Alkynes geometric isomers

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

Relatively pure alkene geometric isomers are prepared by stereoselective reduction of alkynes. 

Internal alkynes were also viable substrates as illustrated by the ethylzincation of 5-decyne which led to the tetrasubstituted alkenyl iodide 71 after iodinolysis, as a single geometric isomer (syn addition) (equation 28)1 4. 

As a general rule, allylic organozinc reagents do not react with unactivated a, ji-disubstituted alkynes. A few exceptions to this trend have been observed in the case of substrates bearing an appropriately located heteroatom such as the homopropargylic tertiary amine 147. The latter reacted regioselectively with allylzinc bromide but led to a 67/33 mixture of two geometric isomers, apparently derived from syn and anti additions (equation 71)82,105. 

A more challenging task is the selective partial hydrogenation (semihydrogenation) of alkynes to yield alkenes. This is a selectivity problem similar to the hydrogenation of dienes in that that the alkyne is hydrogenated preferentially in the presence of an alkene. The possibility of the formation of geometric isomers from nonterminal acetylenes raises the problem of stereoselective semihydrogenation. 

An intersecting RCM is the alkyne variant called ring closing alkyne metathesis (RCAM). In this case, two alkynes, usually methyl terminated, are converted to the cycloalkyne. Unlike RCM, RCAM does not produce geometric isomers. Using this methodology, several constrained diamino-suberic analogues can be produced (Scheme 28.15).45... 

Exo cyclisations of vinyllithiums onto phenyl-substituted alkynes 392 and 394 are also syn stereospecific, and give the sort of stereodefined dienes 393 and 395 of value in Diels Alder reactions.180 6-Exo cyclisations are also possible but are much slower, though they still give geometrically pure products (in contrast to 6-exo cyclisations onto silyl alkynes see below). Vinyllithiums cyclise onto alkyl-substituted alkynes (such as 396) only in the presence of TMEDA, and they do so very slowly. Nonetheless, a single geometrical isomer of the product 397 is obtained. 

The naming system for alkynes is the same as for alkenes, except that alkynes do not have geometrical isomers. The triple bond only allows a single group to extend off of the carbon atoms attached by a triple bond, so no cis- or trans- isomers are possible. 

Alkynes contain triple bonds. Although they are also not able to rotate about the triple bond, they don t form geometrical isomers because the molecule has 180° bond angles at either end of the triple bond. 

The treated alumina with HCl still attached is added to a solution of an alkyne (1-phenylpropyne) and an addition reaction occurs to produce two geometrical isomers of an alkene. One results from cis addition of HCl to the triple bond, and one from trans addition. 

Why do alkanes and alkynes, unlike alkenes, have no geometric isomers ... 

Bombykol is a well-known pheromone, first isolated from Bombyx mori L. Bombykol the three related isomers were synthesized by cross-coupling reaction. Three alkenylboronates or boronic acids (35 and 36) required for the coupling are prepared by starting from two alkynes. The stereoselective syntheses of ( )- and (Z)-l-alkenylboronic acids or esters have been discussed in Section 2.2.1. Halogenation of alkenylboronic acid 34 with either iodine or bromine provides ( )- and (Z)-haloalkenes [71]. The palladium-and base-assisted coupling of the Cj-boronate 32 or 33 with the C,-halide 35 or 36 stereoselectively provides bombykol and its three geometrical isomers (Scheme 2-29) [72]. 

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... 

Finally, a novel three-component radical cascade reaction involving isonitriles has just been published [6]. In this paper, aromatic disulfides, alkynes, and isonitriles have been reported to react under photolytic conditions to afford -arylthio-substituted acrylamides 49 or acrylonitriles 50 in fair yields as mixtures of the E and Z geometric isomers (Scheme 21). The procedure entails addition of a sulfanyl radical to the alkyne followed by attack of the resulting vinyl radical on the isonitrile. A fast reaction, for example, scavenging by a nitro-derivative (route a) or f-fragmentation (route b), is necessary in order to trap the final imidoyl radical, since addition of vinyl radicals to isonitriles seems to be a reversible process. The reaction provides very easy access to potentially useful poly-functionalized alkenes through a very selective tandem addition sequence. 

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