Carbocyclizations migratory insertion

Rhodium-catalyzed, silane-initiated cascade cyclization of 1,6,11-triynes 83 was proposed to occur via a silane-initiated cascade carbocyclization to form the silylated bicyclic triene (Z,Z)-In. / -Migratory insertion of the silylated G=G bond into the Rh-G bond of (Z,Z)-In followed by / -hydride elimination from frans-lln could then form 84a. Alternatively, cisitrans isomerization of (Z,Z)-In followed by / -migratory insertion of the silylated G=G bond into the Rh-G bond of resulting isomer ( ,Z)-In could form cis-Wn. Subsequent / -silyl elimination from m-IIn would form unsilylated tricycle 84b (Scheme 21). 

All these protocols allow us to form a new carbocycle in a bimolecular process. The cyclization involving two different molecules besides CO has been realized, involving carbapalladation of norbornene, migratory insertion of CO, and subsequent intramolecular Heck-like attack at thiophene residue. Thallium acetate is required as electrophilic co-catalyst (Scheme 18). ... 

A proposed mechanism for this reaction is shown in (Scheme 24). Insertion of the less hindered olefin moiety of a diene into the [Y]-H bond forms G-I. Car-bocyclization of G-I gives G-II, and the subsequent cr-bond metathesis with a hydrosilane yields the product and regenerates Cp 2YH THF. The observed high catalyst activity of the Cp 2YMe and Cp 2LuMe complexes relative to Ni(0) and Rh(I) complexes for this carbocyclization is ascribed to the Lewis acidity of the metal center and the presence of an open coordination site. These features favor both /3-migratory insertion and a-bond metathesis over oxidative addition and reductive elimination processes that are preferred in the Ni(0) and Rh(I) catalyst systems. 

Scheme 97) illustrates the proposed mechanism of this novel process. This reaction is believed to proceed through a series of metallacycles. Thus, it is clearly different from that of the CO-SiCaT reaction, which is a stepwise process involving sequential carbocyclizations. The proposed mechanism includes (i) selective coordination of the diyne moiety of enediyne 216 to the active Rh catalyst species, forming metallacycle N-I [2-1-2-l-M]) (ii) insertion of the olefin moiety of 216 into the Rh-C bond forms the fused 5-7-5 tricyclic rhodacycle N-II ([2-I-2-I-2-I-M]) (iii) coordination of CO to the Rh metal followed by migratory insertion of CO into the Rh—C bond gives 5-8-5 rhodacycle N-III or N-IV ([2-I-2-I-2-I-1-I-M]) and (iv)... 

VCPs have served as valuable five-carbon components in various cycloaddition reactions. Usually, they form six-membered metallacycles upon oxidative cycliza-tion with transition metals. Then, migratory insertion of unsaturated molecules, followed by reductive elimination, furnishes the carbocycles. In particular, intermolecular [5-1-2] annulation of VCPs with alkynes, alkenes, and allenes has been studied extensively (Scheme 2.39) [57]. In addition, VCP-cyclopentene rearrangement has been well documented [56 ]. 

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