Hydrido alkene complexes, structures

Both in situ infrared and multinuclear NMR under less severe conditions have been used to gain mechanistic insights. For the hydroformylation of 3,3-dimethyl but-l-ene, the formation and hydrogenolysis of the acylrhodium species Rh(C()R)(C())4( R=CH2CH2Bur) can be clearly seen by IR. NMR spectroscopy has also been very useful in the characterization of species that are very similar to the proposed catalytic intermediates. We have already seen (Section 2.3.3, Fig. 2.7) NMR evidence for equilibrium between a rhodium alkyl and the corresponding hydrido-alkene complex. There are many other similar examples. Conversion of 5.3 to 5.4 is therefore well precedented. In the absence of dihydrogen allowing CO and alkene to react with 5.1, CO adducts of species like 5.6 can be seen by NMR. Structures 5.11 and 5.12 are two examples where the alkenes used are 1-octene and styrene, respectively. 

The next step involves binding of the alkene to 2, forming an 18-electron hydrido-alkene complex that can have several structures, two of which are shown as 4 and 5. Theoretical calculations at the DFT-B3LYP level19 by Jiao and co-workers demonstrated that Structure 4 is about 4 kcal/mol more stable than 5 (probably due to less steric hindrance), although only 5 has the requisite geometry (Co, H, and the double bond carbon atoms all coplanar) for a direct 1,2-insertion step (step c).20 Overall, therefore, steps a and b constitute a ligand substitution via a D mechanism. 

One noteworthy feature of this asymmetric reaction is that the products, 3 and 4, are of opposite absolute configuration. This can be explained by a kinetic resolution between 6 and 7 (Scheme 11.6). Insertion of the aikene 1 into the Pd-Ph bond followed by -hydrogen elimination forms hydrido-alkene complex 6 or its diastereomer 7. It is likely that complex 6 has a preferable structure for further aikene insertion and )8-hydrogen elimination processes, giving the major product 3, while diastereomer 7 readily releases the aikene to give 4. Brown and coworkers [11-13] reinforced this mechanism by NMR spectroscopy and mass specdometry. 

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