Ethane elimination

Step (a) could be the analog of the (n, n ) rearrangements of cyclopropyl ketones , and step (b) is that of the molecular ethane elimination from azomethane and probably occurs from the cw-isomer of the parent compound with yields of 15-25 %. Further studies on this and similar molecules would be of great value. 

These reactions ate all due to the 8a-hydrogen in 1,8 -DHN (329), and replacement of this hydrogen by a methyl group (R = CH3) gives a thermally stable 1,8a-DHN which does not undergo methyl shifts or ethane elimination reactions50 (Scheme 34). 

The higher reactivity of nickel complexes than the palladium congeners was examined theoretically. Fig. 9.1 illustrates a schematic diagram for the orbital correlation on the ethane elimination from cw-M(CH3)2(PH3)2 complexes (M = Ni, Pd) [8,10]. This scheme presumes the least motion process that maintains the C2v symmetry of precursor complex throughout the reaction. The C-M-C angle becomes narrow while the P-M-P angle is gradually extended as the reductive elimination proceeds. Finally, ethane is eliminated with formation of a linear M(PH3)2 complex. 

Fig. 9.1. Schematic diagram for the orbital correlation on the ethane elimination from cis-M(CH3)2(PH3)2 complexes (M = Ni, Pd) [8,10].

The other important trend has been observed for diphosphine-coordinated complexes. Thus the rate of path (b) is significantly affected by the chelation size of ligand. For example, the rate of ethane elimination from cA-NiMe2(diphosphine) complexes markedly increases in the order [dppe < dppp < dppb] [9]. Similar trends have been reported for c 5-PdR(S-r-Bu)(diphosphine) complexes [23a] and cA-Pd(CH2TMS)(CN)(diphosphine) [26]. Table 9.3 lists the kinetic data for the latter complexes. The most reactive is the DIOP complex bearing a seven-membered ring, whose rate constant is about 4760 times greater than that of... 

In the case of white phosphorus, the reaction proceeds through the following elementary steps (i) ethane elimination, (ii) P-P bond activation at iridium forming the Ir(m) hydride [(triphos)IrH(77 -P4)] 921, (iii) isomerization of 921 to [(triphos)Ir( j -P4H)] 922 with selective delivery of the hydride atom to one of the coordinated P-atoms, and (iv) hydrogenation of the teraphosphidohydride ligand with PH3 generation and formation of the known cyclo-Vj, complex [(triphos)Ir( -P3)] 923. 

FIGURE 9.56 Protonation of 3-pentanone and ethane elimination after hydrogen transfer on the alkyl chain in positive ehemical ionization. 

Athene formation requires that X and Y be substituents on adjacent carbon atoms By mak mg X the reference atom and identifying the carbon attached to it as the a carbon we see that atom Y is a substituent on the p carbon Carbons succeedmgly more remote from the reference atom are designated 7 8 and so on Only p elimination reactions will be dis cussed m this chapter [Beta (p) elimination reactions are also known as i 2 eliminations ] You are already familiar with one type of p elimination having seen m Section 5 1 that ethylene and propene are prepared on an industrial scale by the high temperature dehydrogenation of ethane and propane Both reactions involve (3 elimination of H2... 

Dehydrogenation (Section 5 1) Elimination in which H2 is lost from adjacent atoms The term is most commonly en countered in the mdustnal preparation of ethylene from ethane propene from propane 1 3 butadiene from butane and styrene from ethylbenzene... 

Ethylene is recycled, some of which is purged, to eliminate the accumulation of ethane. The olefins are distilled into even-carbon-number fractions... 

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

Schanke CA, LP Wackett (1992) Environmental reductive elimination reactions of polychlorinated ethanes mimicked by transition-metal coenzymes. Environ Sci Technol 26 830-833. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

Nucleophihc substitution of sodium thiolate on 2-(perfluorohexyl)ethane iodide leads to extensive elimination (10-20%) and formation of (perfluorohexyl)ethylene (3), Equation 2. 

The olefin yields are higher with 2-(perfluoroalkyl) ethane iodides than their hydrocarbon analogues apparently because of the strongly electron withdrawing fluorine making the hydrogens more acidic and prone to elimination. 

Oxidative addition of a silyl-protected 4-(bromomethyl)phenol precursor to (tme-da)Pd(II)Me2 (tmeda = tetramethylethylenediamine), followed by ethane reductive elimination, resulted in formation of the benzylic complex 16 (Scheme 3.10). Exchange of tmeda for a diphosphine ligand (which is better suited for stabilizing the ultimate Pd(0) QM complex), followed by removal of the protecting silyl group with fluoride anion, resulted in the expected p-QM Pd(0) complex, 17, via intermediacy of the zwitterionic Pd(II) benzyl complex. In this way a stable complex of p-BHT-QM, 17b, the very important metabolite of the widely used food antioxidant BHT20 (BHT = butylated hydroxytoluene) was prepared. Similarly, a Pd(0) complex of the elusive, simplest /)-QM, 17a, was obtained (Scheme 3.10). 

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