Alkaline Earth Catalysis of Alkynyl Alcohol Hydroalkoxylation/Cyclization
摘要:
Heavier alkaline earth bis(trimethylsilyl)amides [Ae{N(SiMe3)(2)}(2)](2) (Ae = Ca, Sr, Ba) are shown to act as effective precatalysts for the regioselective intramolecular hydroalkoxylation/cyclization of a wide range of alkynyl and allenyl alcohols. In the majority of cases, cyclization of alkynyl alcohols produces mixtures of the possible endo- and exocyclic enol ether products, rationalized as a consequence of alkynylalkoxide isomerization to the corresponding allene derivatives. Cyclization rates for terminal alkynyl alcohols were found to be significantly higher than for substrates bearing internal alkynyl substituents, while the efficacy of cyclization was in general found to be determined by a combination of stereoelectronic influences and the thermochemical viability of the specific alkaline earth metal catalysis, which we suggest is determined by the individual M-O bond strengths. Kinetic studies have provided a rate law pertaining to a pronounced catalyst inhibition with increasing [substrate], indicating that turnover-limiting insertion of C-C unsaturation into the M-O bond requires the dissociation of substrate molecules away from the Lewis acidic alkaline earth center.
Intramolecular Heck Couplings and Cycloisomerizations of Bromodienes and Enynes with 1′,1′-Disubstituted Methylenecyclopropane Terminators: Efficient Syntheses of [3]Dendralenes
作者:Stefan Bräse、Hanno Wertal nee Nüske、Daniel Frank、Denis Vidović、Armin de Meijere
DOI:10.1002/ejoc.200500330
日期:2005.10
atom-economic, palladium-catalyzed cycloisomerization. The vinylpalladium halide intermediate generated by initial carbopalladation of the 1,6-enyne 10-H with in situ generated phenylpalladium iodide also underwent the same cyclization cascade to yield the correspondingly phenyl-substituted cyclic [3]dendralene 41-Ph (21 %). The palladium-catalyzed cycloisomerization of the alk-1-ene-7,12-diyne 33-H gave the
Carbocyclic ring expansion reactions via free radical pathways - part III
作者:Jack E. Baldwin、Robert M. Adlington、Rajinder Singh
DOI:10.1016/0040-4020(92)85014-6
日期:1992.1
The freeradical ring expansion methodology recently described1, leading to the formation of medium sized ring ketones from cyclohexanone precursors has been applied to different side chains and ring sizes. In addition precursors based on lactone rings have been prepared and subjected to radical forming reaction conditions.