Butadiene insertion

D. Butadiene Insertion Into the Allyl-Ni11 Bond of the... 

Fig. 6. Selected geometric parameters (A) of the optimized structures of the key species for the feasible r 2-tra7is-butadiene insertion into the r -syw-allyl-Ni11 bond along 2b—> 7b. Free energies (AG, Ain kcalmol-1) are given relative to the favorable r 3-syn,r l(Cl),A-cis/r[2-trans-butadiene stereoisomer of the precursor 2b.

Scheme 3. Competing paths for formation of all-t-CDT, c, -CDT, c,c,t-CDT and all-c-CDT commencing from the initial coupling product species 2b. Butadiene insertion along 2b —> 7b preferably takes place into the r 3-allyl-Nin bond of 2b. (NB Only one of the four possible stereoisomers is displayed for each of the given species 2b and 7b, respectively.)...

The complete branch for formation of bis(allyl),A-cA-dodecatrienediyl-Ni11 forms is shown to be disabled, because of (i) the unfavorable coupling of two czj-butadienes along Tb -> 2b together with a slow isomerization via r 3-antiy]1 (C3) isomers of TSisoPb], which prevents a sufficient concentration of rx -anti,r l(Cl),A-cis precursors 2b, and (ii) owing to a kinetically impeded butadiene insertion into the p3-ararz-ailyl-Nin bond along 2b 7b. Consequently, the all-c-CDT production route is entirely precluded. 

The initial coupling ri3,r 1(C1)-octadienediyl-Nin compound is likely to represent the critical species that connects the alternative reaction channels. The concentration of the respective species 2a and 2b and their reactivity when undergoing subsequent butadiene insertion or reductive elimination, which is accompanied by intramolecular C-C bond formation, represent important factors for the regulation of the C8 Ci2 product ratio. 

Tellurium sources, 22-24 Thermodynamics in cyclo-oligomerization, 185-186 butadiene insertion, 187-188 reductive elimination, 193, 194 selectivity control, 212 polysilane isomerisation, 158-160 see also Stability Thermolysis, 135, 136, 158 THF (tetrahydrofuran), 97, 150, 153 Thio-Wittig reaction, 37 Tin, 121... 

A chiral Mt -O73-butenyl) species will give, depending on its structure and thus on the orientation of the incoming monomer, a new Mt ( -butenyl) species of the same chirality as the previous one (and hence an isotactic diad) or of the opposite chirality (and hence a syndiotactic diad) it is obvious that the tacticity may concern only 1,2-polymers of non-substituted or substituted butadiene and 1,4-polymers of terminally monosubstituted and symmetrically disubstituted butadiene. The mode of the formation of the butenyl group of the same or opposite chirality with respect to the preceding butenyl group is shown, for 1,3-butadiene insertion, in Figure 5.4 [7],... 

As in the other cases of C-C bond formation catalyzed by transition metal complexes, the critical step is to form the initial metal-carbon bond, which is butadiene insertion into a Rh-H bond to form a Rh-allylic species. 

In each case the titanium remains in the tetravalent state throughout the whole polymerization, indicating the adequate stability of the organometallic compound to 50 °C. Furthermore, only one benzyl anion is transformed by butadiene insertion into the butenyl end group of the growing chain. 

Quite analogously, the trans selectivity of the allylnickel iodide [Ni( / -C3H5)I]2 can be explained, but with the butadiene insertion as the rate-determining step in the catalytic cycle [71-73]. 

Scheme 6 Formation of catalytically active species and pathway of butadiene insertion (CN coordination number)...

Insertion of dienes into M-H bond or M-alkyl bond affords r -allylic complexes or its )7 -alken-J7 -yl resonance form. The allylic complex may further undergo insertion of other unsaturated compounds such as alkene or diene into the unsubstituted or substituted terminal of the allyhc ligand. If successive butadiene insertion takes place, polymers with internal unsaturated bonds are produced as will be described later. A nickel-catalyzed reaction of butadiene with 2 mol of HCN affords adiponitrile, an important feedstock in polymer synthesis (Eq. 1.15). 

Scheme 1.29 illustrates the 1,3-butadiene insertion processes to give complexes with linear or branched chain attached to the metal. 

Scheme 1.29. Two modes of 1,3-butadiene insertions into M-R bond.

The olefin insertion into the syn-q -allyl-Ni" bond of 2, 2 gives rise to the q q, A- rans,-decatrienyl-Ni" isomers of 5 and bis(q )-allyl,A-frans,-dodecatrienediyl-Ni" isomers of 11, respectively, in an exergonic, irreversible process. These isomers, where the allylic groups preferably adopt the mode, are the thermodynamically favorable forms of the decatrienyl-Ni" and dodecatrienediyl-Ni" complexes, which furthermore represent the active precursor species for their decomposition into Cjo- and Ci2-olefins, respectively. Butadiene insertion, although kinetically disfavored (see above), is thermodynamically favorable when compared with ethylene insertion. This leads to strongly stabilized bis(q ),A-trans,-dodecatrienediyl-Ni" species, which act as a thermodynamic sink in the catalytic reaction course, and hence is well suited for experimental isolation and characterization [3a, 6b, 26,27]. 

Scheme 14 Butadiene insertion Markovnikov and anti-Markovnikov mechanism...

The insertion of an olefin into a C—M bond is a critical step in some olefin dimerization and polymerization reactions (Section IV,D), but studies of this reaction have not been very fruitful. The insertion of isobutylene into the C—Ti bond of CH3TiCl3 to give a neopentyl derivative is one of the few straightforward examples of this reaction (133). Olefin, diene, and acetylene insertion in acylcobalt compounds have been reported by Heck in an elegant series of papers sununarized in his review of this area (3). These insertions are often quite complex, as illustrated by the butadiene insertion. 

Previous kinetic studies of the [NiaCOaC j-allyOal-catalysed polymerization of butadiene suggest that the active metal species is [Ni(I)( /-aIIyl)] but studies of the [Ni(I)( y-l-MeC3H4)]-butadiene system indicate that if the complex concentration is sufficiently high the undissociated dimeric form acts as the catalyst. N.m.r. studies indicate that butadiene insertion into the Ni—( 7-allyl) bond is the first step in the polymerization and follows a first-order rate law with respect to the catalyst. The polymerization then proceeds by similar multiple insertions in the propagation step of the reaction. 

Chain propagation If k = rates of butadiene insertion, statistics of placement = f ( kj / )... 

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