a new type of Fischer carbene complexes featuring an extra
double bond in the carbon chain, in the expectation that it
might lead to the discovery of new types of cascade
processes. We envisioned that carbenes 1 (Figure 1) could
serve as precursors of 1-metalla-1,3,5,7-tetraenes, upon
typical cycloaddition reactions. Therefore, we set out to
investigate their behavior in [2+2], [3+2], and [4+2]
cycloaddition reactions.
Table 2. Synthesis of Isooxazoles 5
The required Fischer carbene complexes 1 were synthe-
sized from the corresponding terminal alkynes through the
conventional methods (see the Supporting Information for
details). At first, we employed the carbene complexes 1a
and 1b, in an attempt to verify whether a possible, and
predictable, tandem cycloaddition/cyclopropanation process
might occur. Indeed, when typical procedures for [2+2]
cycloadditions with alkynyl carbene complexes were used,4
homonaphthalenes 3a-d were formed in a diastereoselective
way and were isolated with excellent yields (Table 1).
compd
M
R
yield,a %
5a
5b
5a
5b
Cr
Cr
W
t-Bu
Bn
t-Bu
Bn
82
73
57c
61
Wb
a Isolated yield based on the starting carbene complex.
At this point we decided to evaluate the scope of the
reaction by introducing a substituent on the terminal olefin.
When the complex 1c was stirred with 3 equiv of 2,3-
dihydrofuran 2a at room temperature the anticipated [2+2]
cycloadduct 4a was obtained (Scheme 1). Subsequent heating
Table 1. Synthesis of Homonaphthalenes 3
Scheme 1. Cycloaddition/Metathesis Sequence on Complex 1b
of the cyclobutene carbene (6a) in a sealed tube for 2 h
triggered the formation of naphthalene 7a instead of the
expected cyclopropanation product. Formation of naphthalene
7a could be explained by a metathesis reaction, a type of
process that has been scarcely observed in Fischer carbene
chemistry.6
This reaction was generalized by using several enol ethers,
chromium and tungsten carbene complexes, and different
substitution in the olefin (R ) Ph, Me) (Table 3, entries
1-12). In all cases the naphthalene derivatives 5 were
isolated in good yields. Identical results were obtained when
Z or E olefin configurations were investigated.
a Isolated yield based on the starting carbene complex. b Reactions with
the W carbenes were carried out at 40 °C. c 80% conversion. d After
hydrolysis of the reaction crude.
Following a similar protocol, [3+2] cycloadditions with
nitrones 4 were used as initiator reactions.5 In this case,
benzoisoxazola derivatives 5 were obtained with very good
yield.
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