Pivalate ligand

Scheme 7 Postulated participation of the pivalate ligand as an internal base...

The direct phenylation of l//-pyrrole (1) using a variety of metal catalysts has been reported and key examples will be discussed herein (Scheme 10.1). Sames et al. employed a rhodium catalyst to obtain 2-phenyl pyrrole (2) in 78% yield. The choice of CsOPiv proved to be crucial in this site-selective transformation as the carbonates and phosphates of alkali metals were ineffective. The authors believe that the mildly basic pivalate ligand catalyzes the rhodium-mediated C—H activation via a... 

Related spectra for the oxalate and terephthalate bridged Mo4-containing compounds supported by pivalate ligands are shown in Fig. 9. Here it is clearly evident that on the EPR time scale, 10 s , the oxalate cation involves complete delocalization of the positive charge over all four Mo atoms. The single electron in the HOMO sees equally all four Mo atoms. Here the presence of two spin active nuclei in the M center is more easily seen. In contrast, the EPR spectmm of the terephthalate radical cation is localized on one M02 center the mixed-valence ion is valence-trapped [34]. 

Bursten BE, Chisholm MH, Clark RJH, Firth S, Hadad CM, Macintosh AM, Wilson PJ, Woodward PM (2002) Oxalate bridged complexes of dimolybdenum and ditungsten supported by pivalate ligands (t-BuC02)3M2( t-02CC02)M2(02C-t-Bu)3. Correlation of the solid-state, molecular and electronic structures with Raman, resonance Raman and electronic spectrtil data. J Am Chem Soc 124 3050-3063... 

A large increase in activity, stability, and selectivity was observed when the pivalate ligand (e.g., 6) is exchanged for a nitrato ligand (e.g., 9). For example, while 6 catalyzed the homo-CM of 10-methyl undecenoate with 160 turnovers and 90% Z-selectivity, 9 catalyzed the same transformation with 850 turnovers and >95% Z-selectivity. Similar TONs and selectivities were seen in a variety of other homo-CM reactions. Furthermore, 9 could be manipulated on the bench-top, as opposed to its air-sensitive predecessor 6. 

A large decrease in activity, stability, and selectivity was seen when the pivalate ligand (e.g., 6) was exchanged for a monodentate (k ) ligand such as iodide (e.g., 8). These types of catalysts were slow to initiate, often catalyzed olefin migration reactions instead of undergoing productive metathesis events, and exhibited a finite shelf life even when stored at low temperatures (—30 C) under an inert atmosphere. 

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