.
Angewandte
Communications
2
a, and 3aa was formed in 50% yield (entry 9 in Table 1).
Interestingly, the reaction could be conducted even at room
temperature (entries 10–12 in Table 1). Comparable results
were obtained in DMF and AcOH, whereas the product yield
decreased slightly in CH Cl .
2
2
The diarylacetylenes 2b–f underwent coupling with 1a
when using AgOAc as the oxidant to selectively produce the
corresponding 2,3-diaryl-1-phenyl-1H-phosphindole-1-ones
3
ab–3af (entries 1–5 in Table 2). 4-Octyne (2g) could also
Scheme 2. Proposed mechanism for silver-mediated annulation of 1a
with 2.
be employed in this reaction (entry 6 in Table 2). Interest-
ingly, the asymmetrical phenylacetylenes 2h–p reacted with
1
1
a in a regioselective manner to produce the 2-substituted
,3-diphenyl-1H-phosphindole-1-ones 3ah–3ap in moderate
bond of an alkyne 2 followed by cyclization on the phenyl
moiety of 1a may occur to form the benzophosphole oxide 3.
In contrast to well-established reactions involving the addi-
tion of P-centered radicals to alkenes, an approach that has
been widely employed for synthesizing organophosphorus
compounds, intermolecular addition to alkynes has been little
to good yields (entries 7–16 in Table 2). Notably, no other
isomers were detected in any of these reactions. One possible
reason for the regioselectivity seems to be the facile
formation of phenyl-stabilized alkenyl radicals at the addition
[
9]
step (Scheme 2). In the reactions with silylacetylenes 2j and
2k, better results were obtained by using Mn(OAc) ·2H O as
3
2
[
9]
explored. In particular, examples of cascade processes
involving successive reactions of the resulting alkenyl radicals,
the oxidant at room temperature (entries 10 and 11 in
Table 2). Under the standard conditions using AgOAc at
1008C, small amounts (ca. 10%) of desilylated coupling
products were detected by GC–MS (entry 9 in Table 2). As
expected, 1a reacted with 1,4-di(prop-1-yn-1-yl)benzene (2q)
[
10]
aside from simple reduction, are so far limited.
Conse-
quently, we investigated the annulation reaction in detail to
provide an unprecedented direct route to benzophosphole
derivatives. Furthermore, it was found that these radical
in a 2:1 manner to form a bis(benzophosphole-3-yl)benzene
III
[10a,11]
[2a,12]
processes can also be promoted by a Mn salt
Ag .
as well as
framework (entry 17 in Table 2).
This type of structure is
I
of interest for application in organic light-emitting diodes and
[
1c,e,g]
In a typical experiment, diphenylphosphine oxide (1a;
.5 mmol) was treated with diphenylacetylene (2a;
.25 mmol) in the presence of AgOAc (1 mmol) in DMF
thin-film photovoltaics.
0
0
Next, the reactions of variously substituted phenylphos-
phine oxides with 2a were examined. The reaction of bis(4-
methylphenyl)phosphine oxide (1b) in the presence of
AgOAc gave both the normal coupling product 3ba and the
unexpected isomer 3ba’ as a 1:1 mixture (entry 1 in Table 3).
This may imply that an unanticipated pathway in the
annulation process intervenes. The formation of 3ba’ may
be rationalized by assuming an attack by alkenyl radical A on
the ipso-carbon atom of one of the 4-methylphenyl rings
(
3 mL) at 1008C for 4 h under N . The oxidative annulation
2
product 1,2,3-triphenyl-1H-phosphindole-1-oxide (3aa) was
obtained exclusively, in 96% yield (entry 2 in Table 1). While
the reaction using the substrates in a 1:1 ratio gave 3aa in
8
0% yield (entry 3 in Table 1), decreasing the amount of
AgOAc (0.5 mmol) substantially reduced the product yield
entry 4). The combination of AgNO3 (0.05 mmol) and
K S O (1 mmol), which has been employed to generate P-
(
[
10h,13]
(Scheme 3a).
The resulting spirocyclohexadienyl radical
2
2
8
[
8b,c]
centered radicals,
was not effective for the present
reaction (entry 5 in Table 1). At 808C, the reaction was
sluggish (entry 6 in Table 1). In AcOH, the reaction also
proceeded smoothly, but the product yield was somewhat low
(
entry 7 in Table 1). Note that the present reaction could be
readily scaled up to a gram scale. Thus, from 1a (6 mmol) and
a (3 mmol), 3aa was obtained in 82% yield (0.93 g, entry 8
in Table 1).
2
As shown in Scheme 2, the present reaction seems to
I
proceed through a radical process promoted by Ag . To obtain
additional support for this mechanism, we conducted the
reaction of 1a with 2a in the presence of radical inhibitors
(
2
see the Supporting Information). As expected, adding
,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) or 2,6-di-tert-
butyl-a-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadiene-1-yli-
dene)-p-tolyloxy (Galvinoxy) severely retarded or completely
suppressed the reaction.
It has been reported that P-centered radicals can be
formed in the presence of Mn(OAc) through homolytic PÀH
3
[
10a]
bond cleavage.
Therefore, we employed Mn(OAc) ·2H O
3 2
(
1 mmol) in the place of AgOAc for the reaction of 1a with
Scheme 3. Possible pathways to 3ba and 3ba’.
1
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 12975 –12979