Cobalt-complexed propargyl alcohols

In the presence of suitable nickel or cobalt complexes, propargyl alcohol trimerizes to a mixture of l,3,5-ben2enetrimethanol [4464-18-0] and 1,2,4-trimethanol [25147-76-6] hen-zene (28). 

The facile acid-promoted dehydration of cobalt-complexed propargyl alcohols [18], via P-pro-ton loss from the derived cations (Scheme 4-50) offers improved chemo-, regio- and stereoselectivity vis a vis the free propargylic alcohols, with a strong preference for the more substituted ( )-ene-yne complex [151,152]. Such Co-mediated dehydration has afforded routes to enantiomerically pure manicone and normanicone (4,6-dimethyl-4-octen-3-one,... 

The chemistry and synthetic utility of cobalt complexed propargyl cations have been demonstrated by Nicholas in an impressive series of papers, and the area was reviewed in 1987.72 More recently, reviews of cluster-stabilized cations73 and propargylium complexes74 have appeared. Two general routes for the synthesis of dicobalt-propargylium complexes have been developed. The most commonly used method is the treatment of an alkynic ether or alkynic alcohol-hexacarbonyldicobalt complex with a Lewis or Bronsted acid [Eq. (7)]. 

Enol silyl ethers undergo alkylation with dicobalt-complexed propargylic alcohols after ionization of the latter with fluoroboric acid. A 1,1,1,3,3,3-hexafluoro-isopropylphosphite ligand on the cobalt exerts its effect by virtue of a weakly (/-donating and strongly rr-accepting behavior. 

The Nicholas reaction, that is, the addition reaction include amines, water, alcohols, trialkylaluminum reagents, of nucleophiles to cobalt-complexed propargylic cations, sodium borohydride, diethylaluminum cyanide, trialkylsilyl... 

As part of his research into the synthesis of oxygen-bridged nine- and ten-membered cycloalkanes, Montana introduced a key carbon fragment via the Nicholas reaction. Boron trifluoride promoted reaction between silyl enol ether 28 and cobalt-alkyne complexed propargyl alcohol 29 provides substitution product 30 in excellent yield. ... 

Cobalt-Complexed Propargyl Cations and Their Reactions. The stability and synthetic utility of hexacarbonyldicobalt complexed cations was first reported in 1971. They are readily formed by protonation of propargyl alcohol complexes or addition of electrophiles (protons, or alkyl or acyl cations) to vinylacetylene complexes. They can be isolated in pure form, usually by precipitation as hexafluorophosphates or tetrafluoroborates, but are more commonly generated in situ and used directly. Most of the early work is due to Nicholas, who has comprehensively reviewed the work up to 1986. A detailed description of the procedure for generating the l-methyl-2-propynyl complex and for its reaction with trimethylsilyloxycyclohexene (eq 51) has been given. ... 

In fact, this approach constituted one of the earliest trials. One is introduction of a chiral ligand like glyphos on cobalt by replacing one of the carbon monoxides to obtain an enantiomerically enriched new metal-alkyne complex. The second approach is use of the propargyl alcohol bearing a chiral auxiliary. Replacement of only one carbon monoxide would lead to a mixture of diastereomers. In both cases, a I I mixture of products was obtained, and each diastereomer was separated before the... 

Jeong and co-workers utilized a cobalt-alkyne complex to enhance enantioselectivity of the addition of bis (homoallyl)zinc to propargyl aldehydes 68 by the exaggeration of steric environment. The reaction provided optically enriched propargyl alcohol 69 in the presence of a chiral ligand and titanium tetra(isopropoxide) in excess. Adduct 69 was subjected to PKR to yield optically enriched bicyclic compounds 70 (Equation (39)). ... 

Substituted phthalides can be prepared in a [2+2+2] cycloaddition of two molecules of methyl propiolate with propargyl alcohol. This cyclotrimerization, which is catalyzed by a cobalt(ll)-DPPE complex in the presence of catalytic amounts of zinc, affords the reaction products in acceptable to good yields (Equation 150) <2005CC4955>. 

Acetylenic cobalt complexes greatly facilitate the heterolytic cleavage of adjacent alcohols or ethers. On treatment with Lewis acids, these complexes afford cobalt stabilized carbenium ions, which can be captured by nucleophiles such as enolates. Jacobi and Zheng have employed chiral boron enolates of Evans s oxa-zolidinone 6.91 (R = i-Pr). After removal of the chiral auxiliary, they obtained anti adds 11.43 with a high selectivity [1677] (Figure 11.9). The reaction can be extended to the boron enolates of related oxazolidinones and to a-branched propargyl derivatives. This reaction has been applied to the synthesis of P-aminoacids after Curtius rearrangement and oxidation of the triple bond [1677]. 

Reactions of alkyne-cobalt complexes. The hexacarbonyldicobalt complex of a propargylic alcohol containing a juxtaposed benzyloxy group suffers elimination of the propargylic hydroxyl on treatment with BFj OEtj. An intramolecular hydride transfer is involved. Complexes of vic-diols give deconjugated alkynones. ... 

The reaction of propargyl alcohols with dicobalt octacarbonyl to give the complex salts 148 (X = BF4 or PF6) and synthetic uses of the latter have been reviewed. The salts react with electron-rich aromatic compounds ArH, such as anisole, phenol or N,N-dimethylaniline, to yield substitution products 149 after oxidative demetallation with an iron(III) or cerium(I V) salt with j5-diketones or j -keto esters the corresponding propargyl-substituted compounds 150 are obtained k Acetone reacts in an analogous fashion to give 151. The action of the cobalt complexes 148 on allylsilanes 152 leads to enynes 153. Indole reacts with the complex 148 (R = H R = R = Me) in the presence of boron trifluoride etherate to give 154, which was converted into 155 by the action of iron(III) nitrate " ... 

Propargyl alcohols (17) are efficiently converted to the tranj-3,4-disubstituted chroman (18) via complexation with Co2(CO)8. The key feature of this one-pot procedure is the cyclisation of a cobalt carbonyl stabilised cation by a trisubstituted alkene <97TL685>. 

A general mechanism is illustrated below to demonstrate the stabilization of propargylic cation from the cobalt complex, in which a general alcohol is used as a representative nucleophile. 

As the cation at the a-position (propargyl position) of acetylene complexes is very stable, the reaction products at the propargyl position with nucleophilic reagents are selectively obtained. Usually, as shown in eq. (17.24), first, the cation is prepared by the reaction of a cobalt complex of propargyl alcohol or propargyl ester with tetrafluoric acid and the nucleophilic reagent reacts with the cation. These reactions are called Nicolas reactions [45,65-67]. 

The Nicholas reaction enables efficient substitution reactions of propargyl alcohols, ethers, and acetates. Prior to the substitution step, dicobalt octacarbonyl reacts with the alkyne to yield cobalt-alkyne complex 1. The resulting organometallic complex reacts with inter- or intramolecular nucleophiles in the presence of a Lewis or protic acid to furnish desired substitution products 2. The cobalt-complexed alkyne can be oxidatively removed after this step or used to further fiinctionalize the Nicholas reaction products. The stereoselective synthesis of chiral products using the title reaction is also possible. ... 

In addition to her studies on the asymmetric Nicholas reaction mentioned previously (13 14), Tyrrell investigated standard intramolecular cyclizations to generate benzopyrans. An important advance in this report is that Tyrrell performed the cobalt complexation, Nicholas reaction, and cobalt decomplexation using a one-pot procedure. The conversion of propargyl alcohol 48 into benzopyran 49 highlights this strategy... 

Another important application of cobalt carbonyl complexes is the Nicholas reaction, i.e., creating stable carbocations from propargylic alcohols... 

Propaigylium complexes of cobalt, obtained by treatment of propargylic alcohols with HBF4-OEt2, have been studied with... 

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