Isobutene, formation

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." ... 

Results of a study of the mechanism and temperature-dependent kinetics of the dehydration of hot compressed liquid water do not corroborate an earlier claim that isobutene formation is catalysed by dissociation of BtfOH at 250 °C.21... 

The following facts are the basis for butene isomerization (I) There is a basic similarity in the composition of alkylates produced from all four butene isomers. (2) Alkylate molecules, once formed, are relatively stable under alkylation conditions and do not isomerize to any appreciable extent alkylate fractions having the same carbon number ore not equilibrated (see Table I). (3) Thermodynamic equilibrium between the butene olefins highly favors isobutene formation at alkylation temperatures. (4) Normal butenes p>roduce only small and variable amounts of normal butane, thus indicating only a small and variable amount of chain initiation from normal butenes. Yet the alkylate composition shows a high concentration of trimethylpentanes and a low concentration of dimethylhexanes. (5) A few of the octane isomers can be explai.ned only by isomerization of the eight-carbon skeletal structure this isomerization occurs while isobutene dimer is in ionic form. For example, 2,3,3- and 2,3,4-trimethylpentanes... 

Figure 5 demonstrates the sensitivity of the primary products of this lumped H-abstraction reaction by varying the probability of methyl substitution, i.e. by varying the relative amount of the different classes of isomers (mono-, di-, tri-, tetra-methyl and so on). While ethylene and 1-butene selectivities decrease with the increase in degree of methyl substitution, methyl radical, 2-butene and isobutene formation is enhanced. 

Figure 4.35. Bimolecular reaction scheme for isobutene formation from n-butene. Formation of intermediate primary carbenium ions is circumvented.

Fujii et al. (1988) reported a mechanism of isobutene formation by the yeast Rhodotorula minuta in a culture medium that contained branched-chain carboxylic or amino acids. It was suggested that the pathway leads over isovalerate that is decarboxylated to isobutene. 

Microbial synthesis of isobutene was first officially identified with yeast Rhodotorula minuta, and later it was discovered that a microsome-associated cytochrome— P450rm—was the enzyme system responsible for isobutene formation. The rate of formation of isobutene from isovalerate is 0.18 imiol/nmolp450nn/ min or 16.4 pL/L/h from a live cell system (Fukuda et al. 1984, 1994). However, this rate is too low for commercial application. Additionally, P450rm is still not a very well understood membrane-bound hemeprotein that is difficult to produce in heterologous hosts (Shiningavamwe et al. 2006). 

It was also discovered that some amino acids had a regulatory effect on the microbial synthesis of isobutene. For example, the addition of L-leucine into the fermentation broth stimulated isobutene formation. This was further accentuated by a synergistic effect of aromatic amino acids (L-phenylalanine, L-tryptophane and l-... 

A cell-free isobutene-forming system from disrupted R. minuta cells was also created (Fujii et al. 1988). Isobutene was produced from a-ketoisocaproate, isovaleryl-CoA and isovalerate where isovalerate gave the best isobutene formation rate of these three with 9.1 nL/L/h. Adding NADPH to the reaction mixture proportionally increased the rate of isobutene production, until a concentration of around 0.1 mM, as did increasing the concentratiOTi of isovalerate until 30 mM. EDTA had no inhibitory effect (Fujii et al. 1988). The formation of isobutene was inhibited by some redox reagents and completely eliminated by the presence of carbon monoxide (Fujii et al. 1989b). 

Another reaction to isobutene is through decarboxylation and subsequent dehydration of 3-hydroxyisovalerate (3-hydroxy-3-methylbutyrate), catalysed by mevalonate diphosphate decarboxylase (MDD, EC 4.1.1.33) (Gogerty and Bobik 2010 Marliere 2010). This enzyme, from the class of carboxy-lyases, is part of terpenoid or ergosterol biosynthesis, and isobutene formation is its side reaction. Even though the MDD family of enzymes is present in many microorganisms, none of them are known to synthesise isobutene (van Leeuwen et al. 2012 Bloch et al. 1959) (Fig. 4). 

This shows us that expressing diverse MDDs in one host can lead to a synergistic effect on the isobutene formation rate, since the two enzymes can perform the two reaction steps complementarily. Unfortunately, the patents did not indicate clear values for productivity or yield. 

Still, this discovery was used to screen homologues of the E. meningoseptica sequence in other organisms for the activity towards isobutanol (Marliere 2011). The genes were expressed in E. coli, and enzymes were partially purified, and some of them showed isobutene formation activity from isobutanol or tert-butanol. There is no clear information on the productivity of isobutene by the enzyme or strain (van Leeuwen et al. 2012). 

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