Thermal reactions of butadiene

These theories assert that the pathway of a chemical reaction accessible to a compound is controlled by its highest occupied molecular orbital (HOMO). For the thermal reaction of butadiene, which is commonly called ground-state chemistry, the HOMO is 2 and lowest unoccupied molecular orbital (LUMO) is photochemical reaction of butadiene, which is known to be excited-state chemistry, the HOMO is 1//3 (Fig. 3.5.6). 

In a subsequent study, Inukai and Kojima determined that the enthalpy of activation in the thermal reaction of butadiene with methyl acrylate was 18.0 ... 

Figure 14. Product yields in the thermal reaction of butadiene at 650°C. Feed butadiene contains 0.55% mole % butenes as impurity.

Figure 16. Second-order kinetics for thermal reaction of butadiene...

Cyclization proceeded in nearly 100% selectivity in the case of thermal reaction of butadiene (1 ), yielding 4-vinylcyclohexene (VCH) for the first step and ethylene, cyclohexene, cyclohexa-diene, and benzene in the secondary steps. Similar highly selective cyclizations were observed for the reactions between butadiene and ethylene, propylene, 1-butene, cis-2-butene, trans-2-butene or isobutylene (1), yielding cyclohexene (HCH), 4-methyl-cyclohexene (MCH), 4-ethylcyclohexene, cis-4,5-dimethylcyclo-hexene, trans-4,5-dimethyIcyclohexene or 4,4-dimethylcyclohexene, respectively. Based on the above information, it can be said that butadiene plays an important role in the formation of cyclic compounds in pyrolysis conditions. 

Fora thermal reaction of butadiene [reaction (A7.1) ] ( ground state chemistry ), the HOMO is (reaction pathways shown in Fig. A7.2). For a photochemical reaction of butadiene [reaction (A.7.2)] ( excited state chemistry ), the HOMO is (reaction pathways shown in Fig.A7.3). 

Thermal cycloadditions of butadiene to 3-bromo- 133 and 3-methoxy-5-methylene-2(5//)-furanones 220 were studied (95TL749). These systems contain substituents at C3 capable of stabilizing also a possible radical intermediate, influencing hereby the rate and/or the course of the reaction. Thus, the reaction of 133 and 220, respectively, with butadiene at 155°C afforded mixtures of the expected 1,4-cycloadducts 221 and 222, respectively, and of the cyclobutane derivatives... 

The trans isomer of 1,2-divinylcyclobutane may be isolated in low yield from the mixture formed by thermal dimerization of butadiene.5 The cis isomer has been prepared by a sequence of reactions.6... 

We may also look at this reaction from the opposite direction (ring closing). For this direction the rule is that those lobes of orbitals that overlap (in the HOMO) must be of the same sign. For thermal cyclization of butadienes, this requires conrotatory motion (Fig. 18.3). In the photochemical process the HOMO is the %3 orbital, so that disrotatory motion is required for lobes of the same sign to overlap. 

In most cases the ring opening goes to completion and there are very few examples of the reverse reaction i.e., the thermal cyclisation of butadienes. The ring opening takes place smoothly in solution or in the gas phase at temperatures between 120° and 200°C. 

Thermal rearrangement of propadienylcyclopropanes to methylenecyclopentenes has been examined in several cases however, selective transformation to the product has not necessarily been easy due to the harsh reaction conditions required for the rearrangement. The first example of this type of reaction was reported by Dewar, Fonken, and co-workers in a paper on the kinetics of the thermal reaction of 3-cyclopropyl-l,2-butadiene (44), and the reaction was found to proceed much faster (activation energy difference 8.2 kcal) than that of the corresponding vinylcyclopropane [25]. Several examples have appeared since this initial work, most of which have dealt with the mechanistic aspect of the reaction, but none of them has reached a synthetically useful level [26]. For example, thermal reaction of 3-(2-methylcyclopropyl)-1,2-butadiene (45) gives a mixture of five products, as shown in Scheme 20 [27]. 

Teplyakov, A. V., Kong, M. J. and Bent, S. F. Diels-Alder reactions of butadienes with the Si(100)-2 x 1 surface as a dienophile vibrational spectroscopy, thermal-desorption and nearedge X-ray-absorption fine-structure studies. Journal of Chemical Physics 108, 4599 1606 (1998). 

Diels-Alder reactions are thermal reactions requiring no catalysts (120). However, over the years both acid- and metal-based homogeneous or heterogeneous catalysts have been developed (121—127). Some catalysts used in Diels-Alder catalyzed reactions of butadiene are Fe(NO)2Cl—(CH3CH2)2A1C1, Pd[P(C H5)3]4, Cu(I) exchanged silica—alumina (128,129), large pore zeolites (130), and carbon molecular sieves. An electrochemical process has also been used to catalyze the self-condensation to vinylcyclohexene (131). When the asymmetric Ni catalyst (4) was used, specificity to the enantomeric (5)-4-vinylcyclohexene (132,133) was observed (26% enantiomeric excess). 

Several reactions might be possible in certain systems. For example, the thermal reaction of fulvene with butadiene can potentially proceed through three different Woodward-Hoffmann-allowed supra-supra cycloadditions ... 

The participation of diradical species in charge transfer reactions has been demonstrated in the most widely recognized example of donor-acceptor interaction—i.e., the Diels-Alder reaction. The two-step nature of this reaction has recently been proposed in the isolation of both four-and six-membered ring products in the thermal and photochemical reaction of butadiene and a-acetoxyacrylonitrile (14, 56). 

The thermal reactions of l-oxa-l,3-butadienes such as acroleine 2-78 with alkenes such as 2-79 usually need relatively harsh conditions (150°C-250°C) [120]. As a side reaction polymerisation of the a,/l-unsaturated carbonyl compound can take place addition of radical inhibitors such as hydroquinone or 2,6-di-ferf-butyl-4-methylphenol can be helpful in avoiding this unwanted transformation. In the described hetero Diels-Alder reaction the cycloadduct 2-80 was obtained which was then transformed into racemic-/3-santalene 2-81 (Fig. 2-22). 

Thermal reactions of acetylene, butadiene, and benzene result in the production of coke, liquid products, and various gaseous products at temperatures varying from 4500 to 800°C. The relative ratios of these products and the conversions of the feed hydrocarbon were significantly affected in many cases by the materials of construction and by the past history of the tubular reactor used. Higher conversions of acetylene and benzene occurred in the Incoloy 800 reactor than in either the aluminized Incoloy 800 or the Vycor glass reactor. Butadiene conversions were similar in all reactors. The coke that formed on Incoloy 800 from acetylene catalyzed additional coke formation. Methods are suggested for decreasing the rates of coke production in commercial pyrolysis furnaces. 

Diradical intermediates may occur in other types of cycloadditions and cycloreversions. Kinetic data for the thermal transformation of ci5,ci5-l,5-cyclooctadiene to butadiene and 4-vinylcyclohexene are consistent with a diradical intermediate the same intermediate may be involved in the reaction of butadiene leading to [2 + 2] adducts and to the Diels-Alder product 4-vinylcyclohexene. - That a Diels-Alder product may arise from a stepwise path is not as unimaginable today as may have been the case just a few years ago. In more elaborate contexts as well, regiochemistry may be successfully rationalized through estimations of the relative stabilities of diradical intermediates. ... 

The approach proposed first by Zamojski was based on the thermal reaction of 1-methoxy-1,3-butadiene with a highly active heterodienophile (butyl glyoxylate), which provided a dihy-drop)Tan derivative—precursor of racemic monosaccharides [3,17]. 

The highly exothermic cesium fluoride catalyzed isomerization of hexafluorobutadiene (9) via 10 to hexafluoro-2-butyne (11) is another example [5] that shows that the fluorine atom is preferably bonded to the carbon atom through the sp3-hybridized orbital rather than through the sp2 orbital [6]. And here again, Bent s rule plays an important role in the isomerization [7]. The thermal reaction of hexafluoro-1,3-butadiene provides perfluorotricyclo[3.2.0.02,6]octane (15) as a stable final product via sequential intra-and/or intermolecular cycloaddition at 200°C [8]. In contrast, the parent hydrocarbon (16)... 

The full scope of the acid-catalyzed one-step conversion of N-tert-bu-tylmethanimine, generated in situ from 1,3,5-tri-tm-butylhexahydro-1,3,5-triazine, to symmetrical 3,5-disubstituted pyridines employing enamines has been detailed.290 The generation of 2,3,5,6-tetrasubstituted pyridines from the thermal reaction of enolizable ketones with hex-amethylphosphoric triamide has been proposed to proceed by in situ generation and subsequent [4 + 2] cycloaddition of /V-methyl-1 -aza-1,3-butadienes with intermediate dimethylamino enamines.30... 

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