Transition metalo carbenoid intermediate

Cascades Relying on Transition Metalo Carbenoid Intermediates. 274... 

Abstract This review gives an insight into the growing field of transition metal-catalyzed cascades. More particularly, we have focused on the construction of complex molecules from acyclic precursors. Several approaches have been devised. We have not covered palladium-mediated cyclizations, multiple Heck reactions, or ruthenium-catalyzed metathesis reactions because they are discussed in others chapters of this book. This manuscript is composed of two main parts. In the first part, we emphasize cascade sequences involving cycloaddition, cycloisomerization, or ene-type reactions. Most of these reaction sequences involve a transition metal-catalyzed step that is either followed by another reaction promoted by the same catalyst or by a purely thermal reaction. A simple change in the temperature of the reaction mixture is often the only technical requirement to go from one step to another. The second part covers the cascades relying on transition metalo carbenoid intermediates, which have recently undergone tremendous... 

While most of the initial studies have involved the transition metal-catalyzed decomposition of a-carbonyl diazo compounds and have been reviewed [3-51], it appears appropriate to highlight again some milestones of these transformations, since polycyclic structures could be nicely assembled from acyclic precursors in a single step. Two main reactivities of metalo carbenoids derived from a-carbonyl diazo precursors, namely addition to a C - C insaturation (olefin or alkyne) and formation of a ylid (carbonyl or onium), have been the source of fruitful cascades. Both of these are illustrated in Scheme 27 [52]. The two diazo ketone functions present in the same substrate 57 and under the action of the same catalyst react in two distinct ways. The initially formed carbenoid adds to a pending olefin to form a bi-cyclop. 1.0] intermediate 58 that subsequently cyclizes to produce a carbonyl ylide 59, that is further trapped intramolecularly in a [3 + 2] cycloaddition. The overall process gives birth to a highly complex pentacyclic structure 60. 

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