ORGANIC-REACTIVITY CONTROL BY MEANS OF NEIGHBORING GROUPS AND ORGANOMETALLICS - A PERSONAL ACCOUNT

This review summarizes the main topic of our research and covers the p eriod of the last 15 years. The prime interest is focused on various w ays of controlling the regio- and stereoselectivity of selected organi c reactions, in particular electrophilic additions, cleavage of cyclop ropane rings, and allylic substitutions by means of neighboring groups and/or transition and non-transition metals. In the first part, the f actors governing the course of electrophilic additions are assessed, c ulminating in the formulation of selection rules for the reactivity of cyclohexene systems, and in a concise synthesis of the natural cardio active drug, strophanthidin. These studies also contribute to a better understanding of the mechanisms of electrophilic additions. The secon d part describes recent developments in the stereo- and regiocontrolle d cleavage of cyclopropane rings by non-transition metals (Tl and Hg), and the reactivity and transmetalation (with Pd) of the primary produ cts. This methodology has resulted in novel routes to unique polycycli c structures, and will have synthetic applications in the near future. Evidence for the stereospecific ''corner'' cleavage of the cyclopropa ne ring has been provided for the first time for Tl and later for Hg. The third part deals with transition metal-catalyzed allylic substitut ion. Evidence for a new ''syn'' mechanism for the formation of the int ermediate (pi-allyl)palladium complex has been provided, which runs co unter to the generally accepted ''anti'' mechanism. A novel method for a Pd-catalyzed allylic oxidation has been developed and employed in t he synthesis of natural sesquiterpenes. The increasing importance of t ransition and non-transition metals for synthetic organic chemistry is demonstrated by their unique reactivity in a number of the papers inc luded in this review.