Allylic substitution defined

Fig. 1.28. Derivation of the kinetic expression for the chemoselectivity of allylic substitution versus bromine addition in the Br /Br2/cyclohex-ene system. The rate constants are defined as in Figure 1.27.

Summary Two catalytic reactions, i.e. silylative coupling (mms-silylation) (SC) catalyzed by complexes containing or generating Ru-H and/or Ru-Si bonds (I, II, V, VI) and cross-metathesis (CM) catalyzed by mthenium-carbene (i.e. 1st and 2nd generation mthenium Grubbs catalyst (ID, IV)) of vinyl and allyl-substituted hetero(N,S,B)organic compounds with conunercially available vinyltrisubstituted silanes, siloxanes, and silsesquioxane have been overviewed. They provide a universal route toward the synthesis of well-defined molecular compounds with vinylsilicon functionality. 

A valuable feature of the Nin/Crn-mediated Nozaki-Takai-Hiyama-Kishi coupling of vinyl iodides and aldehydes is that the stereochemistry of the vinyl iodide partner is reflected in the allylic alcohol coupling product, at least when disubstituted or trans tri-substituted vinyl iodides are employed.68 It is, therefore, imperative that the trans vinyl iodide stereochemistry in 159 be rigorously defined. Of the various ways in which this objective could be achieved, a regioselective syn addition of the Zr-H bond of Schwartz s reagent (Cp2ZrHCl) to the alkyne function in 165, followed by exposure of the resulting vinylzirconium species to iodine, seemed to constitute a distinctly direct solution to this important problem. Alkyne 165 could conceivably be derived in short order from compound 166, the projected product of an asymmetric crotylboration of achiral aldehyde 168. 

Le Carrer-Le Goff and co-workers have reported an expeditious synthetic route to a-substituted statines by the well-defined metal-mediated allylation of V-protected a-amino aldehydes followed by the ozonolysis of the double bond (Scheme 11.3).16 Ozonolysis of the double bond in 13, carried out in dichloromethane/methanol, at -78°C in the presence of sodium hydroxide permits the intermediate ozonide to be converted directly to the statine methyl ester 14.17... 

Coupling reactions between allylic halides and alkynylcoppers have been rather heavily investigated as routes to stereochemically defined polyenyl natural products. The CuCl-catalyzed reaction of acetylenic Grignard reagents with chlorine substituted allylic chlorides has been employed in the synthesis of stereochemically pure 1,3,5-undecatrienes... 

Many substituted benzyl and benzoyl esters were considered along with some alkyl esters. After screening, the choice came down to either allyl or 4-nitrobenzyl ester, both of which can be removed by well-defined methodology. The decision was made to use 4-nitrobenzyl ester for the commercial synthesis because of the crystalline properties it conferred on the isolated intermediates. 

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