1,2-dimethylcyclohexene, reaction

In Problem 5 17 (Section 5 13) we saw that acid catalyzed dehydration of 2 2 dimethyl cyclohexanol afforded 1 2 dimethylcyclohexene To explain this product we must wnte a mecha nism for the reaction in which a methyl shift transforms a secondary carbocation to a tertiary one Another product of the dehydration of 2 2 dimethylcyclohexanol is isopropyhdenecyclopentane Wnte a mechanism to rationalize its formation... 

What products would you expect from reaction of 4,4-dimethylcyclohexene with NBS ... 

A similar reaction the with rran.v-isomer 3b gave c -3,5-dimethylcyclohexene (4) with very high diastereoselectivity. Accordingly, the stereochemistry of this substitution is anti. Deuterium labeling experiments using the 1-deuterio or 3-deuterio derivative of 3 a showed that the ratio of SN2 /SN2 with lithium dimethylcuprate was about 50 50, while the ratio with lithium cyano(methyl)cupratc was >96 4. 

The triethylsilane/trifluoroacetic acid reagent system reduces alkenes to alkanes in poor to excellent yields depending largely on the ability of the alkene to form carbocations upon protonation. Under these conditions the more substituted olefins are reduced in better yields and styrene double bonds are reduced in high yields.127,202,207,221-228 The reduction of 1,2-dimethylcyclohexene with this reagent gives a mixture of cis- and trans- 1,2-dimethylcyclohexane.229 The formation of the trifluoroacetate esters is a side reaction.205,230... 

More recently Hartog and Zwietering (103) used a bromometric technique to measure the small concentrations of olefins formed in the hydrogenation of aromatic hydrocarbons on several catalysts in the liquid phase. The maximum concentration of olefin is a function of both the catalyst and the substrate for example, at 25° o-xylene yields 0.04, 1.4, and 3.4 mole % of 1,2-dimethylcyclohexene on Raney nickel, 5% rhodium on carbon, and 5% ruthenium on carbon, respectively, and benzene yields 0.2 mole % of cyclohexene on ruthenium black. Although the cyclohexene derivatives could not be detected by this method in reactions catalyzed by platinum or palladium, a sensitive gas chromatographic technique permitted Siegel et al. (104) to observe 1,4-dimethyl-cyclohexene (0.002 mole %) from p-xylene and the same concentrations of 1,3- and 2,4-dimethylcyclohexene from wi-xylene in reductions catalyzed by reduced platinum oxide. 

In a detailed study of the reduction of the xylenes in the liquid phase on a 5% rhodium on carbon catalyst, Siegel and Ku (105) showed that both 1,2- and 2,3-dimethylcyclohexenes are formed from orlAo-xylene (Fig. 19). The initial (extrapolated) ratio, l,2-/2,3-, lies between 0.5 and 1 but rises as the reaction proceeds. If the initial distribution of cyclo-alkenes was random as previously postulated (97) the ratio should be 0.5. 

In comparison, the ratio of 2,4-/l,3-dimethylcyclohexene obtained from m-xylene is close to unity (1.2) and the ratio changes little during the course of the reaction. An initial random distribution should yield equal amounts of these isomeric cycloalkenes and the relative constancy of the ratio is consistent with the fact that, in competition with one another, 1,3- and 2,4-dimethylcyclohexene are reduced at comparable rates. The failure to observe the other postulated dimethylcyclohexenes is to be expected, because none would have substituents attached to the double bond in the cycle. Consequently, because of their expected greater reactivity in competition, their maximum concentration should be no more than a few per cent of the most reactive of the cycloalkenes actually observed in these experiments. 

Very few data are available concerning the stereochemistry of these additions. Hydration of 1,2-dimethylcyclohexene was found to be nonstereoselective.27 In contrast, predominant or exclusive exo-syn addition was shown to take place in the reaction of 2,3-dideuteronorbomene with water and alcohols 28... 

Fig. 39. Reaction scheme for the hydrogenation of 1,2-dimethylcyclohexene by a Horiuti—Polanyi mechanism as proposed by Siegel [221].

In the hydrogenation of 1,2-dimethylcyclohexene over a platinum catalyst, the suprafacial addition product is formed. Assuming that the mechanism of this hydrogenation is as shown in Figure 11-2, what conditions must be put on the stereochemistry of each of the postulated steps in order that the overall reaction be suprafacial ... 

A surface-mediated (SiC>2 or AI2O3) addition of hydrazoic acid, generated in situ from Me3SiN3 and CF3SO3H, to 1-methylcyclohexene and 1,2-dimethylcyclohexene has been reported69. The reaction obeys the Markovnikov rule and is therefore believed to proceed via the initial protonation of the double bond to generate a carbocation. This mechanism is also supported by the observed non-stereospecificity69. 

Reaction of 1,2-dimethylcyclohexene with the ethylene glycol acetal of acrolein in methylene chloride in the presence of 25 mol % of BF3.0Et2 at -78 to -10°C for 2 hours gives a 70% yield of the cycloadduct 1 in a formal 2k + 2% intermolecular cycloaddition. All of the evidence for this and related reactions, however, indicates a stepwise mechanism for the formation of 1. 

Diphenylmethane does not undergo cycloaddition reactions on direct excitation in the presence of electron acceptors (Gilbert and Lane, 1981). Reaction only occurs when the radiation is absorbed into the intermolecular charge-transfer band. With maleic anhydride [2 + 2] cycloaddition to one of the phenyl rings of diphenylmethane occurs and then the cyclohexa-1,3-diene so created reacts thermally with a further mole of the anhydride. Benzonitrile forms a number of cycloaddition products (Cantrell, 1977), e.g. with 1,2-dimethylcyclohexene [130], [131] and [132] are formed (Scheme 29). Products... 

The hydrogenation of 1,2-dimethylcyclohexene (17 Pt02, HOAc, 1 atm) yields trans-(19) as well as cis-1,2-dimethylcyclohexane (18), but Ir and Os in t-butyl alcohol are much more selective (equation lb). The addition of hydrogen to (17) competes with isomerization to 1,6-dimethylcyclohexene (20), which not only reacts more rapidly but yields trans (19) as well as the cis product (18) via syn addition of the two hydrogen atoms (equation 17). In the Pt-catalyzed reaction, the intermediate (20) can be observed, although its steady-state concentration remains low (0.21% of the 1,2-isomer 17) because of its... 

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... 

The reactions of dimethylcyclohexene derivatives substituted in the allylic position 24 are primarily governed by steric factors. 

Problem 7.4 Unlike the reaction in Problem 7.3, addition of HCl to 1,2-dimethylcyclohexene yields a mixture of two products. Show the stereochemistry of each, and explain why a mixture is formed. 

The Staudinger ketene cycloaddition was utilized as the key reaction in the synthesis of a number of bakkane natural products in the laboratory of A.E. Greene. Dichloroketene was generated in situ from trichloroacetyl chloride by zinc-copper alloy in the presence of phosphorous oxychloride. The [2+2] cycloaddition between dichloroketene and 1,6-dimethylcyclohexene gave the product in high yield and excellent regio- and diastereoselectivity. The cycloadduct was successfully converted to (+)-bakkenolide A. 

The reaction of cyclohexene oxide with LiBr-HMPA in refluxing benzene leads exclusively to cy-clopentanecarbaldehyde, but, like other enolizable aldehydes, the product is not indefinitely stable to the reaction conditions. The rearrangement of 1,2-dimethylcyclohexene oxide, although much slower, gives only the ketone (218) as shown in equation (120). This result is especially difficult to rationalize by any mechanism other than one requiring a bromohydrin intermediate. 

In this reaction the C=C double bond acts as a nucleophile, and in most acyclic or monocyclic molecules oxygen is added with equal facility to the top or bottom face of the alkene. Oxiranes are the most reactive ethers, and are readily susceptible to nucleophilic attack, which results in ring opening. This is in contrast to diethyl ether (ethoxyethane), which is inert even in the presence of, for example, LiAlH4. Likewise, 1,2-dimethylcyclohexene (27) reacts with peroxy acids to give cis-1,2-dimethylcyclohexene oxide (28). 

Catalytic cyclopropanation of alkenes with diazomalonates is sometimes carried out with copper powder, but it appears that copper(I) halide/trialkyl phosphite complexes (for a procedure see Houben-Weyl Vol. E19b, p 1113), bis(acetylacetonato)copper(II), " ° and tet-raacetatodirhodium can be employed more advantageously (Table 13, entries 7-9). For the cyclopropanation of styrene with dicyclohexyl diazomalonate, however, copper(I) triflate was the catalyst of choice, while intramolecular C —H insertion at the cyclohexyl ring took place in the presence of tetraacetatodirhodium. A detailed comparison of copper catalysts for the cyclopropanation of cyclohexene, 1-methyl- and 1,2-dimethylcyclohexene, (Z)- and ( )-hept-2-ene with dimethyl diazomalonate, including competitive reaction pathways such as allylic C-H insertion and carbene dimer formation, is available. The catalyzed interaction between diazomalonic esters and enol ethers leads to cyclopropanes in some cases (e.g. ethoxymethylenecyclohexane to dimethyl 2-ethoxyspiro[2.5]octane-l,l-dicarboxylate ) and to different products in other cases (e.g. 1-methoxycyclohexene, 2-methoxy-3,4-dihydro-2/7-pyran ). This behavior is attributed to the occurence of stabilized dipolar intermediates in these reactions. 

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