Cycloaddition of butadienes

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... 

Construct a diagram like Figure 9.18 for (a) the Hiickel closure of butadiene to cyclobutene and (b) the Hiickel and Mobius cycloadditions of butadiene and ethylene. 

The phospholium salts (1) are available from the McCormack cycloaddition of butadiene derivatives with phosphonous dihalogenides (Route A/Scheme 2) [26-28], from quatemization of chlorophospholenes (3) with alkylhalogenides (Route B/Scheme 2) [29] or from the reaction of alkylphospholenes (4) with bromine or chlorine (Route C/Scheme 2) [29], In the latter case, the halogen reacts selectively with the phosphorus atom. 

The nickel-catalyzed [4 + 4]-cycloaddition of butadiene to form cyclooctadiene was first reported by Reed in 1954.90 Pioneering mechanistic and synthetic studies largely derived from the Wilke group advanced this process to an industrially important route to cyclodimers, trimers, and other molecules of interest.91-94,943 95,96 While successful with simple dienes, this process is not useful thus far with substitutionally complex dienes as needed in complex molecule synthesis. In 1986, Wender and Ihle reported the first intramolecular nickel-catalyzed [4 + 4]-reaction of... 

Fitzgerald, D. R. and Doren, D. J. Functionalization of diamond(lOO) by cycloaddition of butadiene First-principles theory. Journal of the American Chemical Society 122,12334 (2000). 

Conical intersections of the 4 + 4-photo-cycloaddition of butadiene with butadiene has been located at the CASSCF/4-31G level.283 The intramolecular 4 + 4-photo-cycloaddition of two pyridones linked by a three-atom chain yields a nearly quantitative yield of head-to-head cycloadducts.284 The photo-irradiation of 2-furan in the presence of 1-naphthalenecarbonitrile through a Pyrex filter produced both the major 4 + 4-cycloadduct and the minor 2 + 2-adduct.285... 

In fact, the cycloaddition of butadiene to ethylene, as well as cycloadditions of similar non-polar dienes to non-polar alkenes seem experimentally to be cases where concerted and stepwise (biradical or biradicaloid) mechanisms compete. We have recently discussed a number of cases, such as the dimerization of butadiene, piperylene, and chloroprene, the cycloadditions of butadiene or methylated dienes to halogenated alkenes, and others, where non-stereospecificity and competitive formation of [2 + 2] adducts indicate that mechanisms involving diradical intermediates compete with concerted mechanisms10). Alternatively, one could claim, with Firestone, that these reactions, both [4 + 2] and [2 + 2], involve diradical intermediates1 In our opinion, it is possible to believe that a concerted component can coexist with the diradical one , and that both mechanisms can occur in the very same vessel 1 ). Bartlett s experiments on diene-haloalkene cycloadditions have also been interpreted in this way12). 

Catalyst for DIels-Alder reaction of ynamlnes. A zero-valent iron species prepared by reduction of iron(III) chloride with isopropylmagnesium chloride serves as a unique catalyst for cycloaddition of butadiene and ynamines (1) to form 1,4-cyclohexadienamines (2). These products are hydrolyzed by mild acid treatment to (3,y-cyclohexenones (3), which are isomerized to either 4 or 5 by catalytic amounts of rhodium catalysts. 

A particularly short and efficient synthesis of the lycopodium al oid (+)-luciduline (303) features as the first step the [4 + 2] cycloaddition of butadiene to optically pure cyclohexenone (299) opposite to the homoallylic methyl group (Scheme 73). To achieve this excellent face discrimination the presence of SnCl4 was requir which, however, provoked an epimerization at C-8a yielding mainly the undesired rmns-fused product (301). Oximation, under epimerizing conditions, easily rectified this problem by channelling both isomers (300) and (301) to a single cij-fused oxime (302) which was converted to the enantiomerically pure alkaloid (303). 

A. Diels-Alder Reaction 1. Cycloadditions of Butadiene Derivatives... 

Photosensitised cycloaddition of butadiene to cyclopentadiene at —I5°C gives primary products from 1,2 addition (55%), 1,4 addition in which cyclopentadiene acts as diene (42%), and 1,4 addition where butadiene acts as diene (3%) . As the sensitiser was benzophenone (Et = 68.5 kcal.mole ), the result is in agreement with the previous discussion. 

Bartlett provided evidence for the accessibility of the biradical path by finding 0.029% vinylcyclobutane in the 4 + 2 cycloaddition of butadiene and ethylene. " Given the temperature of the reaction, this represents roughly a 10 kcal/mol... 

Let us briefly review the mechanistic information that is available from our previous MC-SCF computations (cycloaddition of two ethylenes [20], as an example of a forbidden reaction and the cycloaddition of butadiene and ethylene [21] and that of fulminic acid plus acetylene [22] as examples of allowed reactions).These results were obtained with ab-initio MC-SCF techniques using minimal STO-3G [23] and extended 4-3IG [24] basis sets. 

Write the frontier molecular orbital analysis for the cycloaddition of butadiene with butadiene when both interact in a suprafacial manner. Is this reaction allowed ... 

Transition metal catalysts can exert an important influence on the regio-selectivity of formal cycloaddition reactions. Cycloaddition of butadiene and XLIV in the absence of metal catalysts takes place at 135° giving a 20 1 mixture of XLV and XLVI. Catalysis by the Ni(acac)2-triphenylphosphine-triethylaluminum system affords XLVI, XLVII as the major product, and only a trace of XLV (Garratt and Wyatt, 1974). A bis(7r-allyl)nickel(0) complex (XLVIII) has been suggested as a reaction intermediate. 

A [4+2] cycloaddition of butadiene with 2-butyne or diphenylacetylene catalyzed by either (dicyclooctatetraene)iron(O) [Fe(cot)2], a combination of iron(III) chloride and isopropylmagnesium chloride, or tris(acetylacetonato)iron and triethylaluminum provides... 

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