Allyl complexes isomerization

The derivative 171 is of interest in this series of complexes since it undergoes isomerization of the alkyne substituent in position 1 to yield 172 and 173. The allyl complex 172 reacts with diphenylarsine to yield predominantly the chelate 174 (X = AsPh2) together with 175 and 176 (98IC1105). Complexes of the types 174 (X = PPh2) and 177 are known (93BSCF673 97BSCF471). 

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. ... 

Thermal insertion occurs at room temperature when R is XCH2CHAr-, at 40° C when R is benzyl, allyl, or crotyl (in this case two isomeric peroxides are formed), but not even at 80° C when R is a simple primary alkyl group. The insertion of O2 clearly involves prior dissociation of the Co—C bond to give more reactive species. The a-arylethyl complexes are known to decompose spontaneously into CoH and styrene derivatives (see Section B,l,f). Oxygen will presumably react with the hydride or Co(I) to give the hydroperoxide complex, which then adds to the styrene. The benzyl and allyl complexes appear to undergo homolytic fission to give Co(II) and free radicals (see Section B,l,a) in this case O2 would react first with the radicals. 

The reactions are accelerated by bromide salts, which are thought to exchange for acetate in the ir-allylic complex. The reactions of acyclic compounds occur with minimal E Z isomerization. This result implies that the TT-allyl intermediate is captured by carbonylation faster than E Z isomerization occurs. 

Related bimetallic systems were also prepared with M = Ni, Pd, Pt or Ag [3-5]. This unprecedented Tl2- t2-SiO bonding mode is also found in the siloxane-substituted complex 4 [6], and may assist the rapid c-Ji-isomerization observed in the allylic complex 5. 

Allylic C/H insertion accompanied by an allylic rearrangement has been observed for carbenoid reactions of ethyl diazoacetate with allylamines (Scheme 23)1S1). Apparently, metal-catalyzed isomerization 117 118 proceeds the C/H insertion process. Although mechanistic details have not yet been unraveled, T)3-allyl complexes... 

In the Rh-BINAP-catalyzed allyl amine isomerization step used in Takasago s Menthol process, the catalyst is inhibited by water through the formation of a hydroxyl-bridged rhodium trinuclear complex [ Rh(BINAP) i(/<2-0H)2]C104 [61]. 

Our calculations show that the isomerization of the silyl-alkyl complex to form a V-allyl complex affords a significant stabilization as summarized in Figure 11. TheiV toil3 isomerization of 9a to the anti silyl-allyi complex, 10a-anti, results in a 9.5 kcal/mol stabilization and isomerization to the syn isomer, lOa-syn, results in a 7.2 kcal/mol stabilization. Isomerization of 9b to the anti silyl-allyi complex, 10b-anti, results in a 6.1 kcal/mol stabilization and isomerization to the syn isomer, lOb-.vyn results in a 5.4 kcal/mol stabilization. High temperature (500 °C) molecular dynamics simulations initiated at the V complex, 9a, reveal that the rj1 to T 3 isomerization has a minimal barrier and occurs in the sub-pico time frame. The inter-conversion between the syn and anti isomers has not been examined since both isomers are stereochemically equivalent, however, we expect the barrier to be small. 

Figure 11. Silyl-allyl complexes as a result of the r)1 to rj3 isomerization from 9a and 9b. The isomerization energies, AEU0, are reported in kcal/mol.

The reaction of but-l-en-3-yl diethyl phosphate with diethylamine produces N,N-diethylpent-3-enamide (86%), indicating that a Ji-allyl complex is involved in the carbonylation reaction. No isomerism to the a,p-unsaturated amides was observed. 

The dissociative mechanism can explain both facts in that the hydrogen removed in the first step may recombine with an isomeric form of the ally lie intermediate to yield the isomeric olefin. Apparently syn and anti 7T-allylic complexes [67, 68) retain their configurations unless each may be converted into a common a-bonded complex in which the nonterminal carbon atoms of the allyl group are connected by a single bond and the isomerization of the intermediate can be represented as in Fig. 11. However, the recombination of the hydrogen atom with the allylic intermediate must be faster than the rate at which it enters the surface pool of... 

Pd-catalyzed reactions proceed via 7i-allyl complexes, which at room temperature isomerize via a ti-o-ti rearrangement. As a consequence, branched as well as Z-and B-linear starting materials yield the same products, with memory effects being minimal at room temperature [16], The isomerization processes of (allyl)Ir complexes are usually slow, and accordingly any memory effects are pronounced. The configurational stability of (allyl) Ir intermediates of the Ir-catalyzed allylic substitution was studied by an investigation of substitutions at nonracemic allylic substrates (Scheme 9.5). 

Subsequently to rhodium coordination with the enyne to form X, oxidative addition with the allyl chloride affords a rhodium-7r-allyl complex. Then isomerization... 

Keto esters such as acetoacetate without a substituent at the a-carbon. namely with two acidic protons, first attack the central carbon of the allenylpal-ladium to form the zr-allyl complex 89. Then intramolecular attack of the enolate oxygen of the 0-keto ester at the rr-ully l system takes place to form the nieihv-lenedihydrofuran 90 as a primary product, which is easily isomerized to the turn 91. The /3-diketone 92 reacts similarly to give the furan 93[40], The reaction can be applied to the synthesis of the phenyIthiomethyl-substituted furan 94. which is useful for the synthesis of natural products such as neoliacine. [41]... 

The reaction is believed to proceed via a c-allyl copper complex, in which the carbon-copper bond is formed at the "/-position, anti to the acetate leaving group. Reductive elimination of copper led to pure "/-substitution. With cyclic aliphatic allylic acetates, the selectivity is generally lower because the o-allyl copper complex can isomerize to the ir-allyl complex with loss of regioselectivity. 

Some of the bicyclo[3.1.0]hexene zirconocene derivatives, especially those containing a phenyl group, rearrange quantitatively to the corresponding 4-vinyl-l-metalla-2-cyclobutene derivatives when heated in toluene to 60-80 °C for several hours (equation 131). The 6,6-dimethyl-2,3-diphenyl-2-zirconabicyclo[3.1.0]hex-3-ene derivative prefers ring-opening to the isomeric j/. -allylic complex upon thermolysis. 

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