6,7-Alkynes selectivity

Palladium-catalyzed reduction of propargyl acetates is possible with Sml2 in the presence of a proton source (Scheme 3.17) [51]. The allene/alkyne selectivity is greatly influenced by the choice of the proton source. Propargyl phosphates were also converted into hydridoallenes by Pd-catalyzed reduction with Sml2 [52],... 

The observation was a significant finding since at the time, when the only synthetic method to reduce alkynes selectively was their conversion by heterogeneous catalytic hydrogenation (Raney nickel) to cis alkenes. The dissolving-metal reduction provided easy access to high-purity trans alkenes since the latter do not readily react further under the conditions used. The efficient reduction of 1-alkynes in this system requires the presence of ammonium ion.196... 

Moreover, it has been shown (Imamura et al. 1995a) that such rare-earth imide or imide-like species exhibit oligomerization activity of alkynes. Selective cyclic dimerization and trimerization of propyne and ethyne to cyclohexadiene and benzene occur at 453 K during the oligomerization, respectively, in which the active catalysts are characterized as rare-earth imides induced by the thermal treatment of R/C. 

I-2+ 2] cycloaddition of alkynes, selectivity among tliree different alkynes can be achieved by using unsymmetrical zirconacyclopentadienes, prepared from oxidative coupling of two different alkynes with Cp2Zr(II), as intermediates to react with the third alkyne in the presence of NiBr2(PPh3)2 [39]. 

Komagawa, S., Saito, S. (2006). Nickel-catalyzed three-component [3-I-2-I-2] cocycliza-tion of ethyl cyclopropyUdeneacetate and alkynes - selective synthesis of multisubstituted cycloheptadienes. Angewandte Chemie, International Edition, 45, 2446-2449. 

A simple heterogeneous nickel-based catalytic methodology is developed for regioselective hydroselenation of terminal alkynes (selective Markovnikov-type addition) and stereoselective hydroselenation of internal alkynes (selective sy -addition) [93]. The catalytic transformation is performed under mild conditions, thus avoiding byproduct formation. 

Three-component coupling with vinylstannane. norbornene (80). and bro-mobenzene affords the product 91 via oxidative addition, insertion, transme-tallation, and reductive elimination[85]. Asymmetric multipoint control in the formation of 94 and 95 in a ratio of 10 1 was achieved by diastereo-differ-entiative assembly of norbornene (80), the (5 )-(Z)-3-siloxyvinyl iodide 92 and the alkyne 93, showing that the control of four chiralities in 94 is possible by use of the single chirality of the iodide 92. The double bond in 92 should be Z no selectivity was observed with E form[86]. 

The presence of formates, oxalates, formic acid, and oxalic acid in the carbonylation of alkynes affects the regioselectivity. Also, the regioselectivity can be controlled to some extent by the proper selection of the ligands. A linear a,... 

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],... 

In a manner analogous to classic nitrile iinines, the additions of trifluoro-methylacetonitrile phenylimine occur regiospecifically with activated terminal alkenes but less selectively with alkynes [39], The nitnle imine reacts with both dimethyl fumarate and dimethyl maleate m moderate yields to give exclusively the trans product, presumably via epimenzation of the labile H at position 4 [40] (equation 42) The nitrile imine exhibits exo selectivities in its reactions with norbornene and norbornadiene, which are similar to those seen for the nitrile oxide [37], and even greater reactivity with enolates than that of the nitnle oxide [38, 41], Reactions of trifluoroacetomtrile phenyl imine with isocyanates, isothiocyanates, and carbodiimides are also reported [42]... 

Products 7a and 7c, with the substituent R a to the carbonyl group, are by far predominantly formed. This regioselectivity is a result of the preferential approach of the alkene 2 to the dicobalthexacarbonyl-alkyne complex 5 from the side opposite to the substituent R of the original alkyne. The actual incorporation of the alkene however is less selective with respect to the orientation of the olefinic substituent R, thus leading to a mixture of isomers 7a and 7c. 

Among the tasks remaining is the replacement of the C-16 hydroxyl group in 16 with a saturated butyl side chain. A partial hydrogenation of the alkyne in 16 with 5% Pd-BaS04 in the presence of quinoline, in methanol, followed sequentially by selective tosylation of the primary hydroxyl group and protection of the secondary hydroxyl group as an ethoxyethyl ether, affords intermediate 17 in 79% overall yield from 16. Key intermediate 6 is formed in 67 % yield upon treatment of 17 with lithium di-n-butylcuprate. 

Although beyond the scope of the present discussion, another key realization that has shaped the definition of click chemistry in recent years was that while olefins, through their selective oxidative functionalization, provide convenient access to reactive modules, the assembly of these energetic blocks into the final structures is best achieved through cydoaddition reactions involving carbon-het-eroatom bond formation, such as [l,3]-dipolar cydoadditions and hetero-Diels-Al-der reactions. The copper(i)-catalyzed cydoaddition of azides and terminal alkynes [5] is arguably the most powerful and reliable way to date to stitch a broad variety... 

The increase of selectivity in consecutive reactions in favor of the intermediate product may be sometimes extraordinarily high. Thus, for example, in the already cited hydrogenation of acetylene on a platinum and a palladium catalyst (45, 46) or in the hydrogenation or deuteration of 2-butynes on a palladium catalyst (57, 58), high selectivities in favor of reaction intermediates (alkenes) are obtained, even though their hydrogenation is in itself faster than the hydrogenation of alkynes. 

Table 1 Selected examples of 5-dialkylamino-3-ethoxycyclopentadienes 60a(b) obtained from 3-dialkylamino-l-ethoxypropenylidenechromium complexes 57 and alkynes in a donor solvent. For details see Scheme 12 [43,44,60,61] ...

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

---