Exo double bond

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

Solvents regularly used in organic reactions are used in heterogeneous catalysis of organic reactions. When solvent information is known, it accompanies other reaction information in each chapter. It must be remembered, however, that the solvent may interact with the catalyst surface and be converted into something undesirable or may combine with or modify one or more of the reactants. The example in Table 1.351 shows the rather minor effect of solvents on the stereochemistry of hydrogenation of the exo double bond in a spatane precursor. 

TABLE 1.3 Influence of Solvent on Hydrogenation of an exo Double Bond... 

Another new example using titanocene as catalyst has been revealed by Malacria and coworkers. Here, a previously unknown combination of radical cyclizahon involving an epoxide-opening of 3-154 and a 3-phosphinoyl-elimination takes place to furnish various pyrrolidines 3-155, bearing a tetrasubstituted exo-double bond, in good yields (Scheme 3.41) [66]. 

Other sporadic examples of [2 + 2] cycloadditions of olefins on the exo double bond of structurally more complex MCPs, such as methylenecyclo-propenes, allylidene-, and alkenylidenecyclopropanes, have been reported. Thus, dicyclopropylideneethane (2) reacted with TCNE (131) to give the [2 + 2] adduct 164 as a minor product, together with the prevalent [4 + 2] adduct 163 (Scheme 76) [39], The same reaction in a different solvent had been previously reported to furnish exclusively the Diels-Alder product (see Sect. 2.1.2) [5]. 

Diarylmethylenecyclopropa[6]naphthalenes 14, unlike their benzene parent counterparts which give cycloaddition reactions at the cyclopropene bridge bond [10a], react on the exo double bond in Diels-Alder cycloadditions (see Sect. 2.1.1) [10b]. The reactions of 14 with the highly electron-deficient acetylenic(phenyl)iodonium triflate 584 give products 586a and 587, which are believed to derive from unstable primary [2 + 2] cycloadducts 585 (Scheme 82) [10b],... 

The addition of thiobenzoyl isocyanate to norbornene and norbornadiene resulted in the thiazinones 230 (R = Ph) and 231, respectively (67CB685). The ter/-butyl- and mesityl-substituted analogs of 230 were prepared by the same method. In the case of a benzyl substituent, the derivative 232 containing an exo double bond was formed (81CB549). 

In our group, we have explored the radical addition of alkyl radicals [97] and anomeric radicals [115] to sugar enones 134 having an exo double bond (see Schemes 46 and 47). Comparison of radical and anionic 1,4-addition, shown here, yields strictly equivalent stereoselectivities. The selectivity also depends on the nature of the entering radical, and reversal of stereoselectivity is observed on going from primary alkyl radicals to the larger t-butyl radical [for analogous observations see also Ref. 116]. 

The reduction of lactam substrates containing proximal exo double bonds may be achieved in high e.e. as demonstrated by the reduction of 3-alkylidene-2-piperidones (Scheme 19)119. Cyclic amino acids may be prepared by, for example, asymmetric hydrogenation of 3 to 4 in up to 79% e.e.120 and the reduction of 5 to 6 in 99% e.e.121. In the latter case a number of chiral diphosphines were screened, and the best results were obtained using BINAP as a ligand with rhodium metal. Several other diphosphines, notably DuPHOS and DIOP, also performed well. The research group which produced... 

In the same series (in this case, the enamines of 2-methylcyclohexanone) one would expect dialkylamines to be intermediate between five- and six-membered ring amines, since in the five-membered ring overlap is favoured because of the increase in exo double-bond character, while in a six-membered ring overlap is disfavoured because of the resistance to increase in the exo double-bond character with a dialkylamine, neither of these factors are operative (Table 9). 

The exo double bond is formed first in polymerizations of a-methylstyrene, but is later isomerized by protonic acid to the more stable endo isomer [14]. Carbocationic polymerizations initiated by protonic acids with extremely basic counteranions, as in triflic acid-initiated polymerizations of isobutene, produce predominantly the unsaturated dimer [285]. The exo dimer forms first and then isomerizes to the more stable endo isomer [Eq. (90)]. 

Lee et al. developed a new method for the construction of the skeleton of HA via the Mn(III)-mediated oxidative radical cyclization of allylic compound 16 derived from 2. The thermodynamically unstable exo double bond product 17 could be easily isomerized to the endo olefin 18 by treating it with triflic acid as reported in the literature (Scheme 4-9). ° ... 

The cyclocarbonylation of vinylic halides (usually the bromide or iodide) is another route to lactones and lactams. 1,2-Substituted vinyl compounds produce lactones where the double bond is incorporated into the ring (equation 27), while 1,1-substituted vinyl substrates give an exo double bond (equation 28). The 1,2-substituted vinyl reactions can be run under ambient conditions (24-72h), producing high yields of... 

The geometry of alkene coordination appears to affect the reactivity toward nucleophiles. 7-Methylenenorbomene reacts with PdCl2 to give the product from Pd-Cl addition across the exo-methylene double bond (equation 37). In the intermediate PdCl2-diene complex, which could not be isolated, the 7-exo double bond would be coordinated so as to lie in the ring plane, while the other aUcenic group would lie perpendicular to the palladium square plane, as is normal. From the structure of the product, trans addition of Pd and Cl occurs. 

The rules that apply to acyclic systems are also applicable to cyclic compounds with an exo-double bond. Thus 3-arylideneflavanones 1 were stereo selectively epoxidized with alkaline hydrogen peroxide and stereospecifically with 3-chloroperoxybenzoic acid20 and potassium hypochlorite21 and, similarly, 6-methoxyaurones 6 were stereoselectively epoxidized with alkaline hydrogen peroxide and stereospecifically with 2-chloroperoxybenzoic acid22. 

To highlight the combined factors of ring size and endo-exo double bonds, we have assembled heats of formation in Figure 7 (data are scanty). On thermochemical grounds we see that endo-6 should be favoured over exo-5 and exo-6 should be... 

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