Unfunctionalized alkenes, stereoselective

Allylic alcohols also cyclopropanate over 100 times faster than their unfunctionalized alkene equivalents. Coordination between the zinc atom and the hydroxyl group in the transition state explains both the stereoselectivity and the rate increase. Unfortunately, while the Simmons-Smith... 

The major drawback of alkene cross-metathesis is the limited ability to control the chemo- and stereoselectivity of the reaction. High yields of the cross-product are obtained by either stoichiometric control or by the use of functionalized alkenes. When unfunctionalized alkenes are used in the reaction, an excess of one of the alkenes must be used in order to get a synthetically useful yield of the... 

Unfunctionalized alkene usually reacts by preferential syn delivery of hydrogen from the less hindered face of the double bond. The degree of stereoselectivity is dependent on the reactant structure, catalysts and reaction conditions. Donor functional groups, particularly hydroxy and amino can be syn directive. 

Diaminocyclohexane [(R,R)- and ( S, S)-enantiomer] forms an imine (SCHIFF base) with 2,5-di-/ rr-butylsalicylaldehyde, which gives a chiral Mn(III) (salen) complex with Mn(II)acetate and oxygen. In contrast to the Sharpless-Katsuki protocol (p 20), this complex effects the stereoselective oxygen transfer (from oxidants, e.g. monopersulfate or NMO) to unfunctionalized alkenes (Jacobsen epoxidation [1], extended by Katsuki [2]) giving rise to enantiomeric oxiranes with 90-98% ee. 

Guided by Marks s report of the samarium-catalyzed hydroboration of alkenes, Molander has developed a samarium-catalyzed protocol for the cyclization/hydroboration of unfunctionalized 1,6-dienes. In an optimized procedure, reaction of 1,5-hexadiene and l,3-dimethyl-l,3-diaza-2-boracyclopentane catalyzed by Gp 2Sm(THF) in toluene at room temperature for 18 h followed by oxidation gave hydroxymethylcyclopentane in 86% yield (Equation (70) R = H, n — ). The transformation was stereoselective, and Sm-catalyzed cyclization/hydroboration of 2-phenyl-1,5-hexadiene followed by oxidation formed /ra/ i--l-hydroxymethyl-2-phenylcyclopentane in 64% yield (Equation (70) R = Ph, n = ). The samarium-catalyzed reactions was also applicable to the synthesis of hydroxymethylcyclohexanes (Equation (70), n=X) but tolerated neither polar functionality nor substitution on the alkenyl carbon atoms. 

---