Metallacycles photolysis

In another study the kinetics and mechanism of an unprecedented T/2-vinyl isomerization of a highly fluorinated tungsten(II) metalla-cyclopropene complex was studied (92). Photolysis of a tungsten(II) tetrafluoroaryl metallacycle 1 and perfluoro-2-butyne results in the formation of the kinetic rf -vinyl complex 2 in which the fluoride is trans to the inserted acetylene and cis to both carbonyl ligands. Upon heating 2 is converted to the thermodynamic rf -vinyl complex 3 in which the fluoride ligand is now cis to the inserted alkyne and trans to one CO and cis to the second CO ligand as shown in Scheme 1. 

Donor-substituted alkynes can insert into the C-M double bond of alkoxycarbene complexes, yielding donor-substituted vinylcarbene complexes [191,192]. In addition to this, photolysis or thermolysis of a-alkoxycyclopropyl carbonyl complexes or a-alkoxycyclobutanoyl complexes can lead to rearrangement to metallacyclic carbene complexes (Table 2.11). This methodology has not been used as extensively for the preparation of carbene complexes as the other methods described above. 

If compound 8 is irradiated (X > 300 nm) in the presence of diazoalkanes, the product pattern shifts dramatically for the diazoacetates and -malonates which have oxygen-containing substituents capable of coordination. At temperatures of about -40 to -20°C (tetrahydrofuran), photolysis predominantly yields mononuclear metallacycles 12 which are not yet accessible by any other means (135, 136). An X-ray diffraction study revealed the strict planarity of the cyclic system (136). The bidentate ligands that wind around the metal arise from metal-mediated carbonyla-tion of the respective carbenes (Scheme 6). 

These MCp2(CH3)2 complexes have also been irradiated in the presence of other substrates to give interesting transformations. Rausch and coworkers (22) showed that photolysis of the MCp2(CH3)2 complexes in the presence of alkynes gave both metallacycles (17) and insertion products (18) [Eq. (16)], as well as numerous organic products. Irradiation of the... 

Stable, isolable metallacycles are also obtained from reaction of complexes that serve as sources of the CpCo fragment (e.g. CpCo(PPh3)2) and alkynes. Upon carbonylation diese typically give high yields of cobalt-complexed cyclopentadienones. Direct reaction of CpCo(CO)2 with alkynes is similarly useful. The cycloaddition of di(t-butoxy)acetylene upon photolysis with CpCo(CO)2 is an example (Scheme 5). In all these systems the final complexes lack coordinated CO, and therefore amine oxides are not suitable reagents for liberating the stable cyclopentadienones. Tetra(t-butoxy)cyclopentadienone is accessible on a preparative scale via controlled electrochemical oxidation. Other oxidants such as Cr have been used as well in other systems. 

---