1,3-Butadiene electrocyclic ring closure

The reaction of 4,4-bis(tnfluoromethyl)-I,3-diaza-1,3-butadienes with certain a,P-unsaturated ketones yields pyrimidine derivatives A two-step mechanism, metathesis-electrocyclic ring closure and metathesis-intramolecular ene reaction, is a plausible explanation for the experimental results (pathway 4, equa-bon 25) [259]... 

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. 

Conjugated heterocumulenes generated in situ by an aza-Wittig reaction are also capable of electrocyclic-ring closure with a subsequent 1,3-H shift. This principle, applied for the first time by Saito et al. on butadiene iminophosphorane 51 (Scheme 28) by treatment with isocyanate and isothiocya-... 

We shall consider first the electrocyclic ring closure of butadiene to cyclobutene... 

Figure 11.6 The orbitals of reactants and products in electrocyclic ring closure of butadiene.

Conlin and coworkers reported a rate constant for the thermal electrocyclic ring closure of the transient l,3-(l-sila)butadiene derivative 8a in cyclohexane at 25 C, k = (1.19 0.06) x 105 s l31. The rate constant corresponds to the inverse of the lifetime of the silene in the dry solvent, as measured by laser flash photolysis of the isomeric silacyclobutene derivative 7a, the stable product of the thermal ring closure reaction... 

Irradiation of s-c/s-2,3-dimethylbutadiene yields both electrocyclic ring closure and double-bond isomerization products (Scheme 28). While the ring closure in. s-c/.s-butadiene, isoprene, 2-isopropylbutadiene, and 1,3-penta-diene is considerably less efficient than s-cis-s-trans isomerization, in s-cis-2,3-dimethylbutadiene it is about 50 times faster (Squillacote and Semple, 1990). This effect is counterintuitive, since the two methyl groups appear to hinder the rotation about the central C—C bond in butadiene. 

Figure 7.20. Comparison of the AO interactions in the photochemical chain abridgement in a polysilane (top) and in the disrotatory electrocyclic ring closure of butadiene (bottom) (by permission from Michl and Balaji, 1991).

Interpretation of the rate constants and Arrhenius parameters for thermal ,Z-isomeriza-tion is not always straightforward mechanistically, however, since sequential electrocyclic ring closure/ring opening reactions can often provide a lower energy route to the same products as formal rotation about a single C=C bond. This can be illustrated by Brauman and coworkers classic study of the thermal isomerization of E,E-, E,Z- and Z,Z-1,4-dideuterio-1,3-butadiene At 637 °C, the E,E- and Z,Z-isomers intercon-... 

Electrocyclic ring closures are reversible, and only those which yield significant amounts of cyclic products at equilibrium have been studied. Most butadiene-cyclobutene equilibria greatly favor the butadienes (see below). An exception is the cyclization of irans, a.s-l,2,3,4-tetraphenyl-l-bromo-l,3-butadiene, which undergoes the predicted conrotatory cyclization to a significant extent. ... 

There is another approach to predicting the stereochemistry of electrocyclic ring closures. One simply looks at the ends of the HOMO to conclude the proper direction for rotation of the bond by creating in-phase interactions during closure. Show that this method also predicts conrotatory closure for butadiene and disrotatory closure for hexatriene. 

FIGURE 18.29 Thermal electrocyclic ring closures of substituted butadienes. 

Electrocyclic reactions are examples of cases where n-electron bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are formed along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HC1 + H system. The reaction is now Mobius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mobius-Hiickel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

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