Ethene isotopically labelled

The [t 4j + t 2j] cycloaddition of alkenes and dienes is a very useful method for forming substituted cyclohexenes. This reaction is known as the Diels-Alder (abbreviated D-A in this chapter) reaction. The transition structure for a concerted reaction requires that the diene adopt the s-cis conformation. The diene and substituted alkene (called the dienophile) approach each other in approximately parallel planes. This reaction has been the object of extensive mechanistic and computational study, as well as synthetic application. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, ethene with butadiene, as demonstrated by isotopic labeling. ... 

W-based catalysts for the metathesis of terminal olefins are comparatively few in number. However, this is partly an illusion because systems such as WClg/EtAlCla/ EtOH, although not effective in the sense of yielding ethene and an internal olefin, cause rapid non-productive metathesis in which the products can only be distinguished from the reactants by isotopic labelling see Ch. 5. 

Based on isotope labeling experiments (TPD, IRRAS) [36-39] it was concluded that the catalytic ethene hydrogenation reaction on surfaces proceeds as a step wise process of hydrogen incorporation. This step wise general mechanism is called the Horiuti-Polanyi mechanism [40] and is shown in Eqs.2.11-2.15 [41]. ... 

Isotopic labeling studies of the co-reaction of ethene and methanol. J Catal 1994 149 458-64. 

R0nning PO, Mikkelsen 0, Kolboe S. Effect of co-feeding ethene or propene on the conversion of methanol over H-ZSM-5. Use of isotopic labeling for mechanistic studies. In Proceedings of the 12th international zeolite conference. 2 1998. p. 1057. 

Seburg and McMahon have performed matrix isolation experiments on both 3-methyldiazirine and diazoethane in an attempt to observe ethyfidene. They were unsuccessful in detecting the desired carbene but did observe isomerization of diazoethylene to 3-methyldiazirine. When matrix-isolated diazoethylene was photolyzed at 8K, ethene and 3-methyldiazirine were observed as the major products in a ratio of 10 1. Additionally, unlike Moore and Pimentel, Seburg and McMahon did not observe isotopic label scrambling when the photolyses were conducted in an Nj matrix. 

As a means of obtaining additional information on the methanol to hydrocarbons reaction over zeohtes we have investigated the reaction between C labeled methanol and ( C) ethene or propene (made in situ from ethanol or isopropanol) over SAPO-34, H-ZSM-5, and dealumlnated mordenlte. The isotopic composition of the reaction products was measured by GC-MS. 

Octa-1,7-diene has been a favourite molecule for testing catalytic activity and for carrying out labelling experiments, because reaction (2) is generally highly selective. The reaction of 1 1 mixtures of normal and 1,1,8,8-J4-octa-1,7-diene gives ethene, the isotopic composition of which 4/2 d/ corresponds closely to... 

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