Catalytic cross-dimerization
is one of the powerful synthetic methods
to produce linear molecules with high atom and step economy. Because
this process involves a carbon–carbon-bond-forming reaction,
enantioselective reactions have also been achieved. The most well-reviewed
area in this field is probably hydrovinylation using ethylene, but
the linear cross-dimerizations using substituted alkenes and alkynes
have also been extensively developed. Not only do the products vary
depending on these substrates, but they mostly differ from hydrovinylation
in the mechanism. This Perspective presents a comprehensive summary
on the basis of these substrates, including their brief historical
background, mechanism, applications to the synthesis of biologically
active compounds or contributions to the total synthesis, and the
state-of-the-art advancements. The controlling factors in the chemo-
and regioselectivities are also discussed.
A CO2 complex of iron(0),
Fe(CO2)(depe)2 (1; depe
= 1,2-bis(diethylphosphino)ethane) has
been prepared by replacement of N2 in
Fe(N2)(depe)2 by CO2.
The X-ray structure analysis
of 1 shows that it has a
η2(C,O)-CO2 ligand in a
trigonal-bipyramidal Fe geometry with
some contribution of an η1(C) mode.
Reaction of 1 with R3SnCl in
Et2O at −78 °C gives the
iron carboxylate complexes
FeCl(CO2SnR3)(depe)2 (R
= Me, (2a), Ph (2b)). The
X-ray
structure analysis of 2b shows that the CO2
fragment bridges between the Fe and Sn atoms
in a
μ-η1(C):η2(O,O‘)-CO2
fashion. Treatment of 1 with Me3SiCl
results in the removal of an
O atom from the CO2 ligand to give a cationic
carbonyliron(II) complex,
[FeCl(CO)(depe)2]+Cl-
(3a), and (Me3Si)2O.
Similarly, 1 reacts with carbon electrophiles such as
MeI and MeOTf
to give the corresponding cationic iron(II) carbonyl complexes
[FeX(CO)(depe)2]+X-
(X = I
(3b), OTf (3c)) and Me2O.
Full elucidation for stable, colorless, and meta-stable colored structures of a new spirobenzothiopyran has been achieved both in the solid state and in a solution. 1',3',3'-Trimethyl-6-nitrospiro[(2H)-1-benzothiopyran-2,2'-indoline] with an ester group as a substituent at the 8-position of 1-sp shows photochromism. The blue-green colored species resulted from UV irradiation (365 nm) in one minute and spontaneously bleaches within one minute in acetone and methanol at 27 degrees C. UV exposure (365 nm) of 1-sp in methanol for 3 h at room temperature results in the growth of deep blue needlelike single crystals of the open form of spirobenzothiopyran, photomerocyanine 1-pmc, whose structures in the solid state and in solution were obtained unambiguously. The X-ray structural analysis of 1-pmc revealed the molecular structure of the zwitterionic photomerocyanine with s-trans,s-trans conformation. 1-pmc is soluble to polar solvents and thermally returns to 1-sp. In the DMSO solution, 1-pmc is found to return slowly to 1-sp (1-pmc gave only 18% of 1-sp for 30 min at 22 degrees C). The detailed NMR studies in DMSO-d(6) including COSY and NOE techniques as well as isotope labeling of the compound showed the structure with s-trans,s-cis conformation in a solution.
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