Isobutylene, loss

Addition to cis- and /n t-2-butene theiefoie yields different optical isomers (10,11). The failure of chlorine to attack isobutylene is attributed to the high degree of steric hindrance to approach by the anion. The reaction intermediate stabilizes itself by the loss of a proton, resulting in a very rapid reaction even at ambient temperature (12). 

The close packing of the isobutylene chain confers on the polymer a high degree of impermeability to gases, but also results in a very lossy rubber. The high hysteresis loss can be utilised in some circumstances to provide good friction in wet conditions. 

The oxidation of8-f-hutyl-l-(2-pyridyl)-2-naphthol illustrates the reaction between a produced cationic center and a tertiary amine (Scheme 29) [40]. The produced pyridinium salt reacts in a basic medium with loss of isobutylene. 

Ionic polymerization may also occur with cationic initiations such as protonic acids like HF and H2SO4 or Lewis acids like BF3, AICI3, and SnC. The polymerization of isobutylene is a common example, shown in Fig. 14.5. Note that the two inductively donating methyl groups stabilize the carbocation intermediate. Chain termination, if it does occur, usually proceeds by loss of a proton to form a terminal double bond. This regenerates the catalyst. 

Formation of XXVII by loss of a proton from the tertiary carbon atom in the neopentyl group of XXVI rather than from the tertiary carbon atom in the secondary isopentyl group is, however, hardly to be expected. Furthermore, the addition of a tertiary olefin to a secondary carbonium ion is also unexpected (compare the results on the copolymerization of n-butylene with isobutylene, page 46). A somewhat more likely combination consists of the addition of the tertiary carbonium ion. XXVIII to fert-butylethylene (XXIX) followed by a 1,3-shift of a... 

Formation of products of rearrangement may be looked upon as occurring by way of loss of hydrogen from a carbon atom which is not adjacent to the carbon atom holding the phosphate radical. This results in the transitory formation of a cyclopropane or cyclobutane ring which then opens to yield the rearranged olefin. Thus, in the copolymerization of isobutylene with 2-butene, the intermediate ester may react in the following ways ... 

The chain continues to grow by addition of isobutylene molecules until chain termination by proton transfer occurs (cf. discussion of titanium tetrachloride mechanism, p. 74). At higher temperatures, the loss or transfer of a proton occurs faster than does the addition reaction and lower molecular weight products are obtained. 

When the ferf-butoxycarbonyl (Boc) carbamate-derived sulfone 5 was subjected to the Bi(OTf)34H20-catalyzed allylation conditions (Scheme 2), the cyclic carbamate 6 was obtained as the major product (36%, diastereoisomeric ratio (dr) = 82 18), along with the corresponding allylation product 7, although in low yield (16%). Such cyclic carbamate resulting from the internal capture of an intermediate (3-silyl cation with the Boc group and concomitant loss of isobutylene has already been reported in the literature [59]. 

Extended life for the zeolite Beta catalyst has been demonstrated in this work using the same, or similar, crude tBA feedstocks to those employed in Table 1. Isobutylene generation has been monitored over ca. 500 to 1000 hours of service, under steady state reactive distillation conditions, without significant losses in activity or changes in product compositions. 

The mechanism most consistent with all the data is an ionic acid opening of the epoxide —apparently where the hydrocarbonyl is used as an acid to attack the epoxide— which is more sensitive to steric effects than to electronic factors. This conclusion may at first appear to be inconsistent with our previous finding that isobutylene reacted with cobalt hydrocarbonyl to give exclusively addition of the cobalt to the tertiary position. The inhibitory effect of carbon monoxide on that reaction, however, indicated that it was probably cobalt hydrotricarbonyl that was actually adding to the olefin and steric effects would be expected to be much less important with the tricarbonyl than with the tetracarbonyl (7) Apparently he feels now that the former reactions really involve the tricarbonyl, loss of CO being important to get the reaction running whereas epoxide attack perhaps involves a tetracarbonyl, steric factors are more important here. 

The Boc group is easily cleaved by brief treatment with trifluoroacetic acid (TFA), CF3COOH. Loss of a relatively stable ferf-butyl cation from the protonated ester gives an unstable carbamic acid. Decarboxylation of the carbamic acid gives the deprotected amino group of the amino acid. Loss of a proton from the fert-butyl cation gives isobutylene. 

Absorption. In the absorption step as much as possible of the olefin should be converted to dialkyl sulfate. The dialkyl sulfates are quite soluble in hydrocarbons above about 40° F, whereas the alkyl acid sulfates are not. Propylene is the preferred olefin, and n-butylenes may be used. Propylene has been used commercially. Although an exhaustive study was not made, the use of a butylene feed containing isobutylene gave poor results. Isobutylene results in a considerable loss of acid and also isobutylene. About 10-25% of the total olefin used in SARP is reacted in the absorber, assuming a net acid consumption of about... 

As mentioned above while sulfoxides are also thermally unstable they tend to undergo rearrangement rather than loss of SO e.g., di-f-butylsulfoxide decomposes readily at 75 °C to give isobutylene, water and 2-methylpropyl 2,2-methylpropane-... 

These results imply the production or modification of acid sites by irradiation, both with y-rays and with heavy particles. The site can hardly be acid, because the reactions (except in one instance) were studied between 149° and 400°, and acid was observed to have a half-life of 1 to 1.5 hour at 100°. A radiation-produced site similar to acid might be more stable in silica-alumina, but some of the above experiments were on pure silica. At —78°, irradiated silica gel catalyzed the polymerization of isobutylene (7d). Since the loss of this catalytic activity on annealing paralleled the loss in acidity, the activity is probably attributable to acid. 

As Eq. 4.4 describes, the initial intermediate is most plausibly the psuedoequatorial iodonium ion 219a having the equatorial sidechain. Displacement by the carbonate oxygen via a six-membered ring transition state results in formation of the oxo-stabilized cation 210b. Loss of isobutylene then produces the c/j-substituted iodocarbonate 220. 

Allyl-/-butyltrithiocarbonates (236), which are obtained from readily available allyl halides by a multi-step reaction with sodium t-butyltrithiocarbonates, can be cyclized with concomitant loss of hydrogen iodide and isobutylene using elemental iodine to afford 1,3-dithiolane-2-thiones (237) ( heme 48) treatment of (237) with pyridine again results in a loss of hydrogen iodide and (238) is formed. Compound (238) can be isomerized using trifluoroacetic acid and (239) is obtained <80JOC2959>. 

In a final example demonstrating the utility of unfunctionalized olefins, Levy, et al.,43 reported a novel adaptation in which the formed C-glycoside cyclizes to an adjacent oxygen with loss of a benzyl group. As shown in Scheme 2.3.17, l-0-acetyl-2,3,4-tri-0-benzyl-L-fucose was treated with isobutylene and an excess of TMSOTf giving the illustrated fused ring C-glycoside. This reaction proceeded in >70% yield in both methylene chloride and acetonitrile. Additionally, borontrifluoride etherate could also be used. The proposed mechanism, shown in Scheme 2.3.17, involves the initial elimination of the... 

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