Allenes from propargylic derivatives

Although the iron-catalyzed synthesis of allenes from propargylic halides was reported by Pasto and coworkers in 1978 [67], little progress was achieved in this field until recently [68]. In 2003, Fiirstner and coworkers discovered propargylic epoxides as valuable substrates for the reaction with Grignard reagents in presence of catalytic amounts of Fe(acac)3 to generate 2,3-allenol derivatives (Scheme 5.24) [69]. 

Molander and Sommers [115] reported a chromium(III)-catalyzed synthesis of allenes from propargyl alcohol derivatives and triaUcylaluminum reagents. When substituted and enantiomerically enriched propargyl alcohols 403 were treated with these reagents in the presence of the chromium complex 404, allenes 405 were obtained in good yields and with high levels of chirality transfer (Scheme 10.138). 

Synthesis.—Vinylidene adamantane (113) has been prepared and its reactions investigated.The synthesis of allenes from propargyl alcohols and their derivatives continues to provide a number of papers dealing with this extremely useful transformation. Ferric chloride catalyses the highly selective formation of allenes... 

Mikami and Yoshida extended the scope of this method considerably by using propargyl phosphates and chiral proton sources [94], The propargylic phosphates thereby have been found to be advantageous owing to their high reactivity towards palladium and the extremely low nudeophilicity of the phosphate group [95]. In some cases, it was even possible to obtain allenes from primary substrates, e.g. ester 194 (Scheme 2.60) [96]. A notable application of this transformation is the synthesis of the allenic isocarbacydin derivative 197 from its precursor 196 [97]. 

The enantioselective synthesis of an allenic ester using chiral proton sources was performed by dynamic kinetic protonation of racemic allenylsamarium(III) species 237 and 238, which were derived from propargylic phosphate 236 by the metalation (Scheme 4.61) [97]. Protonation with (R,R)-(+)-hydrobcnzoin and R-(-)-pantolactone provided an allenic ester 239 with high enantiomeric purity. The selective protonation with (R,R)-(+)-hydrobenzoin giving R-(-)-allcnic ester 239 is in agreement with the... 

Treatment of the propargylic alcohol 144, readily prepared from condensation between benzophenone (143) and the lithium acetylide 101, with thionyl chloride promoted a sequence of reactions with an initial formation of the chlorosulfite 145 followed by an SNi reaction to produce in situ the chlorinated and the benzannulated enyne-allene 146 (Scheme 20.30) [62], A spontaneous Schmittel cyclization then generated the biradical 147, which in turn underwent a radical-radical coupling to form the formal [4+ 2]-cycloaddition product 148 and subsequently, after a prototropic rearrangement, 149. The chloride 149 is prone to hydrolysis to give the corresponding 11 H-bcnzo h fluoren-ll-ol 150 in 85% overall yield from 144. Several other llff-benzo[fc]fluoren-ll-ols were likewise synthesized from benzophenone derivatives. 

Similarly, the allenic derivatives 120 and 121 have been obtained from propargylic alcohols as shown in Scheme 7.34. °... 

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

Displacement rearrangements involving propargyl derivatives allenes from a-acetoxyalkynes [136]. 

In another related and well-known [3,3]-sigmatropic shift usually performed under thermal conditions, the propargyl-Claisen rearrangement,62 silver salts were also able to catalyze the reaction. Silver tetrafluoroborate and hexafluoroantimonate proved to be the best catalysts for this reaction, leading quantitatively to allenic p-ketoesters when starting from propargyl ethers derived from p-ketoesters (Scheme 3.41).63 64... 

This reaction illustrates the stabilising effect exerted by the sulfonium centre on the adjacent double bonds. Isomeric allenes (32) may be similarly obtained from propargylic sulfonium salts (33) (Scheme 18). The allene (32), by treatment with diethyl sodiomalonate (34), followed by addition and rearrangement of the derived sulfonium ylide (35), affords the methyl sulfide (36) (Scheme 18). 

Additives (cosolvents) which serve as ligands have great influence on the reducing power of Smlj. Allylic and propargylic derivatives are reduced via it-allylpalladium species, and the proton source has important effects on the generation of allenes or alkynes. Chiral allenic esters are obtained when pantolactone delivers a proton to racemic organosamarium species derived from 4-phosphato-2-alkynoic esters. ... 

Marshall and co-workers have demonstrated that allenylindium [277] and allen-ylzinc [276] reagents can be formed in situ from propargyl mesylate 389 and that these reagents react enantioselectively with aldehydes without additional Lewis acid catalysis (Scheme 11-28). The allenylzinc and allenylindium reagents 392 and 391 are derived from the allenylpalladium intermediate 390 via metathesis with Et2Zn and Ini, respectively. Allenylpalladium intermediate 390 in turn is derived from (7 )-389 via invertive displacement of the mesylate functionality of 389 with Pd(0). 

Reactions with —OH Groups and Epoxides.—The formation of A -l,2-oxaphos-pholen derivatives from propargylic alcohols and phosphorus trichloride has been studied in detail. Intermediate phosphites (24) and allenic phosphonates (25) are described, and the A -l,2-oxaphosphoIen is produced in the final stage, as shown. Improved conditions have been outlined for the preparation of allylic bromides (26) from allylic alcohols and phosphorus tribromide. Related reactions of primary alcohols with the complex of phosphorus trichloride and DMF lead to the chloride (27) 22 addition of zinc bromide to the reaction results in the formation of alkyl bromides, but an attempt to extend this exchange to the preparation of cyanides was not successful. ... 

Allenes. Alkynyl allenes are available from 8 2 displacement of propargylic derivatives. - ... 

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. 

Four reports have appeared recently on the synthesis of a -allenic alcohols as the major products from addition of propargyl derivatives to carbonyl compounds /3-acetylenic alcohols are co-products. Propargyl trimethylsilane (25 ... 

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). 

Closely related to both allyl carbenoids and the allenyl carbenoids discussed above, propargyl carbenoids 101 are readily generated in situ and insert into zirconacycles to afford species 102 (Scheme 3.27), which are closely related to species 84 derived from allenyl carbenoids [65], Protonation affords a mixture of allene and alkyne products, but the Lewis acid assisted addition of aldehydes is regioselective and affords the homopropargylic alcohol products 103 in high yield. Bicydic zirconacyclopentenes react similarly, but there is little diastereocontrol from the ring junction to the newly formed stereocenters. The r 3-propargyl complexes derived from saturated zirconacycles are inert towards aldehyde addition. 

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