Isomerizations anti-syn

The migrating group retains its configuration. Some reaction conditions can lead to syn-anti isomerization at a rate exceeding rearrangement, and when this occurs, a mixture of products is formed. The reagents that have been found least likely to cause competing isomerization are phosphorus pentachloride and p-toluenesulfonyl chloride.283... 

The inversion barrier for syn/anti isomerization of H2Si=NH is only 5.6 kcal mol-1, whereas the internal rotation energy is 37.9 kcal mor1 (SOCI level of calculation). The rotation barrier can be equated to the ir-bond strength. The inversion transition state has an even shorter SiN bond length of 153.2 pm. The symmetry is C2V.9,10... 

C. Syn-Anti Isomerization and Enantioface Conversion of the Terminal Allylic Group in the Octadienediyl-Ni" Complex... 

The interconversion between octadienediyl-Ni11 stereoisomers may involve two different processes namely the syn-anti isomerization and also the enantioface conversion of one or both terminal allylic groups (Fig. 4). The isomerization of the allylic group is connected with two aspects firstly the interconversion of its syn and anti configuration and secondly the inversion of its enantioface.36 On the other hand, the process of enantioface conversion is not accompanied by alternation of the allylic configuration. 

Cyclic imines do not have the problem of syn/anti isomerism and therefore, in principle, higher enantioselectivities can be expected (Fig. 34.8). Several cyclic model substrates 6 were hydrogenated using the Ti-ebthi catalyst, with ee-val-ues up to 99% (Table 34.5 entry 5.1), whereas enantioselectivities for acyclic imines were <90% [20, 21]. Unfortunately, these very selective catalysts operate at low SCRs and exhibit TOFs <3 h-1. In this respect, iridium-diphosphine catalysts, in the presence of various additives, seem more promising because they show higher activities. With several different ligands such as josiphos, bicp, bi-... 

The stereochemistry of the process was examined by analysis of the products resulting from trapping the lithioallene from a chiral allenylcarbamate with Me3SiCl (Eq. 9.11). Sequential lithiation with BuLi followed by addition of Me3SiCl at -78°C afforded a 75 25 mixture of the syn and anti adducts in 70% yield. On the other hand, deprotonation with LDA at -78 °C in the presence of excess Me3SiCl gave rise to the syn adduct as the sole product in 70% yield. It could therefore be surmised that (1) lithiation proceeds with retention of stereochemistry and (2) syn/anti isomerization of the putative allenyllithium intermediate at -78 °C is slower than silyla-tion (Eq. 9.12). 

The biscyclohexane-annelated bicyclopropylidene 22 a has been observed to undergo a reversible syn-anti isomerization under Znl2 catalysis [110] to end up as a 1 3 mixture ofsyn- and anti-isomers [16] (Scheme 22). 

Scheme 22. Photoinitiated reversible reorganization of 2,2-diarylmethylenespiropentanes 113 and Znl2-promoted reversible syn-anti isomerization of bicyclopropylidenes 22a [16,109]...

Isatin-3-oxime reacts with isocyanates to give carbamoyloximino-isatins (75).290 Irradiation of AT-inethylisatin-3-oxime causes syn-anti isomerization,291 while A -acetylindazolone is obtained from irradiation of AT-acetylisatindioxime.291 Treatment of this dioxime with acetic... 

The reaction of 3-ketoacids with allyl carboxylates is also believed to proceed via a palladium enolate intermediate.126 Less than complete stereospecificity is also observed in these reactions (equation 163). Interestingly, the bicyclic lactone substrate employed to ascertain the stereointegrity of this reaction, in addition to being incapable of any syn-anti isomerization, cannot epimerize the starting material by car-boxylate attack at the metal. The observed stereochemical leakage could be due to epimerization of the intermediate allyl complex (equation 164) or reductive elimination of an allylpalladium enolate (retention) (equation 165). 

Significant progress has been made in enantioselective C—C bond formation in ir-allylpalladium chemistry, where the source of the chirality is the phosphine ligand on the palladium. A number of potential difficulties must be addressed in order for this approach to result in synthetically useful optical yields. Racemization can occur by a variety of mechanisms. Intermediates not bearing substituents at both allyl termini can undergo syn-anti isomerization, which accomplishes racemization (equation 347). 

McCarty CG (1970) syn-anti Isomerizations and rearrangements. In Patai S (ed) The chemistry of the carbon-nitrogen double bonds. Wiley, London, chap 9 Bjorgo J, Boyd DR, Watson CG, Jennings WB (1974) J Chem Soc Perkin Trans 2 757 Johnson GP, Marples BA (1984) Tetrahedron Lett 25 3359... 

They have obtained unambiguous evidence that the photochemical process is completely regiospecific. They have further concluded that this process must be controlled by stereoelectronic factors because the bond (R — C) which is anti to the nitrogen lone pair in oxaziridine 79 migrates more easily than the bond (R"— C) which is syn to the lone pair. Syn-anti isomerization by nitrogen inversion in oxaziridines 79 was rigorously excluded under the reaction conditions (r.t.). 

Aakermark B, Vitagliano A (1985) Reactivity and syn-anti isomerization of (r 3-geranyl)-and (r 3-neryl)palladium complexes. Evidence for electronic control of the regiochemistry of nucleophilic addition. Organometallics 4 1275-1283... 

B.1 Proline syn—anti Isomerization, Implications for Protein Folding... 

Syn-anti isomerization N-terminal to prolyl peptide fragments is one of the rate limiting steps in protein folding [1], Prolyl fragments exist essentially either in the syn or anti form in native proteins, but as an equilibrium... 

More attention has been devoted, particularly in recent years, to the direct GC separation of hydrazones of carbonyl compounds. Phenylhydrazones of aldehydes can be separated successfully in a column packed with SE-30 on Chromosorb W at temperatures ranging from 120 to 190°C [50]. Korolczuk et al. [51] considered this problem in more detail. They described the separation of phenylhydrazones of 27 aldehydes and ketones using different temperature programmes and studied the influence of the initial temperature on the retention of the derivatives. The analysis time is less than 15 min for carbonyl compounds with up to 11 carbon atoms with programming at 10°C/min in the range 150—280°C. Some derivatives provide two peaks which can be ascribed to their syn—anti isomerism, although even the decomposition of the derivatives cannot be eliminated as a cause, particularly with the use of a metallic column. 

The GC separation of 2,4-dinitrophenylhydrazones has been studied by a number of workers [52-55], Non-polar stationary phases of the SE-30 and SF-96 type were utilized for this purpose at temperatures of 200—250°C, mostly with temperature programming. Retention indices of the 2,4-DNPHs of some carbonyl compounds on OV-type stationary phases are presented in Table 5.5. Using columns with a higher separation efficiency, some 2,4-DNPHs provide two peaks. The discussion of whether these artifacts are caused by thermal decomposition or syn-anti isomerization of the derivatives seems to favour the latter. The ratio of the areas of the peaks of the two derivatives depends on the polar-... 

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