Reagents isobutene

The /-butoxycarbonyl group (Boc, "t-box ) has been eMens vely used in peptide synthesis, and Boc derivatives of many amino acids are commercially available. The customary reagent for the preparation from the amine is t-butyl azidoformate in water, dioxane/water, DMSO, or DMF. The cleavage by acids of medium strength proceeds with concomitant loss of isobutene and carbon dioxide (L.A. Carpino, 1957, 1973 see section 4.1.2.2). 

Polarization also occurs in coupling and disproportionation reactions of Grignard reagents with alkyl halides. The vinyl protons of isobutene produced in the reaction of t-butylmagnesium chloride with t-butyl bromide show A/E polarization as do the methyl protons of isobutane (Ward et al., 1970). Similar results arise in the reaction of diethyl-magnesium with organic halides (Kasukhin et al., 1972). 

In this chapter we will be concerned mainly with the formation of high polymers of isobutene, and with the fundamental studies aimed at the elucidation of this reaction. There is no doubt that many useful hints of fundamental interest are to be found in the patent literature, but in most cases the purity of the reagents and the reaction conditions are so ill defined, that no conclusions can be drawn from findings reported in patents. The ratio of scientifically valuable to dubious information in patents is so small that detailed survey of them, in the hope of discovering sound information, should be a most unrewarding occupation. This is more or less true of all chemical patents, but especially so in this particular field where minute traces of impurities can effect catastrophic changes in the reaction pattern. For this reason attention has been confined to work published in the scientific literature. 

A group of workers at the institute for Macromolecular Chemistry at Brno, under the leadership of Vesely, began to report in 1955 on investigations into the polymerisation of isobutene. Most of this work has been done with aluminium chloride as catalyst at -78°. The technique used in the earliest work was rather crude, but it was later refined so as to ensure a reasonable degree of dryness. A commendable feature of this work is the attention given to the purity of the reagents and the specification - for some of them, at least - of the nature and concentration of impurities. 

Kinetic studies on the polymerisation of isobutene at low temperatures by titanium tetrachloride in various solvents form the subject of a series of papers by Plesch and his co-workers [9, 10, 13, 28, 32, 33, 71, 77, 80, 81]. The reactions were followed in an apparatus approximating to an adiabatic calorimeter by means of the temperature rise accompanying the polymerisation. In the early studies moisture was not rigorously excluded from the systems, but later [81] an elaborate vacuum technique was evolved and all reagents were carefully purified and dried. Titanium tetrachloride was also used as catalyst by Okamura and his collaborators [79] in a series of studies concerning the effects of solvent, catalyst, and co-catalyst on the DP of polyisobutene. 

Depending on the trapping reagents, various reactions have been observed an ene reaction between propene, isobutene, and 2231 or 27,34 a [2 + 4] cycloaddition between butadiene and 12,21 22,31 and 27,34 or both an ene reaction and a [2 + 4] cycloaddition between 2,3-dimethylbutadiene and 2231 or 27,34 and between piperylene or hexadiene and 22.31 Some of these reactions are summarized in Scheme 8. By contrast, 27 does not react with the C = C double bond of an alkene such as CH2 = CH-OMe.34... 

We have a choice of reagents for the -butyl cation a halide with Lewis acid catalysis, and -butanol or isobutene with protic acid catalysis. The least wasteful is the alkene as nothing is lost. Protonation gives the r-butyl cation and two r-butyl groups are added in one operation.3... 

Isobutene, reagent grade, was obtained from Phillips Company. 

A plausible intermediate of this olefination is the titanium-methylene sjtecies 4, which is formed from 1 by removal of AlMe2Cl with a Lewis base, from 2 by fragmentation with elimination of isobutene, and from 3 by a-elimination and release of methane. However, none of these three routes to titanium-carbene complexes of type 4 proved to be generally applicable. Consequently, the use of these reagents in synthesis is essentially limited to the transfer of a methylene unit 18]. From a synthetic viewpoint, a general and easy route to substituted titanium-alkylidene species and their use in carbonyl olefinations would be more desirable. 

The preparation of /m-butyl ethers requires a large excess of isobutene (bp -6.9 °C) and the inconvenience and potential hazard of a pressure vessel or a sealed tube in some cases, ten-Amyl (T m) ethers have the same lability as tert-butyl ethers towards acids, and they are prepared the same way, by reaction of the alcohol with 2-methyl-l-butene (bp 31 °C) in the presence of trifluoroborane etherate, but they require only a slight excess of the reagent [Scheme 4.128].228 Deprotection of rm-butyl and fer/-amyl ethers can be accomplished by treatment with a catalytic amount of TBSOTf. When a stoichiometric... 

Again neglecting any enthalpic contribution from the CH2 reagent, we find for the isobutene 1,1-dimethylcyclopropane and allene methylenecyclopropane (3 24)... 

Ionisation is followed by different types of interactions, depending upon the presence in the system of a monomer (e.g., isobutene), of a non polymerisable olefin (e.g., 2,4,4-trimethylpentene-2), or of no further reagent. In the latter instance the ion pair produced in reaction (iii) collapses by an alkylation reaction involving the migration of an alkyl group onto the cation, leaving a more halogenated aluminium alkyl, e.g. ... 

Stereoselective oxidation of alkenes. The reagent oxidizes alkenes at room temperature to give esters of ciV-diols in 50 70% yield. Thus isobutene (1) is converted into 1,1-dimethylethylene bis(trifluoroacetate) (2). 

The polymerization of isobutene initiated by aluminum chloride has been found to be too rapid for systematic kinetic investigation (14, 15). One usually effective means of slowing down fast reactions is to cool the reagents to very low temperatures. It was hoped that experiments at ultralow temperatures might slow down the polymerization reaction and thus open the door for systematic kinetic studies. 

However, the reagent is not stable enough to be shipped safely because it decomposes more or less rapidly into isobutene, carbon dioxide and HF, thus developing autogenous pressure in containers. This has led SNPE and its subsidiary ISOCHEM to manufacture and react BOC-F on site, thus offering low cost protected amino compounds. 

In amino acid chemistry r-butyl ethers are also used successfully for the protection of the serine and threonine hydroxy groups. They are formed by the addition of the alcohols to isobutene and are cleaved with TFA and HBr/AcOH. t-Butyl trichloroacetimidate is also a convenient reagent for their introduction.However, the preparation of O-f-butyl serine and threonine themselves requires several steps including methyl orp-nitrobenzyl esters as intermediates. 

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