Intermolecular reactions high-pressure conditions

In contrast to the hydrazones mentioned above, a,/ -unsaturated N -sulfonyl imines react as electron-deficient diene component in aza Diels-Alder reactions. In addition to several investigations dealing with their intermolecular cycloadditions under thermal and under high pressure conditions [231-234], Boger s... 

Diels-Alder reactions of furans were the subject of a recent review (1986). Their intermolecular versions profit notably from high-pressure conditions (Section 4.1.4.2.1) or the use of benzynes as dienophiles (Section 4.1.7). 

When furfuryl diester 15 was reacted with an equimolar amount of DMAD under high pressure (Skbar, 7days), the product 18 was isolated in 25% yield (Scheme 4) [10]. Interestingly, product 18 was isolated even in the presence of an excess of DMAD in the reachon. This result was explained that bo intramolecular and intermolecular reactions had been promoted under the high-pressure conditions. Thus, intramolecular Diels-Alder (IMDA) reaction of one of the tethered furans occurred inihally with the cyclobutene Jt-bond of 15 and led to the formation of the adduct 16. The remaining furan enhty in product 16 was then free to react with DMAD to form the oxanorbomadiene adduct 17 and reaction stopped at that stage. However, product 18 was finally isolated rather than 17 as a result of secondary retro-DAof 17. 

We should point out that the calculations involved in Example 12.6 assume ideal gas behavior. At the conditions specified (1 atm, relatively high temperatures), this assumption is a good one. However, many industrial gas-phase reactions are carried out at very high pressures. In that case, intermolecular forces become important, and calculated yields based on ideal gas behavior may be seriously in error. 

Determination of reaction mechanisms by combining the observed intermediates in a catalytic cycle. To do this, it is often necessary to measure under different conditions - that is, variable temperature NMR. The use of high-pressure NMR cells is crucial in order to measure under the real catalytic conditions. The EXSY experiment helps to unravel exchange pathways, both intra-and intermolecular. 

FT reactions that give mainly oxygenated hydrocarbons from synthesis gas. Intramolecular CO insertion into a metal-hydride bond is thermodynamically unfavorable under mild conditions. However, under high pressure and temperature, this reaction may be kinetically favorable. It may also take place by an intermolecular mechanism. 

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