Metal-allyl systems

Unlike many other metal-allyl systems, allylsilanes are rcgio-stable at inormal temperatures, with 1,3-sigmatropic shifts occurring at a significant Irate only at temperatures in excess of 300°C. 5-Trimethylsilylcyclo-Ipentadiene provides an exception to this generalization, but it can still be lhandled quite readily. 

The deprotonation of alkenes by organometallic reagents affords allyl species. As the simplest example of delocalized organometallic systems, the alkali metal allyl system has been studied in solution and the solid state in quite some detail this work has been further supported by theoretical studies. Allyl species are usually very reactive undergoing complex rearrangement reactions, and often, the reaction products cannot be directly characterized. Instead, they are often identified by their reaction products. 

Trost and Hachiya [140] studied asymmetric molybdenum-catalyzed alkylations. Interestingly, they noticed that the regioselectivity of this transformation performed with a non-symmetric allylic substrate varied according to the nature of the metal Pd-catalyzed substitutions on aryl-substituted allyl systems led to attack at the less substituted carbon, whereas molybdenum catalysis afforded the more substituted product. They prepared the bis(pyridylamide) ligand 105 (Scheme 55) and synthesized the corresponding Mo-complex from (C2H5 - CN)3Mo(CO)3. With such a catalyst, the allylic... 

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. 

It is more difficult to study equilibria between transition metal allyl compounds and bases, olefins, etc. In the case of Zr (allyl) 4 and pyridine, a valency change occurs as shown by Eq. (8), and the process is irreversible. The polymerization is considered to be preceded by displacement of one allyl group by the monomer (12) as shown in Eq. (1). In the methyl methacrylate/Cr(allyl)3 system it was not possible to detect any interaction between the olefin and catalyst with infrared radiation, even with equimolar concentrations because of the strong absorption by the allyl groups not involved in the displacement processes. Due to the latter, evidence for equilibrium between monomer and catalyst is less likely to be found with these compounds than with the transition metal benzyl compounds. 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

The polymerization of conjugated dienes with transition metal catalytic systems is an insertion polymerization, as is that of monoalkenes with the same systems. Moreover, it is nearly generally accepted that for diene polymerization the monomer insertion reaction occurs in the same two steps established for olefin polymerization by transition metal catalytic systems (i) coordination of the monomer to the metal and (ii) monomer insertion into a metal-carbon bond. However, polymerization of dienes presents several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of o type for the monoalkene polymerizations, while it is of the allylic type in the conjugated diene polymerizations.174-183... 

Allylmetallic reagents The ally] anions obtained by reductive metallation of ally I phenyl sulfides with lithium l-(dimethy amino)naphthalenide (LDMAN, 10, 244) react with a, 3-enals to give mixtures of 1,2-adducts. The regioselectivity can be controlled by the metal counterion. Thus the allyllithium or the allyltitanium compound obtained from either 1 or 2 reacts with crotonaldehyde at the secondary terminus of the allylic system to give mainly the adduct 3. In contrast the allylcerium compound reacts at the primary terminus to form 4 as the major adduct. 

A new electrolysis system comprising two metal redox couples, Bi(0)/Bi(III) and A1(0)/A1(III), has been shown to be effective for electroreductive Barbier-type allylation of imines [533]. The electrode surface structure has been correlated with the activity towards the electroreduction of hydrogen peroxide for Bi monolayers on Au(III) [578], Electroreductive cycliza-tion of the 4-(phenylsulfonylthio)azetidin-2-one derivative (502) as well as the allenecarboxylate (505) leading to the corresponding cycKzed compounds (504) and (506) has been achieved with the aid of bimetallic metal salt/metal redox systems, for example, BiCh/Sn and BiCh /Zn (Scheme 175) [579]. The electrolysis of (502) is carried out in a DMF-BiCh/Py-(Sn/Sn) system in an undivided cell by changing the current direction every 30 s, giving the product (504)in 67% yield. 

Asymmetric allylic substitutions are widely applied in organic synthesis, using various metal complexes, chiral ligands, nucleophiles and allyl systems [39]. Although Pd is often the metal of choice, this is not the case for monosubstituted allylic substrates, where most Pd catalysts predominantly produce the achiral linear product. In contrast. Mo, W and Ir catalysts preferentially give rise to the desired branched products and, in recent years, a number of very effective Ir catalysts for various substrates have been developed [40]. Since, to the best of our... 

Finally, the hybridization of the carbon atom also has a marked effect on its willingness to attach to the transition metal. Allyl or benzyl halides undergo oxidative addition faster than aromatic or vinyl halides. The least reactive are alkyl halides which require the use of nickel(O)9 complexes or highly active catalyst systems.10 If we start from an optically active substrate, then the oxidative addition usually proceeds in a stereoselective manner. 

During the history of a half century from the first discovery of the reaction (/) and 35 years after the industrialization (2-4), these catalytic reactions, so-called allylic oxidations of lower olefins (Table I), have been improved year by year. Drastic changes have been introduced to the catalyst composition and preparation as well as to the reaction process. As a result, the total yield of acrylic acid from propylene reaches more than 90% under industrial conditions and the single pass yield of acrylonitrile also exceeds 80% in the commercial plants. The practical catalysts employed in the commercial plants consist of complicated multicomponent metal oxide systems including bismuth molybdate or iron antimonate as the main component. These modern catalyst systems show much higher activity and selectivity... 

The oxidation of propene to acrolein has received much attention for several reasons. Firstly, the process is of industrial importance in itself, and it is also a suitable model reaction for the even more important, but at the same time more complicated, ammoxidation. Secondly, propene oxidation is, in many aspects, representative of that of a class of olefins which possesses allylic methyl groups. Last, but not least, the allylic oxidation is a very successful example of selective catalysis, for which several effective metal oxide systems have been discovered. The subject has therefore attracted much interest from the fundamental point of view. 

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