Organic Letters
Letter
component, such as allyl trimethylsilane (entries 2 and 9),
comparable yields are obtained.
AUTHOR INFORMATION
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Corresponding Author
ORCID
Of considerable interest are entries 14 and 15 because with 3-
buten-1-ol the double bond geometry of the β-double bond was
inverted. Probably, the coordination of the oxygen to the cobalt
may be relevant which was observed for other oxygen-bearing
functionalities in the past as well.3b
In order to reduce the coordination of the oxygen and invert
the double bond of the β-double bond toward an E-configured
geometry, we repeated entry 15 with the TBS-protected alcohol
7 (Scheme 3).
The reaction resulted in a product mixture, consisting of the
“undesired” product 8a, which still had the Z-configured β-
double bond and two new products which had not been
observed in the previous reactions. First, we identified the
conjugated 1,3-diene 8b, which could result from a β-hydride
elimination “the other way” from the proposed intermediate A,
as was observed from ruthenium-catalyzed Alder-ene reactions.8
Second, an unusual 1,5-diene, product 8c, was observed, which
results from an additional double bond shift from 8a to 8c.
Despite our extended knowledge in double bond isomerization
and translocation with cobalt and nickel based catalysts,9 we
were not able to shift the ratio of 8a:8c by applying our
catalysts. Accordingly, a different mechanism might be in action
and we intend to investigate this phenomenon in more detail in
the future.
The final set of experiments was conducted with the
ruthenium-based catalyst system (compare Scheme 2) to
compare the results and to illustrate the straightforward
compatibility of the catalyst systems. Accordingly, a number
of the 1,3-butadiynes were reacted with terminal alkenes
according to the protocol reported by Lee.6 The results are
summarized in Table 2.
All ruthenium-catalyzed reactions gave exclusively the
branched product 4, where the carbon−carbon bond formation
takes place at C3. In contrast to Lee et al. the subjected 1,3-
diynes needed longer reaction times and in the case of the aryl-
substituted alkenes higher reaction temperatures were needed
to reach complete conversion of the starting materials.
Unfortunately, 3-buten-1-ol gave no complete conversion
even at 60 °C or with a higher catalyst loading. In the case
of the TBS-protected alcohol 7 (entry 10), we were able to
isolate the desired product as a single isomer.
In conclusion, we were able to apply the cobalt- and
ruthenium-catalyzed Alder-ene reaction of unsymmetrical silyl-
substituted 1,3-butadiynes with terminal alkenes in a comple-
mentary fashion. While the α-double bond configuration is
controlled by the reaction mechanism, the configuration of the
β-double bond can be controlled by the substituent on the
alkene component. Those substrates without a donor
functionality prefer the E-configuration, whereas the Z-
configuration is generated when 3-buten-1-ol is applied.
Accordingly, the usefulness of the Alder-ene reaction is
considerably increased via our new protocol.
Notes
The authors declare no competing financial interest.
REFERENCES
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge on the
Synthesis, analytical data, NMR spectra (PDF)
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Org. Lett. XXXX, XXX, XXX−XXX