5- Ethyl-l-methylcyclohexene

Let us now consider the analogous hydroborations/oxidations of chiral derivatives of 1-methylcyclohexene, namely of racemic 3-ethyl-l-methylcyclohexene (Figure 3.26) and of enantiomerically pure a-pinene (Figure 3.27). 

During the addition of a racemic chiral dialkylborane to a racemic chiral alkene a maximum of four diastereomeric racemic trialkylboranes can be produced. Figure 3.31 illustrates this using the example of the hydroboration of 3-ethyl-l-methylcyclohexene with the cyclic borane from Figure 3.30. This hydroboration, however, has not been carried out experimentally. This should not prevent us from considering what would happen if it were performed. 

At the beginning of Section 3.4, we wondered whether 3-ethyl-l-methylcyclohexene could also be hydroborated/oxidized/hydrolyzed to furnish the cis, trans-con 11 gured alcohol. There is a solution (Figure 3.32) if two requirements are fulfilled. First, we must rely on the assumption made in Section 3.4.3 that this atkene reacts with the cyclic borane in such a way that the reagent control of stereoselectivity exceeds the substrate control of the stereoselectivity. Second, both the alkene and the borane must be used in enantiomerically pure form. 

Figure 3.32 showed the reaction of our enantiomerically pure chiral cyclic dialkylborane with (Vi )-3-ethyl- l-methylcyclohexene. ft took place relatively slowly with the rate constant k6 The reaction of the same dialkylborane with the isomeric. S -alkene was shown in Figure 3.33. ft took place considerably faster with the rate constant ky The combination of the two reactions is shown in Figure 3.34. There the same enantiomerically pure borane is reacted simultaneously with both alkene enantiomers (i.e., the racemate). What is happening In the first moment of the reaction the R- and the 5-alkene react in the ratio k6 (small )/ 5 (big). The matched pair thus reacts faster than the mismatched pair. This means that at low conversions (< 50%) the trialkylborane produced is essentially derived from the 5-alkene only, ft has the stereostructure E. Therefore, relative to the main by-product F, compound E is produced...

BBN attacks the C=C double bond of 3-ethyl-l-methylcyclohexene according to Figure 3.20 exclusively from the side that lies opposite the ethyl group at the stereocenter. Consequently, after oxidation and hydrolysis, a fra s,fra s-configured alcohol is produced. The question that arises is Can this diastereoselectivity be reversed in favor of the cis,trans isomer The answer is possibly, but, if so, only by using reagent control of stereoselectivity (cf. Section 3.4.4). 

The faces on both sides of a C=C, C=0, or C=N double bond can have different geometrical relationships to each other (Figure 3.23). If they can be converted into each other by means of a rotation of 180° about a two-fold intramolecular axis of rotation, then they are homotopic. If they cannot be superimposed on each other in this way, they are stereoheterotopic. Stereoheterotopic faces of a double bond are enan-tiotopic if they can be related to each other by a reflection otherwise they are di-astereotopic. Let us illustrate this with examples. The faces of the C=C double bond of cyclohexene are homotopic, those in 1-methylcyclohexene are enantiotopic, and those in 3-ethyl-l-methylcyclohexene are diastereotopic. 

Nonafluoro-l-methylcyclohexene can be oxidized with potassium permanganate to give 2,2,3.3,4.4,5,5-octafluoro-6-oxoheptanoic acid,38 while pcrfluoro(1-ethyl-6-isopropyleyclo-hexene) yields perfluoro(2-methyldecane-3,8-dione).31)... 

A quantitative comparison of metals in the hydrogenation of vinyl ethers has been made, The extent of hydrogenolysis in hydrogenation of l-ethoxy-3-methylcyclohexene decreased in the order Pt Os > Rh Ir > Pd > Ru U24e)-, in the case of ethyl 4-methyl-1-cyclohexenyl ether, the order was Pt Ir > Rh > Os Ru Pd (124d). In ethanol, ketal formation is a... 

A further example of the use of 2//-thiopyrans as surrogates for m-substituted dienes involves the use of the protected 3,4-dihydio-3-(3-oxobutyl)A//-thiopyranA-onc, 3-[2-(2-methyl-l,3-dioxolan-2-yl)ethyl]-4-[tris(l-methy-lethyl)silyl)oxy-2//-thiopyran 328 as an equivalent of l-ethenyl-2-methylcyclohexene in Diels-Alder reactions. The thiopyran reacted with various maleimides to yield the endo cycloadducts and with methyl propenoate to give the exo adduct under either thermal or Lewis-acid-catalyzed conditions. In the latter case concomitant release of the protected ketone functions occurs, acid-catalyzed cyclization of which generates a fused cyclohexenone ring (Scheme 67). Desulfurization, preferably before the aldol cyclization, leads to derivatives of 2,3,4,4a,5,6,7,8-octahy-dro-4a-methylnaphthalenes < 1997CJC681 >. 

High catalytic activities, with turnovers of up to 9(X) cycles min , is displayed in the transfer hydrogenation of a,p-unsaturated ketones, such as benzylideneacetone and chalcone, using 2-propanol and catalytic amounts of [Ir(3,4,7,8-Me4-phen)COD]Cl (phen = 1,10-phenanthroline COD = 1,5-cyclo-octadiene) in a weakly alkaline medium. Other Ir-chelated complexes are also active catalysts in this reaction, with over 95% selectivity for the 1,4-reduction mode. Divalent lanthanide derivatives, such as Sml2 or Ybh in stoichiometric quantities, in THF and t-butyl alcohol or methanol reduce ethyl cinnamate and cinnamic acid to give the saturated derivatives. " Similarly, 3-methylcyclohexenone is reduced to 3-methylcyclohexen-l-ol in 67% yield, but a,p-unsaturated aldehydes are nonselectively reduced with these systems. 

Pentamethylene dibromide allowed to react with Mg in abs. tetrahydrofuran under argon, stirring continued 2 hrs. at room temp., ethyl acetate-[1- G] in tetrahydrofuran added dropwise, stirred 12 hrs. and allowed to stand 24 hrs. at ca. 20° 1-methylcyclohexanol-fl- G] (Y 77% purity 87%) refluxed 3 hrs. with pentachlorophenol j -methylcyclohexene-l- G (Y 90% containing ca. 18% startg. m.) passed repeatedly at 350° through a Pt-on-Al203 catalyst until Hg-elimination ceases -> toluene-[1- G] (Y 84-87% purity 97-99%). A. Rieker, K. Scheffler, and E. Muller, A. 670, 23 (1963). 

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