Steric interactions metalation

Copper is clearly the most selective metal-ion catalyst. Interestingly, proton catalysis also leads to high selectivities. This is a strong indication that selectivity in this catalysed Diels-Alder reaction does not result from steric interactions. 

Catalytic asymmetric hydrogenation was one of the first enantioselective synthetic methods used industrially (82). 2,2 -Bis(diarylphosphino)-l,l -binaphthyl (BINAP) is a chiral ligand which possesses a Cg plane of symmetry (Fig. 9). Steric interactions prevent interconversion of the (R)- and (3)-BINAP. Coordination of BINAP with a transition metal such as mthenium or rhodium produces a chiral hydrogenation catalyst capable of inducing a high degree of enantiofacial selectivity (83). Naproxen (41) is produced in 97% ee by... 

For all the olefins studied, alkyl-, fluoro-, or chloro-substituted, three binary, mononuclear species were observed. It now seems that it is a general property of Ni and Pd atom-olefin reactions at cryogenic temperatures to form complexes that have a maximum coordination of three olefin molecules per metal atom, regardless of the electronic or steric attributes of the substituent(s). As intimated previously, the absence of higher stoichiometry species, even for unsubstituted ethylene, is, most probably, the result of steric interactions (54). 

In the case of olefin metathesis, the selectivity in initiation products can be understood in terms of minimization of the steric interactions in the metal-lacyclobutane intermediates (vide supra), which are governed by the relative position of the substituents the metallacyclobutane with substituents in pos-... 

Similar metallacyclic species have been proposed as intermediates in other reactions of metal compounds with diazoalkanes (57,55). The presence of unfavorable steric interactions in an intermediate such as 53 could well explain the failure to observe any reaction of diaryldiazoalkane with 45 (55). 

Metal phthalocyanines functionalized with four helicenes (62) have also been reported to form chiral columnar aggregates.76 In chloroform solutions of these metal phthalocyanines aggregation into columns occurred upon addition of ethanol, as was observed by UV-Vis spectroscopy. CD spectroscopy revealed that the chromophores within the columnar aggregates are in a chiral environment, implying that the chirality of the peripheral helicenes has been transferred to the supramolecular aggregates. These phthalocyanines stack with a typical intermolecular distance of 3.4 A, and calculations have indicated that to allow this distance the two phthalocyanine moieties have to be rotated because of the bulkiness of the helicenes. It can easily be imagined that a phthalocyanine provided with both R and S helicenes cannot stack in such a defined manner because of the steric interactions between the nonconform helicenes. 

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