Metal allyl mechanism

Another difference between the two mechanisms is that the former involves 1,2 and the latter 1,3 shifts. The isomerization of 1-butene by rhodium(I) is an example of a reaction that takes place by the metal hydride mechanism, while an example of the TT-allyl complex mechanism is found in the Fe3(CO)i2 catalyzed isomerization of 3-ethyl-l-pentene. " A palladium acetate or palladium complex catalyst was used to convert alkynones RCOCSCCH2CH2R to 2,4-alkadien-l-ones RCOCH= CHCH = CHCHR. ... 

A detailed study of the mechanism of the insertion reaction of monomer between the metal-carbon bond requires quantitative information on the kinetics of the process. For this information to be meaningful, studies should be carried out on a homogeneous system. Whereas olefins and compounds such as Zr(benzyl)4 and Cr(2-Me-allyl)3, etc. are very soluble in hydrocarbon solvents, the polymers formed are crystalline and therefore insoluble below the melting temperature of the polyolefine formed. It is therefore not possible to use olefins for kinetic studies. Two completely homogeneous systems have been identified that can be used to study the polymerization quantitatively. These are the polymerization of styrene by Zr(benzyl)4 in toluene (16, 25) and the polymerization of methyl methacrylate by Cr(allyl)3 and Cr(2-Me-allyl)3 (12)- The latter system is unusual since esters normally react with transition metal allyl compounds (10) but a-methyl esters such as methyl methacrylate do not (p. 270) and the only product of reaction is polymethylmethacrylate. Also it has been shown with both systems that polymerization occurs without a change in the oxidation state of the metal. 

The metal-alcoholate mechanism is well established for allylic alcohol epoxidation in the presence of Ti and V catalysts. [41, 51, 52, 111-113], In principle, it can provide a viable pathway also for catalysis by a Re complex. In fact, allylic alcohols may add, at least formally, to either an oxo-Re or peroxo-Re moiety (e.g. of 5a or 5b) in a process which is referred to as metal-alcoholate binding this mechanism gives rise to metal-alcoholate intermediates. We identified four intermediates of alcohol addition to di(peroxo) complexes two resulting transition states, S-8 and S-9b, are shown in Figure 11. All metal-alcoholate intermediates he significantly higher in energy (by 10-22 kcal/mol) than 5b + propenol, except the... 

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. ... 

Scheme 7.15] or S -type mechanism [Equation (7.9)]. Depending on the nature of the nucleophile and catalyst employed, the subsequent nucleophilic substitution of the metal can follow either via a-elimination [path A, Equations (7.8) and (7.9), Scheme 7.15], via an SN2 reaction (path B) or via an SN2 -type reaction (path C). For reasons of clarity, only strictly concerted and stereospecific SN2- or SN2 -anti-type mechanistic scenarios are shown in Scheme 7.15. The situation might, however, be complicated if, e.g., the initial S l -anti ionization event is competing with an Sn2 -syn reaction. Erosion in stereo- and regioselectivity can be the result of these competing reactions. Furthermore, fluxional intermediates such as 7t-allyl Fe complexes are not shown in Scheme 7.15 for reasons of clarity. These intermediates are known for a variety of late transition metal allyl complexes and will be referred to later. Moreover, apart from these ionic mechanisms, radicals might also be involved in the reaction. So far no distinct mechanistic study on allylic substitutions has been published.

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 ... 

At low temperatures the intermediate /3-allylrhodium(III) hydride PF3 complex was detected by 1H and 19 F NMR spectroscopy and provided supportive evidence for the isomerisation of alkenes via an allyl-metal-hydride mechanism (285, 293). 

The fundamental differences between these two mechanisms are that 1) the jr-allyl metal hydride mechanism involves a 1,3-hydrogen shift while the metal hydride addition-elimination mechanism involves a 1,2-hydrogen shift and 2) the hydrogen shift in the Jt-allylhydride mechanism proceeds in an intramolecular fashion while that in the metalhydride addition-elimination mechanism proceeds in an intermolecular fashion. 

A crossover experiment using 3 and 4 under standard conditions demonstrated the intramolecularity of this hydrogen shift. Intramolecularity of the isomerization and the 1,3-hydrogen shift strongly indicates that the reaction proceeds via the Jt-allyl metal hydride mechanism as depicted in Scheme 12.1. 

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. 

The mechanism for the carbonylation of allylic substrates is considered to be composed of the following processes (a) oxidative addition involving allylic C-0 bond cleavage to form r 3-allyltransition metal complexes (b) CO insertion into the metal-allylic bond and (c) nucleophilic attack to liberate carboxylic acid derivatives (Scheme 6). 

Isomerization of allylic and homoallylic alcohols is also catalyzed by the zwitterion-ic Rh(I) complex (sulphos)Rh(COD), with sulphos = [03S(C6H4)CH2C(CH2PPh2)3], in water/n-octane to give the corresponding aldehyde or ketone in high yields and chemoselectivity. A jr-allyl metal hydride mechanism was proposed on the basis of various independent experiments in both homogeneous and biphasic systems [3a]. 

This system is analogous to the HMo(T7 -C3H5)(diphos)2 system developed by Osborn et al. 17), which provided the first direct observation of the TT-allyl-hydride exchange mechanism proposed for the 1,3-hydride shifts found in many metal-catalyzed olefin reactions. Presumably, this hydride undergoes a fluxional rearrangement of the diphos ligands in addition to the metal(allyl)(hydride) <= metal (propene) interconversion. 

Allyl Mechanism The second common mechanism involves allyl intermediates and is adopted by those metal fragments that have two 2e vacant sites but no hydrides. It has been established for the case of Fe3(CO)i2 as catalyst, a system in which Fe(CO)3, formed by fragmentation of Ae cluster on heating, is believed to be the active spedes. The cluster itself is an example of a catalyst precursor. As a 14e spedes, Fe(CO)3 may not have an indepen-... 

Isomerization of olefins by transition-metal complexes is one of the most important goals in organometallic chemistry [6, 7]. For the topic considered here [8], two principal mechanisms can be distinguished (Scheme 5.2) (a) metal hydride addition-elimination mechanism (alkyl mechanism) [9], and (b) reaction via a it-allyl metal hydride intermediate (allyl mechanism) [10]. 

Reaction Mechanism-. The preferential anfi-Markovnikov or 2,1-addition to styrene can be attributed to the organization of the transition state to N-C bond formation. Factors that stabilize the developing anionic charge upon the atom adjacent to the metal center in the transition state to N-C bond formation will be expected to lower the activation energy of the insertion step [98,99]. In the case of the 2,1-insertion of styrene into the Ca-N bond the phenyl group may stabilize the adjacent anionic center. In the case of a 1,2-insertion, no such stabilization exists. The product distribution for the hydroamination of dienes can be explained by invoking a metal allyl complex upon addition of M-NR2 to the diene. 

Alkenes.—Deuterium exchange of propene with MeOD homogeneously catalysed by complexes of platinum, rhodium, and nickel can be monitored by microwave spectroscopy. The results show considerable incorporation of deuterium at C-2, a result which cannot be accommodated by the 7t-allyl-metal hydride mechanism for exchange/isomerization of olefins. However, the n- or /i -allyl-metal hydride mechanism for olefin isomerization has received some useful supporting evidence. The compound (54) can be generated... 

The regiochemical and stereochemical outcome of this reaction is explained by a double Sn2 substitution of the metal and subsequently of the nucleophile with inversion of the configuration in each step without the intermediacy of an q -allyliron complex (a-allyl mechanism) (Scheme 4-197). 

---