Alkenyl, terminal substituents

Handedness Inversion in Azobenzene-doped CLCs In 2002, Ruslim and Ichimura reported that the compound 19 (Fig. 5.16) can induce the helix inversion when doping into LC hosts having alkenyl substituents [73]. Upon UV irradiation, the handedness of the cholesteric helix changed from right-handed to left-handed, whieh is most fikely to be the consequence of the competition between different interconvertible rotational species of the chiral alkyl chain relative to the azobenzene core. It is also worth noting here that the handedness inversion was only observed in LC hosts having alkenyl terminal substituents due to the alkenyl-selective interaction... 

XRD samples 638 f alignment defects, displays 756 alignment materials, displays 732 aliphatic chains 125,395 alkanes, solvents 883 alkanoic acids, esterification alkanols, esterification 106 alkenes, solvents 883 alkenyl, terminal substituents alkyl functionalization, arenes alkyl groups... 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

A fluorine substituent at the terminal position of a 1-alkene is the most shielded of simple alkenyl fluorines, with Z isomers being slightly more shielded (upheld) than E isomers (Scheme 3.37). 

Bicyclic nitrones (132) were formed from the reaction of alkenyl carbonyl compounds (131) with hydroxylamine. The reaction requires the presence of the terminal olefinic electron withdrawing ester group CC Et. Also, the product(s) of reaction are shown to depend on the space filling capacity of substituents R1 — R4... 

It is observed that insertion into a zirconacyclopentene 163, which is not a-substituted on either the alkyl and alkenyl side of the zirconium, shows only a 2.2 1 selectivity in favor of the alkyl side. Further steric hindrance of approach to the alkyl side by the use of a terminally substituted trans-alkene in the co-cyclization to form 164 leads to complete selectivity in favor of insertion into the alkenyl side. However, insertion into the zirconacycle 165 derived from a cyclic alkene surprisingly gives complete selectivity in favor of insertion into the alkyl side. In the proposed mechanism of insertion, attack of a carbenoid on the zirconium atom to form an ate complex must occur in the same plane as the C—Zr—C atoms (lateral attack 171 Fig. 3.3) [87,88]. It is not surprising that an a-alkenyl substituent, which lies precisely in that plane, has such a pronounced effect. The difference between 164 and 165 may also have a steric basis (Fig. 3.3). The alkyl substituent in 164 lies in the lateral attack plane (as illustrated by 172), whereas in 165 it lies well out of the plane (as illustrated by 173). However, the difference between 165 and 163 cannot be attributed to steric factors 165 is more hindered on the alkyl side. A similar pattern is observed for insertion into zirconacyclopentanes 167 and 168, where insertion into the more hindered side is observed for the former. In the zirconacycles 169 and 170, where the extra substituent is (3 to the zirconium, insertion is remarkably selective in favor of the somewhat more hindered side. 

In the first step of the catalytic cycle a coordinatively unsaturated Pd(0) species - which is formed in situ from Pd(OAc)2 and PPh3 -inserts into the alkenyl-I bond of 8 to give 42 (syn addition). Next an insertion of the terminal olefin into the cr-alkenyl-C-Pd bond forms the six-membered ring in 43. The stereochemistry can be explained by 41 reaction of the si-face of the exomethylene group involves a nearly coplanar orientation of the Pd-C bond and the C-C-zrbond. The siloxy substituent is placed pseudo-equatorially. 

The procedure described here provides a convenient method for the conversion of esters to Z-alkenyl ethers.5 The results in the Table show the wide applicability and high Z selectivity of the process. As the substituents R1 or R3 become bigger, or R2 becomes smaller, higher Z selectivity is observed. The stereochemistry of the isomers (Table, cases 1-10) was determined by 13C NMR.8 Since isomerization of alkenyl ethers has been reported to take place under GLC conditions, the remaining Z/E ratios were measured by 1H NMR (200 MHz) analysis. Esters having terminal double bonds reacted to afford the corresponding alkenyl ethers in about 50% yield (cases 7 and 9). Esters with internal double bonds gave better yields and the stereochemistry of double... 

Additions of Si-H bonds to alkynes occur under similar conditions and with the same catalysts as hydrosilation of alkenes. Free radical addition to terminal alkynes gives cis products by a stereospecific terminal trans addition . Supported platinum catalysts give trans products by a terminal cis addition . Chloroplatinic acid catalyzed additions to terminal alkynes give mixtures of trans-1-alkenylsilanes and trans-2-alkenylsilanes in a ratio ranging from 1 1 to 1 5 depending on the substituents on silicon . Addition of SiH2Cl2 to CH2=CHC(CH3)3 gives trans-1-alkenyl- and bis(trans-l-alkenyl)silane products, but no (2-alkenyl)silane °. Internal alkynes react more slowly than terminal alkynes, and even reactions catalyzed by chloroplatinic acid require heat. 

Why does the (EBTHI)Zr system induce such high levels of enantioselectivity in the C-C bond formation process It is plausible that the observed levels of enantioselection arise from minimization of unfavorable steric and torsional interactions in the complex that is formed between 3 and the heterocycle substrates (Scheme 3). The alternative mode of addition, illustrated in Fig. 1, would lead to costly steric repulsions between the olefin substituents and the cyclohexyl group of the chiral ligand [6]. Thus, reactions of simple terminal olefins imder identical conditions results in little or no enantioselectivity. This is presumably because in the absence of the alkenyl substituent (of the carbon that bonds with Zr in i) the aforementioned steric interactions are ameliorated and the olefin substrate reacts indiscriminately through the two modes of substrate-catalyst binding represented in Fig. 1. 

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