Butadiene insertion reactions

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... 

The main path of the palladium-catalyzed reaction of butadiene is the dimerization. However, the trimerization to form /j-1, 3,6,10-dodeca-tetraene takes place with certain palladium complexes in the absence of a phosphine ligand. Medema and van Helden observed, while studying the insertion reaction of butadiene to 7r-allylpalladium chloride and acetate (32, 37), that the reaction of butadiene in benzene solution at 50°C using 7r-allylpalladium acetate as a catalyst yielded w-1,3,6,10-dodecatetraene (27) with a selectivity of 79% at a conversion of 30% based on butadiene in 22 hours. 

The rate also varies with butadiene concentration. However, the order of the rate dependence on butadiene concentration is temperature-de-pendent, i.e., a fractional order (0.34) at 30°C and first-order at 50°C (Tables II and III). Cramer s (4, 7) explanation for this temperature effect on the kinetics is that, at 50°C, the insertion reaction to form 4 from 3, although still slow, is no longer rate-determining. Rather, the rate-determining step is the conversion of the hexyl species in 4 into 1,4-hexadiene or the release of hexadiene from the catalyst complex. This interaction involves a hydride transfer from the hexyl ligand to a coordinated butadiene. This transfer should be fast, as indicated by some earlier studies of Rh-catalyzed olefin isomerization reactions (8). The slow release of the hexadiene is therefore attributed to the low concentration of butadiene. Thus, Scheme 2 can be expanded to include complex 6, as shown in Scheme 3. The rate of release of hexadiene depends on the concentra-... 

In practice, butadiene is present in large excess, so that the insertion reaction (3 — 4, see Scheme 2), which depends only on the concentration of ethylene, becomes the rate-determining step. 

In this codimerization reaction, the predominant complex is 3, which should lead to the ethylene-butadiene codimerization product. If the ethylene in complex 3 is displaced by butadiene to form 7 before the insertion reaction takes place, then a C8 or higher olefin could be formed... 

Reactions a and b in Scheme 8 represent different ways of coordination of butadiene on the nickel atom to form the transoid complex 27a or the cisoid complex 27b. The hydride addition reaction resulted in the formation of either the syn-7r-crotyl intermediate (28a), which eventually forms the trans isomer, or the anti-7r-crotyl intermediate (28b), which will lead to the formation of the cis isomer. Because 28a is thermodynamically more favorable than 28b according to Tolman (40) (equilibrium anti/syn ratio = 1 19), isomerization of the latter to the former can take place (reaction c). Thus, the trans/cis ratio of 1,4-hexadiene formed is determined by (i) the ratio of 28a to 28b and (ii) the extent of isomerization c before addition of ethylene to 28b, i.e., reaction d. The isomerization reaction can affect the trans/cis ratio only when the insertion reaction d is slower than the isomerization reaction c. 

In most of the reactions of heteroatom-substituted carbene complexes with alkynes the first event is insertion of the alkyne into the carbon-metal double bond. If vinylcarbene complexes undergo insertion reactions with alkynes, (1,3-butadien-l-yl)carbene complexes result (Figure 2.27). 

Metal-Halogen Compounds. An unusual example of the addition of a metal halide to a conjugated diene has been reported. The complex formed from palladium chloride and butadiene has been shown to be a dimer of 1-chloromethyl-7r-allylpalladium chloride, (85). Whether this is a true insertion reaction or some type of ionic reaction has not been determined, but its close analogy with the olefin-palladium chloride insertion reaction mentioned above would suggest an insertion mechanism for the diene reaction also. 

Although bis(phosphite) carbyne complex Cp[P(OMe),]2Mo=C(c-Pr) is incapable of undergoing carbonyl insertion reactions, it adds 1 equivalent of HCl in ether forming the ring-opened -butadiene complex Cp[P(OMe),l(Cl)Mo( 4-butadiene) in 15% yield, and P(OMe)j in equal amounts (equation 108)158. Careful analysis of the reaction using... 

The metal carboxylate insertion mechanism has also been demonstrated in the dicobaltoctacarbonyl-catalyzed carbomethoxylation of butadiene to methyl 3-pentenoate.66,72 The reaction of independently synthesized cobalt-carboxylate complex (19) with butadiene (Scheme 8) produced ii3-cobalt complex (20) via the insertion reaction. Reaction of (20) with cobalt hydride gives the product. The pyridine-CO catalyst promotes the reaction of methanol with dicobalt octacarbonyl to give (19) and HCo(CO)4. 

A 1,4 addition seems to occur with dienes like 1,3-butadiene derivatives and benzene. The formation of a yellow compound by reaction of SiO with benzene results in a compound stable in air up to 500 °C. The appearance of Si-H vibrations in the IR spectrum suggests an insertion reaction in the C-H bond, similar to the reaction of SiF2 with benzene. 

Although bis(phosphite) carbyne complex Cp[P(OMe)3]2Mo=C(c-Pr) is incapable of undergoing carbonyl insertion reactions, it adds 1 equivalent of HCl in ether forming the ring-opened f/ -butadiene complex Cp[P(OMe)3](Cl)Mo( / -butadiene) in 15% yield, and P(OMe)3 in equal amounts (equation 108) . Careful analysis of the reaction using two equivalents of HCl reveals the presence of the metal hydride complex Cp[P(OMe)3]2Cl2MoH as the main products (70%), and free butadiene. It was furthermore shown that the two molybdenum complexes are not interconvertible under the reaction conditions and both the yields and products ratio are invariant with temperature in the range of -40 °C to room temperature and the amount of added HCl (1 or 2 equivalents). 

Bu NC yields related imine complexes (calix[4]OMe)Ta() -Bu N=CR2). Although butadiene is -bonded to the metal in Ta / -Bu -calix[4]-(0Me)(0)3 () " -C4H6), it behaves as ifitwere bonded in the migratory insertion reaction... 

An analogous insertion reaction into the allyl-ruthenium bond occurs when (jt-C3H5)Ru(CO)3Cl is treated with butadiene at 70-80 °C in hydrocarbon solution. 

For the insertion reaction, two different mechanistic possibilities exist. As was first suggested by Cossee and Arlman [34, 35], the rf - or i/ -coordinated butadiene, acting as an electrophile in each case, can undergo a nucleophilic attack by the butenyl anion in its ir-bonded structure. Simultaneously with the C-C bond formation from the butadiene an i/ -coordinated butenyl anion is regenerated as the chain end, while the polybutadienyl chain has been elongated by a further C4 unit with one new double bond (cf Scheme 3). 

During a very short initiation period the cation [Ni(Ci2H 9)] , which is present mainly in the thermodynamically more stable syn form b, reacts via the less stable but more reactive anti form a with insertion of butadiene into the anti--polybutadienyl complex c. As a result of the very rapid anti-syn isomerization this complex also exists in equilibrium (cf. Kf) with the more stable syn complex d, which must be regarded as the stable storage complex under the conditions of polymerization. With butadiene the polybutadienyl-butadiene complexes e and f are formed as the actual catalysts. By the much higher reactivity of the less stable anti complex e, the formation of cis units is catalyzed. Since all the equilibria can be assumed to be rapid, the insertion reaction of butadiene (/ 2c) has to be taken as the rate-determining step in the catalytic cycle. Thus, the catalytic activity is dertermined thermodynamically by the concentration of the -c/5-butadiene complex in the anti form e and kinetically by its reactivity k2c-... 

This means that the cis-trans selectivity depends solely on the difference between the free standard enthalpies AGg - AGf of the transition states for the insertion reaction with the anti and the syn form of the polybutadienyl cw-butadiene complex e and f, respectively, independently of their concentration ratio in the anti-syn equilibrium K. To explain the cis-trans selectivity of 93 % cis and 4 % trans attributed to the ligand-free allylnickel(II) cation, a stability difference between the transition states of about 1.9 kcal mol (8 kJ mof ) must be assumed, which can easily arise from the different steric conditions for the necessary coordination of the next double bond in the anti and the syn forms e and f of the catalyst complex. 

Instead of the coordination of the next double bond in the growing chain, one hard ligand or anion can coordinate to the nickel, in the axial position either above or below the plane of the complex, and give the necessary energetic support for the insertion reaction with the // -c/ -coordinated butadiene see Structure 3 in Scheme 6. 

The entrance into the catalytic cycle from complex 5 may occur via a small equilibrium concentration of Ni-(la)-(MVN) complex6 (pathA, Schemes) and/or via oxidative addition of HCN to generate the species Ni-[la]-HCN, 7 (pathB). In either event, formation of the hydridoalkene complex Ni-[1]-(MVN)(H)(CN), 8, occurs and is followed by an insertion reaction to produce the (ri -benzyl)nickel cyanide intermediate 9. Although this allyl-type species has not been directly detected, the exclusive formation of the branched nitrile supports its intermediacy. Analogous intermediates have been postulated in the hydrocyanation of 1,3-butadiene with NilPlO-o-tolylljjj or Ni[P(OEt)3]4 and in the hydrocyanation of styrene with Ni[P(0-p-tolyl)3]4. Examples of other nick-el-benzyl complexes exhibiting similar allylic interactions in the solution and solid state are also known. 

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). 

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]. 

---