Photochemical CO elimination

An attractive approach to cyclocarbon molecules required the preparation of the higher carbon oxides 6-8 followed by either thermal or photochemical CO-elimination (Scheme 2) [41, 521. The direct synthesis of 6-8 by oxidative macrocyclization of 9 was not possible, since all attempts to prepare the latter... 

Photochemistry of Rhenium(I) Complexes A. Photochemical CO Elimination Reaction of a Re(I)... 

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

Photochemical reductive elimination of H2 from dihydrides serves as a route to generating reactive intermediates. Photolysis of Ru(PPh3)3(CO)H2 in benzene yields Ru(PPh3)3CO. Ultrafast measurements found that hydride loss and Ru(PPh3)3CO formation is complete in 6 ps. Photolysis ofboth Ru(CO)2(PPh2)2H2 and Ru(CO)3(PPh3)2 in gas matrices resulted in the formation of the same Ru(CO)2(PPh3)2 intermediate. ... 

The ruthenium compound gives an object lesson in the merits of combining matrix with time-resolved solution studies. Photochemical reductive elimination of H2 may be followed by the decay of the characteristic i (Ru-H) bands in the infrared spectrum or by the growth of three intense visible bands at co. 740, 540, and 460 nm. Although the product lacks characteristic infrared features, the circumstances of its formation and its response to matrix dopants imply that it is the 16-electron species Ru(dmpe)2, presumably with a square planar skeleton. 

Photoactivated substitution on M(CO>5L systems has been reported by Wrighton and by Dahlgren and Zinck. The photochemical reaction proceeds by two paths that yield different products L elimination (path A) and CO elimination (path B), as shown in Scheme 7.9. 

The evidence for this scheme comes from several observations. The reaction is promoted rather than inhibited by CO, so that the initial photochemical event is not CO elimination. The photochemical behavior is unusual because the quantum yield changes if the light source is pulsed on and off. Tanaka and co-workers reported that the rate decreases by a factor of 10 when the pulse rate is changed from 10 s to 1 s, and the phenomenon was confirmed by Goldman et al. In Scheme 7.20, this is accounted for by competition between return to reactants and the photoactivated insertion step. Goldman and co-workers have identified the various octahedral species by low temperature NMR. 

Practical thermal and photochemical routes to methylene generally involve elimination of stable neutral molecules, among them N2 and CO. 

Note that not all cyclopropenones eliminate CO photochemically. Thus, the first cyclohexynes had to be generated by pyrolysis of the precursor cyclopro-penone. °°... 

M-1s-1) to form RhCl(PPh3)2 and RhCl(H2C=CH2)(PPh3)2, respectively however, reaction with H2 to form the dihydride is much slower, (1 x 10 M-- -s-- -). Also described are flash photolysis studies of the dinitrogen species IrCl(N2) (PPI13) 2 and the dihydride H2lrCl(C0)(PPh3)2- In both cases, the transient IrCl(PPh3)2 is formed. These results indicate that CO labilization from the Ir(lII) dihydride is a facile photochemical pathway and the photo-reductive elimination of H2 is a more complicated mechanism than previously inferred. 

Vaska s complex trans-IrCl(CO)(PPh ) has served as an important model for mechanistic investigation of catalytically relevant reactions such as the oxidative addition and reductive elimination of small molecules(15). The latter processes have also been the subject of some photochemical investigation. For example, the reductive elimination of H2 depicted in Equation 5, which is a relatively slow thermally activated process (k = 3.8 x 10- s l in 25° benzene solution (15)), has been shown to occur readily when the dihydride complex was subjected to continuous photolysis with 366 nm light(16). However, Vaska s compound itself was reported to be... 

Intramolecularly alkoxy base-stabilized bis(germylene)-, 54, or (germylene)(silylene)-, 55, iron complexes, of type IV, were obtained from the photolysis of digermyl or germylsilyl iron complexes (equation 43)14. These are the same type of complexes noted above for base-free tungsten analogs146 involving a-elimination subsequent to photochemical elimination of CO as noted below. 

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