Cross-metathesis with Allylic Derivatives

This category of coupling usually involves two type I olefins, and proceeds in good yield but with variable stereoselectivity. 

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A subsequent publication by Blechert and co-workers demonstrated that the molybdenum alkylidene 3 and the ruthenium benzylidene 17 were also active catalysts for ring-opening cross-metathesis reactions [50]. Norbornene and 7-oxanorbornene derivatives underwent selective ring-opening cross-metathesis with a variety of terminal acyclic alkenes including acrylonitrile, an allylsilane, an allyl stannane and allyl cyanide (for example Eq. 34). 

Cross-Metathesis vs Silylative Coupling of Vinylsilane with Allyl-substituted Hetero(N,S>B)oi g nic Derivatives... 

Vinylsilanes undergo productive cross-metathesis (CM) and silylative coupling (SC) with allyl-substituted (N, B)functionalized alkenes to yield l-silyl-3,Ar, -substituted propenes with preference (for V-derivatives) and exclusive formation (for boronates) of the f-isomer. 

Isomerization of the C-allyl glycoside 302 provides the corresponding C-(l-propenyl) derivative 303 as a major E-isomer. Cross metathesis proceeds in high yield only with the Grubbs II catalyst to deliver the homodimer 304, transformed to C-disaccharide analog 305 of the mycobacterial motif. 

On the other hand, although well-defined or in-situ initiated metallacarbenes are inactive for selfmetathesis of vinyl-substituted silanes and siloxanes, we revealed recently a high catalytic activity of Grubbs catalyst in cross-metathesis of vinyltrialkoxysilanes and vinyltrisiloxanes with styrene, 1-alkenes and selected allyl ethers and other derivatives [10-12]. 

While vinylsilanes undergo productive cross-metathesis (Mo and Ru carbenes) with allyl-substituted functionalized alkenes, their effective transformation with derivatives containing a fimctionalized group attached directly to a carbon -carbon double bond can be achieved only via silylative coupling catalyzed by metal complexes containing (or generating) M-H and/or M-Si bonds (M = Ru, Rh, Ir). 

A number of research groups have utilized the strategy outlined above to synthesize benzene ring-containing compounds. For example, Kotha successfully employed it for the synthesis of phenylalanine derivatives [36]. Starting with aUcyne 87 and the aUcene allyl acetate, cross-enyne metathesis afforded diene 88. This readily underwent a Diels-Alder reaction with dimethyl acetylenedicarboxylate (DMAD) to afford cyclohexene 89. Aromatization of these compounds with DDQ subsequently furnished the phenylalanine 90 in good yield (Scheme 17.17). 

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