Carbon-metal bond, diene insertion

Dicumyl chloride 61 p-Dicumyl methyl ether 10 Dielectric constant 123 Diene insertion, carbon-metal bond 165... 

A number of transition metal complexes react with alkenes, alkynes and dienes to afford insertion products (see Volume 4, Part 3). A general problem is that the newly formed carbon-metal bond is usually quite reactive and can undergo a variety of transformations, such as -hydride elimination or another insertion reaction, before being trapped by an electrophile.200 Usually, a better stability and lower reactivity is observed if the first carbometallation step leads to a metallacycle. It is worthy to note that the carbometallation of perfluorinated alkenes and alkynes constitutes a large fraction of the substrates investigated with transition metal complexes.20015... 

The attachment of a slightly polar carbon-metal bond to a symmetric or quasisymmetric diene is in correspondence with respect to symmetry (pseudosymmetry) and, according to the correspondence principle, should proceed relatively rapidly. It is interesting to note that as far back as 30 years ago Stearns and Forman [64] and Szwarc [65] drew an analogy between the elementary act of diene insertion on the carbon-metal bond and the Diels-Alder reaction. Moreover, taking into account the fact that dienes can take part in the diene synthesis only in the cisoid conformation [66] it should be assumed by analogy... 

Carbon-metal bond, diene insertion 165 Cationation 1, 4, 7-10, 30, 37, 43-46 Cationogen 5 Chain conformation 280... 

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... 

It is clear that each polymerisation step involves the insertion of the coordinated conjugated diene into the active metal-carbon a bond, becoming a n bond owing to this insertion, which by subsequent monomer coordination reverts to a a bond. 

The presence of two double bonds greatly increase the number of the interconnected effects determining the configuration of an added monomeric unit. In this case the stereochemistry may already be established during the transition complex formation between the monomer and the centre or after the insertion of the diene into the metal—carbon bond. The realization of either of these possibilities is determined by the specific driving force of addition, composed of the electronic, steric, and isomeric states of the diene. 

This reaction constitutes a special type of process in which a hydrogen atom and a nucleophile are added across the diene with fonnation of a carbon-hydrogen bond in the 1-position and a carbon-Nu bond in the 4-position. Some examples of such reactions are hydrosilylation [12-18], hydrostannation [19,20] amination [21,22], and addition of active methylene compounds [21 a,23,24], These reactions are initiated by an oxidative addition of H-Nu to the palladium(0) catalyst, which produces a palladium hydride species 1 where the nucleophile is coordinated to the metal (Scheme 8-1). The mechanism commonly accepted for these reactions involves insertion of the double bond into the palladium-hydride bond (hydride addition to the diene), which gives a (jr-allyl)palladium intermediate. Now depending on the nature of the nucleophile (Nu) the attack on the jr-allyl complex may occur either by external trans-aVtBck (path A) or via a cw-migration from palladium to carbon (path B). 

An interesting cyclization reaction was reported that involved the reaction of dienes, diynes, or ene-ynes with transition metals to form cyclopentenone derivatives in the presence of carbon monoxide.363 in a simple example, ene-yne 444 was heated with dicobalt octacarbonyl and CO to give a 68% yield of 445.364 jjjj transformation has become an important synthetic tool known as the Pauson-Khand reaction.365 jhe mechanism probably involves insertion of the alkene (or alkyne) into the transition metal bond, which is why it is presented in this section. Formally, it is a [2+2+l]-cycloaddition, but the accepted mechanism is the one proposed by Magnus,364 and shown in Figure 13.8.366 n has been stated that further study is required to... 

Because the reaction of insertion can proceed only via the metal-carbon a-bond, at the first stage, the calculations were aimed at finding the manner in which the coordinating diene can stabilise the o-stmcture of the active centre. The calculations of prereaction complexes showed that both piperylene and isoprene (like butadiene [79]) can stabilise the o-form of the centre if they occupy the coordination site in the Nd sphere (Figure 3.1c), which becomes... 

In the polymerisation of piperylene regioselectivity, addition of 1,4-units according to head-to-tail type, is caused by the fact that only the double bond of diene bearing no methyl substituent can react with the metal-carbon o-bond of the active centre. Involvement of the double bond of piperylene bearing the methyl substituent in the reaction of insertion is considerably hampered because of the steric repulsion between the methyl groups of the terminal unit of the growing chain and the diene. The smaller difference in energy between the two possible o-forms of the terminal unit in the polymerisation of piperylene, as compared to butadiene, is responsible for the increased lifetime of the active centre in the o-form, in which the Nd atom is linked to the atom. This results in an increased content of trans-1,2- and trans-1,4 units in polypiperylene. 

Many examples of alkene and alkyne insertion into metal-carbon bonds can also be found in the section on homogeneous catalysis. Other recent examples include the insertion of conjugated dienes into palladium-allyl bonds, olefin arylation in the presence of palladium acetate, and the reaction of ethylene with arylmagnesium halides in the presence of nickel chloride. Reaction of isocyanates with nickel-ethynyl compounds... 

The propagation reaction proceeds via insertion into these carbon-transition metal bonds after the diene has been coordinated as a Ti-complex ... 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

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