Substituted allenes

The acylation of Wittig reagents provides the most convenient means for the preparation of allenes substituted with various electron-withdrawing substituents. The preparation of o-allenic esters has been accomplished by the reaction of resonance-stabilized phosphoranes with isolable ketenes and ketene itself and with acid chlorides in the presence of a second equivalent of the phosphorane. The disadvantages of the first method are the necessity of preparing the ketene and the fact that the highly reactive mono-substituted ketenes evidently cannot be used. The second method fails when the a-carbon... 

Allene-substituted lactams or cyclic imines are useful intermediates in the synthesis of indolizine derivatives. While the former are stable and need a Pd(0) catalyst and the presence of phenyl iodide to react < 1997TL6275>, the latter are produced in situ and react immediately (Scheme 37) <2001JA2074>. 

Efforts have been made to find stereoselective routes which provide disubstituted azetidines. Palladium catalysed cyclization of an enantiomer of allene-substituted amines and amino acids gives the azetidine ester 2 and a tetrahydropyridine in variable yield and ratio, depending on the substituents and conditions <990L717>. The (TRIS)- and (253I )-isomeis of the substituted azetidine-2-carboxylic acids 3 (R = COjH) are obtained in several steps from the corresponding 3 (R = CHjOSiMejBu ) which, in turn, is produced in high yield by photochemical intramolecular cyclization <98HCA1803>. 

Allenic alcohols with a variety of allene substitution patterns form 2-alkenyltetrahydropyrans by a 6-exo ring closure upon cyclization with silver or mercury salts (equation 91 ).209 The organosilver intermediate undergoes protiodemetallation under the reaction conditions, while the vinylmercury... 

Electrophilic substitution of propargylsilanes is an important route to allenes. Cycliza-tion gives allene-substituted cyclic compounds as shown in equations 103 and 104. 

The use of gold catalysts is becoming more important in preparative organic chemistry and several examples have been provided where 4-allene-substituted /3-lactams 234 are cyclized in a short time and in high yield in the presence of gold(m) chloride to the carbapenem 235 (R3, R4 = H or R3, R4yf H) Equation (26) <2005AGE1840>. 

Yamamoto described a cascade cyclization reaction to prepare allene-substituted isochromenes 39 (Scheme 5.18).68 Diynones 38, when submitted to the action of AgSbI V, (5mol%), formed a benzopyrylium intermediate Z (identified by NMR spectroscopy), which could undergo a 1,4-Michael-type addition with alcohol nucleophiles to produce allenylisochromenes 39 (Scheme 5.18). 

Allene-substituted malonates undergo a Au-catalysed Ag-assisted cyclisation to 3,6-dihydropyran-2-ones unlike with Pd catalysts which promote carbocyclisation, here nucleophilic attack of the methoxycarbonyl group on the allene occurs (Scheme 28) <07OL2235>. 

The anomalous hydride reduction of hexamethyl-Dewar benzene oxirane has been reported. With AIH3, a-allene substituted oxiranes give a- and / -allenyl alcohols. ... 

Allenes. Substituted allenes are prq phosphates. Interestingly, enol triflates ten protected aminoalkenylphosphates." Sil> ... 

Allenes. Substituted allenes are prepared from ketones via elimination of the enol phosphates. Interestingly, enol Inflates tend to give alkynes. The method can be applied to protected aminoalkenyl phosphates. Silyl enol ethers" also undergo elimination. 

Some synthetic applications of 1,4-difunctionalization of various 1,4-dienes are cited here. Stereoselective 1,4-difunctionalization of the 1,3-cyclohexadiene 217 using Pd(OAc)2 and O2 in DMSO afforded nearly equal amounts of 1,4-adduct 218 and addition-elimination product 219 [97]. Pd(II)-promoted intramolecular reaction of the allene-substituted 1,3-cyclohexadiene 220 gave the triene 223 regio-and stereoselectively. The reaction may be explained by nucleophilic attack of the... 

The additives of water can serve as mechanistic probes and aid in obtaining true mechanistic understanding in some organocatalytic reactions [123]. The water is nucleophile in palladium-catalyzed oxidative carbo-hydroxylation of allene-substituted conjugated dienes [124]. This is an example of Pd- catalyzed oxidation leading to C -C bond formation in water with subsequent water attack on a (rr-allyl) palladium intermediate. The different effect of the water concentration on the intra- and extra-diol oxygenations of 3,5-di-tert-butylcatechol with O2, catalyzed by FeC in tetrahydrofuran-water indicates that the intermediates for two reactions are different (model for Catechol-2,3-dioxygenases) [125]. 

Backvall has reported the gold(III)-catalyzed cycloisomerization of allene-substituted malonic esters to form p,Y-unsaturated 6-lactones [115]. For example, treatment of allenyl malonate derivative 76 with a catalytic 1 3 mixture of AUCI3 and AgSbF in acetic acid at 70 °C led to isolation of 8-lactone 77 in 99% yield (Eq. (12.41)). The transformation was restricted to substrates that contained a terminally disubstituted allenyl moiety and presumably occurs via nucleophilic attack of the carbonyl oxygen atom on a gold-complexed allene followed by acetate-mediated demethylation and protodeauration. 

The general apphcation of Toste s chiral phosphine ligated Au(I) systems for cyclohydroamination was extended to the synthesis of isoxazolidines (Scheme 15.81) and pyrazolidines (Scheme 15.82) by using protected allene-substituted aUcoxyamines and hydrazines, respectively, as substrates [299]. The nature of the N-protecting group was found to have a substantial effect on reactivity and selectivity. Protecting groups explored in this transformation focused on carbamates and sulfonylamides. 

However, for the cyclization of the oxyanion substituted enyne-allene, substitution causes both the Myers-Saito and Schmittel cyclizations to switch their product formation preferences from diradicals to polar intermediates, as established by The stabilization of the oxyanion-... 

V.2.2 Palladium-Catalyzed Allylic, Propargylic, and Allenic Substitution with Nitrogen, Oxygen, and Other Groups 15-17 Heteroatom Nucleophiles... 

V.2.2.1 ALLYLIC, PROPARGYLIC, AND ALLENIC SUBSTITUTION WITH HETEROATOM 1847... 

Heating of 132 with diphenylketene at 110 °C for nine days affords thebis-cycloadduct . Thermolysis of the furane derivative 133 generates the allene substituted ketene 134, which cyclizes to form the cyclobutenone derivative 135 in 40 % yield . 

Finally, a-, y-, and 5-allene-substituted p-toluenesulfonamide bearing a tethered nitrogen nucleophile undergo a palladium(0)coupling/cyclization reaction with aryl iodides and carbon monoxide to form pyrrolines 49, pyrrolidine 50, and/or piperidine 51 (63]. Also in this case, the reaction works at 20 atm of carbon monoxide in presence of Pd(PPh3)4, K2CO3 as a catalytic system and involves a it-allylpalladium intermediate generated by the... 

In 2010, Malacria and coworkers developed a novel Au-catalyzed cyclization of 1,6-enyne 9, furnishing allene-substituted tetrahydrofuran 11 in excellent yield. The proposed mechanism involves an initial enyne cyclization to generate the cationic gold-vinyl complex 10 and subsequent unexpected [l,5]-hydride shift. Deuteration experiments support the [l,5]-hydride shift and rule out the inter-molecular scrambling. This cascade reaction is unusual because the cyclization precedes followed by an uncommon [l,5]-hydride shift (Scheme 12.5) [9]. 

Scheme 12.5 Au-catalyzed synthesis of allene-substituted tetrahydrofuran. O...

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