Syn/anti isomers

Dichloroneopentylsilene is formed in situ by reaction of trichlorovinylsilane with LirBu [1], The [2+2] cycloaddition to imines yields Si-dichloro functionalized 2-silaazetidines in a preparative scale [2], When aldimines are used as trapping agents for the silene, the resulting SiN-four membered ring compounds are isolated as syn/anti-isomers (syn/anti 2/1). 

Instead of a phenylhydrazone group of Schiff structure. Cl of n-glu-cose (D-arabino-hexose) phenylosazone may have a ring or azo structure unsuitable for formazan reaction. In this connection, Hardegger and Schreier have stated that, if all the structural possibilities are taken into account, their number may be as high as one hundred. In the case of d-glucose phenylosazone, these may be represented by nine fundamental compounds, of which the formulas IX, XI, XII, XV, and XVI—and all of the isomers syn, anti, a, and /3) derivable from them—are unsuitable for formazan reaction. 

Tine force field was then used to predict the results for fhe addition of the E and Z isomers c Ihe enol boronate of butanone (R = Me) to ethanol (R = Me). The relevant transitio. Iructures are shown in Figure 11.34. A Boltzmann distribution, calculated at the ten perature of the reaction (—78°C), predicted that the Z isomer would show almost complel syn selectivity syn anti = 99 1) and that the E isomer would be selective for the an product anti syn = 86 14). These results were in good agreement with the experunenti... 

Protonation of the anion [SN2] by acetic acid in diethyl ether produces the thermally unstable sulfur diimide S(NH)2. Like all sulfur diimides, the parent compound S(NH)2 can exist as three isomers (Scheme 5.5). Ab initio molecular orbital calculations indicate that the (cis,cis) configuration is somewhat more stable than the (cis,trans) isomer, while the (trans,trans) isomer is expected to possess considerably higher energy. The alternative syn,anti or E,Z nomenclatures may also be used to describe these isomers. The structures of organic derivatives S(NR)2 (R = alkyl, aryl) are discussed in Section 10.4.2. 

Hantzsch s proposal is consistent with characteristic infrared vibration frequencies of syn- and a fr-diazoates, suitably labeled with 15N, measured by Kiibler and Liittke (1963) and by an X-ray structure determination of a sy -diazoate by Alcock et al. (1980b). It is appropriate, therefore, to replace the syn/anti by the (Z/E) nomenclature for isomers (Sec. 7.1)... 

In 1980 and 1982, Callot and co-workers reported that Rh(Por)l catalyzed the reaction between alkenes and ethyl diazoacetate to give syn cyclopropoanes as the major products (Eq. 25). " This was unusual as most transition metal catalysts for this reaction give the anti isomers as the predominant products. Kodadek and co-workers followed up this early report and put considerable effort into trying to improve the syn/anti ratios and enantioselectivity using porphyrins with chiral substituents. 

The diastereoselectivity observed in simple systems led to investigation of enantiomerically pure aldehydes. It was found that the E- and Z-2-butenylboronates both exhibit high syn-anti diastereoselectivity with chiral a-substituted aldehydes. However, only the Z-isomer also exhibited high selectivity toward the diastereotopic faces of the aldehyde.38... 

Normally, the addition of C-nucleophiles to chiral a-alkoxyaldehydes in organic solvents is opposite to Cram s rule (Scheme 8.15). The anti-Cram selectivity has been rationalized on the basis of chelation control.142 The same anti preference was observed in the reactions of a-alkoxyaldehydes with allyl bromide/indium in water.143 However, for the allylation of a-hydroxyaldehydes with allyl bromide/indium, the syn isomer is the major product. The syn selectivity can be as high as 10 1 syn anti) in the reaction of arabinose. It is argued that in this case, the allylindium intermediate coordinates with both the hydroxy and the carbonyl function leading to the syn adduct. 

For many catalytic cyclopropanations, the stereoselectivity describing the stereochemical relation between substituents at the carbenoid and those at the double bond is not very pronounced. EjZ or syn/anti ratios of ca. 1-3 in favor of the less congested isomer may be considered normal (for examples see Tables 6 and 7). The stereochemical outcome can be expected to be governed by the nature of the olefin, the diazo compound and the catalyst. 

Figure 7.22 cis, trans, syn, anti isomers for TTP DTPA-bis amides. (Adapted with permission from Figure 7 of Caravan, P. Ellison, J. J. McMurry, T. J. Lauffer, R. B. Chem. Rev., 1999, 99(9), 2293-2352. Copyright 1999, American Chemical Society.)... 

A related catalyst 20 was used in the diastereoselective carbonyl-ene reaction between ethylidenecyclohexane, ethylidenecycloheptane, or 2-methyl-2-butene and trifluoroacetaldehyde (21, R = CF3, Equation (ll))19 or methyl glyoxylate (R = C02Me).20 The best results were obtained when X = Br 63-85% yields are obtained with syn/anti ratios of 95 5 or better, and ee s of the. qw-isomer of 74-89%. 

In a related publication, Kobayashi and his team reported on Zr-catalyzed asymmetric Mannich reactions that utilize the more electron-rich oxygenated ketene acetals shown in Scheme 6.28 [93], A noteworthy aspect of this study was that the levels of syn/anti diaste-reocontrol proved to be dependent on the nature of the alkoxide substituent whereas the (3-TBS acetals predominantly afforded the syn isomer, the OBn derivatives afforded a larger amount of the anti isomer. As before, the presence of an additive, this time 1,2-dimeth-ylimidazole (DMI), proved to be important with regard to the level of Ti-facial selectivity. The phenol activating group can be removed by the same procedure as reported previously, with essentially identical degrees of efficiency (see Scheme 6.27). 

These products obviously arose from metallation at unsubstituted ring positions in the starting tetrahalobenzene. To force metallation at the carbon-bromine bonds, they used 2,6-dibromo-3,5-difluoro-p-xylene which, with magnesium in THF gave mainly mono-adduct. With butyllithium, only the bis-adduct was obtained (15%). No mention was made of syn/anti isomers of the bis-adducts. 

A Mukaiyama-type aldol reaction of silyl ketene thioacetal (48) with an aldehyde with large and small a-substituents (e.g. Ph and Me), catalysed by boron trifluoride etherate, gives mainly the iyn-isomer (49), i.e. Cram selectivity. For the example given, changing R from SiBu Me2 to Si(Pr )3 raises the syn preference considerably, which the authors refer to as the triisopropylsilyl effect. Even when the and R groups are as similar as ethyl and methyl, a syn. anti ratio of 5.4 was achieved using the triisopropylsilyl ketene thioacetal. 

C——C— (where X was an atom having an unshared pair of electrons) there was no evidence of the formation of any but the syn isomer, while in the absence of any 5,6-chain the syn anti ratio was approximately 60 40. 

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