Allyl mechanism

In addition, a 532 (visible) or 355 (UV region) nm laser-induced photoisomerization of allylic alcohols to aldehydes catalyzed by [Fe3(CO)i2] or [Fe(CO)4PPh3] was developed by Fan [176]. In this reaction, key intermediates such as the 7i-allyl hydride species [FeH(CO)3(q -C3H3ROH)] (R = H, Me) were detected by pulsed laser FTIR absorption spectroscopy. These results strongly support the 7i-allyl mechanism of photoisomerization of allyl alcohols. 

Scheme 18 a- vs. tc-allyl mechanism in Fe-catalyzed allylic substitutions... 

Scheme 22 Fe-catalyzed allylic substitution via tc-allyl mechanism [67, 68]...

Double bond migration occurs either by the Jt-allyl mechanism (abstraction-addition) or by the Horiuti-Polanyi mechanism (addition-abstraction). Pd is thought to favor Jt-allyl and Pt Horiuti-Polanyi mechanisms. 

The presence of stable V-O-Si bonds also limits the possibility of insertion of O in the activated organic molecule and the stable coordination limits the possibility of alkene activation through an allylic mechanism. Both these factors are important in controlling the consecutive transformation of propylene. 

The two mechanisms may result in substantial and characteristic differences in deuterium distribution. The metal hydride addition-elimination mechanism usually leads to a complex mixture of labeled isomers.195 198 208-210 Hydride exchange between the catalyst and the solvent may further complicate deuterium distribution. Simple repeated intramolecular 1,3 shifts, in contrast, result in deuterium scram-bling in allylic positions when the ir-allyl mechanism is operative. ... 

Clark and Cook 71) disproportionated [1-14C] propylene and [2-14C] propylene over cobalt oxide-molybdate-alumina catalyst. At 60 °C their results were consistent with those reported Mol and coworkers, confirming the four-center mechanism. At temperatures above 60 °C, double-bond isomerization activity of the cobalt-molybdate catalyst became a factor and at 160 °C nearly one-half of [l-l4C] propylene had isomerized to [3-14C] propylene prior to disproportionation. The authors note that at temperatures where isomerization does not occur, the possibility of a jr-allyl intermediate appears to be excluded however, at higher temperatures, the 77-allyl mechanism cannot be so easily dismissed. 

FIG. 4. The w-allyl mechanism postulated to explain initial exchange of S hydrogen atoms in an isolated trimethylene unit on Pd (6). 

The tr-allyl mechanism also accounts for the results if one accepts the rather strained tr-allyl (C1-C2-C3) as readily participating. So far none of the model compounds seemed to distinguish clearly the two mechanisms. However, Roth et al. (11) suggested that l-methylbicyclo[3,3,0]octane (III) should show this distinction since edge-on rollover is impossible in this case but the tr-allyl mechanism should still be feasible. Quinn et al. (14) synthesized this compound and found initial exchange of only 11 hydrogens (and not 13) with a maximum in the d1 isomer at lower temperatures (Table III) on Pd catalysts. These results... 

Figure 7.2 Isomerization by the allyl mechanism. Note that 7.6 to 7.7 is an oxidative addition reaction, as shown in 7.1.

Olefin isomerizations can follow two different mechanisms, depending on whether or not the metal species involved contains an M—H bond. Nickel and palladium complexes, but also iron and rhodium, can induce isomerization via a 77-allyl mechanism ... 

Isomerization in the absence of a hydrido complex or added hydrogen is frequently and incorrectly assigned an / -allylic mechanism. This facile assignment overlooks the possibility of hydrido complexes being formed from reactions with alcohols or other solvents or reagents during the reaction. 

The (7-allyl mechanism was supported by the fact that yr-allyl complex 35 is quite Linreactive with respect to c/j-migration whereas complex 36 reacts fast in the same process (Scheme 8-9) [55]. The low reactivity of yr-allyl complex 35 can be explained by the disfavored conversion of this isomer to its cr-allyl complex because of the change of the substituent R from an equatorial to an axial position. For complex 36, on the other hand, formation of the o-allyl complex should be facile because the R group will become equatorial in this complex. 

The principle of the r-allyl mechanism is illustrated in Scheme 1. The catalytic process is initiated by coordination of the terminal olefin to the metal followed by activation of the aliphatic C H-bond, affording the three-carbon arrangement in TT-bonding to the metal. The metal-attached hydride has thus two positions to which it may be transferred a and y), the c-position being nonproductive and the y-position leading to the internal olefin. It follows from Scheme 1 that the jS-C-H entity is not affected. 

A nice model for the r-allyl mechanism has been reported by Bdnnemann [18] for the pair of nickel complexes 1 and 2 forming a temperature-dependent equilibrium (eq. (8)). 

With the higher olefins, where other possible species may exist, the situation is naturally more complicated, but there is rarely any definite evidence for 7r-allylio intermediates (butene isomerization over ruthenium may be an exception). This is not to say that they do not exist, but rather that the observations may be rationally interpreted without them. It is worth noting that the predictive value of ir-allylic mechanisms is somewhat small. 

The square planar complex 73 undergoes P-elimination via liberation of L to form a transient equilibrium of the iminium-rhodium hydride o-complex 74a and the Jt-complex 74b, Eq. 16. These complexes 74a and 74b represent a unique nitrogen-triggered mechanism that is different from either the hydride addition-elimination pathway or the 7i-allyl mechanism resulting in the intramolecular 1,3-hydrogen shift. 

In a similar study, intramolecular carbohalogenation of alkenes with allylic halides led to different product mixtures when mediated by either the tin reagent or the palladium(O) complex30. Thus, an organometallic palladium-allyl mechanism is supported. 

The formation of both erythro and threo products from suitable allylic alcohols containing CD2OH groups implies that an -allylic mechanism is involved in their decomposition (equation 3). 

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