Organometallic chemistry: 1,2-migration to form zirconates, titanates and benzene derivatives, palladium -catalyzed cyclizations, and synthesis of natural products
Abstract
Treatment of zirconacyclopentenes by alkynylmetals results in a Zr-promoted alkyl 1,2-migration on a carbon-carbon triple bond to form a novel organozirconate in a highly chemo, regio and stereoselective manner in high yields. The structure of the zirconate was established as a zirconocene moiety attached to a 1,5-diene moiety by three C-Zr bonds. In addition to the formation of 1,5-dienes and 1,5-iodoenynes, the reactivity of this zirconate was promising: regioselective protonolysis of 2 C-Zr bonds, apparent alkynylzirconation, formation of a novel six membered zirconacarbocycle. Treatment of zirconacyclopentadienes with alkynyllithium results in a novel reaction to prepare regioselectively pentasubstituted benzene derivatives in moderate to high yields. Unsymmetrical zirconacyclopentadienes yielded benzene derivatives with good regioselectivity (from 83:17 to 93:7). Treatment of titanacyclopentenes with alkynylmetals results in a novel reaction to prepare 1,2,3 trisubstituted benzene derivatives regioselectively (≥94:6). The palladium-catalyzed cyclizations are other approaches to form carbon-carbon bonds. The tandem carbopalladation carbonylative esterification of 1,1 disubstituted alkenes gave optimum yields by using DMF, MeOH, and H2O with catalyst activation by oxygen, and the cyclization could be achieved diastereoselectively via 1,4-asymmetric induction. Highly selective carbonylative carbopalladation of alkynes via zipper-mode or dumbbell-mode cascade cyclizations was studied with the termination step achieved via C, N and O nucleophilic trapping. The α-epi-vetivol protected by MOM was synthesized diastereoselectively (>95:5) in 7.3% yield through 9 steps via a diastereoselective carbopalladation cyclization with 1,4-asymmetric induction as the key step. A novel metal-organic approach was developed to synthesize exocyclic alkenes such as Z-γ-bisabolene in a highly stereoselective manner (>98%) using the Ru-catalyzed ring closing metathesis.
Degree
Ph.D.
Advisors
Negishi, Purdue University.
Subject Area
Organic chemistry
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