13CO labelling

Other miscellaneous imines that underwent photoreaction with chromium alkoxycarbenes include iminodithiocarbonates [33],the mono-N-phenylimine of benzil and the bis-JV-phenyl imine of acetoin [20]. By preparing the chromium carbene complex from 13CO-labeled chromium hexacarbonyl, /J-lactams with two adjacent 13C labels were synthesized [34]. 

The seminal synthesis of this type of complex, specifically TpPt(H)2Me (347), barely a decade ago, resulted from stirring TpPtMe(CO) (150) in 1 1 acetone/H20 for 24 h,117 a process that was subsequently patented.118 A modified, base-catalyzed, variation of this process was independently applied to the synthesis of Tp Pt(H)2Me (151) from Tp PtMe(CO) (348).66 The initial step of this conversion is believed to be analogous to the WGS reaction, viz. nucleophilic attack (H20 or OH) at the carbonyl inducing loss of CO a process supported by 13CO labeling studies that confirmed loss of 13C02.117 Resultant traces of carbonic acid then facilitate protonation of the intermediate [TpxPtMeH] anion. 

Figure 2. Undecoupled 13C NMR Fourier transform spectra of Co(III)-transferrin-CO3. (A) 13CO -labeled transferrin (B) no label (C) after adding H13COf to the preparation used for A. The line at 104 ppm is caused by 13C03 specifically bound to transferrin and is 14 Hz wide. The linewidth of free H13C03 at 96 ppm is 7 Hz (32).

C-carbonyl-labeled amino acids permitted some of the 13CO resonances to be assigned (Wang and Shortle, 1996). In many cases, the resonances for the ft segment displayed the typical upheld shift... 

Many of the questions regarding the mechanism of the reaction shown in Eq. (22.24) have been answered by the use of 13CO. The results have shown that when the entering carbon monoxide is 13CO, there is no 13CO in the acyl group. Instead, the labeled CO is in a position that is cis to the acyl... 

During steady-state isotopic transient kinetic analysis, the 12CO was switched to 13CO and the carbon-containing adsorbed and gas phase species were monitored in the IR as they exchanged from the 12C to the 13C label. Particular attention was made to those species that exchanged on a timescale similar to that of the exchange of the product C02, as that species could be a likely intermediate to the water-gas... 

When the reaction is run in the presence of 13CO and stopped after one half-life, not only is the label incorporation into recovered starting material negligible, as predicted by the mechanism, but the label incorporation into H2Os2(CO)s exceeds that required by the mechanism. This shows that it is undergoing carbonyl exchange under the reaction conditions and suggests that such dinuclear species are much more labile than their mononuclear counterparts. 

Two homogeneous metal complex water-gas shift catalyst systems have recently appeared 98, 99). The more active of these comes from our Rochester laboratory (99, 99a). It is composed of rhodium carbonyl iodide under CO in an acetic acid solution of hydriodic acid and water. The catalyst system is active at less than 95°C and less than 1 atm CO pressure. Catalysis of the water-gas shift reaction has been unequivocally established by monitoring the CO reactant and the H2 and C02 products by gas chromatography The amount of CO consumed matches closely with the amounts of H2 and C02 product evolved throughout the reaction (99). Mass spectrometry confirms the identity of the C02 and H2 products. The reaction conditions have not yet been optimized, but efficiencies of 9 cycles/day have been recorded at 90°C under 0.5 atm of CO. Appropriate control experiments have been carried out, and have established the necessity of both strong acid and iodide. In addition, a reaction carried out with labeled 13CO yielded the same amount of label in the C02 product, ruling out any possible contribution of acetic acid decomposition to C02 production (99). 

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