Dienes reaction modes

The ring closure of a diene to a cyclobutene can occur with rotation of the two termini in the same conrotatory) or opposite disrotatory) directions. For suitable substituted compounds, these two reaction modes lead to products with different stereochemistry. 

The fact that organosamarium allyl complexes of the type Cp 2Sm(CH2CH=CHR) can arise from the treatment of Cp 2Sm or [Cp 2Sm(/r-H)]2 with a variety of olefin and diene substrates makes samarium chemistry more intriguing. The reaction modes are illustrated in Scheme 18. These allylsamarium complexes 55 react with C02 to afford the carboxylate products 56, which participate in monometallic/bimetallic interconversions (Equation (10)). Carbon disulfide and 0=C=S also insert into carbon-samarium bonds, which form only monometallic species.29... 

The competitive silyl migration to either a vinyl or phenyl group in aryl- and vinyl-disilanes has been investigated by Ishikawa and coworkers133. With 2,3-dimethylbuta-l,3-diene, two modes of ene reaction are possible for 238 to give 543 and 544. Another ene... 

A cw-1-substituent on a diene increases the strain energy of the cis conformation required for the 1,4-cycloaddition so that this reaction mode becomes difficult and other oxygenations such as [2+2]- or ene-addition, take place (Sch. 24). 

It is well known that the thermal decomposition of a bicyclo[1.1.0]butane gives the corresponding 1,3-butadiene probably via the Woodward-Hoffmann allowed concerted pathway, while photolysis of cyclobutene also provides 1,3-butadiene as a major product. A remarkable difference in the reaction modes between the silicon and carbon analogs of the bicyclo[1.1.0]butane/cyclobutene system would partially be a consequence of the fact that the formation of the silicon analog of a conjugated diene is highly undesirable. 

In presence of dienes (Scheme 1), heterodienes, and heterotrienes (Scheme 2) rapid cycloadditions take place which prevent other reaction modes of the silylenes (insertions, polymerization, or HX-elimination-polymerization). Although a concerted [l+4]-cycloaddition is symmetry-allowed a stepwise mechanism (Scheme 3) via a three-membered intermediate is prefered or at least partly be competing to account for the formation of double-bond isomers. Ususally the formal [4+l]-cycloadducts (allylsilane-type) are the main products while the isomers with vinylsilane-units are side-products (< 30 %). Exceptions are... 

By the nature of its molecular mechanism, the carbonyl-insertion reaction represents a typical reaction mode of o alkyltransition metal complexes. Formation of the new C—C cr-bond takes place during a 1,2-alkyl-migration step, transforming an alkylmetal carbonyl moiety [cts-M(CO)R] into an acylmetal unit (M—COR) (89). In general, (s-cir-diene)-zirconocene complexes 5 appear to exhibit a substantial alkylmetal character (90). Therefore, it is not too surprising that some members of this class of compounds [in contrast to most other dienetransition metal complexes (97)] react with carbon monoxide with C—C bond formation (45). However, as demonstrated by X-ray structural data for 5 (Tables V... 

Thus, we investigated in detail the factors influencing intramolecular addition modes in the radical cyclopolymerization of some unconjugated dienes, including diallyl and dimethallyl dicarboxy-lates, acrylic and methacrylic anhydrides, and allyl -substituted acrylates and discussed the selectivity of reaction mode of intramolecular hh or ht addition in terms of the thermodynamical standpoint. The present article gives a summary of our recent work including published papers(18-23). 

With the achievement of high stereochemical control resulting from nearly exclusive endo topography in the Lewis acid catalyzed reactions interest next turned to the effect of chiral Lewis acid complexes on the enantiofacial selectivity of the cyclocondensation reaction. For example, consider the cycloaddition of benzaldehyde to diene (8) under the influence of a chiral catalyst. Assuming a pericyclic reaction mode and high endo selectivity, a mixture of two possible products can result. Compound (64) is desig-... 

The synthetic importance of normal Diels-Alder reactions rests on the great variability of the diene and dienophile components. Almost all additions occur in a stereospecific way. There are only a few examples which show a nonstereospecific reaction mode <91TH 62i-oi, 94TH 62i-03>. Furthermore, in many cases Diels-Alder reactions are highly regioselective, 1-substituted and 2-substituted... 

The free radicals produced by the irradiation of the sulphonyl iodides can also be trapped by double bonds in a reaction akin to those described for the sulphonyl chlorides (Section I.A). Thus the irradiation of p-toluenesulphonyl iodide (11a) in the presence of acrylonitrile and butadiene affords the adducts 15 and 16, respectively. In the absence of an alkene or diene, irradiation affords the anhydride 17 and the disulphone 1826. Truce and coworkers27 have also utilized this reaction mode in a study of the light-induced addition of sulphonyl iodides to allenes. 

In such a situation, but without an additional substituent on the Pd-substituted cyclopropane ring, the reaction mode along pathway B normally predominates. This is illustrated by the Heck coupling reaction of bicyclopropylidene (50) with iodobenzene, which under normal Heck conditions gave the phenyl-substituted diene 78 in up to 78% isolated yield or, in the presence of a dienophUe, the corresponding Diels-Alder product in excellent yield (see Sects. IV.2.2.1, IV.2.1.2, and The coupling of 50 with iodobenzene in... 

This conversion of an acyclic bisdiene to a cychzed enediene constitutes a new Pd-mediated reaction mode, which can be rationalized as outlined in Scheme 7. Com-plexation of bisdiene 17 following oxidative cyclization (i.e., oxidative addition with intramolecular coupling of the diene moieties) affords palladacycle 20. Protonation in an... 

Insertion and addition are definitely two of the most fundamental reaction modes for both carbenes and silylenes. While the most convenient method for trapping carbenes involves their addition to double bonds to give stable cyclopropane derivatives, the corresponding method for trapping silylenes is generally complicated by the fact that most of the silacyclopropanes are unstable species and therefore undergo various secondary processes. In practice, with the exception of alkynes and coqjugated dienes, addition reactions are rarely employed to intercept silylenes. 

In THF, the representative procednre for the hydrobo-ration of dienes used BH3-SMe2, utilizing the borane to diene addition mode BH3-SMe2 (0.14 mL, 1.4680 mmol) was added dropwise to a solution of a diene dissolved in THF (1 mL) pre-cooled to —40 °C. The reaction mixtnre was left to stir under nitrogen at 0 °C for 1 h to give a clear solution, which, depending on the diene used, contained a white precipitate. 

The popularity and usefulness of this process are due in part to its synthetic versatility. The 4jt- and 27t-components of this reaction may be varied widely, thus allowing for considerable structural diversity (substitution on the diene and the dienophile intermolecular vs. intramolecular reaction mode, heteroatom substitution in the diene [25] or dienophile [26], etc.). Moreover, the peri-, regio-, and stereoselectivity of the cycloaddition can often be predicted because of the extensive development of theoretical models such as the Woodward-Hoffmann rules [27] and frontier molecular orbital (FMO) theory [28]. 

In the case of (i) we can predict, because of our previous findings, that the only distinguishable rotation modes are conrotation and disrotation. Each mode has two formal possibilities, and hence there is a maximum of four possible polyenic products when the substituents a are all different. In reaction (3.25), for example, it is clear that the extrusion of sulphur dioxide is accompanied by the outward disrotatory motion of the methyl groups so as to form tranj,tra s-hexa-2,4-diene. The alternative disrotatory process, in which the methyl groups rotate inward, is of higher energy and would have resulted in the c/s.cis-product. Conversely, the conrotatory motions each would have yielded d5,traHS-hexa-2,4-diene reaction (3.25) is therefore highly stereospecific. 

The 1-ADPM rearrangement and the alternative reaction modes of 1-substituted-1,4-dienes via radical-cation intermediates described above have an obvious mechanistic interest. However, the yield of products in these reactions is considerably lower than in the triplet-sensitized photoreactions of these compounds, with the exception of oxime ethers 39e and 40c therefore, the synthetic utility of these reactions is limited. However, this inconvenience can be overcome using alternative electron-acceptor sensitizers. Recent results show that replacing DCA by dicyanodurene (DCD) as an electron-acceptor sensitizer and biphenyl as a co-sensitizer, results in a considerable increase in the yields of 1-ADPM photoproducts.Thus, for example, DCA-sensitized irradiation of 39c, for 2.5 h, affords cyclopropane 41c in 15% yield, whereas using DCD as the sensitizer, this compound is obtained in 60% yield after... 

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