Alkenes systems

For acyclic allylic substrates die situation is mote complex, since a larger number of reactive conformations, and betice corcesponding transition states, compete. Hius, mediyl ciimamyl derivatives 163 tX= O.Acj, upon treatment witli litliiiim dimetliylcuprate, mainly gave tlie S 2 substitution product 166 fentry 1, Tab. 6.6 and Sdieme 6.34) [80]. Hie preference for die S 2 product is expected, since de-conjugation of die alkene system is electronically imfavorable. 

Two RCM reactions were employed in a new and efficient route to a key chiral intermediate, isoquinuclidine 150, in the synthesis of alkaloid (-F)-catharanthine <06AG(I)5334>. The first RCM makes use of chiral enone 151, derived from L-serine, to generate a chiral dihydropyridinone 152. Intramolecular alkene metathesis of dialkenyl piperidine 153 generates 150, which represents the first example of the use of RCM in the generation of an azabicyclo[2.2.2]alkene system. 

Stereoselective Murai-type couplings can be carried out on conjugated alkene systems. Such a procedure was employed in the synthesis of the elaborated lactone G (Equation (197)).160... 

Rhodium catalysts have also been used with increasing frequency for the allylic etherification of aliphatic alcohols. The chiral 7r-allylrhodium complexes generated from asymmetric ring-opening (ARO) reactions have been shown to react with both aromatic and aliphatic alcohols (Equation (46)).185-188 Mechanistic studies have shown that the reaction proceeds by an oxidative addition of Rh(i) into the oxabicyclic alkene system with retention of configuration, as directed by coordination of the oxygen atom, and subsequent SN2 addition of the oxygen nucleophile. 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

As a consequence of this model, it should be foreseeable that increasing allylic A strain - arising from employment of a Z alkene system, for example - should favor transition state 167 even more, giving higher levels of E selectivity for the... 

Conjugation with an electron-withdrawing group substantially lowers the energy of the lowest unoccupied molecular n-orbital, which results in less negative reduction potentials for the alkene system. The class of compounds is referred to as activated alkenes, Polarographic half-wave potentials for some activated alkenes in aprotic solvents are listed in Table 3.3... 

Acetylenic precursors employed in the syntheses of sugars may be divided into three groups (a) aldehydes (usually in the form of acetals), (b) alkyl alkynyl ethers, and (c) alkynols or alkynediols. Some of them are commercially available (for example, 2-butyne-l,4-diol), and others are prepared by Grignard-type reactions between 1-alkynylmag-nesium halides or lithium alkynes and suitable aldehydes, ketones, or epoxides. In this way, the synthesis of substrates having the desired number of carbon atoms, as well as the necessary functional groups, can be achieved. The next step consists in partial saturation of the triple bond to afford the desired cis- or trans-alkene. ct.s-Alkene systems... 

The irradiation of benzenes with alkenes provides a fascinating array of photochemical reactions, not least because it converts the aromatic substrates into polycyclic, non-aromatic products. In principle, benzene can undergo reaction across the 1,2-(ortho). 1,3-(meta), or 1,4-(para) positions the 1,3-cycloaddition is structurally the most complex, but it is the predominant mode of reaction for many of the simplest benzene/alkene systems. The products are tricyclic compounds with a fusion of two five-membered rings and one three-membered ring, and an example is the reaction of benzene with vinyl acetate (3.411. For monosubstituted benzenes there can be a high... 

The extensive delocalization and aromatic character of pyridones, pyrones, etc. are shown by their chemical shift and coupling constant values (Table 8). By contrast, pyrans and thiins show chemical shifts characteristic of alkenic systems (Table 9). For these and for rings containing only a single endocyclic bond (Table 10), H NMR spectroscopy offers a most useful tool for structure determination. 

Acyclic perfluoroenols are strongly destabilized relative to their cyclic counterparts 132 the result is general for alkene systems.133... 

The diselenides-mediated reaction can be extended to a mixed alkene system comprising 2-methoxypropene and acrylonitrile (Scheme 6.5). The intermolecular C-C bond-forming processes (arrow 2 and 3) are sequenced by 5-exo cydization (arrow 4) onto the newly formed C-C double bond [11]. 

Scheme 4.56). The two-bond, F-F coupling constant observed in such conjugated systems is much smaller than that of the nonconjugated alkene systems. 

The catalyst + aliphatic n-alkene system, for all of the olefins from propene to 1-decene, give almost 40,000 possible conformations. The most stable conformations for each of the olefins were selected (around 1,700) and their transition states were optimized at IMOMM level. The calculated enantiomeric excesses are shown in Fig. 13. Calculations are able to reproduce the observed increase in ee for short chains, and the presence of a ceiling value after which the increase in enantioselectivity is much smaller, in excellent agreement with experiment. 

First reported in 1977 [25], Feringa s overcrowded alkene systems account for the most abundant and significant contribution to this area [26]. Chirop-tical switch 10 (Scheme 2) [27] was the first example of this kind of molecular rotor. 

It will help at this point to realize that the molecule consists of three spin systems with the insulation point at C-6 (see Section 3.5.2). The alkyl system consists of H-l, H-2, H-3, H-4, H-5, and H-9 an alkene system consists of H-7 and H-8 and another alkene system consists of H-10. The alkyl system accounts for the multiplets at the right of the spectrum, and the alkenes account for the multiplets at the left side. It will also help to reiterate that the protons of an alkyl CH2 group will be diastereotopic in the presence of a chiral center. As will be the protons of an alkene=CH2 group. 

The resulting <7 alkyl bond in such complexes is very reactive, especially towards carbon-carbon bonds. Thus an alkene in the reacting system will lead to coordination followed by migratory inser tion into the palladium-carbon a bond. This process is like hydrometallation and is called carbo palladation as carbon and palladium are attached to the ends of the alkene system. There is nc change in oxidation state during this process, although the ligands (often phosphines) must dissociate to allow coordination of the alkene and associate to provide a stable final 16-electron product. 

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