Transition metal catalyzed alkyne substrates catalysts

In an interesting contrast to the analogous silicon and germanium systems, catalytic addition of the unactivated hexamethyldistannane to unsaturated substrate readily occurs with the common palladium catalysts Pd(PPh3)4 or Pd(dba)2. In fact, prior to 1991 this was the only distannane investigated for addition reactions. The first example of transition metal-catalyzed double stannylation of unsaturated molecules, palladium-catalyzed cis addition of hexamethyldistannane to 1-alkynes, was reported in 1983 by Mitchell and co-workers.66 More recently, the system has been extended to include other substrates and both the hexaethyl- and the hexabutyldistannanes. All examples reported involve the use of a Pd(0) complex as the catalytic species. 

Transition metal-catalyzed [2 + 2 + 2] cocyclization of two molecules of an alkyne with an alkene is a powerful method for forming 1,3-cyclohexa-dienes [29-31]. These compounds are of course valuable partners for Diels-Alder reactions [32]. Through the right choice of substrates, both [2 + 2 + 2] and [4 + 2] cycloadditions can be performed in a single chemical operation [33]. Indeed, the reaction of electroneutral diyne 14 with electron-deficient maleimide 15 in the presence of lmol% of a Ru(I) catalyst exclusively afforded the highly symmetrical 1 2 adduct 17 in 74% yield (Scheme 9). 

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. 

The thermal Diels-Alder reaction of electron-rich 1,3-dienes with electron-deficient alkynes affords the corresponding cyclohexa-1,4-dienes, aromatization of which gives the corresponding benzenes. As the thermal Diels-Alder reaction proceeds in a concerted fashion that is best described by the Woodward-Hoffmann rules, it is difficult to use electronically neutral substrates. On the other hand, as the transition metal-catalyzed Diels-Alder reaction may proceed in a stepwise fashion, electronically neutral substrates can be employed. For the transition metal-catalyzed Diels-Alder reaction, cobalt-based complexes are the most frequently employed catalysts [38], although some rhodium-based complexes have also been used [39]. 

The transition-metal-catalyzed intramolecular hydroarylation of alkynes has emerged in recent years as a powerful tool for the atom-economical synthesis of carbocyclic and heterocyclic compotmds [101-104], including coiunarin derivatives [105-109]. In this context, Kitamura and coworkers have described the high-yield preparation of several angelicin derivatives 50 by intramolecular hydroarylation of 4-benzofuranyl alkynoates 49 using Pd(OAc)2 as catalyst (Scheme 23) [110]. The starting materials 49 are easily accessible substrates that can be generated by the condensation of... 

Silylformylation, defined as the addition of RsSi- and -CHO across various types of bonds using a silane R3SiH, CO, and a transition metal catalyst, was discovered by Murai and co-workers, who developed the Co2(CO)8-catalyzed silylformylation of aldehydes, epoxides, and cyclic ethers [26]. More recently, as described in detail in Section 5.3.1, below, alkynes and alkenes have been successfully developed as silylformylation substrates. These reactions represent a powerful variation on hydroformylation, in that a C-Si bond is produced instead of a C-H bond. Given that C-Si groups are subject to, among other reactions, oxidation to C-OH groups, silylformylation could represent an oxidative carbonylation of the type described in Scheme 5.1. 

A drawback of the copper(l)-catalyzed variant of this reaction is the presence of the transition metal, which is potentially toxic for hving cells. Therefore, ring-strained cyclooctyne derivatives have been introduced as substrates for the azide-alkyne cycloaddition whose reaction with the azide does not require a transition metal catalyst (Figure 16). The driving force of the corresponding copper-free click reaction stems for the release of ring strain associated with the conversion of the cyclooctyne s triple bond into a double bond. Fluorine substituents make the system even more reactive. ... 

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