Internal Addition of Nucleophiles

3 UnsyivmetricallY Placed Substituents in Stoichiometric Electrophilic Multihapto-Complexes 593 

Cycloheptadienyliron complexes also frequently show examples of internal nucleophile addition [214,215,224—227]. Steric effects are conventionally cited to explain the tendency of cycloheptadienyl complexes to give internal addition products (whereas cydohexadienyl complexes react at the termini). There is kinetic evidence to support this, as cycloheptadienyl complexes have been shown to be less reactive than cydohexadienyl complexes [223]. The CH2 of the cydohexadienyl complexes, and the CHjCHj of the cydohepadienyl complexes fold out away from the metal in these structures, and in the cydoheptadienyl case, each CH2 blocks a terminus of the Jt system. NudeophUic attack is displaced to the internal positions, forming ij, r] products in competition with the 1 products from the normal addition pathway [214,215,224—230]. With larger metals [Ru(CO)3 and Os(CO)3], this effect becomes more pronounced, and the products can predominate [230,231]. The nature of the nudeophile can also influence the preferred pathway (see Section 14.4). With soft nudeophiles, cydoheptadienyl complexes show the terminal addition pathway, while hard nudeophiles add internally [232]. Further 

The main purpose of this chapter is to discuss the directing effects of functional groups in these reactions. As one would expect, steric effects direct internal nucleophile addition to the less hindered of the electrophilic centers as in the formation of 49. With a C-1 OMe substituent, a addition has been proposed [235]. Internal nucleophile addition to the less hindered of the electrophilic centers has also been reported for 50 [236]. Other examples give mixtures [237]. On the other hand, 1,4-dimethyl substitution on the tj cydoheptadienyl ligand shows regiocon-trol dominated by the C-1 Me group (y selectivity ipso to the C-4Me group) [229]. 

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