Trans orientation

The inversion of the substituent and the lone-pair on nitrogen takes place slowly at room temperature and has been extensively studied (77AHC(2l)207, 77IZV2266, 77IZV1687). The inversion takes place by a buckling of the ring and the mixtures obtained are those with the trans orientation predominant (77IZV2266). 

The addition of secondary amines to acetylenes is most applicable to the synthesis of conjugated acyclic enamines (50,171,172). Particularly the addition to acetylenic esters and sulfones has been investigated (173-177) and it appears that an initial trans addition is followed by isomerization to more stable products where the amine and functional group are in a trans orientation (178). Enamines have also been obtained by addition of secondary amines to allenes (179). 

A closely related procedure for preparing thietane dioxides is the one-step conversion of cyclic a-amino ketoximes (222) to 2-(co-cyanoalkyl)-3-dialkylaminothietane dioxides (226), with trans-orientation of the substituents249 (equation 86). 

Looked at in a different way, the orientation of gauche conformations is always very low, and the oriented chains are predominantly in the trans conformation so that the increase in the trans/gauche ratio is accompanied by a rapid rise in trans orientation. In fact, it has been pointed out 27) that to a very good approximation the overall orientation... 

In Fig. 11, experimental results are shown for the PET films, together with two calculated relationships between B20/a2 and P20o°v< rall> based on the alternative hypotheses that the NMR anisotropy arises either from wholly trans oriented material, according to Equation (40b) or from wholly gauche oriented material. It is very satisfactory to note that the experimental results are very close to the first hypothesis. 

Fig. 11. B20/a2 plotted against P2(0)overa together with the theoretical boundaries (A) for wholly gauche oriented material (B) for wholly trans oriented material. Draw ratios (a) 2 1 (b) 2.5 1 (c) 5 1 for one way drawn biaxially oriented film (d) 3.25 1 for uniaxal drawn rod. Reproduced from Polymer by permission of the publishers, Butterworth Co (Publishers) Ltd. (C)...

To determine the potentiality of catalyst 72a, various alkenes with methyl, ethyl and iso-propyl substituents in cis or trans orientations relative to the aryl substituent were hydrogenated (Scheme 47). In general, (E)-alkenes produced better results than their cis isomers. 

Some examples of alkylation reactions involving relatively acidic carbon acids are shown in Scheme 1.3. Entries 1 to 4 are typical examples using sodium ethoxide as the base. Entry 5 is similar, but employs sodium hydride as the base. The synthesis of diethyl cyclobutanedicarboxylate in Entry 6 illustrates ring formation by intramolecular alkylation reactions. Additional examples of intramolecular alkylation are considered in Section 1.2.5. Note also the stereoselectivity in Entry 7, where the existing branched substituent leads to a trans orientation of the methyl group. 

Diels-Alder reactions are stereospecific with respect to the E- and Z-relationships in both the dienophile and the diene. For example, addition of dimethyl fumarate and dimethyl maleate with cyclopentadiene is completely stereospecific with respect to the cis or trans orientation of the ester substituents. 

C36H60O30,C6H5O3S -Na+-10 H20 Cyclomaltohexaose - sodium ben-zenesulfonate, decahydrate (CDXBZS)275 P212121 Z = 2 D = 1.47 R = 0.07 for 2,894 intensities. This is a channel-type structure, with a conformation very similar to that of the potassium acetate280 and Methyl Orange complexes.281 All of the pyranose residues have the 4Cl conformation, and the primary alcohol groups are in the gauche-trans orientation. 

The 14-membered macrocycle 1,4,8,11-tetraazacyclotetradecane (cyclam or [14]aneN4), unlike cyclen, is capable of encircling most transition metal ions and in the case of Co111 the trans configuration is much preferred by comparison with the folded cis isomer. Electrochemical reduction of A,v-[Co(cyclam)(OI I)2]+ in 3M NaOH leads to rapid isomerization to the trans form, and the relative stabilities of the trans and cis isomers of the di- and trivalent complexes were determined from a thermodynamic cycle.702 This preference for trans orientation of the non-macrocyclic donors has enabled the isolation and investigation of many Co complexes without the complications of isomerization. Some novel examples include /r[Pg.61]

In some cases, planar chain structures become impossible when replacing a hydrogen atom on the backbone of a macromolecule by a bulky group. Packing problems arise due to steric hindrance if the trans orientation of each bond is... 

We have recently investigated X-ray structure of 3-methoxycarbonyl-[l,4,2] diazaphospholo[4,5-a]pyridine as first example of molecular structure determination for [l,4,2]diazaphosphole ring [84], The ester substituent lies strictly in the molecular plane with carbonyl group in the trans orientation with the formal C=P bond. Endocyclic P-N and P-C bonds are averaged between respective single and double bond lengths. 

This ring system could be prepared by thionation of benzil monohydrazone 172 with Lawesson s reagent (LR) followed by cyclization. The reaction gives 2,3-diphenyl-7,8,9,9a-tetrahydro-27/,677-pyrido[2,TA][l,3,4]thiadiazine 173 in a diastereoselective way, with the hydrogens at 2- and 9a-position in trans orientation. Benzil monohydrazone 172 was obtained by condensation of benzil 170 with 1 equiv of iV-aminopiperidine 171 (Scheme 23) <1998TA1531>. 

Remarkably, Claver et al. showed that in a square planar rhodium carbonyl chloride complex, two bulky phosphite ligands (65) were able to coordinate in a trans orientation.214 Diphosphite ligands having a high selectivity for linear aldehyde were introduced by Bryant and co-workers. Typical examples are (67)-(70).215,216... 

The isomerization barrier of 15.0-20.0 kcal mol-1 (AG ) can be considered to be large enough to allow isolation and characterization of bis(q3-<2 /),A- nms-dodecatrienediyl-Nin stereoisomers of 7b41 as reactive intermediates in the stoichiometric cyclotrimerization process. Furthermore, the trans orientation of the two allylic groups gives rise to an insurmountable barrier for reductive elimination for these cases, which prevents these species from readily leaving the thermodynamic sink via a facile reductive elimination. The isolated intermediates clearly constitute dead-end... 

The stereochemistry of the hexitols affects the manner of ring fusion, as is seen in Figs. 3 and 4. In mannitol, sorbitol (gulitol) and iditol, the l,4 3,6-dianhydro rings are m-oriented, whereas in the dulcitol, allitol and altritol (talitol) series the 1,4 3,6-rings are trans-oriented. This makes the ring system in this latter series almost planar although in the case of dianhydrodulcitol and allitol there is considerable strain in the molecules. 

D-manno, /3-D-xylo, a- and /3-D-lyxo, a- and /3-D-ribo, and a-D-ara-bino)278 on the other hand, it was observed that, when the fluorine atom and the substituent at C-2 are ci.s-oriented, the JF,C value is numerically higher (231.6-236.8 Hz) than when they are trans -oriented (224.7-228.46 Hz). 

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