Frans-Cyclohexene

As previously mentioned, double bonds in relatively small rings must be cis. A stable trans double bond326 first appears in an eight-membered ring (frans-cyclooctene, p. 104), though the transient existence of frans-cyclohexene and cycloheptene has been demonstrated.327 Above about 11 members, the trans isomer is more stable than the cis.160 It has proved possible to prepare compounds in which a trans double bond is shared by two cycloalkene rings (e.g., 86). Such compounds have been called [m. n]betweenanenes, and... 

Both cis and frans-cyclohexene have been synthesized, but only one of them can be isolated. Electrophilic addition of ROH to one isomer occurs spontaneously, while addition to the other isomer occurs only in the presence of a strong acid, such as sulfuric acid. Calculate the energy of protonation for each isomer (cyclohexene —> protonated cyclohexene, trans-cyclohexene —> protonated trans-cyclohexene), and identify the more reactive isomer. Also examine electrostatic potential maps. Suggest an explanation to account for both the reactivity difference and the structural changes. (See also Chapter 7, Problems.)... 

The stability oiadamantene (54) should correspond roughly to that of frans-cyclohexene. While the transient intermediacy of frans-cyclohexene during the photolysis of cyclohexene has been postulated118>, no evidence for the intermediacy oi54 during the photolysis of 1-adamantylacetone is... 

Based on MM2 results, predictions have been made for the structure of frans-cyclohexene (43a) and frans-cycloheptene (39a). rrans-Cyclohexene... 

Verify this with handheld molecular models.) frans-Cyclohexene might resanble the structure shown in Fig. 7.4. There is evidence that it can be formed as a very reactive short-lived intermediate in some chemical reactions, bnt it is not isolable as a stable molecule. 

Methyl-6-(W,/V-dimethylamino)-frans-4-nitro-frans-5-phenyl-cyclohexene... 

With cyclohexene epoxides, the ring opening is frans-diaxial. 

The energy differences seem to be somewhat unrealistic, specifically if one considers that 15, 16, and 17 can be regarded as analogues of trans-cyclo-pentene, /rani-cyclohexene, and respectively, as frans-cycloheptene.12 As has... 

Benzenediol, see Hydroquinone p-Benzenediol, see Hydroquinone Benzeneformic acid, see Benzoic acid Benzene hexachloride, see Lindane Benzene hexachloride-a-isomer, see a-BHC Benzene-crs-hexachloride, see p-BHC Benzene-y-hexachloride, see Lindane a-Benzene hexachloride, see a-BHC p-Benzene hexachloride, see p-BHC 6-Benzene hexachloride, see 8-BHC y-Benzene hexachloride, see Lindane frans-a-Benzene hexachloride, see p-BHC Benzene hexahydride, see Cyclohexane Benzene methanoic acid, see Benzoic acid Benzene methanol, see Benzyl alcohol Benzene tetrahydride, see Cyclohexene Benzenol, see Phenol... 

Ru(0)(biqn)(tmtacn)](C10 )2 and [Ru(0)(diopy)(tmtacn)](C10 )2 (biqn=C2 symmetric 1,T-biisoquinoline, diopy=(R,R)-3,3 -(l,2-dimethylethylenedioxy)-2,2 -bipyridine) aremadefrom [RuCl(L)(tmtacn)] + (L=biqn, diopy) and(NH )2[Ce(N03)J with Li(ClO ). Electronic and IR spectra were measured (v(Ru=(0) bands lie at 760 and 795 cm" respectively). The (diopy) complex is paramagnetic with 2.88 B.M. As stoich. [Ru(0)(biqn)(tmtacn)] + and [Ru(0)(diopy)(tmtacn)] VCH3CN they oxidised alkenes (styrene, cis and fran.y-P-methylstyrenes, fran -stilbene, nor-bomene, cyclohexene) to mixtures of aldehydes and epoxides. Conttary to expectation the (diopy) complex did not effect enantioselective epoxidations except with fran -stilbene, for which a moderate e.e. of 33% was observed [623]. 

At this point, it is noteworthy that the spectrum of cyclohexane adsorbed at low temperature on Pt(lll) also changes in the 200-230 K region to give a closely similar spectrum [the match is rather better with the cyclohexane spectrum from Land et al. at 200 K (222) than with that obtained by Bussell et al. at 230 K (221)]. We conclude therefore that it is very probable that the first dissociation step of cyclohexane on Pt(lll) gives frans-di-cr-cyclohexene, the temperature of conversion and the possibility of adjacent... 

A special case involves the thermal decomposition of 3,4-dinitrofuroxan (104). The cycloreversion is already observed at room temperature and the nitroformo-nitrile oxide could be trapped with electron-deficient nitriles. The cycloadditions with styrene, phenylacetylene, frans-stilbene, and cyclohexene, however, led to complex mixtures of products that could not be separated (104). In the related case of a furoxan with an a-hydrogen adjacent to the sulfonyl group, the reaction was proposed to proceed according to course (b) (Scheme 6.7). 

Subsequent work by Bedos,equally applicable to cyclohexene oxide (Eq. 917), which yielded tla-corresponding jS-haloethers on treatment with methyl, ethyl, or n-propyl halides at 160-190 . The stereochemical course of the reaction was of course not explored at the time, but may be supposed to lend t-n a frans-ooufigur tiou. 

Very little work has been carried out on the kinetics of the PdCl2 only system. This is probably because of the slowness of the reaction. A stereochemical study using 3,3,6,6-cyclohexene-d4, however, provided evidence for frarw-acetoxypalladation in this system (5). For example the reaction scheme for formation of 3-cyclohexen-l-yl acetate is shown in Equation 22 (from Ref. 5). This scheme involves frans-acetoxypallada-tion and ci -Pd(II)-H (D) eliminations and additions. This agrees with the exchange kinetics which also suggest frarw-acetoxypalladation. 

Exercise 11-16 Starting with cyclohexene, show how you could prepare each of the following compounds a. the epoxide of cyclohexene, b. c/s-cyclohexane-1,2-diol, and c. frans-cyclohexane-1,2-diol. 

In the second example, suprafacial approach of a dienophile to the 2,5 carbons of frans,trarcs-2,4-hexadiene is seen to lead to a product with two methyl groups on the same side of the cyclohexene ring ... 

Essentially all Diels-Alder reactions are one-step processes. If the reactions are stereogenic, they often occur with predictable stereochemistry. For example, cw,fran.s-l,4-disubstituted 1,3-butadienes afford cyclohexenes with a trans-(X) arrangement of the substituents at C3 and C6 (Figure 15.14, top). In contrast, trans,trans- 1,4-disubstituted 1,3-butadienes afford cyclohexenes in which the substituents at C3 and C6 are cis ( ) with respect to each other (Figure 15.14, bottom). Accordingly, Diels-Alder reactions exhibit stereospecificity with regard to the diene. 

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