Cascade reactions hydride termination

Rhodium carbonyl complexes also catalyze the cascade cyclization/hydrosilylation of 6-dodecene-l,l 1-diynes to form silylated tethered 2,2 -dimethylenebicyclopentanes. For example, reaction of ( )-85 with dimethylphenylsilane catalyzed by Rh(acac)(CO)2 in toluene at 50 °G under GO (1 atm) gave 86a in 55% yield as a single diastereomer (Equation (56)). Rhodium-catalyzed caseade cyclization/hydrosilylation of enediynes was stereospecific, and reaction of (Z)-85 under the conditions noted above gave 86b in 50% yield as a single diastereomer (Equation (57)). Rhodium(i)-catalyzed cascade cyclization/hydrosilylation of 6-dodecene-1,11-diynes was proposed to occur via silyl-metallation of one of the terminal G=G bonds of the enediyne with a silyl-Rh(iii) hydride complex, followed by two sequential intramolecular carbometallations and G-H reductive elimination. ... 

Terpene synthases, also known as terpene cyclases because most of their products are cyclic, utilize a carbocationic reaction mechanism very similar to that employed by the prenyltransferases. Numerous experiments with inhibitors, substrate analogues and chemical model systems (Croteau, 1987 Cane, 1990, 1998) have revealed that the reaction usually begins with the divalent metal ion-assisted cleavage of the diphosphate moiety (Fig. 5.6). The resulting allylic carbocation may then cyclize by addition of the resonance-stabilized cationic centre to one of the other carbon-carbon double bonds in the substrate. The cyclization is followed by a series of rearrangements that may include hydride shifts, alkyl shifts, deprotonation, reprotonation and additional cyclizations, all mediated through enzyme-bound carbocationic intermed iates. The reaction cascade terminates by deprotonation of the cation to an olefin or capture by a nucleophile, such as water. Since the native substrates of terpene synthases are all configured with trans (E) double bonds, they are unable to cyclize directly to many of the carbon skeletons found in nature. In such cases, the cyclization process is preceded by isomerization of the initial carbocation to an intermediate capable of cyclization. 

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