Mechanism alkyne reduction with

This reaction typifies the two possibilities of reaction routes for M-catalyzed addition of an S-X (or Se-X) bond to alkyne (a) oxidative addition of the S-X bond to M(0) to form 94, (b) insertion of alkyne into either the M-S or M-X bond to provide 95 or 96 (c) C-X or C-S bond-forming reductive elimination to give 97 (Scheme 7-21). Comparable reaction sequences are also discussed when the Chalk-Harrod mechanism is compared with the modified Chalk-Harrod mechanism in hydrosily-lations [1,3]. The palladium-catalyzed thioboratiori, that is, addition of an S-B bond to an alkyne was reported by Miyaura and Suzuki et al. to furnish the cis-adducts 98 with the sulfur bound to the internal carbon and the boron center to the terminal carbon (Eq. 7.61) [62]. 

The electron transfer to the acetylenic bond forms the frans-sodiovinyl radical 20 that, after protonation, produces tram radical 21. At low temperature (—33°C) in the presence of excess sodium, the conversion of the trans radical to sodiovinyl intermediate 22 is slightly more rapid than the conversion of the tram radical to the cis radical 23 (21 —> 22 > 22 —> 23). As a result, protonation yields predominantly the trans alkene. However, low sodium concentration and increased temperature lead to increasing proportion of the cis alkene. Although other dissolving-metal reductions are less thoroughly studied, a similar mechanism is believed to be operative.34 Another synthetically useful method for conversion of alkynes to trans alkenes in excellent yields is the reduction with CrS04 in aqueous dimethylforma-mide.198... 

Coupling of 1-alkyne 410 with 1-alkene 411, catalysed by CpRu(cod)Cl in aqueous DMF, affords the diene 414 as an ene-type product in good yield. One explanation of the reaction is the formation of the (71-al ly I )(> 2-al kync)i n termediate from the 1 -alkene and insertion of the alkyne [161]. However, formation of the ruthenacyclopentene 412, subsequent /1-elimination to form 413, and reductive elimination offer a more easily understandable mechanism. A formal synthesis of altemaric acid (415) was achieved by this reaction [162],... 

In a polarographic measurement one may, in favourable cases, determine the potentials, reversibility and electron equivalents for a given cathodic process. When combined with an analysis of products, these usually provide insights into the gross mechanism of reduction. By using Ej data (Table 7) as limits, one can arrange reductions in which some group may be altered cleanly before the triple bond is touched, or vice versa. At the same time, data comprise an approximate electro-philicity scale of alkynes towards cathodic electrons, that is, a sort of solution electron affinity . The practical and theoretical uses of these data appear in several places in this chapter. 

The mechanism of the Sonogashira cross-coupling follows the expected oxidative addition-reductive elimination pathway. However, the structure of the catalytically active species and the precise role of the Cul catalyst is unknown. The reaction commences with the generation of a coordinatively unsaturated Pd species from a Pd " complex by reduction with the alkyne substrate or with an added phosphine ligand. The Pd " then undergoes oxidative addition with the aryl or vinyl halide followed by transmetallation by the copper(l)-acetylide. Reductive elimination affords the coupled product and the regeneration of the catalyst completes the catalytic cycle. 

The mechanism of the aluminium hydride reductions with LiAlff4 or RedAl involve a tram hydroalumination helped by coordination of A1 to the triple bond and external nucleophilic attack. The regioselectivity of the hydroalumination is again determined by silicon the electrophilic A1 attacks the alkyne on the carbon bearing the silyl group (the ipso carbon). 

An addition to an alkene can form up to two new chiral centers, and a reaction that occurs with only a syn or only an anti addition mechanism will give a product with predictable stereochemistry. Conversion of alkynes to alke-nes can also occur with either syn or anti stereoselectivity. When these alkyne reductions are taken in combination with alkene addition reactions, target molecules with a wide variety of stereochemical relationships can be prepared. 

Generally proposed mechanisms of the transition metal-catalyzed [2+2+2] cycloaddition of alkynes are shown in Schemes 19.1 and 19.2. Two alkynes react with the transition metal complex to generate metallacyclopentadiene A. The subsequent [4+2] cycloaddition of A with the alkyne affords metallabicyclo[2.2.0]heptadiene B. Reductive elimination affords the desired benzene (Schemes 21.1). Alternatively, insertion of the alkyne into A leading to metallacycloheptatriene C followed by reductive elimination also affords the desired benzene (Schemes 19.1). The mechanism via the intermediates A and B is confirmed in the CpCo(I)-phosphine complex-catalyzed [2+2+2] cycloaddition of alkynes by the theoretical calculation [5]. 

A mechanism for this cyclization has been proposed (Scheme 8.26). It initiates with the oxidative addition of aldehyde C-H bond to the Rh(I) cationic species to form a hydroacylrhodium A. The insertion of the carbon-carbon double bond of the allene moiety leads to oxo-rhodacycle B, which upon stereoselective 1,3-niigration of carbon-Rh bond gives C. The subsequent insertion of alkyne gives the rhodacy-cle D, which upon reductive elimination releases the final bicyclic ketone and the cationic active Rh(I) species. The stereoselective axial/center chirality transfer is in accordance with the proposed mechanism. Moreover, supporting this mechanism, an experiment with the deuterium-labeled substrate aldehyde C(0)-D affords the cyclic compounds with a deuterium on the alkene moiety. 

The proposed mechanism involves the initial formation of Co(l) from Co(II) by reduction with Zn. Coordination of the alkyne and the alkene to Co(l) to form a metalocyclopentene intermediate could be followed by sequential p-hydride elimination/reductive elimination to provide the conjugated diene. 

Finally, Cramer and co-workers described an alternative route to classical Pauson-Khand reaction for the synthesis of cyclopentenones. The proposed procedure involves a reductive Ni -catalyzed [3+2] cycloaddition between aryl enoates and internal alkynes. More interestingly, the use of a chiral NHC led to a highly enantioselective reaction [eqn (10.38)]. Note that with unsymmetric alkynes the reaction is also regioselective. A plausible mechanism was proposed with the hypothesis that facial-selective coordination and incorporation of the enoate are controlled by a single chiral side chain of the carbene. 

It was found [99JCS(PI )3713] that, in all cases, the formation of the deiodinated products 38 and 39 was accompanied by formation of the diynes 40 which were isolated in 60-90% yield. The authors believed that the mechanism of deiodination may be represented as an interaction ofbis(triphenylphosphine)phenylethynyl-palladium(II) hydride with the 4-iodopyrazole, giving rise to the bisftriphenylphos-phine)phenylethynyl palladium(II) iodide complex which, due to the reductive elimination of 1 -iodoalkyne and subsequent addition of alk-1 -yne, converts into the initial palladium complex. Furthermore, the interaction of 1-iodoalkynes with the initial alkyne in the presence of Cul and EtsN (the Cadiot-Chodkiewicz reaction) results in the formation of the observed disubstituted butadiynes 40 (Scheme 51). 

Since activation of the N-H bond of PhNHj by Ru3(CO)i2 has been reported to take place under similar conditions [306], it has been proposed that the reaction mechanism involves (i) generation of an anUido ruthenium hydride, (ii) coordination of the alkyne, (iii) intramolecular nucleophilic attack of the nitrogen lone pair on the coordinated triple bond, and (iv) reductive ehmination of the enamine with regeneration of the active Ru(0) center [305]. 

The proposed reaction mechanism (Scheme 7-2) comprises (1) oxidative addition of ArSH to RhCl(PPh3)3 to give Rh(H)(Cl)(SPh)(PPli3)n, (2) coordination ofalkyne to the Rh complex, (3) ris-insertion of alkyne into the Rh-H bond with Rh positioned at terminal carbon and H at internal carbon, (4) reductive elimination of 16 from the Rh(III) complex to regenerate the Rh(I) complex. 

Abstract Significant advances have been made in the study of catalytic reductive coupling of alkenes and alkynes over the past 10 years. This work will discuss the progress made in early transition metal and lanthanide series catalytic processes using alkyl metals or silanes as the stoichiometric reductants and the progress made in the use of late transition metals for the same reactions using silanes, stannanes and borohydrides as the reductant. The mechanisms for the reactions are discussed along with stereoselective variants of the reactions. 

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