Synthetic applications of metallate rearrangements

Abstract

Abstract

Transition Metal‐Catalyzed Carbon−Carbon Bond Formation with Grignard Reagents − Novel Reactions with a Classic Reagent

“…[14] This process has been utilized successfully in the alkylation of α-haloalkylmetals (e.g., M ϭ Mg, [15] Zn, [16] B, [17] or Cu [18] ) and allows the facile introduction of an alkyl group to the organometallic reagents. This process enables complex metallic reagents to be prepared from relatively simple and easily accessible organometallic species.…”

Tandem Insertion of Halocarbenoids and Lithium Acetylides into Zirconacycles: A Novel Rearrangement to Zirconium Alkenylidenates by β‐Addition to an Alkynyl Zirconocene

“…The first proposed step is the insertion of a 1,1-dihalo-1-lithio species into 10 to form the ring-expanded six-membered zirconacycle 16 via 1,2-metallate rearrangement of 15. [9,14] After addition of lithiated phenylacetylide to afford the ate complex 17 ring closure occurs by 1,2-bond migration to give intermediate 18. From previous work [15] we know that the 1,2-rearrangement of the neutral intermediate 16 is slow at À78 8C, hence the requirement for the formation of 17 before rearrangement (Scheme 4).…”

“…[10] Since then a wide variety of carbenoids have been used, [11] the regiochemistry of insertion into unsymmetric zirconacycles studied, [12] and the chemistry applied to natural product synthesis. [13] A reasonable mechanism for the carbenoid insertion is by initial donation of an electron pair to the 16-electron zirconium center to form an 18-electron zirconate complex 2 followed by a 1,2-metallate rearrangement [14] to give ring-expanded zirconacycle 3. A special case of carbenoid insertion into zirconacycles is that of 1-lithio-1,1-dihalo species, in which the zirconacycle ring expansion is followed by a ring closure with loss of a second halide (Scheme 2).…”

Lithium Carbenoids

“…The electrophilic nature of α-lithiated cyclic unsaturated ethers, in particular dihydrofuran 106 and dihydropyran 108 which are readily available by deprotonation, becomes evident from the reaction with alkyllithium compounds: In a nucleophilic substitution, the alkyl group R is introduced under complete inversion of configuration at the carbenoid center to give the ring-opened products 107 and 109. After protonation, they serve as useful intermediates in natural product synthesis (equation 58) 261,355 . The yield in this reaction can be improved substantially, when the ring opening is catalyzed by CuCN.…”

“…When warmed up to temperatures higher than −40 • C, a Fritsch-Buttenberg-Wiechell rearrangement takes place to give the alkyne 112 (equation 60). Below that temperature, the lithium compound 111 maintains its nucleophilic reactivity 355 . …”