A Alkoxyethyl complexes

Treatment of 22,10 with one equivalent of LiHBEt3 in THF at -80°,followed by removal of solvent and pentane extraction of the residue, afforded the a-alkoxyethyl complexes 23,24 in 75-95% yields. The corresponding iron ethyl complexes were not present as judged, by NMR. Compound 24b, previously reported as the analogous NaBH -ethanol reduction product mixed with the corresponding ethyl complex 8 (63), represents the only other known Group 8... 

We were concerned that synthesis of a,6-dialkoxyethyl complexes 16 and 17 would be limited by overreduction of 13 and 14. Both appended a-alkoxyethyl (30,63) and 6-alkoxyethyl (16,64) groups separately undergo facile reductive cleavage to ethyl ligands. Model studies were accordingly carried out in order to first establish conditions favoring generation of 6-alkoxyethyl and a-alkoxyethyl compounds via monohydridic reduction of suitable substrates. 

It has long been postulated that these facile reactions occur via formation of a cationic intermediate (upon acid-catalyzed loss of ROH) which can be formulated either as a a-bonded alkylcobalt carbonium ion or a cobalt(III)-olefin m complex. Recently, firm kinetic evidence has been obtained for the occurrence of an intermediate in the acid-catalyzed decomposition of 2-hydroxy- and 2-alkoxyethyl-cobaloximes [94]. Thus, while 2-hydroxyethylcobaloxime decomposes with strictly first-order kinetics in mildly acidic H2SO4/H2O mixtures, the alkoxy derivatives show a substantial lag followed by a first-order decay which is slower than that for the hydroxyethyl complex. In strongly acidic mixtures (//q < —5) all compounds show a rapid burst of absorbance change, followed by a slower first-order decay which is identical for all compounds whether measured spectrophotometrically or manometrically. These observations support the mechanism shown in Eqn. 56. 

Strong nucleophiles such as Co complexes and soft nucleophiles such as R3P, RS and RSe undergo. 5, 2 reactions with alkyl-Co complexes (the reverse process of Scheme 100, i), while strong bases such as OH" cause an E2 reaction with jS-cyano-, -hydroxy- or -alkoxyethyl-Co complexes (reverse process of Scheme 100, ii). Redox reactions also cause homolytic cleavage of the Co—C bond. For example [Ir Cl p" and Au salts generate alkyl radicals and a Co species by outer sphere single electron oxidation (Scheme 102). 

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