Alkenes equilibration

The AcBr-2AlX3 (X = Cl, Br) complexes display high activity in the alkylation of adamantane with alkanes to form poly alkylated adamantanes (Cn < C < C33) and bisadamantylalkanes (C23 < C < C50)119 [Eq. (5.70)]. The suggested pathway includes the 1-adamantyl cation and alkyl cations generated by hydride removal by the superacidic complexes. The 1-adamantyl cation then alkylates alkenes equilibrating with the alkyl cations. Various transformations may follow, resulting in the formation of additional products. 

These compounds have been studied for a long time and there is a great deal of recent evidence from UV studies photoelectron spectra and ionization potentials ", Raman spectroscopy alkene equilibration microwave spectroscopy, X-... 

The tertiary alcohol leaving group, the acid catalyst, and the 50 50 mixture all suggest El rather than E2. There is only one proton that can be lost and, as there is very little difference between the isomeric alkenes, equilibration probably gives the 50 50 mixture. 

The ratio of isomeric alkenes obtained from a primary alcohol such as 1-butanol depends on the relative rates of dehydration by Mechanism 5.3 and the E2 process in Problem 5.21. However, the strongly acidic reaction conditions also promote alkene equilibration, which increases the proportion of 2-butenes at the expense of 1-butene. The mechanism of this equilibration is based on principles that we will consider in Chapter 6. 

When terf-pentyl alcohol was reacted with acetic anhydride and perchloric or tetrafluoroboric acids, the reaction proceeded via the more stable trisubstituted alkene (21) under thermodynamic control and the reaction product was mainly a 2,3,4,6-tetramethylpyrylium salt (25). However, with terf-pentyl chloride, acetyl chloride and aluminum chloride or antimony pentachloride, the diacetylation occurred under kinetic control the alkene equilibration proceeds very fast with these Lewis acids so that the product (26) is derived from the more reaetive but less stable disubstituted alkene (22) 63,64 formulas below, the intermediate monoaeetylation products... 

The reversibility of alkene protonation leads to alkene equilibration... 

Trans stereochemistry of the alkene product is established during the second reduction step when the less hindered trans vinylic anion is formed from the vinylic radical. Vinylic radicals undergo rapid cis-trans equilibration, but vinylic anions equilibrate much less rapidly. Thus, the more stable trans vinylic anion is formed rather than the less stable cis anion and is then protonated without equilibration. 

A related palladium(O)-catalyzed epimerization of y-aziridinyl-a,P-enoates 244 was also reported by Ibuka, Ohno, Fujii, and coworkers (Scheme 2.60) [43]. Treatment of either isomer of 244 with a catalytic amount of Pd(PPh3)4 in THF yielded an equilibrated mixture in which the isomer 246 with the desired configuration predominated (246 other isomers = 85 15 to 94 6). In most cases the isomer 246 could be easily separated from the diastereomeric mixture by a simple recrystallization, and the organocopper-mediated ring-opening reaction of 246 directly afforded L,L-type (E)-alkene dipeptide isosteres 243. 

Scheme 2.23 provides some examples of conjugate addition reactions. Entry 1 illustrates the tendency for reaction to proceed through the more stable enolate. Entries 2 to 5 are typical examples of addition of doubly stabilized enolates to electrophilic alkenes. Entries 6 to 8 are cases of addition of nitroalkanes. Nitroalkanes are comparable in acidity to (i-ketocslcrs (see Table 1.1) and are often excellent nucleophiles for conjugate addition. Note that in Entry 8 fluoride ion is used as the base. Entry 9 is a case of adding a zinc enolate (Reformatsky reagent) to a nitroalkene. Entry 10 shows an enamine as the carbon nucleophile. All of these reactions were done under equilibrating conditions. 

Monocyclic Phosphoranes. - Further studies on the mechanism and stereochemistry of the Wittig reaction have been conducted by a combination of 1H, 13C and 3 P n.m.r.2k 25. The results show that at -18°C both ois and trans diastereomeric oxaphosphetans (e.g. 17 and 18) may be observed and their decomposition to alkenes monitored by n.m.r. Evidence was presented to suggest that during this process oxaphosphetan equilibration involving the siphoning of (17) into (18) occurred in competition with alkene formation. 

There are reasons for believing that this common product mixture from each of the two alkenes—(67) and (70)—does not arise from equilibration of these starting materials before addition proper takes place. It could well be that the higher degree of TRANS stereoselectivity observed (p. 318) for addition at lower temperature, and with higher concentration of HBr, results not from the intervention of a cyclic bromonium radical (71), but from slower rotation about the central carbon-carbon bond. Relatively rapid H transfer (by the higher concentration of HBr) could then take place to the less hindered side of (69) or (73), leading to preferential TRANS additional overall. 

In cyclic alkenes, where such equilibration of radical intermediates cannot occur, there is a preference, but not an exclusive one (except for cyclohexenes), for overall TRANS addition. 

Kinetic studies show that insertion (the enantioselection step) is very rapid, and that the rate-determining step is the hydrogenolysis of the M-C bond. Nonetheless, under H2-starving conditions, there is evidence that fi- I elimination can be competitive with hydrogenolysis. />-H elimination of the alkyl intermediate gives back the starting alkene and, through an equilibration process, it... 

The contrast between the lack of enantioselectivity in Scheme 39 and the moderate to excellent diastereoselectivity seen with alcohol nucleophiles in Schemes 19 and 33 can be attributed to the difference in leaving groups (diphenyl phosphate vs diethyl phosphate) and to the differences in the radical cations themselves, all of which impinge on the rate of equilibration of the contact alkene radical cation/anion pair. 

In general the activity of transition metal complexes for alkene isomerisation is low in the presence of carbon monoxide, but HCo(CO)4 is an exception to this rule. Depending on conditions, full equilibration of the alkene isomers is obtained. 

In the first few minutes of a batch reaction using 1-alkenes an extremely fast reaction therefore may take place, which is the direct hydroformylation of 1-alkene, but after an equilibration to the internal isomers has taken place the reaction slows down considerably. 

Initially the polymer molecular weight distribution obeys a Poisson distribution, typical of a chain growth reaction without chain transfer. Since the reactions are reversible, at a later stage, also the equilibration between the polymers becomes important and a broad distribution of molecular weights is obtained. As can be seen from Figure 16.5 the presence of linear alkenes causes chain termination (chain transfer) and thus low molecular weights are produced if the cycloalkenes are not sufficiently pure. 

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

However, (TMS)3Si radicals are found to add to a variety of double bonds reversibly and therefore to isomerize alkenes [19]. An example is shown for the interconversion of ( )- to (Z)-3-hexen-l-ol and vice versa by (TMS)3Si radicals (Reaction 5.1). Figure 5.1 shows the time profile of this reaction under standard experimental conditions (AIBN, 80 °C). The equilibration of the two geometrical isomers is reached in ca 10 h, and the percentage of Z/E = 18/82 after completion corresponds to an equilibrium constant of = 4.5. The difference in the stability of the two isomers in 2-butenes, i.e., AG°( -isomer) - AG° (Z-isomer) = — 3.1kJ/mol, corresponds to K = 3.5, since... 

Elsewhere, Heaney et al. (313-315) found that alkenyloximes (e.g., 285), may react in a number of ways including formation of cyclic nitrones by the 1,3-APT reaction (Scheme 1.60). The benzodiazepinone nitrones (286) formed by the intramolecular 1,3-APT will undergo an intermolecular dipolar cycloaddition reaction with an external dipolarophile to afford five,seven,six-membered tricyclic adducts (287). Alternatively, the oximes may equilibrate to the corresponding N—H nitrones (288) and undergo intramolecular cycloaddition with the alkenyl function to afford five,six,six-membered tricyclic isoxazolidine adducts (289, R = H see also Section 1.11.2). In the presence of an electron-deficient alkene such as methyl vinyl ketone, the nitrogen of oxime 285 may be alkylated via the acyclic version of the 1,3-APT reaction and thus afford the N-alkylated nitrone 290 and the corresponding adduct 291. In more recent work, they prepared the related pyrimidodiazepine N-oxides by oxime-alkene cyclization for subsequent cycloaddition reactions (316). Related nitrones have been prepared by a number of workers by the more familiar route of condensation with alkylhydroxylamines (Scheme 1.67, Section 1.11.3). 

Surprisingly, the critical experiment has been done infrequently over the last one-half of a century The requirements for an experiment that truly speaks to the issue at hand are that one be able to see the results of addition of both spin states of a single carbene, and these requirements rarely have been met. For example, the direct irradiation of methyl diazomalonate leads to the stereospecific addition expected of a singlet carbene, whereas the photosensitized decomposition of the diazo compound leads to formation of the triplet carbene and loss of the stereochemical relationship originally present in the reacting alkene. Rotational equilibration in the intermediate seems to be complete, as it makes no significant difference whether cis or trans alkene is used as starting material (Scheme 7.9). ... 

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