Allenes, cycloadditions with imine

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

The [2 + 2]-cycloaddition reactions of l,3-di-tert-butylallene-l,3-dicarbonitrile (go) with imines afford azetidines [60]. The nitrogen atom of the imine was attached to the central carbon atom of the allene to give 2-methyleneazetidines. 

A number of cycloadditions of imines or imino compounds with a variety of alkenes, including allenes (88HCA1025), vinyl ethers (88TL547), methyl acrylate (86ZOR636), ketene acetals (87JOC365) and electrophilic alkenes (85T1953), afford functionalized azetidines. 

The metallocene complex 27 containing a M=X double bond undergoes overall [2 + 2] cycloaddition with an internal alkynes to give heterometallacyclobutenes (28) [77], A formal [2 + 2] cycloaddition of CpjZr (=N Bu)(thf) with imine affords a 2,4-diazametallacyclobutane, whose further reaction with imines results in an imine metathesis reaction [78] azametallacyclobutene is an intermediate in the Cp2Zr(NHR)2-assisted hydroamination of alkynes and allene [79],... 

The methods for generating acyl ketenes (Scheme 7-V) and their subsequent in situ participation in [4 + 2] cycloadditions with a wide range of hetero- or olefinic and acetylenic dienophiles (Scheme 7-VI), including acyl ketenes,185 186,197 carbonyl compounds, 86-188 nitriles,1874,189,191 isocyanates and isothiocyanates,1864,190,191 ketenes,191 imines,1864,1874,191,192 carbo-diimides,l87c 190,191,193 ynamines,194 ketene acetals,1864,195 enol ethers,1864,191,196 and V-sulfinylamines197 have been extensively reviewed.5,9,12 Two reports have detailed the 4-n- participation of allenic ketones in [4 + 2] cycloaddition reactions [Eq. (51)].198,199... 

The mechanism of the group 4 metal-catalyzed hydroamination of alkynes (Scheme 4) and allenes has been thoroughly investigated in detailed kinetic and mechanistic [41 6] as well as computational studies [47, 48]. The catalytically active species is believed to be a metal imido complex, which undergoes a reversible, rate-determining [2 + 2]-cycloaddition with an alkyne, respective allene, to yield an azametallacyclobutene species. Subsequent protonolysis leads to the imine hydroamination product. Isolation and characterization of intermediate azacyclobutene species, which are catalytically competent themselves, is a strong support of the mechanism [41,43,49-51]. Although no direct experimental [52, 53]... 

Reversible pyridine dissociation yields the non-Lewis base stabilized imido complex [Cp(NHAr) Ti=NAr] (54). This coordinatively unsaturated species undergoes a [2 + 2] cycloaddition reaction with allene to give an azatitanocyclobutane (55), which is then protonated to the /mamido complex. Elimination of enamine occurs followed by isomerization to the energetically more favorable imine. 

Less frequently applied are [3 + 2] and [2 + 2] cycloadditions of oxygen-substituted allenes [102-104], Battioni et al. described only a limited number of [3 + 2] cycloadditions of phenyloxy- and methoxyallene with diphenyldiazomethane (157) and the nitrile imine derived from diphenylhydrazonoyl chloride (159) (Scheme 8.40) [102], Both 1,3-dipoles exclusively attack the terminal C=C bond, furnishing cycloadducts 158 and 160. Padwa et al. reported [3 + 2] cycloadditions of methoxyallene 145 with two nitrones which afforded isoxazolidines in low yield [103]. 

Common reactions of the ylide include (i) [2,3]-sigmatropic rearrangement of allylic, propargylic, and allenic ylides (ii) [l,2]-shift (Stevens rearrangement) (iii) 1,3-dipolar cycloaddition of the ylide generated from carbonyl compounds or imines with dipolarophiles, usually G=G or C=C bonds and (iv) nucleophilic addition/elimination, leading to the formation of epoxides or cyclopropanes (Figure 2). 

Alkenes, perfluoro-cycloadditions to benzyl azide, 60, 35 pyridine imines and ylids, 60, 36 Alkenes, perfluoro-, as heterocyclic precursors, 59, 10 Alkylation, free-radical, of 1,3-dimethyluracil, 55, 227 A-Alkylation, [ 1,2,4]triazolo[ 1,5-u)-pyrimidines, 57, 110 Alkynes, photocycloaddition to uracils, rearrangement with HCNO elimination, 55, 149 Alkynes, ethoxy-, cyclaoddition to hexafluoroacetone azine, 60, 32 Alkynes, perfluoro-, as heterocyclic precursors, 59, 10 Allenes... 

Aromatic and aliphatic acyl isocyanates participate in a similar range of [4 + 2] cycloadditions although [2 + 2] and simple addition reactions often are observed. The acyl isocyanate substituents may determine or alter the observed course of the reaction, and the substituent effects have been detailed in extensive reviews.7,71 Observed [4 + 2] cycloadditions of acyl isocyanates with selected olefins, p-quinones, allenes, the carbon-carbon double bond of ketenes, electron-rich acetylenes, imines, dianils, ethy-lenediimines, enamines, enol ethers, ketene acetals, carbodiimides, azirines, and vinyl sulfides have been described.7 0 The reaction of aromatic acyl isocyanates with carbodiimides is not a simple, direct [4 + 2] cycloaddition but proceeds by a kinetic [2 + 2] cycloaddition followed by a subsequent rearrangement to provide the observed [4 + 2] cycloadduct [Eq. (40)].97... 

A new class of chiral bifunctional thiourea catalysts derived from trans-2-amino-l-(diphenylphosphino)cyclohexane was developed by Jacobsen and Fang in order to be applied to a highly enantioselective synthesis of a wide range of 2-aryl-2,5-dihydropyrrole derivatives. This strategy was based on a [3-1-2] cycloaddition between an A-phosphinoyl imine and an allene in the presence of TEA and water as additives. High yields combined with excellent enantioselectivities of up to 98% ee were observed in all cases of substrates, as shown in Scheme 6.19. 

Scheme 6.19 [3 -E 2] Cycloadditions of allenes with iV-phosphinoyl imines.

Allene insertion/nucleophile incorporation can also be combined with 1,3-dipole cycloaddition as an efficient protocol for the synthesis of various fused polycycles. In 2005, Dondas et al. used this strategy to synthesize various nitrogen- or oxygen-containing fused polycycles such as 323 through intermediate azomethine ylides, azomethine imines, and nitrones [112] (Scheme 6.85). 

Also for the diastereoselective aza-MBH reaction (Scheme 7.10) between chiral A-thiophenyl imines and methyl vinyl ketone, PTA was found to be an active catalyst. The products were obtained in good yields (>80%) with excellent diastereoselectivities (>99% de, Ar = 4-F3CC6H4), albeit at long reaction times (up to 5 days) [52]. PTA is also an effective catalyst in the [3 + 2] cycloaddition of activated allenes with A-thiophosphoryl imines to give 3-pyrrolines [53]. 

Zhang B, He Z, Xu S, Wu G, He Z (2008) Nucleophilic phosphine-catalyzed [3 -I- 2] cycloaddition of allenes with JV-(thio)phosphoryl imines and acidic methanolysis of adducts iV-(thio)phosphoryl 3-pyrrolines a facile synthesis of free amine 3-pyrrolines. Tetrahedron 64 9471-9479... 

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