Butadiene thermal reaction

More functionalized 5,6-dihydro-2H-pyran-derivatives 71 and 72 have been prepared [26] by cycloaddition of 1 -methoxy-3-trialkylsilyloxy-1,3-butadienes 69 with t-butylglyoxylate (70) (Scheme 5.6). Whereas thermal reactions did not occur in good yields because of the decomposition of the cycloadducts, application of pressure (10 kbar) allowed milder conditions to be used, which markedly improved the reaction yields. The use of high pressure also gives preferentially en Jo-adduct allowing a stereocontrolled synthesis of a variety of substituted 5,6-dihydro-2H-pyran-derivatives, which are difficult to prepare by other procedures. 

The alternate approach of Dewar and Zimmerman can be illustrated by an examination of the 1,3,5-hexatriene system.<81,92> The disrotatory closure has no sign discontinuity (Hiickel system) and has 4n + 2 (where n = 1) ir electrons, so that the transition state for the thermal reaction is aromatic and the reaction is thermally allowed. For the conrotatory closure there is one sign discontinuity (Mobius system) and there are 4u + 2 (n = 1) ir electrons, so that the transition state for the thermal reaction is antiaromatic and forbidden but the transition state for the photochemical reaction is aromatic or allowed (see Chapter 8 and Table 9.8). If we reexamine the butadiene... 

Nitrosoimines can undergo thermal reaction, a unimolecular, two-step mechanism has been proposed, as shown in Scheme 3.22 [193]. In this mechanism, a concerted electrocyclization is envisioned to form the strained four-membered ring in 41, followed by a presumably forbidden, but highly exothermic, deazetization to give 41. The electrocyclic ring closure is, at first glance, a 4-electron process, analogous to the cyclization of butadiene [194] or acrolein [194, 195]. This would be expected to involve rotation around the C=N bond coupled with C-O bond formation. 

The simplest example of an electrocyclic reaction involving 4n electron system is the thermal opening of cyclobutenes to 1,3 butadienes. The reaction can be done thermally or photochemically and under either conditions, it is stereospecific. 

The recent syntheses of this compound have made it available for thermal studies (Wiberg and Lampman, 1963 Frey and Stevens, 1964 Srinivasan, 1963). Above 190° C, bicyclobutane isomerizes to butadiene. The reaction is predominantly homogeneous but there is some evidence for a small heterogeneous component of the reaction. The results of Srinivasan et al. (1965) yield the Arrhenius equation... 

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]. 

If we analyze the case of hexatriene to cyclohexadiene conversion, the situation is just the reverse. The thermal reaction should be disrotatory and the photochemical reaction conrotatory. The butadiene belongs to (4ri)n system and hexadiene to (4n +2)rc system, and a generalization of the systems may be attempted. 

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). 

In the thermal reaction the [4 + 2] or Diels-Alder adduct is the major product, whereas in the photochemical reaction [2 + 2] cycloadditions dominate. Because the photochemical additions are sensitized by a ketone, C6H5-COCH3, these cycloadditions occur through the triplet state of 1,3-butadiene and, as a result, it is not surprising that these cycloadditions are stepwise, nonstereospecific, and involve diradical intermediates. 

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 ... 

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). 

The participation of Diels-Alder type intermediates in polymerization was considered by Hill et ah (26) in 1939 as a result of the elucidation of the structures of the butadiene homopolymer and the butadiene-methyl methacrylate copolymer resulting from thermal polymerization in emulsion. The considerable amount of alternating 1,4 and 1,2 structures in the homopolymer and the predominantly 1,4 structure of the butadiene in the copolymer which contained more than 50% alternating units of butadiene and methyl methacrylate led to the proposal that the reaction proceeded through a Diels-Alder dimer complex or activated complex. Chain initiation involved a thermal reaction in which the activated com-... 

In a thermal reaction hexafluoroacetone [65] and several trifluoroacetones [66] also react with butadienes in a straightforward fashion. 

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). 

A case where both effects are important is the ring opening of the cyclobutene radical cation l +.13 This reaction was studied experimentally by Bally and coworkers, who found that, in contrast to the thermal reaction, only the trans-1,3-butadiene radical cation, trans-2 +, is formed in a matrix isolation experiment.14,15 This surprising result can be explained by considering the electronic states of the species involved in the reaction, which are shown in Fig. 1. 

---