Ligands electron-poor pyridines

Moderate to good enantioselectivities were obtained for nearly all examples, but the products from 83a-c could be recrystallized to higher enantiomeric purity. Addition of iodine was critical for catalysis as was the use of a ligand with electron-poor para-fluorophenyl groups on the phosphorous atom. Substitution at the 3 position of the pyridine ring was described as being difficult for both the quinolines and pyridine systems. The resulting hydrazine derivatives could be easily converted to piperdines by reduction with Raney nickel or under Birch conditions. 

More recently, Chiu and coworkers have developed a Ni complex containing a tetradentate pyridine/NHC ligand (complex 18, Eq. 22) which catalyzes the Suzuki reaction at catalyst loadings between 1 and 3 mol% [56]. Aryl iodides, bromides, and chlorides with both electron-rich and -poor aryl rings were compatible. However, electronically poor or electronically neutral aryl bromides performed much better than did electron-rich aryl bromides. It was also found that the use of 2 equivalents of PPh3 was crucial to achieving high yields with aryl chlorides. 

Lautens and coworkers described recently a Rh/Pd-catalyzed domino synthesis of aza-dihydrodibenzoxepines (Scheme 4.28) [50]. The Rh-catalyzed hydroary-lation of 3-chloro-2-vinylpyridines 136 with 2-hydroxyphenylboronic esters 137 afforded the intermediate 138. Subsequent Pd-catalyzed C-O coupling provided the aza-dihydrodibenzoxepines 139 in good yields. The one-pot domino process worked well with electron-poor vinyl pyridines. However, the reaction did not proceed to full conversion under domino conditions with electron-rich vinyl pyridines. An asymmetric variant of this reaction was also described using both a chiral diene and an achiral phosphine as ligands, producing products in moderate to good yields with excellent enantioselectivities. This reaction represented the first example of an asymmetric multimetal-catalyzed reaction. 

General behaviors are summarized in Table 1. For ligands such as carbon monoxide, isocyanides (CNR), carbynes, nitrosyl NO+ and, although to a smaller extent, nitriles (NCR), which are not only a donors but also good jr acceptors, the net electron-donor ability is modest or poor and the Pl parameter has a high value. However, ligands that are not involved in jt-bonding, such as ammonia (NH3) and pyridine, exhibit lower Pl values that, nevertheless, are usually not... 

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