Cyclizations of butadiene

Electi ocyclic reactions are examples of cases where ic-electiDn 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 fomred 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 HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

Oxidative cyclization of butadiene and trapping with a nucleophlla... 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

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. 

Mechanistic studies of the nickel-catalyzed cyclization of butadiene have been carried out. The formation of various cyclic compounds catalyzed by nickel complexes is explained via the intermediacy of ir-allylic nickel complexes 11 and 12. 

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. 

Examples of both oxidation and reduction have been found. Breil, Heimbach, Kroner, Muller and Wilke (130) have studied the cyclization of butadiene. Three different cyclic trimers of butadiene have been obtained depending on the catalyst system. These are summarized in Table 10. 

The Ni-catalyzed cyclizations of butadiene and acetylene opened a fruitful field of cycloaddition of various unsaturated compounds to afford various cyclic compounds. These cyclizations are now understood by the formation of metallacycles as intermediates (eq. 1.8). 

Cyclization of butadiene catalysed by Ni(0) catalysts proceeds via 7r-allylnickel complexes. At first, the metallacyclic bis-7i-allylnickel complex 6, in which Ni is bivalent, is formed by oxidative cyclization. The bis-7r-allyl complex 6 may also be represented by cr-allyl structures 7, 8 and 9. Reductive elimination of 7, 8 and 9 produces the cyclic dimers 1, 2 and 3 by [2+2], [2+4] and [4+4] cycloadditions. Selectivity for 1, 2 and 3 is controlled by phosphine ligands. The catalyst made of a 1 1 ratio of Ni and a phosphine ligand affords the cyclic dimers 1, 2 and 3. In particular, 1 and 3 are obtained selectively by using the bulky phosphite 11. 1,2-Divinylcyclobutane (1) can be isolated only at a low temperature, because it undergoes facile Cope rearrangement to form 1,5-COD on warming. Use of tricyclohexylpho-sphine produces 4-vinylcyclohexene (2) with high selectivity. 

Kiji J, Okano T, Nomura T, Saiki K, Sai T, Tsuji J (2001) Mechanistic studies on Pd-catalyzed telomerization and co-cyclization of butadiene amphiphilicity of bis-7i-allylpalladium intermediate in the presence of phosphine ligand. Bull Chem Soc Jpn 74 1939-1945... 

This reasoning was used by the author in 1961 to rationalize the ubiquitous photochemical cyclization of butadienes to cyclobutenes here it was noted that the excited state has a high 1,4-bond order. The same reasoning was applied 6,12) to understanding the key step of cyclohexadienone rearrangements (vide infra). Still another example is the decreased central bond order in the excited state of stilbene which, as Daudel has noted 13), is in accord with photochemical cis-trans interconversion. 

Diene Cyclization. In 1952 Reed (157) discovered the catalytic dimerization of butadiene with Reppe catalyst in the presence of acetylene. Important results were obtained by Wilke (200) in the cyclization of butadiene with a nickel(0) catalyst. With bis-7r-allylnickel, biscyclo-i,5-octadienenickel, or cyclododecatrienenickel, he obtained the trimerization of butadiene to cyclododecatetraene while, with a catalyst of the type Ni(PR3)4, in which perhaps one coordination site cannot be replaced, he obtained the dimerization to cycloocta-l,5-diene. The mechanism of these reactions, in which 7r-allyl systems can be in equilibrium with o--7r-allyl systems (Figure 7), have been proved by Wilke and co-workers who isolated the intermediate compounds. It is worth noting that all these catalysts have ligands of weak -acceptor character which are labile and do not prevent butadiene from coordinating. The presence of weak t acceptors on the nickel tends to favor the structure of the diene, as was emphasized by Mason (112). 

As discussed in Chapter 12, electrocyclic reactions may proceed in a conrotatory or disrotatory fashion, that is, the n system cyclizes in an antarafacial or suprafacial manner, respectively. Since there is only a single component, it should be counted according to the general component analysis for a thermally allowed reaction, that is, Ti rn = conrotatory and = disrotatory. Thus, the cyclization of butadienes... 

The cyclization of butadiene, as illustrated, involves the coupling of the 7r-allylnickel moiety. Various cyclic 1,5-dienes can also be synthesized by the coupling reaction of allyl dibromides using Ni(CO)4 via a 7r-allyl complex. 

The best known example is the cyclization of butadiene and acetylene 121 14°). Butadiene forms cyclooctadiene and cyclododecatriene by the catalytic action of nickel, iron, and other metal complexes. By an experiment using an iron complex with deuterated butadiene, it was proved that no hydrogen shift takes place in the cyclization reaction 70>. 

This type of cyclization of butadiene with heteropolar double bonds seems to be a general reaction. Tsuji and Ohno reported that isocyanates react with conjugate diene to form 1-substituted 3,6-divinylpiperidone. For example, isoprene and phenyl isocyanate gave tram and cis-3,6-diisopropenyl-l-phenyl-2-piperidone in the presence of palladium phosphine complexes, 67>. 

USB Catalyst in polymerization of acetylene to benzene and styrene, trimerization of ethynyl compds, cyclization of butadiene. 

It is well known that Ni(0) catalyzes the cyclodimerization and cyclotrimer-ization of butadiene to form COD or CDT. On the other hand no cyclization of butadiene occurs with Pd(0) catalyst. Pd(0)-catalyzed dimerization of butadiene to form 1,3,7-octatriene (1) is the main reaction. 

Figure 5.1. Rudimentary correspondence diagram for cyclization of Butadiene. (The orbitals are labeled as in Fig. 19 of ref. [1] their irreps in are given in parenthesis)...

The central role assigned by OCAMS to nuclear motion suggests that the alternative modes of desymmetrization capable of allowing polyene cyclization might profitably be described as symmetry coordinates with different irreps in C y. b for disrotation and U2 for conrotation. These are illustrated for the cyclization of butadiene in (a) and (b) respectively of Fig. 5.4. 

The question posed in the preceding paragraphs as to the need for a reevaluation of the concept of allowedness , can be dismissed as a non-problem as long as it is taken as axiomatic that, for an orbital symmetry analysis to be of any use, the symmetry elements [retained along the pathway] must bisect bonds made or broken in the process . In contrast to the allowed conrotatory cyclization of butadiene to cyclobutene, in which the C2 axis bisects a newly formed cr bond, the only bond bisected by the axis in its conversion to bicyclobutane is the one between C2 and C3, which is essentially single in both the reactant and the product. 

The crucial moment in the formulation of the systematic theory of pericyclic reactions is undoubtedly represented by the advent of the so-called Woodward-Hoffmaim rules [20-22], on the basis of which it was possible to explain and to rationalize the remarkable stereospecificily of these reactions. This specificity manifests itself in the predominant formation of only one stereoisomer as well as by the dramatic change of the preferred reaction mechanism depending on whether the reaction proceeds under the conditions of thermal or photochemical initiation. Thus, e.g., the thermal cyclization of butadiene to cyclobutene proceeds by the conrotatory mechanism, while for the photochemical reaction the disrotatory reaction is prefened. 

Figure 1 Occupation numbers of natural orbitals in thermally allowed cyclization of butadiene to cyclobutene in dependence on the value of the generalized reaction coordinate cp.

Figure 11 The partitioning of the modified More O Ferrall diagram with the corresponding reaction path for thermally forbidden disrotatoiy cyclization of butadiene to cyclobutene. The extent of electron reorganization measured by the value of the fimctional L is -0.48 along this reaction path.

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