Cis-irans isomers

This diene, prepared according to A. E. Montagna and D. H. Hirsch [US Patent 2,902,722 (1959)], is practically a pure (ranr-isomer, whereas the commercially available reagent is a cis-irans-isomer mixture. 

Both cis- and irans- 1,2-dibromoethylenes react with metal acetylides, but with different reactivity. For the competitive reaction of cis- and trans- fl-dibromoethylenes with the zinc acetylide 314 shows that the irans isomer is more reactive than the cis isomer[228,229]. It was also found by a competitive reaction with the zinc acetylide 317 that the irans vinyl monobromide 315 is more reactive than the cis isomer 316[230]. 

The second subject is concerned with cis and irans isomers. The trans isomer has the higher value (except for azobenzene) and the larger max.- This win be apparent from the data in Table XIII. 

The ratio of intensities of ion peaks [M — Br]+/M+ for cis-1,2-dibromocyclo-hexane is 1 to 55, while for its irans-isomer the ratio is 64 to 5. The difference is due to anchimeric assistance of the second bromine atom (Scheme 5.11) in diaxial conformation) [16]. 

There are now only three isomers because cis-irans and trans-cis geometries are identical. 

Note that American chemists would not italicize cis and Irans (or fac and mer) when not port of name, nor hyphenate irans isomer." See also Footnotes 33 and 34.]... 

Silicon-directed stereoselective syn addition of the hydroxy group to the olefinic double bond occurs intramolecularly in a TiCLt-or TsOH-catalyzed cyclization of vinylsilanes 139 bearing a hydroxy group207. The reaction gives mainly the irans-isomer (trans cis = 80-90 20-10) (equation 116)208. It is noteworthy that no cyclization is observed in the absence of the silyl substituent under the same conditions. 

Here we are primarily concerned with the fact that this ortho -adduct may occur in the form of two diastereomers. The diastereomers are formed as a 57 43 cis/trans-mixtme in the absence of A1C13, but a 95 5 cis/trans-mixture is obtained in the presence of A1C13. In the latter case, thus, one is dealing with a Diels-Alder reaction that exhibits a substantial simple diastereoselectivity (see Section 11.1.3 for a definition of the term). Here, the simple diastereoselectivity is due to kinetic rather than thermodynamic control, since the preferentially formed ds-disubstituted cyclohexene is less stable than its irans-isomer. 

In the tetrahydrofuran compounds there is opportunity for the existence of cis and irans isomers. They have been separated81 in the case of XL and XLII, the cis-form of which forms an anhydride. Evidence for the existence of the cts-form of 2,5-dishydroxymethyltetra-hydrofuran (XXXVIII) is found in the behavior of either its dichloro-or di-toluene-p-sulfonyl derivative with methanolic ammonia. From this reaction the dicyclic compound, 8-oxa-3-aza6fcyeZo [3,2,1] octane (XLIII) is obtained which can have been formed only from the m-form of the dicarbinol.82 An example of the addition of bromine to the furan... 

Cis-trans isomers (geometric isomers) are diastereomers thai dilTer in their cis-irans arrangement on a ring or double bond. 

The structures of the cis- and iran -isomers of Pt(PEt3)2Cl4 have been determined [176]. Table 3.26 shows the bond lengths depend slightly on the Irans-ligand. 

The thermal route has little control over the outcome of the reaction in terms of the possible isomers (cis, Irans) and is limited to those transition metal precursors that readily withstand the harsh reaction conditions. It does not have the potential for a general synthetic route. 

The couplings of vicinal protons in 1,2-disubstituted alkenes lie in the range 6- 12 Hz for cis protons (dihedral angle O ) and 12-17 Hz for trans protons (dihedral angle 180°), thus also following the Karplus-Conroy equation. Typical ejtamples are the alkene proton AB systems of coumarin (16a) (cis) and irans-cinnamic acid (16b), and of the cis-trans isomers 17a and b of ethyl isopentenyl ether, in addition to those in problems 3, 4, 9, 11 and 29. 

Another interesting case of metal-assisted ring-opening of Py has been reported by Wolczanski using Nb complexes. As shown in Eq. 6.26, the low-valent (silox)3Nb fragment coordinates Py in the ti (N,C) mode to yield (siloxljNblri fN.Cl-NCjHj), which readily undergoes C-N insertion by thermolysis in benzene at rather moderate temperatures (70 °C) [56]. The stoichiometry of the reaction yields 0.5 eq. of Py and 0.5 eq. of (silox)3Nb=CHCH=CHCH=CHN =Nb(silox)3 as a thermodynamic mixture of cis,cis-, trans,cis-, irans,trans- and cis,trans-isomers. 

When pure rrcinj-PtCU(Bu3P)2 is placed in solution with a trace of Bu3P, isomerization occurs to give a mixture of cis and Irans isomers. Provide a plausible mechanism. 

---