Allenes from propargyl acetates

CRABBE AlleneSynthesis Synthesis of terminal allenes from propargylic acetates. 

Nantz, M. H. Bender, D. M. Janaki, S. A convenient terminal allene synthesis from propargylic acetates. Synthesis 1993, 577-578. 

Another, albeit less-frequently employed option for a copper-mediated reduction of propargylic electrophiles to allenes relies upon the use of a copper hydride, for example, Stryker s reagent [(Ph3P)CuH]6. This reagent was applied by Brummond and Lu147 to the synthesis of the structurally complex precursor 197 for a potent antitumor agent, ( )-hydroxymethylacylfulvalene, from propargyl acetate 196 (Equation (11)). 

Allenic alcohols,3 In the presence of Sml2 and Pd[P(C6H3),]4, sec- and tert-propargylic acetates add to ketones to give allenic alcohols as the only or major product. A mixture of allenic and homopropargylic alcohols is formed from reactions of primary propargylic acetates. 

An alternative, but related, route to allenic titanium reagents from propargylic esters has been reported recently. Reaction of titanocene dichloride with BuMgCl and Mg yields a reactive titanocene intermediate, formulated as Cp2Ti. This reduced Ti species reacts in situ by oxidative addition to propargylic acetates. The allenyltitanium reagents thus produced add to aldehydes and ketones, as expected, to afford homopropargylic alcohols (Table 9.27) [43]. 

Some synthetically important allenylmetallics, such as allenylzinc and allenylin-dium reagents, are prepared from allenylpalladium intermediates. These reactions are discussed in appropriate sections of this chapter. This section covers the reactions of allenylpalladium compounds without further transmetallation. Allenylpalladium complexes can be prepared from propargylic halides, acetates, carbonates, mesylates, alcohols and certain alkynes [83-87], The allenylpalladium compound prepared from 3-chloro-3-methyl-l-butyne has been isolated and characterized spectroscopically (Eq. 9.106) [83], It was found to couple with organozinc chlorides to produce homologated allenes quantitatively (Eq. 9.107). 

Similarly, the benzannulated enyne-allenes 172 and 173 were prepared from the propargylic acetates 171 by cuprate addition or by Pd-catalyzed addition of arylzinc chloride (Scheme 20.35) [49]. The presence of a butyl group and a p-anisyl group at the allenic terminus of 173a and 173b permits competition between a formal ene reaction and a formal Diels-Alder reaction leading to 174 and 175, respectively. 

The formation of chiral allenes from centrochiral propargylic alcohol derivatives and organocopper reagents, e.g., the conversion of (/ )-l-cthynylpentyl acetate to allene 928. 

Fujisawa et al. prepared allenic acids with high levels of diastereoselectivity in the rearrangements of silyl ketene acetals derived from propargyl glycolates (Scheme 146) [142]. The authors argue that the stereoselectivity is principally due to a diaxial-hke interaction between the pseudo-axial H and OTMS substituents that is present in the chair-like transition state. 

Similar to alkynes, allenes can be converted to alkenes without over-reduction of the olefin. Thus, good yield of terminal olefin 18 was obtained from aUene 17 (Scheme 8). Allene 20 could in turn be prepared from propargyl cabonate 19 under slightly modified reaction conditions. For comparison, reactions using the corresponding propargyl acetates were much less satisfactory (Scheme 9). 

The reaction of propargylic chiral acetals with a catalytic copper reagent (RMgX/5% CuX) provides the expected alkoxy allenes in quantitative yield (Table 3)61. The diastereomeric excess is highly dependent on the size of the ring of the acetal and on the type of substituents it contains. The best diastereomeric excess is 85% with the acetal derived from cyclooctanediol. The use of lithium dimethylcuprate results in 1,2-addition lo the triple bond and the resulting lithium alkenyl cuprate bearing a cyclic acetal does not eliminate even at reflux temperature ( + 35°C). 

Allenylboranes can also be prepared from lithiated propargyl chloride [20]. As noted above, these intermediates react with acetic acid to afford allenes (Table 9.11). 

Rearrangements of propargyl esters with silver salts were first mentioned by Zakharova in the mid-1940s.49 He described the conversion of 3-chloro-3-methyl-but-l-yne into a mixture of acetates in which the allenic acetate, l-acetoxy-3-methylbut-1,2-diene, was the major compound (Scheme 3.30). Although this product could arise from a silver assisted SN2 reaction, it could also be produced from the substitution product through rearrangement, probably catalyzed by silver ions. 

A new preparative method for allylic indium(m) reagents via a reductive transmetallation of 7r-allylpalladium(n) or 7T-allylnickel(n) complexes with indium(i) salts is reported. This method enables the use of a wide variety of allylic compounds, such as allylic chlorides, acetates, and even allylic alcohols, in combination with Pd or Ni catalysts.43-50 7r-Allylpalladium(ii) resulting from the addition of arylpalladium(n) to allene is also transformed by metallic indium to the corresponding allylindium.51-54 Similarly, propargylindium(m) can be prepared from the corresponding propargyl alcohol derivatives.55-58... 

The addition of Br2, Cl2 and BrCl to the simple allene has been investigated by Peer (1962) and discussed by recent reviewers (Griesbaum, 1966 Taylor, 1967). The primary products of addition of bromine, irrespective of the solvent used is the 1,2-dibromide which adds a second bromine molecule at a much slower rate than allene. The nature of the products resulting from addition of chlorine depends on the solvent in inert media the 1,2-diohloride is formed, together with some propargyl chloride 92, whereas in acetic acid the solvent-incorporated adduct 93... 

C-H bond fission and the production of ethynyl radicals. Butadiyne and vinyl acetate are formed when the T -shaped ethyne dimer is irradiated at 193 nm in argon or xenon. The dynamics of the photodissociation of propyne and allene have been studied. The H2 elimination from propyne is a minor route for propyne dissociation and the major path identified in this study is loss of the alkyne hydrogen. A study of the photodissociation dynamics of allene and propyne has been reported and this work has demonstrated that allene gives rise to a propargyl radical while propyne yields the propynyl radical. Other research has examined the photodissociation of propyne and allene by irradiation at 193 nm. ... 

Several new aspects of organoalane chemistry have been described this year. Orthoesters react with alanes, prepared from a variety of allyl and propargyl halides, to give the corresponding 0,y-unsaturated acetals (Scheme 45). Interestingly reactions involving acetals rather than ortho esters lead exclusively to the formation of allenic ethers (160). ... 

The acyl migration of benzylic acetates was also applied for the synthesis of indenes (equation 87). Isomeric indenes were also obtained as minor products. The transformation was actually shown to proceed via allenes, which could be independently prepared by Ag(I)-catalyzed 1,3-acetate migration from the same starting materials. Propargylic sulfides and dithioacetals undergo similar transformations as propargylic carboxylates to give indene derivatives with Au(I) or Au(III) catalysts. 

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