Allenyl sulfone

Recently, Hiroi and co-workers reported a palladium-catalyzed asymmetric transformation of chiral 2-alkynyl sulfmates 142 into allenyl sulfones 145 (Scheme 4.38) [58], Treatment of 142 with Pd(OAc)2 in the presence of a phosphine ligand afforded allenylsulfones 145 with high stereospecificities (73-89%) in good yields, probably through intermediates 143 and 144. 

Because of the usually high temperatures required to rearrange propargyl sulfi-nates to allenyl sulfones, a report about such palladium-catalyzed transformations seems to be promising [106], Even at low temperatures the synthesis and isomerization of trifluoromethanesulfinate 51 are possible if, during the reaction of the ter-... 

Several trivial but highly useful reactions are known to convert one acceptor-substituted allene into another. For example, the transformation of allenic carboxylic acids is possible both via the corresponding 2,3-allenoyl chlorides or directly to 2,3-allen-amides [182,185], Allenylimines were prepared by condensation of allenyl aldehydes with primary amines [199]. However, the analogous reaction of allenyl ketones fails because in this case the nucleophilic addition to the central carbon atom of the allenic unit predominates (cf. Section 7.3.1). Allenyl sulfoxides can be oxidized by m-CPBA to give nearly quantitatively the corresponding allenyl sulfones [200]. The reaction of the ketone 144 with bromine yields first a 2 1 mixture of the addition product 145 and the allene 146, respectively (Scheme 7.24). By use of triethylamine, the unitary product 146 is obtained [59]. The allenylphosphane oxides and allene-... 

Based on nucleophilic addition, racemic allenyl sulfones were partially resolved by reaction with a deficiency of optically active primary or secondary amines [243]. The reversible nucleophilic addition of tertiary amines or phosphanes to acceptor-substituted allenes can lead to the inversion of the configuration of chiral allenes. For example, an optically active diester 177 with achiral groups R can undergo a racemization (Scheme 7.29). A 4 5 mixture of (M)- and (P)-177 with R = (-)-l-menthyl, obtained through synthesis of the allene from dimenthyl 1,3-acetonedicar-boxylate (cf. Scheme 7.18) [159], furnishes (M)-177 in high diastereomeric purity in 90% yield after repeated crystallization from pentane in the presence of catalytic amounts of triethylamine [158], Another example of a highly elegant epimerization of an optically active allene based on reversible nucleophilic addition was published by Marshall and Liao, who were successful in the transformation 179 — 180 [35], Recently, Lu et al. published a very informative review on the reactions of electron-deficient allenes under phosphane catalysis [244]. 

The Diels-Alder product 324 resulting from the diene 322 and the allenyl sulfone 323 can be used together with potassium tert-butoxidc directly in a Ramberg-Back-lund reaction [333]. In this case, a diene permanently in the cisoid conformation is regenerated to make the polycyclic compounds 326 and 327 available by the same Diels-Alder/Ramberg-Backlund sequence. 

Allenes containing an allyl unit at a suitable position within the molecule can undergo an intramolecular ene reaction. Thus, allenyl sulfones 423, prepared by oxidation of the corresponding sulfoxides, lead to ring closure products 424 on heating [371]. 

Whereas enyne 429 is formed in excellent yield from allenyl sulfone 428 as a stable product of 1,4-elimination of water [118], short-lived butatrienones 431 can only be characterized by argon matrix infrared spectroscopy after 1,2-elimination of HX from precursors 430 by flash vacuum pyrolysis [373, 374]. 

Allenyl sulfones can also readily accept the nucleophilic attack of alkyl lithium [104—106]. No migration of the C=C bond was observed. R2CuLi reacts with 1,2-alle-nylsulfones similarly [107-109]. 

In case of 3-substituted 1,2-allenyl sulfones, e.g. 210, the carbocupration reaction exhibits Z-selectivity, indicating that the nucleophile attacks from the less hindered... 

The reaction of 1,2-allenyl sulfones with (-(-ephedrine led to enantiomerically pure 1,3-oxazolidines 216 via the formation of a conjugated enamine and subsequent intramolecular Michael addition [115]. 

Denmark and Harmata studied the reaction of 1,2-allenyl sulfones and 2-prope-nols under the catalysis of 5 mol% of sodium alkoxide affording 2-alloxy-2-propenyl sulfone 217, which can be converted to 2-oxo-5-alkenyl sulfone 218 by treatment with 1.5equiv. of KH in HMPA, a carbanion-accelerated Claisen rearrangement [116, 117]. 

With 3-substituted-l,2-allenyl sulfones, the 1,4-addition reaction with alcohols also afforded E-isomers, which can be converted to different diastereomeric 3,4-di-substituted-2-oxoalkenyl sulfones 220 highly stereoselectively under different conditions [118]. 

The reaction of 1,2-allenyl sulfones with EtOH in the presence of a catalytic amount of EtOK afforded 2-ethoxy-2-alkenyl sulfones 221, which can be easily alky-... 

In the above reaction, the migration of the C=C bond was observed. With the introduction of a substituent to the 3-position of 1,2-allenyl sulfones, the C=C bond did not migrate [97] and the expected ( )-228 was isolated. 

Dimethylpyridine N-oxide and 1,2-allenyl sulfone in CHC13 at room temperature gave the 1,3-dipolar cycloaddition product 229, which subsequently underwent a [l,5]-sigmatropic rearrangement and a migration of the C=C bond to afford 231. The [2 + 2]-cycloaddition of the C=N bond in 231 with the terminal bond in propadie-nyl sulfone would deliver the tricyclic product 232 [123]. 

Allenyl sulfones can also readily undergo hydrohalogenation reactions with MX (M = Na, Li X=C1, Br, I) to afford 2-haloallylic sulfones 236 [87, 126]. The reactivity depends greatly on the substitution pattern of the allene moiety. 

Like the nitrone-olefin [3 + 2]-cycloaddition, the nitrone-allene [3 + 21-cycloaddition also takes place regioselectively to furnish methylene-substituted isoxazoli-dine derivatives. The substituents of the 3- and 4-positions of the cycloadducts 72a and 72b are disposed cis in contrast, the reaction of 70 with allenyl sulfone 71c gives rise to the trans-cycloadduct 72c exclusively (Table 12.4). On treatment with base or heating, methyleneisoxazolidines 72 readily rearrange to isoxazo-lines via a 1,3-hydrogen shift. 

In contrast to allenyl sulfones, allenyl phenyl sulfoxide failed to react with the Danishefsky s diene even at an elevated temperature [116]. Introduction of an electron-withdrawing nitro group on the aromatic ring, however, lowered the LUMO energy level and facilitated the cycloaddition, providing phenol 134. 

A new type of triaryl phosphine-functionalized imidazolium salt containing cations such as (6) has been prepared. Palladium complexes of (6) generated in situ have been used successfully in Heck-type reactions of aryl halides with acrylates and of 4-bromotoluene with styrene derivatives.34 The first Heck-type reaction of aryl halides with allenes has been reported. 1,3-Double arylations were observed with 3-substituted-l,2-allenyl sulfones, while 1-monoarylation was favoured with 3,3-disubstituted-l,2-allenyl sulfones.35 It has been shown that the a-arylation of methane-sulfonamides (7) may be achieved using palladium catalysis reaction proceeds through the sulfonamide enolates.36 It is also reported that palladium cross-coupling of alkynes with /V - (3 - i odophe n y I an i I i ncs) may lead to the formation of substituted carbazoles.37... 

Palladium catalysts rearrange chiral 2-alkynyl sulfinates 64 into chiral allenyl sulfones 65 [43]. 

More recently, substituted allenyl sulfones were also engaged in Pd/Ag-catalyzed Heck-type reactions. Treated with palladium tetrakis(triphenylphosphine) and a mixture of silver and potassium carbonate in DMF at 80°C, such allenyl sulfones... 

Treatment of the allenyl sulfones 112 with KO Bu in BuOH at room temperature provides the oxocenes 113 or 114 in high yields (Scheme 25) <20010L3385>. 

Scheme 26 Palladium-catalyzed asymmetric transformation of chiral 2-alkynyl sulfinates into allenyl sulfones...

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