Hydrogenation cyclohexenes, stereoselective

Similarly to peroxycarboxylic acids, DMDO is subject to cis or syn stereoselectivity by hydroxy and other hydrogen-bonding functional groups.93 However a study of several substituted cyclohexenes in CH3CN —H20 suggested a dominance by steric effects. In particular, the hydroxy groups in cyclohex-2-enol and... 

These types of catalysts, [Cp LnCH(SiMe3)2], are also used to hydrogenate substituted methylenecyclopentenes and cyclohexenes in good to very good dia-stereoselectivities, especially when the substituent is in the a-position to the al-kene (Tables 6.10 and 6.11). However, the presence of functional groups such as amine or ether is detrimental to catalysis. 

Some rhodium complexes are more promising in the hydrogenation of arenes. The pentamethylcyclopentadienyl rhodium complex [ RhCUCi -CsMes) ], for instance, is stereoselective in the reduction of CgD6, yielding the all-m product almost exclusively, without the formation of cyclohexene.179... 

In the hydroboration/oxidation reaction of 2-cyclohexen-l-ol derivatives the regio- as well as the stereoselectivities can be steered in either of two directions by use of different reagents and reaction conditions. Dialkylboranes, e.g., 9-BBN, give mainly tn rs-l,2-diols, whereas with 1,3,2-benzodioxaborole ( catecholborane") in the presence of the rhodium hydrogenation catalyst Rh(PPh3)jCl the trans-1,3-diols become predominant. The cis- 1,2-diols are usually only formed in trace amounts, but the cis-l,3-diols are always produced as byproducts in 10 — 20% yield (D.A. Evans, 1988). 

The stereoselective fram-diazidation occurred with cyclohexene to give fram-l,2-diazidocyclo-hexane (1), hydrogenation of the adduct 1 on platinum dioxide gives pure tram- 1,2-cyclohex-anediamine 28. The same method was applied to the preparation of irans-(5a,6/ )-diazido-steroids, for example, 38. Benzonorbornene underwent cxo-cw-addition73. 

A comparison of the proportion of the saturated stereoisomers, formed under the same conditions (Pt02, AcOH) from the isomeric xylenes and the derived dimethylcyclohexenes, leads to the proposal that the reaction proceeds through the desorbed cyclohexenes. Low concentrations of the intermediate cyclohexenes were detected later. The effect of the metal on the stereoselectivity of hydrogenating o-xylene follows closely the effect of the metal on the hydrogenation of 1,2- and 1,6-dimethylcyclo-hexene. The highest selectivity for the conversion of o-xylene to the cis isomer is given by iridium... 

Stereoselectivity in cyclic allylic alcohols depends on the ring size and on the reagent used. 2-Cyclohexen-l-ol, on treatment with peroxybenzoic acid, gives a cis epoxide, probably because of the hydrogen bond of the hydroxyl hydrogen in the transition state. If the acetate of the alcohol is treated with peroxybenzoic acid, the trans epoxide predominates over the cis epoxide in a ratio of 4 1 (equation 276) [298]. 

Carbohydrate-derived enolones are a particular case of chiral cyclohexenes. Addition of tert-butyl radicals occurs with high stereoselectivity trans to the C-5 substituent at both x- and /f-anomers8I5. The primary adduct radical rearranges through transfer of the benzoyl group and final hydrogen atom abstraction to yield the product as a mixture of isomers at C-2. Treatment of the mixture with pyridine leads to the formation of one enolone wdth elimination of benzoic acid. 

Chloro-l,3-dienes are stereoselectively formed via c/.v-halocarborative addition of al-kenes to alkynes in the presence of catalytic amounts of palladium(II) chloride, lithium chloride and oxygen46. The reaction proceeds via cis chloropalladation of the alkyne and c/.v-orientated alkene insertion, followed by /(-hydrogen elimination. Alternatively, 1.4-dienes are obtained if, for steric reasons, a. nn-periplanar /(-hydrogen atom is not available to form the conjugated diene (e.g., after m-orientated insertion of cyclohexene)46. 

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