 |
CYCLOPROPENES:
VERSATILE BUILDING BLOCKS FOR ORGANIC CHEMISTRY
DIASTEREOSELECTIVE Pd-CATALYZED HYDRO- AND METALLASTANNATION
OPTICALLY ACTIVE CYCLOPROPANES
FIRST ENANTIOSELECTIVE HYDROSTANNATION
DIRECT Pd-CATALYZED ARYLATION OF CYCLOROPENES
SILA MORITA-BAYLIS-HILLMAN REACTION OF CYCLOPROPENES
The
third area of our interest focuses on the development of novel
transition metal-catalyzed transformations involving strained
ring systems. We
have found highly stereo- and regioselective transition
metal-catalyzed hydro-, sila-, and stannastannation of
cyclopropenes, which proceeded very rapidly at temperatures as
low as –78oC to produce up to pentasubstituted
cyclopropane derivatives in very good yields. It
was shown that the addition across the double bond of
cyclopropene is generally controlled by steric factors and
proceeds from the least hindered face. The directing effect of
alkoxymethyl substituents in the hydrostannation reaction of
3,3-disubstituted cyclopropenes was also demonstrated.
Cyclopropylstannanes were converted into the corresponding
cyclopropyllithium derivatives or cyclopropyl halides with
retention of configuration. This methodology represents a
powerful approach toward a wide variety of highly substituted
stereodefined cyclopropylstannanes, important building blocks [Tetrahedron
2004, 60, 3129 (review)]
unavailable by other methods. [J.
Am. Chem. Soc. 2002,
124, 11566]
[go
to contents]
We
have also developed catalytic, highly enantioselective
hydroboration of cyclopropenes.
Thus, a variety of 2,2-disubstituted cyclopropyl
boronates have been synthesized via
this method with high degrees of diastereo- and
enantioselectivity. It was demonstrated that ester and
alkoxymethyl substituents serve as effective directing groups in
the hydroboration reaction. The directing effect was found to be
necessary in achieving high degrees of enantiomeric induction.
Selected cyclopropylboronic derivatives were successfully
employed in the Suzuki cross-coupling reaction to produce the
corresponding optically active arylcyclopropanes in good yields.
[J. Am. Chem. Soc. 2003, 125, 7198]
[go
to contents]
Recently,
our group demonstrated first examples of catalytic
enantioselective hydrostannation reaction.
Thus, optically active 2,2-disubstituted
cyclopropylstannanes were efficiently obtained via the
rhodium-catalyzed hydrostannation of cyclopropenes.
This reaction was shown to be very general with respect
to substituents at C-3 and displayed good functional group
compatibility. It
was demonstrated that facial selectivity of hydrostannation is
entirely controlled by steric factors and proceeds from the
least hindered face. This
methodology is interesting not only from the fundamental point
of view, as it represents the first example of catalytic
enantioselective hydrostannation of a C=C double bond, but also
as a very efficient approach to optically active
cyclopropylstannanes, invaluable building blocks for organic
chemistry. [J.
Am. Chem. Soc. 2004, 126,
3688], [J.
Org. Chem. 2007, 72, 8910]
[go
to contents]
We
have also demonstrated the first examples of direct
palladium-catalyzed arylation and heteroarylation of
cyclopropenes. This method allows for efficient synthesis of
various tetrasubstituted cyclopropenes, including nonracemic
cyclopropenes, which are not available via known asymmetric
cyclopropenation methods. Mechanistic studies suggest that,
among several alternative pathways, including: Heck-type (A),
cationic path (B), C-H activation (C), or cross-coupling
protocols (D); electrophilic path B is the most viable pathway
for this transformation. [J.
Am. Chem. Soc. 2005, 127, 3714]
[go
to contents]
Our group developed a Sila Morita-Baylis-Hillman reaction on cyclopropenes, which features nucleophilic addition of electron rich phosphines to the double bond of silylcyclopropene followed by a 1,3-Brook rearrangement. This novel transformation provides a general, efficient, and expeditious route to 1-(silyloxymethyl)cyclopropenes starting from easily available
1-silylcyclopropenes. [J.
Am. Chem. Soc. 2007, 129, 14868]
[go
to contents]
|
|
