Deuterium reaction + alkenes

Another series of closely related reactions for which transition-state calculations have greatly helped in providing an understanding of the observed trends is the addition to deuterium-substituted alkenes. Szwarc and co workers (Feld et al., 1962) have determined secondary deuterium isotope effects for methyl and trifluoromethyl radicals by comparing the rate of addition to a terminal alkene with the rate for the deuterium-substituted alkene (25). Isotope effects for cyclopropyl radical addition have been measured by Stefani and coworkers (1970). For these three radicals a small inverse isotope effect (kJkK)... 

Strongest evidence for a rapid equilibration of the a-halo sulfone with its a- and a -anions comes from reactions performed in D2O. If such reactions are interrupted before they are complete, the recovered a-halo sulfones are almost fully deuterated at both the a- and a -positions. Furthermore, the alkene products are also essentially fully deuterated, this time at the vinylic positions. When allowed to run to completion, this forms the basis of a very useful preparative method for deuterium-labeled alkenes (Scheme 8 and equation 24). Not only is a high percentage incorporation possible, but the source of deuterium is very cheap. 

The kinetic hydrogen isotope effects which have been observed in the hydrogen-ene reactions of deuterium-labelled alkenes with singlet oxygen, tV-pheny I triazolincdione, nitrosopentafluorobenzene, formaldehyde, methyl chloroacrylate, and ethyl propiolate as the enophiles, and the stereoselectivity of the reactions, imply the initial formation of a complex between the alkene group and the enophile. The various products which have been identified in the metalloene reactions can be rationalised on die basis of a similar mechanism (Scheme 9-2).67... 

When alkene is in excess, the reaction in a constant volume system stops when the deuterium is used up, and the deuterium alkene ratio decreases continuously as the reaction proceeds. Unreacted but partially exchanged alkene remains at the end, and the deuterium number of the alkanes M is less than two, the alkane-d<, becoming a major product (see Figure 7.9 for the propene-deuterium reaction on Pt/pumice ). Contrarily when deuterium is in excess, the final alkane deuterium number is greater than two, the alkene-rfo falls to near zero, and the alkane-d2 becomes the major product (see also Figure 7.9). Similar but less complete results were seen with propene ° and ethene on a variety of supported platinum catalysts. 

There are few reports of alkene-deuterium reactions on bimetallic catalysts, but those few contain some points of interest. On very dilute solutions of nickel in copper (as foil), the only product of the reaction with ethene was ethene-di it is not clear whether the scarcity of deuterium atoms close to the presumably isolated nickels inhibits ethane formation, so that alkyl reversal is the only option, or whether (as with nickel film, see above) the exchange occurs by dissociative adsorption of the ethene. Problems also arise in the use of bimetallic powders containing copper plus either nickel, palladium or platinum. Activation energies for the exchange of propene were similar to those for the pure metals (33-43 kJ mol ) and rates were faster than for copper, but the distribution of deuterium atoms in the propene-di clearly resembled that shown by copper. It was suggested that the active centre comprised atoms of both kinds. On Cu/ZnO, the reaction of ethene with deuterium gave only ethane-d2. as hydrogens in the hydroxylated zinc oxide surface did not participate by reverse spillover. ... 

The Ramberg-Backlund rearrangement of a-halo sulfones is frequently carried out with t-BuOK. This reaction provides a useful route to deuterium-labeled alkenes (eq 43). t-BuOK is presumably the active base in a modification which involves the direct conversion of a sulfone into an alkene with KOH and a mixture of r-BuOH/CCLt. The base converts the sulfone to the... 

Oxidation of ethylene in alcohol with PdCl2 in the presence of a base gives an acetal and vinyl ether[106,107], The reaction of alkenes with alcohols mediated by PdCl2 affords acetals 64 as major products and vinyl ethers 65 as minor products. No deuterium incorporation was observed in the acetal formed from ethylene and MeOD, indicating that hydride shift takes place and the acetal is not formed by the addition of methanol to methyl vinyl etherjlOS], The reaction can be carried out catalytically using CuClj under oxygen[28]. 

It should be noted that the selective reduction of phenylacetylene and diphenylacetylene to either the ds-alkene or the alkane was achieved using LiAlH4 in the presence of FeCk or NiCk as a catalyst [90, 91]. However, deuterolytic workup of the reaction mixtures gave deuterium incorporations <26%, indicating that these reagent systems are not well suited for the synthesis of vinyl- or alkylaluminum compounds from alkynes. 

The reaction was carried out with eOEt in EtOD, and (11) re-isolated after half-conversion to (13) it was found to contain no deuterium, i.e. no (14) nor did the alkene (13) contain any deuterium, as might have been expected by elimination from any (14) formed. This potentially favourable case thus does not proceed by an ElcB pathway of the form described above though we have not ruled out the case where k2 k, i.e. essentially irreversible carbanion formation. 

Deuterium-labeling studies pointed to the operation of a nonstandard Chalk-Harrod mechanism for these reactions involving a silyl-alkene insertion step.133... 

The observation of a primary solvent deuterium isotope effect (kH/fa>) = 2-4 on the specific acid-catalyzed hydrolysis of vinyl ethers provides evidence for reaction by rate-determining protonation of the alkene.69 Values of kHikD 1 are expected if alkene hydration proceeds by rate-determining addition of solvent to an oxocarbenium ion intermediate, since there is no motion of a solvent hydron at the transition state for this step. However, in the latter case, determination of the solvent isotope effect on the reaction of the fully protonated substrate is complicated by the competing exchange of deuterium from solvent into substrate (see above). 

In this case ylide complexes are not observed and therefore the reactions are very simple. When L 2-methylpyridine or acetonitrile, the product was shown to be (XII) rather than (XIII). Complex (XII) could be characterised directly by lU and 13C NMR spectroscopy or, more simply, treated with triphenylphos-phine to release the alkene. Figure 1 shows the 13C XH NMR spectrum of the released alkene (together with 2-methylpyridine), which clearly shows 1 1 1 triplets for carbon atoms C1 and C due to coupling to deuterium as expected for the alkene from (XII) but not from (XIII). In addition, the 2H 1H NMR spectrum shows approximately equal integration for deuterium at C1 and at C1 ... 

The aforementioned deuterated derivatives were prepared by way of reduction of a ketone, aldehyde, or ester with sodium borodeu-teride, or by deuteroboration of an alkene. An interesting reaction, perhaps eventually applicable to direct deuteration of polysaccharides, was reported by Koch and Stuart413 and by them and their coworkers,41b who found that treatment of methyl a-D-glucopyranoside with Raney nickel catalyst in deuterium oxide results in exchange of protons attached to C-2, C-3, C-4, and C-6. In other compounds, some protons of CHOH groups are not replaced, but the spectra may nevertheless be interpreted with the aid of a- and /3-deuterium effects. 

The gas-phase reactions of the fulvene radical cation with neutral 1,3-butadiene, alkenes and 2-propyl iodide have been investigated by Russell and Gross131a using ICR mass spectrometry. Unlike ionized benzene, ionized fulvene undergoes no C—C coupling with 2-propyl iodide. On the basis of deuterium and 13C labelling, the reaction of ionized fulvene with 1,3-butadiene was suggested to occur by [6 + 4] cycloaddition to yield tetrahydroazulene radical cations. Cycloadditions of neutral fulvene were also studied in this work. 

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