Propargylic compounds complexes

The rhodium(II)-catalyzed reaction of propargyl compounds 169 and diazo compounds 170 gave corresponding functionalized allenes 171 together with cydopro-penes 172 (Scheme 3.87) [126]. Rh2(pfb)4, where pfb represents perfluorobutyrate, was found to be an excellent catalyst for preparing the allenes 171. An analogous rhodium(II) complex, Rh2(OAc)4, afforded mainly 172 with only a trace amount of 171 (<5%). 

The reaction of alkynyllithium compounds with alkoxycarbene tungsten complexes leads to anionic propargyl tungsten complexes (Figure 2.33 see also Figure 3.9). These intermediates are stable at low temperatures and react upon Lewis acid catalysis with aldehydes or A -sulfonyl imines to yield five-membered heterocycles [280]. Oxidative methoxycarbonylation [375] of the intermediate vinyl tungsten complex, followed by elimination of methanol leads to pyrroles or furanes (Figure 2.33 Entry 6, Table 2.22). 

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. 

From a mechanistic viewpoint, the Pd(0)-eatalysed reactions of propargylic compounds so far discovered can be classified into four types I IV The allenyl complexes 5 undergo three types of transformations depending on reactants. Type I reactions proceed by insertion of unsaturated bonds to the n-bond between Pd and the sp2 carbon in 5. Type la is the insertion of alkenes to the palladium-carbon n-bond, and the 1,2,4-alkatrienes are formed by /f-elimination. Alkynes insert to form the alkenylpalladium 6, which undergoes various transformations such as insertion of unsaturated bonds and anion captures. 

Carbonyl Allylation and Propargylation. Boron complex (8), derived from the bis(tosylamide) compound (3), transmeta-lates allylstannanes to form allylboranes (eq 12). The allylboranes can be combined without isolation with aldehydes at —78°C to afford homoallylic alcohols with high enantioselectivity (eq 13). On the basis of a single reported example, reagent control might be expected to overcome substrate control in additions to aldehydes containing an adjacent asymmetric center. The sulfonamide can be recovered by precipitation with diethyl ether during aqueous workup. Ease of preparation and recovery of the chiral controller makes this method one of the more useful available for allylation reactions. 

The Pd(0)-catalyzed reactions of propargylic compounds so far discovered can be classified into four types, I, II, III, and TV, from a mechanistic viewpoint. The allenyl intermediate complex 8 undergoes three types of transformation, depending on reactants. The reactions of Type I proceed by insertion of unsaturated bonds into the a-bond between palladium and sp carbon in 8. This a-bond has a reactivity similar to the a-bond formed by the oxidative addition of alkenyl halides to Pd(0) in the Heck reaction [3]. Therefore, reactions similar to those observed in the Heck reaction are expected to occur witli the intermediate 8. Alkenes and carbon monoxide are known to insert into the palladium-carbon a-bond. The allene derivatives 9 are formed by these reactions (Scheme 11.3). 

The Pd(0)-catalyzed reactions of propargylic compounds can be understood by the following mechanistic considerations. The first step in the catalytic reactions is the oxidative addition of a propargylic compound to Pd(0) species to form an inteimediate complex. By this oxidative addition, Pd(0) is oxidized to Pd(II). The intermediate Pd(II) complex undergoes further reactions with other reactants. Complex formation by stoichiometric reactions of propargylic chlorides 2 and 4 with Pd(Ph3P)4 has been studied, and the o-allenylpalladium 3 and the propargylpalladium (or c7-prop-2-ynylpalladium) 5 were isolated as yellow powders (Scheme 11.2) [2]. The allenylpalladium chloride 3 is... 

The Pd(0)-catalyzed reactions of propargyl compounds so far discovered can be classified into several types from a mechanistic viewpoint [1], The a-allenylpalla-dium complexes 1 as intermediates undergo three types of transformations depending on reactants. 

Allenes 169 and alkynes 170 are prepared by hydrogenolysis of propargyl compounds with several hydrides. Triethylammonium formate is used most conveniently under mild conditions [45]. Chromium tricarbonyl-complexed phenylallene 172 was prepared from the carbonate 171 [46]. The alkyne 174 was obtained selectively from the propargyl formate 173 having an amino group [47]. 

Although superficially similar, propargyl compounds do not form t -complex intermediates, but give t -allenic complexes. As part of a catalytic cycle, these can undergo typical reactions, such as coupling (Schemes 9.82 and 9.83), ° ° reduction by formate, alkene insertion and carbonylation (Scheme 9.84). 

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