Sterically bulky groups

The stereoselective reactions in Scheme 2.10 include one example that is completely stereoselective (entry 3), one that is highly stereoselective (entry 6), and others in which the stereoselectivity is modest to low (entries 1,2,4, 5, and 7). The addition of formic acid to norbomene (entry 3) produces only the exo ester. Reduction of 4-r-butylcyclohexanone (entry 6) is typical of the reduction of unhindered cyclohexanones in that the major diastereomer produced has an equatorial hydroxyl group. Certain other reducing agents, particularly sterically bulky ones, exhibit the opposite stereoselectivity and favor the formation of the diastereomer having an axial hydroxyl groi. The alkylation of 4-t-butylpiperidine with benzyl chloride (entry 7) provides only a slight excess of one diastereomer over the other. 

In summary, the leaving groups which coordinate strongly to copper give the y-alkylation product even in the presence of a sterically bulky substituent at the y-position. 

Using sterically bulky groups in the ortho positions of the phenyl rings in ArNC(H)NHAr, such as Ar = 2,6-Me2-QH3, led to formation of dinudear and trinuclear complexes. This suggests that steric factors can prevent the formation... 

There was a substantial electronic influence with electron-withdrawing substituents decreasing enantioselectivity. Interestingly, steric bulkiness at this remote part of the molecule was found to be highly effective for asymmetric induction. The bulky ANM group provided 97% ee with a high isolated yield. 

Sterically bulky ligands have been used increasingly to enforce low coordination numbers (as low as two) in cr-bonded compounds. A review on the chemistry of compounds containing the tris(trimethylsilyl)methyl and related bulky groups is available.154... 

The use of sterically bulky ligands to provide kinetic stability to compounds of the heavier group 2 elements has become widely practiced, and has facilitated the synthesis of diorganyl complexes of various types. Advances in the organometallic chemistry of bonded compounds of calcium, strontium, and barium have been reviewed. 

Prior to the development of polymerisation catalysts based on the bis(imino)pyridine framework, the bis(imino)pyridine moiety was widely incorporated into macrocycles [163, 164], As an extension of this design strategy to polymerisation applications, several groups have been attempting to incorporate sterically bulky bis(arylimino) pyridine units into macrocycles. In a similar fashion, the introduction of the bis(imino)pyridine unit into polymeric chains in which polymerisation-active metal centres are bound has been the subject of study. 

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