Isobutene mechanism

The reduction product arising either from the decomposition1 of r-BuMgBr into isobutene and HMgX or through a six-membered Meerwein Ponndorf-Vcrlcy-like mechanism is also obtained (about 20% yield). 

Fig. 41. Proposed mechanisms for the reactions (a) Y + propene, (b) Y + cis-2-butene, (c) Y + 1-butene, (d) Y + isobutene. Note that the mechanism for Y + trans-2-butene is similar to that for Y + cis-2-butene and so is not shown. Double-sided arrows indicate resonance structures. See text for details.

Highly alkylated l-chloro-2-(trimethylsilyl)cyclopentenes 44, which are of interest as possible cyclopentyne precursors, were prepared by reacting 3-chloro-3-methyl-l-(trimethylsilyl)but-l-yne (45) with 1,1-dialkylated or 1,1,2-trialkylated ethylenes in the presence of titanium tetrachloride35. Because of the low S/v 1 reactivity of 45, the yields of the products were moderate. The stepwise [3 + 2]-cycloaddition mechanism discussed above was proven by the isolation of the intermediate acyclic adduct (in 74% yield) when 45 and isobutene were reacted in the presence of BCI3. Under these conditions, the intermediate 46 could be trapped by Cl since BCI4 is more nucleophilic than TiC.15 (equation 16). 

Tebbe and co-workers reported the first olefin metatheses between titanocene-methyli-dene and simple terminal olefins [13]. They showed cross-metathesis between isotopically labeled isobutene and methylenecyclohexane to be catalyzed by titanocene-methylidene. This process is referred to as degenerate olefm metathesis as it does not yield any new olefin (Scheme 14.6). The intermediate titanacyclobutane has been isolated and characterized [14], and its stability [15] and mechanism of rearrangement [16] have been investigated. 

Typical monomers which polymerise through cationic mechanisms are isobutene, styrene, a-methylstyrene, vinyl ethers and vinyl carbazoles. At present, about 100 vinyl monomers are known that can be polymerised by the cationic initiators. 

In 1965 Chmelir proposed another mechanism for the polymerisation of isobutene catalysed by AlBr3 in heptane solution. If the reaction system is dried it is found that upon addition of BF3 or TiCl4, no polymerisation takes place and only addition of water can bring about polymerisation. 

The polymerisation of isobutene is so far a repeatable, but not a reproducible reaction. Following on from the work of Seymour et al., in this department, and work at ICI and in America, we are attempting to elucidate the reaction mechanism of this polymerisation. 

The important feature was the recognition that because the relation between k and [TiClJ was rectilinear, the number of concentration terms on each side of the equilibrium had to be the same, which ruled out the involvement of ion-pairs. Our work opened up the new field of binary ionogenic equilibria (BIE) which was to prove an essential preliminary to unravelling the mechanism of the polymerisation of isobutene by A1C13. We also showed... 

Tater, Gantmakher and Medvedev [43] revived the Hunter-Yohe mechanism with the addition that they supposed it to operate only in polar solvents, the idea being that the zwitterion would thus be stabilised by solvation. The experiments of Colclough and Dainton [36] with the system styrene-stannic chloride-nitrobenzene, and those of Tongworth, Plesch and Rutherford [44] with isobutene-titanium tetrachloride-various alkyl chloride solvents, then showed that in these systems a co-catalyst is required and that therefore the HY-MG mechanism cannot be operative. It has in fact now been abandoned by its originators, who have conceded that in the styrene-SnCl4-EtCl system a co-catalyst is required [45]. 

This is, of course, the Hunter-Yohe mechanism, involving a zwitterion, but with this monomer it does not seem implausible in particular, the objection that it involves a large charge separation, which has some force in the case of a sterically hindered hydrocarbon such as isobutene, is probably not valid here, especially in view of the flexibility which... 

There remains, of course, the question why apparently isobutene (and perhaps other aliphatic olefins) do not polymerise by the pseudo-cationic mechanism - or do so much less readily than, say, styrene. Probably the short answer lies in the relative stabilities of the esters in the polymerisation conditions, (e.g., perchlorate stabilised by co-ordination of styrene). The long answer will have to be based on a detailed understanding of all the factors which determine this stability and thus govern the equilibrium between ester and ions. 

For isobutene polymerized in ethyl chloride by SnCl4 and H20 a transfer mechanism involving the catalytic complex has been suggested on kinetic grounds [4] ... 

Thus, our experiments with isobutene show that for this monomer the Gantmakher and Medvedev theory is not applicable they also show that for isobutene CH2C12 is not a cocatalyst to TiCl4. However, it was still possible that the polymerisation of styrene at the lowest water concentration was due not to residual water, but either to co-catalysis by the solvent or to direct initiation by the Gantmakher and Medvedev mechanism. However, since we found the molecular weight to be independent of both the water and the TiCl4 concentration and the rate at low water concentration to be independent of the TiCl4 concentration, these alternatives appeared unlikely. 

H20, temperature -30 °C to -95 °C) the initial rate of polymerisation of isobutene is proportional to the concentration of monomer, that of styrene proportional to the square of the monomer concentration [13], so that the same mechanism cannot apply to both monomers. Further, in the very system in which the Gantmakher and Medvedev mechanism would be most plausible styrene + SnCl4 + nitrobenzene [10] - the polymerisation is found to be of first order in monomer and not, as Gantmakher and Medvedev predict, of second order, and moreover depended on the presence of water. 

In the field of cationic polymerisation a notoriously intractable problem is the mechanism of initiation by aluminium halides. Despite much work on the polymerisation itself, there are few studies of the initiation mechanism. Existing theories are shown to be inadequate to explain the most characteristic features of the reactions when a solution of an aluminium halide in an alkyl halide is introduced into a solution of isobutene, there ensues a fast polymerisation which generally stops at incomplete conversion, and the number of moles of polymer formed is much smaller than the number of moles of initiator these features are found over a very wide range of conditions. 

When isobutene is added to a solution of AlBr3 in MeBr or of A1C13 in alkylchloride at -20 °C to -80 °C, the rate of increase of conductivity is of second order with respect to the A1 compound, which supports our suggested initiation mechanism. 

Various mechanisms have been proposed to explain the aforementioned laboratory results. The following low-temperature (atmospheric) sequence based on isobutene as the initial fuel was first proposed by Leighton [2] and appears to account for most of what has been observed ... 

Jr., A Comprehensive Kinetic Mechanism for CO, CH20, and CH3OH Combustion, Int.J. Chem. Kinet. 39, 109-136 (2007) Held, T., The Oxidation of Methanol, Isobutene, and Methyl tertiary-Butyl Ether, No. 1978-T, PJi.D. Dissertation, Princeton University, Princeton, NJ, 1993 Burgess, D. R. F., Jr, Zachariah, M. R., Tsang,... 

From a kinetic point of view, the formation of methacrolein and methacrylic acid occurs through two parallel reactions. This has been explained by hypothesizing the mechanism illustrated in Figure 14.4, elaborated starting with reactivity studies on isobutene, methacrolein, and methacrylic acid, and from Fourier... 

In decomposing oxonium ions the situation is quite different, i.e., the preference for alkene loss is much less emphasized and aldehyde (ketone) loss is gaining importance. The observed changes are in good agreement with the postulated mechanism of the onium reaction. [143,167] The alternative pairs of oxonium ion plus alkene and aldehyde plus carbenium ion may be formed with a preference for the first one, because APA is comparatively small (20-60 kJ mol" ) or even zero, e.g., for the acetone/isobutene pair [143,166,167]... 

Thermochemical parameters estimated by semiempirical AMI calculations have been found to support the proposal that isobutene formation on gas-phase thermolysis of iV-methyl-A-phenyl-fert -butylsulfenamide and morpholinyl-ferf -butylsulfenamide occurs by a unimolecular mechanism involving a four-centre cyclic transition state and co-formation of the corresponding thiohydroxylamines." ... 

Thermal treatment of (=SiO)Hf(CH2Bu )3 at increasing temperatures leads to the successive evoluhon of neopentane, isobutene and isobutane as well as several alkanes varying from Cj to C5. Polyisobutenes are also formed on the surface. The mechanism by which such decomposition occurs suggests a succession of y-H eliminations with formahon of neopentane followed by P-methyl transfer and formation of isobutene and [Hf]-Me (Scheme 2.14). This isobutene is reinserted into [Hf]-Me with formahon of isopentene and [Hf]-H. 

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