Geometrical isomers control

This was ihe first investigation of stereoselective formation of geometric isomers of tertiary amines. The authors suggested most enamine systems should be subject to considerable stereochemical control. 

The isomer distribution of the nickel catalyst system in general is similar qualitatively to that of the Rh catalyst system described earlier. However, quantitatively it is quite different. In the Rh system the 1,2-adduct, i.e., 3-methyl-1,4-hexadiene is about 1-3% of the total C6 products formed, while in the Ni system it varies from 6 to 17% depending on the phosphine used. There is a distinct trend that the amount of this isomer increases with increasing donor property of the phosphine ligands (see Table X). The quantity of 3-methyl-1,4-pentadiene produced is not affected by butadiene conversion. On the other hand the formation of 2,4-hexadienes which consists of three geometric isomers—trans-trans, trans-cis, and cis-cis—is controlled by butadiene conversion. However, the double-bond isomerization reaction of 1,4-hexadiene to 2,4-hexadiene by the nickel catalyst is significantly slower than that by the Rh catalyst. Thus at the same level of butadiene conversion, the nickel catalyst produces significantly less 2,4-hexadiene (see Fig. 2). 

The hydrosilylation of l,4-bis(trimethylsilyl)but-3-en-l-yne (141) was beautifully controlled and four different isomeric products could be prepared independently with 93-96% selectivity by a proper choice of geometric isomers of 141 and transition metal catalysts [113]. One of the four products from the reaction of 141 with 132p was allene 142, which was obtained as a mixture (142 143 = 96 4) in 93% yield (Scheme 3.73). 

During the dehydration of pentoses in acidic solution, the cis and trans 2-enes (homologs of 44 and 46 see p. 177) react rapidly to form the 3-enes, 94a and 94b. Anet8 has presented evidence for the existence of cis and trans forms of the 6-carbon 2-enes. The relative proportion of the geometrical isomers 94a and 94b is controlled by the ratio of the cis and trans forms of the reacting 2-enes and by the rate of interconversion through the tautomer, 72. The trans form (94a) is unable to cyclize, whereas the cis isomer (94b) could readily cyclize to 28 and this could be dehydrated to 2-furaldehyde (27). Formed by extended enolization of 94, the diene 72 has the electronic arrangement needed for cyclization to afford 75a, a tautomer of re-... 

Cyclopropane formation occurs from reactions between diazo compounds and alkenes, catalyzed by a wide variety of transition-metal compounds [7-9], that involve the addition of a carbene entity to a C-C double bond. This transformation is stereospecific and generally occurs with electron-rich alkenes, including substituted olefins, dienes, and vinyl ethers, but not a,(J-unsaturated carbonyl compounds or nitriles [23,24], Relative reactivities portray a highly electrophilic intermediate and an early transition state for cyclopropanation reactions [15,25], accounting in part for the relative difficulty in controlling selectivity. For intermolecular reactions, the formation of geometrical isomers, regioisomers from reactions with dienes, and enantiomers must all be taken into account. 

BINOL-derived titanium complex was found to serve as an efficient catalyst for the Mukaiyama-type aldol reaction of ketone silyl enol ethers with good control of both absolute and relative stereochemistry (Scheme 8C.24) [57]. It is surprising, however, that the aldol products were obtained in the silyl enol ether (ene product) form, with high syn-diastereoselec-tivity from either geometrical isomer of the starting silyl enol ethers. 

Harding, L. P., Jeffery, J. C., Riis-Johannessen, T., Rice, C. R., Zeng, Z. T., Anion control of the formation of geometric isomers in a triple helical array. Dalton Trans. 2004, 2396-2397. 

A recently developed application of this method in the field of polymers involves the synthesis of three geometrical isomers by polyaddition of diethynyl compounds, based on transition-metal-catalyzed dimerization of arylacetylenes [5], This reaction goes highly regio- and stereoselectively and is controlled by appropriate choice of the catalyst, affording polymers with ( )-, (Z)- and gem-vinylene linkages with selectivity over 92 %, as illustrated in Scheme 12. 

However, the reaction with IN3 is largely non-stereospecifio and the geometric isomers of 51 are obtained in the ratio 2 1 under kinetic control. The formation of large amounts of the cis isomer of 51 has been tentatively explained in terms of cis oriented ion-pairs (see Scheme 2) but it may be a serious argument against the hypothesis of cyclic io-donium ions (Storr, 1970). The addition mechanism is probably more complicated than suggested and the idea of the formation of 52, although reasonable, requires further support. 

In this chapter we shall talk about reactions similar to the ones on the previous page and we shall be interested in how to control the geometry of double bonds. Geometrical isomers of alkenes are different compounds with different physical, chemical, and biological properties. They are often hard to separate by chromatography or distillation, so it is important that chemists have methods for making them as single isomers. 

Synthetic analogues of this compound, such as the trienes, are also effective at arresting insect development, providing that the double bond geometry is controlled. The Z,E,E geometrical isomer of the triene is over twice as active as the , ,E-isomer, and over 50 times as active as the ,Z,Z- or Z,E,Z-isomers. 

How can the Z selectivity in Wittig reactions of unstabilized ylids be explained We have a more complex situation in this reaction than we had for the other eliminations we considered, because we have two separate processes to consider formation of the oxaphosphetane and decomposition of the oxaphosphetane to the alkene. The elimination step is the easier one to explain—it is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state (as in the base-catalysed Peterson reaction). Addition of the ylid to the aldehyde can, in principle, produce two diastere-omers of the intermediate oxaphosphetane. Provided that this step is irreversible, then the stereospecificity of the elimination step means that the ratio of the final alkene geometrical isomers will reflect the stereoselectivity of this addition step. This is almost certainly the case when R is not conjugating or anion-stabilizing the syn diastereoisomer of the oxaphosphetane is formed preferentially, and the predominantly Z-alkene that results reflects this. The Z selective Wittig reaction therefore consists of a kinetically controlled stereoselective first step followed by a stereospecific elimination from this intermediate. 

The recorded reports of the synthesis of enamines which can exist as geometric isomers are generally characterized by the absence of discussion of the stereochemical constitution of the products. It is likely that, where possible, mixtures of stereoisomers are obtained when employing the general procedures, whose composition is the result of thermodynamic control. 

A general catalytic method to achieve the complete control of cis trans selectivity is not available. Tlie distribution of the geometrical isomers can, however, be appreciably modified by using phosphites [(RO) ] ligands on copper in olefin cyclopro pa nations the isomeric ratio will depend on the R group. 

From 1976 to 1978, tremendous efforts were spent on the isolation of a major component (in a pure form) of the sex pheromone of the female cockroach, Periplaneta americana. The potency of the compound isolated, periplanone B, was truly amazing (threshold limits of 10 pg). It was considered to be a promising agent to control this pest. However, only trace amounts were isolated from natural sources (a total of 0.2 mg isolated from 75 000 specimens). Even the use of modern methods of instrumental analysis did not lead to the elucidation of its complete structure. With the help of these methods it was possible only to ascertain the basic structural features, as shown in formula 59a (Scheme 1.16). The problem of its stereochemistry remained unanswered. This most difficult part of the problem was finally solved only after a total synthesis of three of four possible geometrical isomers of periplanone Comparison of the spectral parameters of synthetic samples with those of the natural periplanone B determined the stereochemistry as shown in structure 59b. A minor component of the same pheromone, periplanone A, was available in even... 

A stereoselective tandem Sakurai-carbonyl-ene reaction for the synthesis of steroid derivatives has been reported [48]. When LtAlCl2 and la were employed in this cyclization, stereochemical control was different. The cyclization product obtained with la is only 19 (Sch. 17), even though the starting material contained all four geometrical isomers use of LtAlCl2 resulted in a mixture of two different stereoisomers in lower yield. 

The important point about substituted enolates is that they can exist as two geometrical isomers, cis or trans. Which enolate is formed is an important factor controlling the diastereoselectivity because it turns out that, in many examples of the aldol reaction, ds-enolates give syn aldols preferentially and frans-enolates give anti aldols preferentially. 

When we first introduced the concept of enantiomers and chirality in Chapter 16, we stressed that any imbalance in enantiomers always derives ultimately from nature. A laboratory synthesis, unless it involves an enantiomerically pure starting material or reagent, will always give a mixture of enantiomers. Here is just such a synthesis of the Japanese beetle pheromone you have just met. You can see the Z-selective Lindlar reduction in use—only one geometrical isomer of the double bond is formed— but, of course, the product is necessarily racemic and therefore useless as beetle bait, because in the original addition of the lithiated alkyne to the aldehyde there can be no control over stereochemistry. If all the starting materials and reagents are achiral, the product must be... 

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