Nucleophilic, migratory aptitudes

After a decade of research the basic principles in the chemistry of decamethylsilicocene (1) seem to be understood. This compound shows the reactivity of a nucleophilic silylene due to the fact that the Tt-bonded pentamethylcyclopentadienyl ligands are easily transferred to a-bonded substituents during the reaction. The steric requirements of these substituents permit reactions with bulky substrates. The migratory aptitude and the leaving-group character of the pentamethylcyclopentadienyl groups... 

Answer. Three aspects of the scheme shown in Figure 13.2 may be examined by orbital interaction theory (a) the binding of CO compared to ethylene (b) the reactivity of coordinated carbonyl compared to ethylene with respect to reactivity toward intramolecular nucleophilic attack and (c) the migratory aptitude of alkyl versus carbonyl groups. 

These two observations were explained by the intermediacy of carbene species. Nucleophilic P-addition generates a vinyliodonium ylide - iodoallene intermediate (83), which loses iodobenzene giving the alkylidenecarbene (84). If either the substituent R or the nucleophile Nu has a high migratory aptitude, a 1,2-shift leads to the rearranged alkyne products. However, when the substituent of the alkylidenecarbene has a y-hydrogen and if both the substituent and nucleophile have a poor migratory aptitude, intramolecular 1,5-C-H insertion takes place to form the cyclopentene derivatives. If a viable external trap is present, intermolecular insertion then occurs preferentially, 7,21,22,24... 

The most investigated ctirbon nucleophiles are p-dicarbonyl or related enolates, generally cyclic in structure and fully substituted (i. e., tertiary) at the nucleophilic carbon. The products of reaction of these enolates with alkynyliodonium salts are p-dicarbonyl compounds, and their formation is mostly dependent upon the migratory aptitude of the substituent on the ethynyliodonium salt. 

As a consequence of the superior migratory aptitude of hydrogen, the parent ethynyliodonium salt gives exclusively rearrangement (alkynylation) products with diverse P-dicarbonyl nucleophiles in 63-78<7o yield (Scheme 3-4) [28]. This reaction represents a very convenient way of introducing the HC = C — functionality into keto and ester P-dicarbonyl compounds. 

The migratory aptitude decreases from p-nitrophenyl over phenyl and / -tolyl to /j-anisyl. ° This substituent effect argues in favor of a two-step mechanism, i.e., nucleophilic addition of the metalated a-carbon onto the aryl //7TO-position to give the bridged intermediate 354 and subsequent scission of the oxygen-aryl bond (Scheme 1-277). 

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