Isobutene elimination

With silyl-substituted oxiranes, dibal-H favors the primary alcohol and Bu 3A1H favors the secondary alcohol. These observations have been interpreted in terms of the timing of the hydride transfer to one of the oxirane carbons. dibal-H, which exists as a Lewis complex in donor media (R3N-A1H(Bu )2, or R20-A1H(Bu )2) acts as a nucleophilic hydride source, which preferentially attacks the least-hindered carbon. With Bu 3A1, complexation with the oxirane oxygen precedes isobutene elimination and the generation of the Al—H bond. A considerable carbocation character is acquired in the transition state, hence formation of the primary alcohol is favored. It is worthy of note that trialkylstannyl-substituted oxiranes are reduced with Red-Al invariably at the oxirane... 

Acrylonitriles with donor substituents such as a methoxy or ter/-butylsulfanyl group in the a position reacted with methylenecyclopropane and bicyclopropylidene at higher temperatures to give [2-f2] cycloadducts 14. The tert-butylsulfanyl group gave rise to the formation of a second product with a 5-thiaspiro[2.4]heptane structure 15 due to the isobutene elimination from the intermediate. ... 

Despite the high selectivity of the reaction, the formation of by-products must be thoroughly monitored since severe specifications are usually imposed on the C4 raffinate (the C4 stream leaving the plant after the isobutene elimination) used later on as a feedstock to alkylation, metathesis or 1-butene extraction (where oxygenates act as poisons). 

An effect of the ester spatial conformation on the cyclo-elimination process has been observed [88] the isobutene elimination is assumed to... 

The simplest monomer, ethylenesulfonic acid, is made by elimination from sodium hydroxyethyl sulfonate and polyphosphoric acid. Ethylenesulfonic acid is readily polymerized alone or can be incorporated as a copolymer using such monomers as acrylamide, aHyl acrylamide, sodium acrylate, acrylonitrile, methylacrylic acid, and vinyl acetate (222). Styrene and isobutene fail to copolymerize with ethylene sulfonic acid. 

The preparation of mono- and di-tm-butylcyclopentadienes 1 and 2 starting from monomeric cyclopentadiene was reported first in 1963 [23]. It was noted that the nucleophilic attack of the cyclopentadienide anion on ferf-alkyl halide has to compete with elimination reaction giving isobutene. The yield of the di- and tri-fer/-butylcyclopentadienes 2 and 3 was therefore reported to be modest to low [23, 24], Recently an elegant improvement for this synthesis using phase transfer catalysis was presented (Eq. 1), but the availability of the tri-substituted derivative... 

The first example of chemically induced multiplet polarization was observed on treatment of a solution of n-butyl bromide and n-butyl lithium in hexane with a little ether to initiate reaction by depolymerizing the organometallic compound (Ward and Lawler, 1967). Polarization (E/A) of the protons on carbon atoms 1 and 2 in the 1-butene produced was observed and taken as evidence of the correctness of an earlier suggestion (Bryce-Smith, 1956) that radical intermediates are involved in this elimination. Similar observations were made in the reaction of t-butyl lithium with n-butyl bromide when both 1-butene and isobutene were found to be polarized. The observations were particularly significant because multiplet polarization could not be explained by the electron-nuclear cross-relaxation theory of CIDNP then being advanced to explain net polarization (Lawler, 1967 Bargon and Fischer, 1967). 

Usually the stronger acids are also the more effective co-catalysts, but exceptions to this rule are known. Trichloroacetic acid, but not the equally strong picric acid, will co-catalyze the system isobutene-titanium tetrachloride in hexane.2 8 Some Lewis acid-olefin systems will not polymerize at all in the absence of a co-catalyst, an example being isobutene with boron trifluoride.2 4 This fact, together with the markedly slower reaction usual with carefully dried materials, has nourished the current suspicion that a co-catalyst may be necessary in every Lewis acid-olefin polymerization. It is very difficult to eliminate small traces of water which could act as a co-catalyst or generate mineral acid, and it may well be that the reactions which are slower when drier would not go at all if they could be made completely dry. 

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

When oligoisobutenes are formed from gaseous isobutene at ambient temperature by BF3 and H20 the initial group is CH3, formed by addition of a proton to the monomer [8]. The predominant terminal groups are double bonds [8] formed by transfer reactions involving elimination of a proton from the growing carbonium ion ... 

If an experiment of this type is performed with an aqueous solution saturated with isobutene and containing 20 mM Cl, a unimolecular rise of conductance of the solution occurs after production ( < 1 ps) of SOi." radicals. At 20 °C the rate constant of this conductance change, which is independent of pH between 4 and 11, is 3.1 x 10" s [46]. These results are explained by reactions (27)- 30), (29) and (30) constituting the actual addition/elimination sequence ... 

Formation of the phosphide 54 can be recognized by the orange color in the reaction solution when LiCMes is added. Addition of Me3CCl causes the subsequent hydrogen substitution for lithium in 54. As LiCl and isobutene are eliminated, the hydrogenated triphosphane 53 is formed [Eq. (9a)]. 

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. 

The presence of tin atoms regularly distributed on the platinum surface isolates the platinum atoms by increasing the distance between two adjacent platinum atoms, as does the copper atoms on a nickel surface [108] or the tin atoms on a rhodium, platinum or nickel surface [106, 109-111]. The presence of tin would thus avoid the hydrogenolysis reaction, leading to a more selective catalyst (Figure 3.37). Indeed, the formation of isobutene from isobutane involves only one platinum atom, with the reaction passing through a simple mechanism of P-H elimination after the first step of C-H bond activation (Scheme 3.26). 

Alkane elimination is a basic photodecomposition mode of highly branched alkanes, e.g., most of the excited neopentane molecules split directly to methane and isobutene. 

The chemistry in path (a) is analogous to that shown for CH3CHO in Scheme 7. Path (b) features intramolecular hydrogen transfer and elimination of 02 and C02. Most likely, the product formation in step (b) also involves a p-peroxide, which eliminates isobutene and Craq03+ via a six-membered transition state. This path has no counterpart in the CH3CHO reaction, where it would not generate stable products. b. 

Modern methods of peptide synthesis began with the solid-phase method introduced by Merrifield299 in 1962 (Fig. 3-15). To begin the synthesis a suitably protected amino acid is covalently linked to a polystyrene bead. The blocking f-butoxycarbonyl (Boc) group is removed as isobutene by an elimination reaction to give a bound amino acid with a free amino group. 

The fragmentation of A-f-butylpyrroles322 appears to be contrary to that observed for other A-alkylpyrroles. The formation of an immonium ion from the molecular ion would require elimination of a methyl radical. This cleavage is not observed but a fragment ion resulting from loss of isobutene is formed instead. The exact mechanism of this cleavage is not known. 

The gas-phase unimolecular pyrolysis of 2,4-dimethylpentane-2,4-diol has been found to occur by eliminative formation of acetone, isobutene, and H20 via a concerted six-membered cyclic transition state (Scheme 10).69 Single-pulse shock tube studies of the eliminative decomposition of ethoxy compounds have also been reported.70... 

TGA (02) of the second compound, MeAl[OSi(OBu-t)3](OBu-t), gives a very similar decomposition profile with a ceramic yield (1017 °C) of 29.5 wt% vs a theoretical value of 29.3 wt% for Al203-2Si02. The decomposition process for both compounds is believed to occur by elimination of isobutene, methane and some water as discussed below for the Zr and Hf analogs. 

The Ti and Hf compounds are monomers, whereas the Zr complex was dimeric. As found by Hrncir and Skiles, the series of M[OSi(OBu-t)3]4 compounds are all moisture-sensitive. TGA studies indicate that the Ti complex decomposes cleanly at temperatures >240 °C and gives a ceramic yield of 25 wt% whereas the theoretical ceramic yield for TiSiC>4 should be 29.07 wt%. The primary gaseous thermolysis products were identified by mass spectroscopy to be isobutene and water. A likely pathway for decomposition appears to involve /1-hydrogen elimination followed by condensation of the resulting Si—OH groups to generate isobutene, water and an oxide network as shown in equations 66 and 67. 

The Steglich Esterification is a mild reaction, which allows the conversion of sterically demanding and acid labile substrates. It s one of the convenient methods for the formation of terf-butyl esters because r-BuOH tends to form carbocations and isobutene after a subsequent elimination under the conditions employed in the Fischer Esterification. 

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