Lewis and protic acid mediated Nicholas reactions of 3-acetoxycyclohept-1-en-4-ynedicobalt hexacarbonyl: site selectivity of nucleophile incorporation

Abstract

Nicholas reactions on the cation derived from the cyclic allylic acetate alkynedicobalt complex 1 favour the γ-site kinetically for most nucleophiles, with increasing amounts of αproducts in cases with greater nucleophilicity. Some regiocontrol in introduction of a specific nucleophilic fragment is possible by using different nucleophiles. Under conditions where reversibility is possible, the thermodynamically favoured site is exclusively γ- .

Generation and Reactions of a Benzodehydrotropylium Ion–Co2(CO)6 Complex

“…In previous work, Nicholas reactions of cations that are allylic in addition to being propargylic to an alkyne–Co 2 (CO) 6 group have been shown to favor attacks at the γ-site [i.e., the terminus remote to the alkyne–Co 2 (CO) 6 unit] over the α-site for the majority of nucleophiles (with a greater selectivity for lower reactivity nucleophiles), for reasons that are not entirely well-understood. This observation has been found to apply to both acyclic and cyclic systems . Here, this situation is much less straightforward, as the γ-site of 3b is benzylic, whereas the α-site is substantially more hindered than in the literature cases.…”

Diversity‐Oriented Synthetic Endeavors of Newly Designed Ferrier and Ferrier‐Nicholas Systems Derived from 1‐C‐Alkynyl‐2‐deoxy‐2‐C‐Methylene Pyranosides

“…In summary, the incorporation of two nucleophiles to the Ferrier‐Nicholas pyranosidic system 15 to generate the bis‐functionalized compounds 62 – 65 , must involve the sequential formation of two different vinylogous Nicholas cations (cations generated from dicobalt complexes arising from propargyl‐allyl ether derivatives),, e.g. A , B , (Scheme ), each one reacting at the γ‐ rather than at the α‐position, with the external nucleophiles to provide bis‐functionalized species C (Scheme ) …”

A Substrate‐Based Approach to Skeletal Diversity from Dicobalt Hexacarbonyl (C1)‐Alkynyl Glycals by Exploiting Its Combined Ferrier–Nicholas Reactivity

“…Some studies dealing with the influence of the nucleophile in its site‐selective incorporation to allylic/propargylic Nicholas‐type cations have appeared 45. 46 In particular, reports from Green’s group on cyclic allylic/propargylic cations have shown that even though the site remote to the alkyne‐Co 2 (CO) 6 moiety (γ‐site, C‐3 substitution in our study) was the kinetically preferred one for most nucleophiles, increasing amounts of α‐products (C‐1 derivatives) were obtained with more nucleophilic reagents 46. Accordingly, the higher nucleophilicity of TMSCN (along with its small size)47 will be responsible for the observed regioselectivity in the formation of 33 .…”