Cyclohexane substituted, reactions

Solvent effects on the rate of the decarbonylation of MeCOMn(CO)5 were examined by Calderazzo and Cotton (50) and are presented in part in Table IV. In general they are very small, and no regular trends can be discerned. This virtual lack of dependence of the rate on the nature of the solvent and very little correlation between the rate and the dielectric constant of the solvent are typical of substitution reactions of metal carbonyls (J). In the light of the foregoing, a qualitative observation that CpFe(CO)2-COMe decarbonylates much more readily on treatment at reflux in nonpolar heptane or cyclohexane than in polar dioxane is somewhat intriguing 219). 

It is found that the rate of substitution reaction between Mn(CO)sBr and As(C6H5)3 varies somewhat with the solvent. The rate constant at 40 °C when the solvent is cyclohexane is 7.44 X 10-8 sec-1, and when the solvent is nitrobenzene it is 1.08 X 10-8 sec-1. In light of the principles described in Chapter 6, what does this observation indicate about the mechanism of the reaction What would you expect a reasonable value for the rate constant to be if the solvent is chloroform See Table 6.7. 

In a one-pot synthesis of thioethers, starting from potassium 0-alkyl dithiocarbonate [36], the base hydrolyses of the intermediate dialkyl ester, and subsequent nucleophilic substitution reaction by the released thiolate anion upon the unhydrolysed 0,5-dialkyl ester produces the symmetrical thioether. Yields from the O-methyl ester tend to be poor, but are improved if cyclohexane is used as the solvent in the hydrolysis step (Table 4.13). In the alternative route from the 5,5-dialkyl dithiocarbonates, hydrolysis of the ester in the presence of an alkylating agent leads to the unsymmetrical thioether [39] (Table 4.14). The slow release of the thiolate anions in both reactions makes the procedure socially more acceptable and obviates losses by oxidation. 

The photochemistry of [Cr(CO)e] has been investigated in several studies. Flash photolysis of cyclohexane solutions of [CrfCO) ] affords two species one has a of 470 nm and a lifetime of 5 ms and the other, = 440 nm, has a lifetime > 1 s. The relationship between photolysed species of [CrfCO) ] and photochemical substitution reactions described in Scheme 4 has been suggested from i.r. and u.v. spectroscopic studies of matrix-isolated species. ... 

In (—)-sparteine-mediated deprotonation and electrophilic substitution reactions, the minor enantiomer is close to the limits of exact determination. Therefore, the influence of the alkyl residue on selectivity was investigated for less efficient (/ ,/ )- ,2-bis(dimethyl-amino)cyclohexane/i-BuLi (equation 11)°. On the base of the isolated corresponding... 

Substitution reactions are not very common for substituted cyclohexane. The substituted carbon in a cyclohexane ring is a secondary centre—in the last chapter, we saw that secondary centres do not react well via either SnI or Sn2 mechanisms (p. 426). To encourage an Sn2 mechanism, we need a good attacking nucleophile and a good leaving group. One such example is shown—the substitution of a tosylate by PhS-. 

We must assume that this holds even for simple unsubstituted cyclohexanes, and that substitution reactions of cyclohexyl bromide, for example, occur mainly on the minor, axial conformer. This slows down the reaction because, before it can react, the prevalent equatorial conformer must first flip axial. 

When bromine is added to cyclohexane, the bromine dissolves as it undergoes a substitution reaction and the product forms as a red-brown precipitate. 

TRIR data were used to follow ligand substitution reactions of photocata-lytically-generated intermediates CpMn(CO)2(CyH), where CyFI = cyclohexane, with L = cyclopentene, thf, furan or pyrrolidine, to form CpMn(CO)2(L).103 Picosecond to microsecond time-scale TRIR was used to follow the photoinduced dynamics of [t 5-C5H4C(0)R]Mn(C0)3, where R = CH(SMe)2 or C(SMe)3.104... 

T. K. M. Shing and L. H. Wan, Facile synthesis of valiolamine and its diastereomers from (-)-quinic acid. Nucleophilic substitution reactions of 5-hydroxymethyl-cyclohexane-l,2,3,4,5-pentol, J. Org. Chem., 61 (1996) 8468-8479. 

More sophisticated examples are provided by substitution reactions, which are influenced by a remote double bond. The 3p-hydroxy group of cholesterol, for example, can be substituted by chloride with PClg or, after tosylation, by methoxide. In both cases almost quantitative yields of p-substituted compounds are observed. All 3P-hydroxy steroids with a 5,6-double bond give these reactions. The homoallylic carbonium ion at G3 and its cyclization after neuttaliza-tion at the y carbon atom have thus been established as well as the thermodynamic preference of equatorial substitution in cyclohexane units (Scheme 3.4.2). 

The framework substitution of Mn(II) is responsible for a variety of phenomena (1) the high selectivity of the MnAPSO-34 toward formation of ethane in the reaction of methanol conversion (231), (2) the catalytic activity of MnAPO-5 in oligomerization of propane (232) and the catalytic activity of MnAPSO-44 in methanol dehydration (233), (3) the high yields in the production of isobutene and isobutene over MnAPSO-11 (234), (4) a sensor ability of MnAPO-5 to detect small molecules as CO, CO2, N,2 and H2O at room temperature (235), (5) MnAPO-5 registered to give 11% yield of p-IPEB and 5.8% yield of m-IPEB at 350°C in isopropylation of ethylbenzene with 2-propanol (236), and (6) cyclohexane (RH) reactions with O2 on MnAPO-5, where the oxidation rates are determined to be proportional to the number of redox-active Mn centers (237) The measurements by H2-O2 reduction-oxidation cycles indicate that these species act as active sites for kinetically relevant elementary steps in alkane oxidation catalytic cycles. 

As discussed in Sect. 1.2, Pd is both selective and versatile. In the absence of any other more reactive functional groups, even alkanes, such as methane, cyclohexane,f ° t and adamantane, can undergo paUadation via C—H activation. As in the other pallada-tion reactions via C—H activation, it is as such a stoichiometric process. In addition to the concerted perpendicular interaction of Pd with the C—H bond shown in Schemes 3 and 4 in Sect. VIII.3.3, the coUinear electrophilic substitution reaction of Pd with the C—H bond, as shown in Scheme 5, has been suggested. However, this mechanism appears to be incompatible with the results observed with adamantane, and this point needs to be clarified further. 

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