Acyclic functionalized alkenes

Nelson, D.J., Li, R. and Brammer, C. (2001) Correlation of relative rates of PdCh oxidation of functionalized acyclic alkenes versus alkene ionization potentials, HOMOs, and LUMOs. J. Am. Chem. Soc.r 123, 1564. 

It was recognized early that efficient olefin cross metathesis could provide new methods for the synthesis of complex molecules. However, neither (la) nor (2a) were very effective at intermolecular cross metathesis owing to poor reaction selectivity (cross vs. intramolecular metathesis) and low E.Z ratios see (E) (Z) Isomers) The advent of more active and functional group tolerant olefin metathesis catalysts recently made cross metathesis a viable route for constructing a large variety of functionalized acyclic alkenes. 

Arynes also undergo transition metal-catalyzed cotrimerization with appropriately functionalized acyclic alkenes. In the presence of a catalytic amount of a palladium-phosphine complex, one molecule of the electrophilic alkenes 231 substituted with either one or two electron-withdrawing groups undergo cotrimerization with two molecules of aryne to form a mixture of dihydrophenanthrene 232 and ortho-alkenyl biaryls 233 (Scheme 12.64) [127]. 

The hydrogenation is usually limited to nonpolar alkenes (terminal and internal cyclic and acyclic alkenes), even though Ti systems have been used to hydrogenate alkenes containing ether and ester functionalities such as vinyl ethers or methyl oleate [42, 45, 59, 62]. 

The most selective - and also most general - titanocene catalyst is complex 35 d, also studied by Buchwald and coworkers. This catalyst was used to hydrogenate a variety of functionalized and unfunctionalized cyclic and acyclic alkenes with excellent ee-values in most cases [46]. Enamines could also be hydrogenated with enantiomeric excesses of 80-90% [47]. However, high catalyst loadings (5-8 mol%) and long reaction times were required to drive the reactions to completion. 

The insertion reaction is stereospecific and syn. Moreover the /S-hydride elimination is also syn. For acyclic alkenes there is free rotation in the organopalladium intermediate so that the more stable /ra .v-alkene is formed. Electron-withdrawing groups in the alkene also increase the rate of the insertion reaction and give higher yields generally, but the reaction is limited to relatively sterically unhindered alkenes. In general, polar solvents such as DMF or acetonitrile are most commonly used. There are several common additives which aid in the reaction. These include lithium or tetraalkylammonium chlorides and bromide, silver salts, or cuprous iodide, but exactly how they function is unknown at present. 

The temporary functionalization of cyclic or acyclic alkenes with a phenylsulfonyl group, followed by [4 -t- 2] cycloadditions of 1,3-bis-oxygenated dienes, opens a regiocontrolled route to annulated or 5-substituted cyclohexenones (Scheme 18). ... 

Related molybdenum catalysts appear to show even more functional group tolerance. To date, the major test of functional group compatibility has been in the synthesis of polymers however, it is anticipated that this activity will persist into acyclic metathesis. Later transition metals are active in the metathesis polymerization of highly functiondized cyclic alkenes. These catalyst systems, which appear to tolerate almost all functional groups, show very low activity for acyclic alkene metathesis. If these systems can be activated, the problems associated with the use of alkene metathesis in the synthesis of multifunctional organics will be solved. 

BBN-H tolerates many functional groups, and this, coupled with its high regioselectivity, allows the clean synthesis of a number of functionalized organoboranes e.g. equation 26), including many derived from unsaturated heterocyclic compounds. It also shows impressive stereofacial selectivity in the hydroboration of cyclic alkenes (e.g. equations 27-29), - and sometimes in the cases of acyclic alkenes. ... 

The functionally related stereospecific fragmentation of P-stannyl and P-silyl cyclic ketones to acyclic alkenes under oxidative conditions has been observed previously, see K. Nakatani and S. Isoe, Tetrahedron Lett., 1984, 25, 5335 M. Ochiai, T. Ukita, Y. Nagao and E. Fujita, J. Chem. Soc., Chem. Common., 1985, 637 S. R. Wilson, P. A. Zucker, C. Kim and C. A. Villa, Tetrahedron Lett., 1985, 26,1969. 

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