762, 750, 699
8.41 (s, 1 H, CHarom), 8.00 (d, 1 H,
J = 5.0, CHarom), 7.42 (m, 8 H, CHarom), 134.5, 132.8, 132.6, 130.1, (Cq), 129.4,
2.82 (m, 4 H, 2 CH2), 2.09 (m, 2 H,
CH2)
205.3 (C=O), 202.1 (C=O), 138.3, 136.0,
calcd for C21H16O2: 1696, 1615,
300.1150;
found: 300.1151
1442, 1253,
1097, 912,
755
128.8, 127.8, 127.7, 127.5 (CHarom), 44.6
(CH2), 41.3 (CH2), 18.3 (CH2)
a
a
8.60 (s, 1 H, CHarom), 8.00 (d, 2 H,
J = 5.0, CHarom), 7.39 (m, 7 H, CHarom), 134.9, 132.8, 130.4, 130.0 (Cq), 129.8,
211.3 (C=O), 201.9 (C=O), 137.5, 136.9,
–
–
2.89 (m, 2 H, CH2), 2.51 (m, 2 H, CH2), 129.0, 127.9, 127.4, 127.1 (CHarom), 45.8
1.91 (m, 4 H, 2 CH2)
(CH2), 39.3 (CH2), 26.9 (CH2), 23.2 (CH2)
11d
11e
7
8
0
8
–
–
–
–
8.31 (s, 1 H, CHarom), 7.93 (d, 2 H,
J = 11.1, CHarom), 7.20–7.56 (m, 7 H,
CHarom), 2.59 (m, 4 H, 2 CH2), 1.65 (m, 129.2, 129.0, 128.6, 128.3 (CHarom), 45.5
208.4 (C=O), 201.4 (C=O), 136.3, 134.0,
133.6, 131.9, 131.0, 130.6, (Cq), 130.1,
calcd for C26H26O2: 1665, 1442,
370.1933;
found: 370.1933
1257, 1071,
802, 757
4 H, 2 CH2), 1.32 (m, 8 H, 4 CH2)
(2 CH2), 29.2 (2 CH2), 29.0 (2 CH2), 24.1
(2 CH2)
a The by-products could not be separated from 11c.
In contrast to 1 and 5, the PDDA of diynones 9 can pro- troduction of a chlorine atom is instructive. If a chlorine
duce two different regioisomeric naphthalenes 12 and 13, atom is present in the aryl group belonging to the chro-
which differ in the aryl group (Ar1 or Ar2) involved in the mophore (Ar2) the selectivity is remarkably increased to
ring closure. On irradiation of the parent compound 9a 4.6:1 (9d) whereas the same substitution in Ar1 inverts the
(Ar1 = Ar2 = Ph) we obtained the naphthalenes 12 and 13 12:13 ratio (9e). This effect is even more pronounced in
in a 2:1 ratio. To explore how the regioselectivity can be case of a CF3 group (9f,g). Here, the selectivity rises to
influenced we introduced different substituents in para 7.3:1 (9f). Obviously, the introduction of electron with-
position of the aryl groups. While methoxy groups have drawing groups in Ar2 increases the regioselectivity in fa-
only marginal influence on the 12:13 ratio (9b,c), the in- vor of naphthalene 12 (Tables 3 and 4).
Synthesis 2005, No. 9, 1445–1454 © Thieme Stuttgart · New York

1448
P. Wessig et al.
PAPER
Table 3 Results of the Irradiation of 9a–k
Reactant
9a
Ar1
Ar2
Products
12a, 13a
12b, 13b
12c, 13c
12d, 13d
12e, 13e
12f, 13f
12g, 13g
14, 15
Yield (%)a
Ratiob
2:1
Ph
Ph
74
87
87
73
54
69
68
52
67
58
64
9b
4-MeOC6H4
Ph
Ph
1.9:1
1.7:1
4.6:1
1:1.5
7.3:1
1:1.7
1.3:1
2:1
9c
4-MeOC6H4
4-ClC6H4
Ph
9d
Ph
9e
4-ClC6H4
Ph
9f
4-CF3C6H4
Ph
9g
4-CF3C6H4
1-Napthc
Ph
9h
Ph
9i
1-Napthc
Ph
16, 17
9j
Mesd
18
–
9k
Ph
Mesd
19, 20
6.5:1
a Sum of products.
b Ratio of products.
c 1-Napth = 1-naphthyl.
d 2,4,6-Trimethylphenyl.
Table 4 Analytical Data of Compounds 12a–g, 13a–g and 14–20
Product 1H NMR (CDCl3/TMS)
13C NMR (CDCl3/TMS)
d
HRMS (EI, 70 eV) IR (KBr)
m/z
(cm–1)
d, J (Hz)
12a
12b
7.89 (m, 10 H, CHarom), 3.22 (m, 2 H,
CH2), 2.33 (m, 2H, CH2)
206.0 (C=O), 148.0, 139.9, 136.7, 136.2 (Cq),
132.0, 130.8, 129.7, 128.4, 128.2, 127.9, 127.6,
125.9, 124.5 (CHarom), 37.5 (CH2), 24.7 (CH2)
calcd for C19H14O: 1711, 1618, 1441,
258.1045;
found: 258.1045
1279, 1105, 875,
750, 700
8.28 (s, 1 H, CHarom), 7.95 (m, 1 H,
CHarom), 7.62 (m, 2 H, CHarom), 7.41 (m, (Cq), 131.9, 131.7, 129.6, 129.2, 128.7, 125.1
2 H, CHarom), 6.99 (m, 2 H, CHarom),
3.86 (s, 3 H, CH3), 2.99 (t, 2 H,
J = 6.03, CH2), 2.86 (t, 2 H, J = 4.89,
CH2)
208.1 (C=O), 157.8, 146.5, 138.5, 136.5, 133.2
calcd for C20H16O2: 1713, 1600, 1420,
288.1150;
found: 288.1151
1231, 1174, 877,
706
(CHarom),123.2 (Cq), 114.1 (CHarom), 55.1 (CH3),
37.0 (CH2), 25.1 (CH2)
12c
8.19 (s, 1 H, CHarom), 7.83–6.81 (m, 9 207.3 (C=O), 159.7, 147.0, 137.5, 135.6, 135.1
H, CHarom), 3.36 (s, 3 H, CH3), 2.90 (m, (Cq), 132.3, 132.1, 131.5, 129.6, 128.7, 128.1,
calcd for C20H16O2: 1709, 1615, 1512,
288.1150;
1246, 1175, 1033,
1017, 883, 856,
844, 756
2 H, CH2), 2.62 (m, 2 H, CH2)
127.6 (CHarom), 123.6, 118.9 (CHarom), 114.3
(CHarom), 103.9 (CHarom), 55.1 (CH3), 44.1 (CH2),
25.2 (CH2)
found: 288.1150
12d
12e
12f
8.35 (s, 1 H, CHarom), 7.83 (m, 1 H,
CHarom), 7.51–7.12 (m, 7 H, CHarom),
2.93 (m, 2 H, CH2), 2.66 (m, 2 H, CH2) (CHarom), 123.0 (Cq), 36.7 (CH2), 25.3 (CH2)
207.9 (C=O), 157.6, 137.8, 135.4, 135.0 (Cq), 132.4 calcd for
1715, 1618, 1598,
1087, 849, 808,
756
(CHarom), 131.2 (Cq), 129.7, 128.8, 127.3, 125.2
C19H13ClO:
292.0655;
found: 292.0655
8.29 (s, 1 H, CHar), 7.95 (m, 1 H,
CHarom), 7.55–7.18 (m, 7 H, CHarom),
2.96 (t, 2 H, J = 5.64, CH2), 2.66 (m, 2 124.3 (Cq), 36.9 (CH2), 25.1 (CH2)
H, CH2)
207.8 (C=O), 145.9, 136.7, 135.0, 133.4, 132.8
(Cq), 131.1, 131.0, 129.1, 125.3, 124.9 (CHarom),
calcd for
C19H13ClO:
292.0655;
1703, 1614, 1488,
1380, 1288, 1163,
1076, 1014, 821,
704
found: 292.0654
8.33 (s, 1 H, CHar), 8.09–7.17 (m, 8 H, 207.2 (C=O), 147.3, 138.6, 136.2, 135.9, 134.5,
CHar), 2.98 (m, 2 H, CH2), 2.66 (m, 2
H, CH2)
calcd for
1629, 1607, 1571,
1438, 1329, 1173,
1140, 1038, 937,
907, 878, 698, 638
133.8 (Cq), 131.7, 130.2, 130.1, 129.6, 128.3,
128.2, 127.8, 126.3 (CHarom), 123.6, 121.5 (Cq),
36.8 (CH2), 25.3 (CH2)
C20H13F3O:
326.0919;
found: 326.0919
Synthesis 2005, No. 9, 1445–1454 © Thieme Stuttgart · New York

PAPER
Naphthalenes via Photo-Dehydro-Diels–Alder Reaction
1449
Table 4 Analytical Data of Compounds 12a–g, 13a–g and 14–20 (continued)
Product 1H NMR (CDCl3/TMS)
13C NMR (CDCl3/TMS)
d
HRMS (EI, 70 eV) IR (KBr)
m/z
d, J (Hz)
(cm–1)
12g
8.30 (s, 1 H, CHarom), 7.98–7.17 (m,
8 H, CHarom), 2.93 (m, 2 H, CH2), 2.66 (Cq), 132.6, 130.2, 132.9, 128.9, 126.1, 125.8,
(m, 2 H, CH2)
206.0 (C=O) 145.8, 141.2, 135.6, 135.2, 134.1
calcd for
C20H13F3O:
1712, 1618, 1597,
1324, 1160, 1107,
1065, 862, 758
125.7 (CHarom), 124.5 (Cq), 36.7 (CH2), 25.0 (CH2) 326.0919;
found: 326.0918
13a
13b
8.29 (s, 1 H, CHarom), 7.96 (m, 1H,
CHarom), 7.61–7.16 (m, 8 H, CHarom),
2.93 (m, 2 H, CH2), 2.64 (m, 2 H, CH2) 125.9, 123.9 (CHarom), 36.9 (CH2), 25.2 (CH2)
207.7 (C=O), 146.0, 137.4, 137.2, 135.7, 134.1
(Cq), 132.7, 130.6, 129.8, 128.7, 128.5, 127.7,
calcd for C19H14O: 1704, 1618, 1600,
258.1045;
found: 258.1044
1441, 1283, 1114,
1010, 755, 702
7.71 (m, 2 H, CHarom), 7.47 (m, 2 H, 206.7 (C=O), 157.8, 146.5, 138.5, 136.5, 133.2
CHarom), 7.15 (m, 4 H, CHarom), 6.79 (s, (Cq), 133.1, 132.8, 131.7, 130.1, 129.7, 128.4,
1 H, CHarom), 3.59 (s, 3 H, CH3), 3.17 (t, 128.2, 125.0, 121.7, 105.6 (CHarom), 55.6 (CH3),
calcd for C20H16O2: 1708, 1600, 1512,
288.1150;
found: 288.1151
1246, 1173, 1110,
844, 754
2 H, J = 6.46, CH2), 2.63 (t, 2 H,
J = 4.25, CH2)
37.6 (CH2), 24.8 (CH2)
13c
13d
13e
13f
13g
14
7.46–7.17 (m, 7 H, CHarom), 6.80 (m, 2 185.5 (C=O), 161.7 (Cq), 135.2, 131.6, 128.2, 127.8 calcd for C20H16O2: 1707, 1617, 1463,
H, CHarom), 3.74 (s, 3 H, CH3), 2.90 (t, (CHarom), 123.5 (Cq), 114.5 (CHarom), 111.5 (Cq),
288.1150;
found: 288.1152
1421, 1374, 1296,
1225, 1031, 816,
757, 703, 625
2 H, J = 6.39, CH2), 2.73 (t, 2 H,
J = 5.64, CH2)
55.4 (CH3), 44.1 (CH2), 25.4 (CH2)
7.83 (m, 2 H, CHarom), 7.57–7.14 (m, 7 206.7 (C=O), 147.8, 138.7 (Cq), 136.4 (CHarom),
H, CHarom), 3.22 (t, 2 H, J = 5.64, CH2), 134.0, 133.7 (Cq), 131.5, 131.1, 131.0, 127.6, 125.7 C19H13ClO:
calcd for
1713, 1613, 1592,
1446, 1313, 1107,
1079, 872, 694
2.65 (m, 2 H, CH2)
(CHarom), 124.9 (Cq), 37.5 (CH2), 24.8 (CH2)
292.0655;
found: 292.0655
7.84–7.14 (m, 9 H, CHarom), 3.19 (m, 2 205.7 (C=O), 147.9, 139.7, 135.5, 135.0, 133.0
calcd for
C19H13ClO:
292.0655;
1616, 1599, 1488,
1280, 1204, 1103,
1085, 1013, 856,
805, 753
H, CH2), 2.61 (m, 2 H, CH2)
(Cq), 131.9, 129.4, 128.2, 126.7, 125.0 (CHarom),
35.5 (CH2), 24.6 (CH2)
found: 292.0655
7.94–7.16 (m, 9 H, CHarom), 3.22 (t, 2
H, J = 5.64, CH2), 2.64 (m, 2 H, CH2) 131.6, 130.9, 130.2, 129.6, 128.4, 127.9, 126.4,
125.2, 125.0, 124.9 (CHarom), 124.8 (Cq), 37.4
206.0 (C=O), 147.9, 140.2, 137.9, 136.8 (Cq),
calcd for
C20H13F3O:
326.0919;
1711, 1619, 1599,
1324, 1163, 1121,
1066, 916, 858,
754
(CH2), 24.8 (CH2)
found: 326.0920
7.91–7.15 (m, 9 H, CHarom), 3.25 (m, 2 205.3 (C=O), 150.3, 141.0, 137.7, 133.9 (Cq),
calcd for
C20H13F3O:
1716, 1607, 1439,
1288, 1172, 1123,
1067, 949, 906,
703
H, CH2), 2.66 (m, 2 H, CH2)
131.9, 131.0, 129.7, 128.9, 127.6, 126.2, 125.9,
124.6 (CHarom), 123.7, 122.2 (Cq), 37.3 (CH2), 24.9 326.0919;
(CH2)
found: 326.0918
8.49 (s, 1 H, CHarom), 8.09 (d, 1 H,
207.8 (C=O), 147.3, 136.5, 135.4, 134.2, 133.8,
calcd for C23H16O: 1711, 1618, 1501,
J = 8.28, CHarom) 7.97 (t, 2 H, J = 7.16, 132.6, 132.0, 130.6 (Cq), 128.7, 128.5, 128.4,
CHarom), 7.48–7.21 (m, 8 H, CHarom), 127.6, 126.4, 126.2, 126.1, 126.06, 125.7, 125.5,
308.1201;
found: 308.1201
1256, 1111, 801,
780
4.30 (m, 1 H, CH2), 2.97 (m, 1 H, CH2), 124.2 (CHarom), 36.8 (CH2), 24.9 (CH2)
2.75 (m, 2 H, CH2)
15
16
17
18
8.81 (m, 2 H, CHarom), 7.89 (m, 1 H,
CHarom), 7.72–7.32 (m, 9 H, CHarom),
3.40 (t, 2 H, J = 5.65, CH2), 2.77 (m, 2 128.0, 127.9, 127.6, 126.9, 126.8, 125.3, 123.7,
H, CH2) 119.6 (CHarom), 37.4 (CH2), 25.2 (CH2)
205.7 (C=O), 150.3, 139.6, 136.5, 134.9, 132.7,
131.6, 130.2 (Cq), 129.8 (CHarom), 129.5 (Cq)128.6, 308.1201;
found: 308.1201
calcd for C23H16O: 1711, 1594, 1440,
1214, 1110, 1062,
746, 699
7.92–7.83 (m, 4 H, CHarom), 7.55–7.00 205.5 (C=O), 148.0, 137.9, 136.7, 134.5, 133.4,
(m, 8 H, CHarom), 3.28 (t, 2 H, J = 6.78, 132.7, 132.6, 132.2 (Cq), 128.5, 128.32, 128.29,
CH2), 2.61 (m, 2 H, CH2)
calcd for C23H16O: 1706, 1615, 1597,
308.1201;
1279, 1178, 1111,
891, 798, 775, 759
128.1, 127.7, 127.1, 126.0, 125.9, 125.6, 125.5,
125.2, 124.8 (CHarom), 37.2 (CH2), 24.9 (CH2)
found: 308.1201
9.13 (s, 1 H, CHarom), 8.76 (d, 1 H,
J = 7.91, CHarom), 7.82–7.30 (m, 10 H, 131.2, 130.9, 130.1 (Cq), 129.8, 128.7, 128.6,
CHarom), 2.99 (m, 2 H, CH2), 2.72 (m, 2 127.8, 127.5, 127.2, 124.3, 123.1, 118.0 (CHarom), found (ESI):
207.8 (C=O), 148.7, 137.6, 134.9, 134.3, 131.5,
calcd for C23H16O + 1715, 1618, 1399,
H+: 309.1274;
1260, 1074, 800,
774
H, CH2)
36.9 (CH2), 25.3 (CH2)
309.1276
8.39 (s, 1 H, CHarom), 8.04 (m, 1 H,
CHarom), 7.51–7.37 (m, 3 H, CHarom),
2.83 (m, 2 H, CH2), 2.76 (m, 2 H, CH2), 126.0, 125.1, 123.6 (CHarom), 36.8 (CH2), 24.9
2.41 (s, 3 H, CH3), 1.82 (s, 6 H, CH3) (CH2), 21.2 (CH3), 19.9 (CH3)
207.9 (C=O), 146.2, 137.4, 136.3, 136.2, 135.5,
134.4, 133.1, 132.8 (Cq), 130.8, 128.8, 128.5,
calcd for C22H20O: 1718, 1617, 1599,
300.1514;
1381, 1277, 1181,
1110, 1007, 905,
850, 750, 672
found: 300.1514
Synthesis 2005, No. 9, 1445–1454 © Thieme Stuttgart · New York

1450
P. Wessig et al.
PAPER
Table 4 Analytical Data of Compounds 12a–g, 13a–g and 14–20 (continued)
Product 1H NMR (CDCl3/TMS)
13C NMR (CDCl3/TMS)
d
HRMS (EI, 70 eV) IR (KBr)
m/z
(cm–1)
d, J (Hz)
19
7.91 (d, 2 H, J = 7.16, CHarom), 7.54–
7.49 (m, 2 H, CHarom), 7.33 (m, 1 H,
CHarom), 7.01 (s, 2 H, CHarom), 3.34 (m, 126.1, 124.1 (CHarom), 37.2 (CH2), 24.9 (CH2), 21.3 found: 300.1514
2 H, CH2), 2.73 (m, 2 H, CH2), 2.40 (s, (CH3), 20.0 (CH3)
205.9 (C=O), 148.2, 139.1, 136.90, 136.86, 135.7, calcd for C22H20O: 1704, 1615, 1598,
131.4, 131.3 (Cq), 128.3, 128.1, 127.8, 127.4,
300.1514;
1446, 1279, 1204,
1099, 877, 750
3 H, CH3), 1.74 (s, 6 H, CH3)
20
8.49 (s, 1 H, CHarom), 7.41–7.21 (m, 6 208.2 (C=O), 148.0, 142.5, 138.4, 137.1, 135.8,
H, CHarom), 2.89 (m, 2 H, CH2), 2.74 (s, 134.5, 132.2 (Cq), 130.7, 129.4, 128.3, 126.9, 120.7 300.1514;
3 H, CH3), 2.70 (m, 2 H, CH2), 2.35 (s, (CHarom), 37.0 (CH2), 25.9 (CH2), 21.8 (CH3), 20.1 found: 300.1514
calcd for C22H20O: 1709, 1600, 1442,
1272, 1104, 1053,
856, 706, 648
2 H, CH3), 1.86 (s, 2 H, CH3)
(CH3), 19.8 (CH3)
After these promising results we were interested to probe
whether the PDDA permits the preparation of 1,1¢-bi-
naphthyls. These compounds have become exceptional
significance as axial chiral ligands for transition metal ca-
talysis.15
The preparation of 1,1¢-binaphthyls by PDDA requires re-
actants already containing a naphthalene residue. As with
compounds 9a–g, the photochemical cyclization may af-
ford two different products, namely 1,1¢-binaphthyls and
phenanthrenes. Both naphthalene derivatives 9h and 9i af-
forded the desired 1,1¢-binaphthyls (14, 16) besides the
phenanthrenes 15 and 17, in which the ratio of these prod-
ucts depends on the position of the naphthalene moiety
(Scheme 5). The structure of 1,1¢-binaphthyl 16 could be
unambiguously determined by X-ray crystal structure
analysis (Figure 116). By comparison of the NMR and MS
data of 16 and 17 with those of 14 and 15 we were also
able to assign their structure unambiguously.
Figure 1 X-ray crystallographic structure of 16
mation of one of the isomers by blocking the ortho posi-
tions of one aryl substituent we prepared compounds 9j,k
and investigated their photochemical behavior. Whereas
9j gave only the expected naphthalene 18 in 58% yield we
isolated after irradiation of 9k besides 19 (56%) also com-
The hitherto discussed approach to influence the regiose-
lectivity of the PDDA is based on different electronic
properties of substituents tethered at the aromatic rings
Ar1 and Ar2. To find out if it is possible to suppress the for-
hν
O
hν
+
O
Ph
Ph
O
O
Ph
9j
18
Ph
O
9h
14
15
Ph
O
Ph
O
Ph
hν
+
O
H
hν
O
+
H
O
O
20 *)
9k
19
9i
16
17
Scheme 6 Photochemical behavior of compounds 9j,k (*) observed
NOE’s)
Scheme 5 Photochemical behavior of compounds 9h,i
Synthesis 2005, No. 9, 1445–1454 © Thieme Stuttgart · New York

PAPER
Naphthalenes via Photo-Dehydro-Diels–Alder Reaction
1451
pound 20 in low yield (9%), which is obviously the result 13a is completely suppressed by the quencher whereas the
of a methyl group migration (Scheme 6). The observed sensitizer has only marginal influence on the reaction rate
NOE’s supporting the structure of 20 are indicated in the and product formation. Consequently, we conclude that
formula. We will provide an explanation for this surpris- the PDDA of 9a starts from a triplet excited state which is
ing finding in the following section.
presumed to be a p→p* state based on quantum chemical
calculations. Obviously, a very efficient ISC takes place
from the initially formed 1(n→p*) state to the 3(p→p*), a
process, which is in agreement with the El-Sayed rules.17
Mechanism
Concerted [4+2] cycloadditions are, according to the
Woodward–Hoffmann rules,18 thermally allowed and
photochemically forbidden and we therefore assume that
the PDDA proceeds stepwise via diradical intermediates.
To characterize the different species along the reaction
pathway between the reactant 9a and products 12a and
Concerning the mechanism of the PDDA the following
questions arose: Which spin state (singlet or triplet) is the
reactive one at the beginning of the reaction? Does the
[4+2] cycloaddition proceed concerted or stepwise?
Which reactive intermediates are involved in the reaction?
To determine the spin state of the reactive excited species, 13a we have undertaken extensive quantum chemical cal-
we used triplet quenchers and sensitizers. This approach culations. For this we used the DFT hybrid functional
demands the accurate knowledge of the triplet energy of B3LYP,19 the basis sets 6-31G*20a–20c and 6-
the reactants. For compound 9a, which was chosen for the 311++G**,20d,e respectively, and the program package
mechanistic investigation, we obtained from phosphores- Gaussian 03.21
cence measurements a triplet energy of 63 kcal/mol. Ac-
cordingly, we irradiated 9a in the presence of
The PDDA starts from the triplet excited state of 9a whose
energy was calculated to be 61.1 kcal/mol above the
benzophenone (ET = 68 kcal/mol) as triplet sensitizer and
ground state, which is in very good agreement with the ex-
2,5-dimethylhexa-2,4-diene (ET = 55 kcal/mol) as triplet
perimentally determined triplet energy. The first step,
consisting in the formation of a C–C bond between two
quencher. We found that the formation of products 12a/
Scheme 7 Initial steps of the PDDA of 9a (B3LYP/6-311++G**//B3LYP/6-31G*, red = triplet, green = singlet, energy differences to each
preceding species are given in parenthesis23)
Synthesis 2005, No. 9, 1445–1454 © Thieme Stuttgart · New York

1452
P. Wessig et al.
PAPER
H (D)
Ph
acetylenic carbon atoms, is very exothermic (39.4 kcal/
mol) and proceeds across a small activation barrier (5.6
kcal/mol). The resulting butadienediyl-diradical 22 seems
to play a central role in the mechanism of the PDDA. Re-
markably, the energy difference between the singlet and
the triplet state of this diradical is small (3.4 kcal/mol),
that is, the singlet and triplet potential energy surfaces are
nearly degenerated at this point. It is well-known that such
states are highly predetermined for spin inversion pro-
cesses.22 Bearing in mind that the activation barrier of the
next step, the attack of one of the radical centers on the op-
posite aromatic ring, is much higher in the triplet state (ca.
23 kcal/mol) than in the singlet state (ca. 8 kcal/mol) it is
very likely that the intersystem crossing (ISC) takes place
at the stage of the diradical 22 (Scheme 7). The thus ob-
tained cyclic allenes 24a,b are tautomers of products 12a,
13a and could undergo two consecutive intramolecular
1,2-hydrogen shifts to give these products. The calculated
activation barriers of the first 1,2-hydrogen shift are rela-
tively high (15–16 kcal/mol) and it is therefore improba-
ble that the reaction takes place via hydrogen shifts.
Whitlock has shown for the thermal DDA that the isomer-
ization of cycloallenes such as 24 occurs through a proto-
nation-deprotonation sequence initiated by the solvent.8
To find out if this applies to the PDDA too, we irradiated
9a in methanol-d1 as deuteron source. The NMR spectra
of the crude reaction mixture obtained on irradiation in
MeOH and MeOH-d1are depicted in Figure 2. These
spectra clearly indicate that the isomerization from 24 to
12a, 13a proceeds through protonation of the allene and
subsequent deprotonation of cation 25 (Scheme 8).
O
O
+H+ (D+)
H
H
Ph
24b
25
+ MeO–
– MeOH
H (D)
O
Ph
13a
Scheme 8 Isomerization of cycloallene 24b to 13a
carbonyl group.24 The introduction of electron-withdraw-
ing groups in para position of phenyl group A (Scheme 7)
increases this reactivity grading. Otherwise, the calcula-
tions revealed only marginal differences between the elec-
tron density of the two phenyl groups. It should be noted
that the observed regioselectivity for 9a (2:1) is correctly
reflected by the calculated difference between the corre-
sponding activation barriers (cf. Scheme 7). The surpris-
ing formation of product 20 becomes understandable in
view of the irradiation results of 9a in the presence of
MeOH-d1. We assume that the cycloallene 26 formed
from 9k is protonated by the solvent analogous to 24b (cf.
Scheme 8) giving the cation 27 (Scheme 9). 27 undergoes
a 1,2-methyl shift to the cation 28, which gives 20 after
deprotonation. This mechanistic proposal is supported by
a model calculation of cation 29 (B3LYP/6-311++G**//
B3LYP/6-31G*17,18). We found a relatively low activation
barrier of 11 kcal/mol via transition state 30 and that the
rearranged cation 31 is more stable than 29 by 21 kcal/
mol. It is very likely that the driving force for such rear-
rangement is higher for cation 27 too, due to steric hin-
drance between the methyl group at the bridgehead atom
and the adjacent phenyl group.
The regioselectivity in the second ring closure reaction
from diradical 22 to cycloallenes 24 could be rationalized
either by different electron density of the two phenyl
groups or by different reactivity of the two radical centers.
Preliminary calculations have indicated that the radical
center located at the b-position of the enone moiety have
a considerably lower electron density than the other radi-
cal center owing to an electron-withdrawing effect of the
Herein we have reported on the first applications of a new
photochemical ring closure reaction, the photo-dehydro-
Diels–Alder reaction (PDDA). By this reaction highly
substituted naphthalenes are accessible in a straightfor-
ward manner. Starting with 3-arylynones, the PDDA pro-
ceeds both inter- and intramolecularly, in which the latter
approach gives better yields and is, therefore, more valu-
able from a preparative point of view. Although the regio-
selectivity in the formation of naphthalenes 12 and 13 is
only moderate (2:1) in case of the parent compound 9a, it
could be increased up to 7:1 upon introduction of elec-
tron-withdrawing groups in appropriate positions. The
preparative scope of the PDDA is impressively demon-
strated by the synthesis of 1,1¢-binaphthyls 14 and 16.
The mechanism of the PDDA was elucidated by DFT cal-
culations. According to the results of these calculations,
the [4+2] cycloaddition proceeds stepwise in which buta-
diene-1,4-diyl diradicals were identified to be the key in-
Figure 2 NMR spectra of the reaction mixture of 12a, 13a, obtained
on irradiation of 9a in MeOH and MeOH-d1
Synthesis 2005, No. 9, 1445–1454 © Thieme Stuttgart · New York

PAPER
Naphthalenes via Photo-Dehydro-Diels–Alder Reaction
1453
Horspool, W. M.; Lenci, F., Eds.; CRC Press: Boca Raton,
2003, 52. (d) Wagner, P. J. In CRC Handbook of Organic
Photochemistry and Photobiology, 2nd ed.; Horspool, W.
M.; Lenci, F., Eds.; CRC Press: Boca Raton, 2003, 58.
(e) Wessig, P. In Radicals in Organic Synthesis; Renaud, P.;
Sibi, M. P., Eds.; VCH-Wiley: Weinheim, 2001.
(f) Wessig, P. In CRC Handbook of Organic Photochemistry
and Photobiology, 2nd ed.; Horspool, W. M.; Lenci, F.,
Eds.; CRC Press: Boca Raton, 2003, 57.
Ph
Ph
Me
Me
Me
Me
+ H+
hν
9k
O
O
Me
Me
26
27
1,2-Me shift
(2) (a) Jones, G. II Org. Photochem. 1981, 5, 1. (b) Griesbeck,
A. G. In CRC Handbook of Organic Photochemistry and
Photobiology, 2nd ed.; Horspool, W. M.; Lenci, F., Eds.;
CRC Press: Boca Raton, 2003, 59. (c) Griesbeck, A. G. In
CRC Handbook of Organic Photochemistry and
Photobiology, 2nd ed.; Horspool, W. M.; Lenci, F., Eds.;
CRC Press: Boca Raton, 2003, 60.
(3) Wessig, P.; Mühling, O. In Synthetic Organic
Photochemistry; Griesbeck, A. G.; Mattay, J., Eds.; Marcel
Decker: New York, 2005, 41–88.
Ph
H
Me
Me
– H+
20
O
Me
28
Me
H
H
Me
Me
(4) The introduction of electron-donating groups, which
facilitate oxidative degradation of aromatic rings, reduce the
Norrish–Yang reactivity of corresponding ketones
dramatically.1c,d
(5) Michael, A.; Bucher, J. E. Ber. Dtsch. Chem. Ges. 1895, 28,
2511.
29
30
31
Scheme 9 Mechanism of the formation of 20
(6) Pfeiffer, P.; Möller, W. Ber. Dtsch. Chem. Ges. 1907, 40,
3841.
termediates. We assume that at this stage the triplet state
of the reactants, which is initially formed after photo-
chemical excitation by ISC, is converted back to the sin-
glet state. The cycloallenes formed after the second C–C
bond-forming step are isomerized in the course of a pro-
tonation-deprotonation sequence assisted by the solvent.
The regioselectivity of the PDDA is presumably based on
different reactivities of the two radical centers of butadi-
ene-1,4-diyl diradicals.
(7) (a) Bucher, J. E. J. Am. Chem. Soc. 1910, 32, 212.
(b) Haworth, R. D.; Sheldrick, G. J. Chem. Soc. 1935, 636.
(c) Haworth, R. D.; Kelly, W. J. Chem. Soc. 1936, 745.
(d) Baddar, F. G.; El-Assal, L. S.; Doss, N. A. J. Chem. Soc.
1959, 1027. (e) Brown, D.; Stevenson, R. J. Org. Chem.
1965, 30, 1759. (f) Klemm, L. H.; Gopinath, K. W.; Lee, D.
H.; Kelley, F. W.; Trod, E.; McGuire, T. M. Tetrahedron
1966, 22, 1797.
(8) Whitlock, H. W.; Wu, E.-M.; Whitlock, B. J. J. Org. Chem.
1969, 34, 1857.
(9) Müller, E.; Sauerbier, M.; Streichfuß, D.; Thomas, R.;
Winter, W.; Zountsas, G. Liebigs Ann. Chem. 1971, 750, 63.
(10) Müller, E.; Odenigbo, G. Liebigs Ann. Chem. 1975, 1435.
(11) Wagner, F.; Meier, H. Tetrahedron 1974, 30, 773.
(12) (a) Chabala, J. C.; Vincent, J. E. Tetrahedron Lett. 1978, 19,
937. (b) Yamaguchi, M.; Shibato, K.; Fujiwara, S.; Hirao, I.
Synthesis 1986, 421. (c) Doubsky, J.; Streinz, L.; Leseticky,
L.; Koutek, B. Synlett 2003, 937.
(13) Yashina, O. G.; Sarwaa, T. W.; Kaigorodowa, T. D.;
Wereschtschagin, L. I. J. Org. Chem. USSR (Engl. Transl.)
1968, 4, 2032; Zh. Org. Khim.; 1968, 4: 2104.
(14) (a) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4156.
(b) Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113,
7277.
Irradiation of ketones 1, 5 and 9
The irradiation of ketones 1, 5 and 9 was performed in methanol so-
lution (400 mL, c = 0.01 M), using a high-pressure mercury arc
lamp (150 W, Heraeus). Light of wavelength below 300 nm was ab-
sorbed using a PyrexTM glass jacket between the lamp and the reac-
tion vessel. The solution was stirred using a Teflon-coated bar
magnet. The reaction was monitored by TLC and aborted when the
reactant had almost completely disappeared (15–20 h). The solution
was concentrated in vacuo and the residue was separated by flash
chromatography on silica gel (40–63 µm, eluent petroleum ether–
EtOAc mixtures: 100:5–100:20).
(15) (a) Miyashita, A.; Yasuda, A.; Takaya, H.; Toriumi, K.; Ito,
T.; Souchi, T.; Notori, R. J. Am. Chem. Soc. 1980, 102,
7932. (b) Takaya, H.; Mashima, K.; Koyano, K.; Yagi, M.;
Kumobayashi, H.; Taketomi, T.; Akutagawa, S.; Notori, R.
J. Org. Chem. 1986, 51, 629.
(16) We thank Dr. B. Ziemer for performing the X-ray structure
analysis of 16. Details of the structure investigation are
available on request from the Cambridge Crystallographic
Data Centre, on quoting the depository number CCDC
266349.
Acknowledgment
We are grateful for financial support from the Deutsche For-
schungsgemeinschaft (We-1850/5-1). Furthermore we thank the
‘Norddeutscher Verbund für Hoch- und Höchstleistungsrechnen’
(HLRN) for generous allocation of computational capacity.
References
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Synthesis 2005, No. 9, 1445–1454 © Thieme Stuttgart · New York

1454
P. Wessig et al.
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