Stereoselective reactions alkynes

This was explained by the involvement of a vinylidene complex that is also in agreement with the migration of the acetylenic hydrogen to C-2 observed by deuterium labeling. The stereoselective reaction requires the use of EtjN and a slight excess of the alkyne. 

In this last section of the chapter we shall leave elimination reactions to look at addition reactions. Alkynes react with some reducing agents stereoselectively to give either the Z double bond or the E double bond. Some of these reactions were described briefly in Chapter 24. 

The central problem in the synthesis of acyclic polyunsaturated compounds (for example, many pheromones) is the construction of an aliphatic chain possessing double bonds of the required configuration in specified positions. The strategy of synthesis in this area could be based either at the identification of unsaturated synthons with double bonds in the needed configuration (as was done in the synthesis of the juvenile hormone, see Section 2.8), or the employment of stereoselective reactions in the process of chain construction, such as carbometallation of alkynes (see Section 2.3.3). 

Reduction of an alkyne to a cis alkene is a stereoselective reaction, because only one stereoisomer is formed. 

Hydrothiolation of 1-alkynes with thiols gives vinyl sulfides with high regio- and stereoselectivities. Reaction of 1-octyne with benzenethiol catalyzed by Pd(OAc>2 gave the Markonikov-type product 110. Isomerization of the double bond occurred and the isomer 111 was obtained when PdCl2(PhCN)2 was used [34]. 

Remarkably, the diiridium(l) complexes show excellent regio- and stereoselectivities for alkyne dimerization and trimerization reactions, giving a single hexadienyne trimer and exclusive formation of head-to-head Z-butenyne (Scheme 29) [112],... 

Takeuchi has shown that the stereoselectivity of alkyne hydrosilylation can be controlled by both the ligand and the solvent (Equation 16.31). addition of the Si-H bond across the alkyne has been observed for the hydrosilylation of hexyne by triethyl-silane in ethanol solvent in the presence of a catalyst generated from the combination of [Rh(COD),] " and PPhj. In contrast, trans addition occurs for the same reaction in the same medium with a neutral rhodium catalyst lacking added phosphine. Trans addition also occurs in the presence of Wilkinson s catalyst in toluene. (Equation 16.32). As discussed in Section 16.3.5.2.4 below, trans addition likely results from the isomerization of a vinyl intermediate. -... 

The ruthenium-catalyzed hydrosilylation/protodesilylation protocol is a useful method for stereoselective internal alkyne (cycloalkyne) reduction to (E)-alkenes [(iJ)-cycloalkenes] and is a complement to the cis selectivity observed in the Lindlar reduction (eqs. (27) and (28)) (184,185). The utility of this reaction sequence is attributed to the fact that many of the knovra methods for transforming ahquies to ( )-alkenes either have poor selectivity or are incompatible with common functional groups. 

The reaction above is an example of a stereoselective reaction. A stereoselective reaction is one in which the reactant is not necessarily chiral (as in the case of an alkyne) but in which the reaction produces predominantly or exclusively one stereoisomeric form of the product (or a certain subset of stereoisomers from among all those that are possible). 

Alkynes can be treated with organometallic reagents to get c/s-carbometalated alkenes in the presence of suitable catalysts. Subsequently, the metal can be substituted by a proton during aqueous workup. Alternatively, the metalated alkene can be treated with an electrophile to introduce a second substituent to the alkene. Catalytic amounts of tris(acetylacetonato)iron allow the reaction of 3-pentynyl ethers with butyllithium at -20 °C in toluene to give 4-methyl-octen-3-yl ethers in high yields (Scheme 4—268). Addition of an electrophile other than before aqueous workup leads in a stereoselective reaction to the products of a syn carbometalation with subsequent displacement of the metal by the electrophile under retention of the configuration. ... 

The ene reaction of an alkyne and an alkene produces a 1,4-diene. An important application, the regio- and stereoselective coupling of 17-(Z)-ethylidene steroids and alkynes to give cholane-type 16,22-dienes, is described in section 4.5.2. 

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

Stereoselective and chemoselective semihydrogenation of the internal alkyne 208 to the ew-alkene 210 is achieved by the Pd-catalyzed reaction of some hydride sources. Tetramethyldihydrosiloxane (TMDHS) (209) i.s used in the presence of AcOH[116]. (EtO)3SiH in aqueous THF is also effective for the reduction of alkynes to di-alkenes[l 17], Semihydrogenation to the d.v-alkene 211 is possible also with triethylammonium formate with Pd on carbon[118]. Good yields and high cis selectivity are obtained by catalysis with Pd2fdba)3-Bu3P[119],... 

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

The reaction of 3-substituted 3-haloallenes with various cuprates, the converse reaction of propargyl derivatives, proceeds in an SN2 manner to form alkynes 69b. Very high anti stereoselectivity is achieved693. 

Thus far, chemists have been able to influence the stereoselectivity of macro-cyclic RCM through steric and electronic substrate features or by the choice of a catalyst with appropriate activity, but there still exists a lack of prediction over the stereochemistry of macrocyclic RCM. One of the most important extensions of the original metathesis reaction for the synthesis of stereochemi-cally defined (cyclo)alkenes is alkyne metathesis, followed by selective partial hydrogenation. 

The addition of (TMS)3SiH to a number of monosubstituted acetylenes has also been studied in some detail. These reactions are highly regioselective (anti-Markovnikov) and give terminal (TMSlsSi-substituted alkenes in good yields. High cis or trans stereoselectivity is also observed, depending on the nature of the substituents at the acetylenic moiety. For example, the reaction of the alkynes 23 and 24 with (TMSlsSiH, initiated either by EtsB at room temperature (method or by thermal decomposition of di-ferf-butyl peroxide at 160 °C... 

The influence of iron(III) salts on coupling reactions of alkynes and aldehydes (Scheme 10, routes B and C) was also explored. In these routes, a new stereoselective coupling of alkynes and aldehydes was unmasked, which led to ( ,Z)-1,5-dihalo-1,4-dienes (route B, Scheme 10) and/or ( )-a,p-unsamrated ketones (route C, Scheme 10) [27]. 

Nickel complexes formed in situ by the reaction of NiCl.S-COD) with the iini-dazolium salts IMesHCl or IPrHCl in the presence KO Bu catalyse the hydrosilylation of internal or terminal alkynes with EtjSiH. Interestingly, Ni tri-butylphosphine complexes are inactive in this hydrosilylation reaction. The monosilylated addition products were obtained with slow addition rates of the alkyne in the reaction mixture and were formed with variable degree of stereoselectivity, depending on the type of the alkyne, the silane and the ligand on Ni [50],... 

---