Cyclohexene regioselective reactions

The reaction is regioselective and sterospecific the tellurium moiety is added exclusively to the terminal carbon of terminal aUcenes, and only frany-adducts are obtained with cyclohexene. The reaction may proceed through the tellurenylation of the olefin (in a rapid equilibrium as demonstrated in separate experiments), forming a telluronium intermediate which is then methanolysed. 

The [t 4j + t 2j] cycloaddition of alkenes and dienes is a very useful method for forming substituted cyclohexenes. This reaction is known as the Diels-Alder (abbreviated D-A in this chapter) reaction. The transition structure for a concerted reaction requires that the diene adopt the s-cis conformation. The diene and substituted alkene (called the dienophile) approach each other in approximately parallel planes. This reaction has been the object of extensive mechanistic and computational study, as well as synthetic application. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, ethene with butadiene, as demonstrated by isotopic labeling. ... 

The regioselectivity of this reaction is excellent (92 8), and the diastereomeric purity of 2 is estimated to be 93% de on the basis of the oxidation of 2 to (5)-2-cyclohexen-1-ol (93% ee). Similarly, the reaction of 2 with acetaldehyde provides (S.iS H- -cyclohexeny ethanol with an enantiomeric purity of 92% cc. Reactions of 2 with other aldehydes, however, have not yet been reported. 

The anti-Markovnikov product was formed with >95% regioselectivity at 35°C. The examples in Scheme 5-21, Eq. (1) show that cyano and hydroxyl functional groups are tolerated by the catalyst, and diphenylphosphine oxide can be added to both C=C bonds in a di-alkyne. The reaction also worked for internal alkynes (Scheme 5-21, Eq. 2). Unusual Markovnikov selectivity was observed, however, for 1-ethynyl-cyclohexene (Scheme 5-21, Eq. 3) [17]. 

Apart from the role of substituents in determining regioselectivity, several other structural features affect the reactivity of dipolarophiles. Strain increases reactivity norbornene, for example, is consistently more reactive than cyclohexene in 1,3-DCA reactions. Conjugated functional groups usually increase reactivity. This increased reactivity has most often been demonstrated with electron-attracting substituents, but for some 1,3-dipoles, enol ethers, enamines, and other alkenes with donor substituents are also quite reactive. Some reactivity data for a series of alkenes with several 1,3-dipoles are given in Table 10.6 of Part A. Additional discussion of these reactivity trends can be found in Section 10.3.1 of Part A. 

The Heck reaction has been applied to synthesis of intermediates and in multistage syntheses. Some examples are given in Scheme 8.9. Entries 1 and 2 illustrate both the (3-regioselectivity and selectivity for aryl iodides over bromides. Entries 3 and 4 show conditions that proved favorable for cyclohexene. These examples also indicate preferential syn Pd-H elimination, since this accounts for formation of the 3-substituted cyclohexene as the major product. 

Relatively polar diphosphine 8 has an elongated P-P bond, and thus exhibits unusual reactivity. The reaction of 8 with acrylonitrile or methyl acrylate proceeds at 50 °C in a chemo- and regioselective manner to afford the 1,2-addition product with the PPh2 group attached at the terminal position (Equation (64)).165 Tetrachlorodiphosphine reacts with cyclohexene to give trans-adduct presumably via an ionic pathway.166... 

A unique system for catalytic silaboration of allenes, in which a catalytic amount of organic halide is used as a crucial additive, has been reported (Equation (86)).232 In the presence of Pd2(dba)3 (5 mol%) with 3-iodo-2-methyl-2-cyclohexen-l-one (10mol%), reactions of terminal allenes with a silylborane afford /3-silylallylboranes in good yields with excellent regioselectivity. It is worth noting that the addition takes place at the terminal C=C bond in contrast to the above-mentioned palladium-catalyzed silaboration. The alkenyl iodide can be replaced with iodine or trimethylsilyl iodide. The key reaction intermediate seems to be silylpalladium(n) iodide, which promotes the insertion of allenes with Si-C bond formation at the central -carbon. 

The vinyloxirane reaction was later extended to methylidene cyclohexene oxide and to related meso derivatives [53]. The effects of the diastereomeric ligands 42 and 43 (Fig. 8.5), derived from (S)-binaphthol and (S, S)- or (R, R)-feis-phenylethyl-amine respectively, were investigated. In the case of kinetic resolution of racemic methylidene cyclohexane epoxide 45 with Et2Zn, ligand 42 produced better yields, regioselectivity, and enantioselectivity than 43 (Scheme 8.27). 

The position selectivity is observed in those cases where a nonsymmetrically substituted diene acts as a dienophile. The more substituted double bond is involved in an ion radical reaction, which develops according to Scheme 6-16. This scheme makes understandable the regioselectivity observed. Such regioselectivity is possible when both the diene and the dienophile are nonsymmetrically substituted. Then dimerization can be of the head-to-head type, with the formation of 1,2-disubstituted derivatives of cyclohexene, or the... 

The chiral anisole derivative 37 has been used in the synthesis of several asymmetric functionalized cyclohexenes (Table 9) [22]. In a reaction sequence similar to that employed with racemic anisole complexes, 37 adds an electrophile and a nucleophile across C4 and C3, respectively, to form the cyclohexadiene complex 38. The vinyl ether group of 38 can then be reduced by the tandem addition of a proton and hydride to C2 and Cl, respectively, affording the alkene complex 39. Direct oxidation of 39 liberates cydohexenes 40 and 41, in which the initial asymmetric auxiliary is still intact. Alternatively, the auxiliary may be cleaved under acidic conditions to afford /y3 -allyl complexes, which can be regioselectively attacked by another nucleophile at Cl. Oxidative decomplexation liberates the cyclohexenes 42-44. HPLC analysis revealed high ee values for the organic products isolated both with and without the initial asymmetric group. 

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