Propargylic alkylation

As described in the previous section, the ruthenium-catalyzed propargylic alkylation of propargylic alcohols with acetone afforded the corresponding alkylated products in high yields with complete selectivity [27]. When an optically active 1 -phenyl-2 -propyn-1 -ol was treated with acetone at room temperature in the presence of la as catalyst, only a racemic alkylated product was obtained [27]. This result... 

The catalytic propargylic alkylation was investigated in the presence of thiolate-bridged diruthenium complexes as catalysts generated in situ from reactions of [Cp RuCl(p2-Cl)]2 with optically active thiols prepared from the corresponding optically active alcohols [27]. Typical results for the reaction of 1-phenyl-2-propyn-l-ol with acetone in the presence of a variety of catalysts are shovm in Scheme 7.19. 

Scheme 7.23 Propargylic alkylated compound as an intermediate for cycloaddition.

This stoichiometric reaction constitutes a new contribution to vinylidene chemistry and a novel method to generate alkenylcarbene ligand from simple propargyl alkyl ethers rather than via activation of cyclopropenes [4] or by stoichiometric activation of butadiene [6[. When linked to a suitable metal-ligand moiety this carbene constitutes an alkene metathesis initiator. 

Stoichiometric Propargylic Alkylation of Propargylic Alcohols Assisted by Cobalt... 

In 1993, Nicholas and his co-worker developed the stereospecific propargylic alkylation of chiral propargylic alcohols 30 with enol silanes 31 by using a stoichiometric amount of [Co2(CO)5L] (L = phosphite), but separation procedures of the produced diastereoisomers are necessary twice on the way to obtain the compounds specifically alkylated at the propargylic position 32 (Scheme 5). In 2001, Montana and his co-worker reported the diastereo-selective Nicholas alkylation of propargylic acetal complexes 33 bearing a chiral auxiliary with various enol silanes 34 (Equation (14)). A high diastereoselectivity is observed, but unfortunately, only low to moderate enantioselec-tivities are achieved in all cases. 

Thus, a synthetic cycle for the formation of enantiomerically pure propargylic alkylated compounds from an achiral propargylic alcohol has been accomplished by starting from the ruthenium-BINAP complex 66 (Scheme 12). This stepwise reaction provides the first synthetic approach to highly enantioselective propargylic substitution reactions. 

Alkynes conjugated to a triple bond are very reactive systems toward hydrotelluration. The reaction occurs in a shorter reaction time when compared to the reactions commented above. In this case, even disubstituted triple bonds give vinylic tellurides in good yields.41,42,139 140 In the case of non-symmetrical diynes, an order of reactivity is established for the hydrotelluration of triple bonds of the enyne systems (terminal > propargylic > alkyl substituted > aryl substituted).140 This order of reactivity is reflected in the preferential formation of the vinylic tellurides shown in Scheme 39. 

As an alternative route to [5]pericyclynone 34, we also prepared octamethyl[5]pericyclyne, 41 [8], and oxidized it to the corresponding ketone (Fig. 9-11) [22]. This approach followed the earlier route outlined in Fig. 9-3, deviating only by the omission of methyl groups on the final propargylic alkylating agent. 

Propargylic esters undergo enantioselective alkylation with aldehydes (R-CHjCHO) to give propargylic alkylated products as a mixture of diastereomers (136), using cooperative catalysis by a copper(I)-BINAP complex and a diarylprolinol TMS ether." ... 

Scheme 14 Copper-catalysed asymmetric propargylic alkylation using Grignard reagents...

The enantioselective propargylic alkylation of propargyUc alcohols with aldehydes [188] or p-ketoesters [189] has recently been accomplished by cooperative catalytic reactitMis using a thiolate-bridged diruthenium complex and a chiral... 

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