Allenes aryl-substituted

In 1994, Badone et al. reported that the Stille coupling of allenylstannane 77 and aryl triflates 78 resulted in formation of various aryl-substituted allenes 79 in moderate to good yield (Scheme 14.18) [39]. The choice of catalyst was certainly a crucial issue in this process for optimizing yield and rate. The best results could be obtained employing a catalyst cocktail of Pd2(dba)3-TFP-LiCl-CuI. Similar Stille coupling reactions with stannylated allenes and aromatic iodides as substrates were described by Aidhen and Braslau [40a] and Huang et al. [40b],... 

In 1998, Saigo et al. [150] discovered the Rh(I)-catalyzed allenic version of the vinylcyclopropane rearrangement. With a subsituent on the cyclopropane ring two different isomers can be formed depending on which of the different allylic C-C bonds in the three-membered ring is broken. Depending on R2, different regioselec-tivites were obtained with 250. Alkyl-substituted substrates lead to 251 and aryl-substituted substrates to 252 (Scheme 15.79) [150]. 

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

The propargyl alcohols react with trivalent phosphorus halides to give allenic phosphorus esters as described in Scheme 3 and Table VI. In the case of aryl-substituted alkynols or highly hindered t-propargyl alcohols which contain no free acetylenic —H, thionyl halides or phosphorus trihalides yield bromo- or chloroallenes [74d], Thionyl chloride also reacts in a similar fashion with a wide variety of unhindered secondary alcohols (structure XV) to give a mixture of the chloroallene and chloroalkyne [74a-d]. 

Aryl-substituted allenes dimerize giving symmetrical and nonsymmetrical bismethylenecy-clobutanes. 

The driving force for conjugation may be a significant factor to stabilize the allene structure in the isomerization of aryl-substituted alkynes145 [Eq. (4.30)146] ... 

As mentioned in Section 3.3.1, allenes 169 are not formed from alkenylpalladium 167. However, aryl-substituted allenes 187 are obtained predominantly by the coupling of aryl bromides with dialkylacetylenes 186 [80]. 

The allene aryl (5-lactones have conveniently been synthesized via the indium-mediated reaction of aldehydes with (<9-methoxycarbonylaryl)propargyl bromide in aqueous ethanol (Equation (55)).246 Indium-mediated ultrasonication reaction between aldehydes and l-bromo-2-butyne gives 1-substituted ( )-2,5-dimethyl-2,5,6-heptatrien-l-ols in moderate to good yields (Scheme 62).247... 

When substituted allylic and propargylic groups are used, rearranged products are often obtained. The extent of rearrangement depends on the substitution at the double or triple bonds. The aliphatic substituents facilitate formation of the allenic products, whereas the opposite is observed for aryl-substituted triple bonds ... 

Gold-catalyzed intermolecular hydroalkoxylations of allenes have received much less attention than their intramolecular counterpart. Nishina and Yamamoto performed the addition of primary or secondary alcohols using 5 mol% each of PhsPAuCl and AgOTf without any solvent and obtained the allylic ethers resulting from attack of the alcohol at the less substituted allenic terminus with moderate to high yield. The best results were obtained for aryl-substituted allenes. Unfortunately, enantiomerically enriched allenes provided only racemic addition products under these conditions. In the presence of a gold(I) catalyst and iV-iodosuccinimide, 2-iodoallylic ethers were obtained in a regioselective and stereoselective manner. 

A further variation of these functionalizations of cyanoarenes is the NOCAS process [14, 15]. As shown in Scheme 14.2, path g, this involves the addition of a nucleophile (which is often the solvent) to the donor radical cation. The thus-formed neutral radical adds to the acceptor radical anion, while rearomatization by the loss of an anion leads again to an overall ipso-substitution. AUenes could be used as the donors in these reactions, as shown recently by Arnold [50]. Accordingly, the irradiation of TCB in the presence of tetramethylaUene (15) in a 3 1 MeCN/MeOH mixture afforded 1 1 1 arene-allene-methanol adduct 16 in 48% yield (Scheme 14.9, central part). Interestingly, the addition of methanol took place exclusively at the central allene carbon, while aromatic substitution occurred through the terminal carbons. co-Alkenols, in which an O-nucleophile and an easily oxidized moiety are both present, could also be used. In the latter case, the initial ET was followed by a cyclization, yielding aryl-substituted tetra-hydrofurans or tetrahydropyrans as the final products via a tandem Ar—C, C—O bond formation [51]. 

As exemplified with 23, the highly regioselective platinum-catalyzed hydroboration of alkoxy-substituted allenes with pinacolborane provided the corresponding 2)-y-alkoxyallylboronates by total anti-Markovnikov addition at the terminal double bond (Equation 12) [45]. Alkyl- and aryl-substituted allenes devoid of an alkoxy group, how-... 

A novel iron-catalyzed, cationic cyclopropane ring opening was reported. Aryl-substituted cyclopropyl acetylenes gave ring-opened allene intermediates by a cationic rearrangement. In some cases, this gave functionalized naphthalene products. 

Disubstituted aryl bromides react with dialkylacetylenes, in the presence of catal)ttic quantities of both Pd(OAc)2 and PPhs, to give the corresponding aryl-substituted allenes in good yield (eq 111).20 ... 

Pd(0)-catalyzed arylation of the aryl-substituted a-allenic alcohols with hypervalent iodonium salts afforded substituted trans-epoxides. Alternatively, arylation of the alkyl-substituted a-allenic alcohols in the presence of K2CO3 afforded syn-diol cyclic carbonates and trans-epoxides in the presence of CS2CO3 (Scheme 5). 

Cobalt(I)-Mediated [2+2+2] Cyclization of Allene-Diynes A Diastereoselective Approach to 11-aryl Steroid Core. 11-Aryl-Substituted Steroid Systems by Co-catalyzed [2+2+2] Cyclization of Allene-Diynes... 

HMPA is also the only dipolar aprotic solvent to be used extensively with organomagnesium compounds. Large effects are observed when HMPA is used as either a solvent or a cosolvent. As examples, HMPA accelerates addition of an allylic organomagnesium compound to aryl-substituted alkenes, addition of (Trignard reagents to Carbon Monoxide, and addition of Propargylmag-nesium Bromide to allylic halides to give allene products. ... 

Aryl-S02Cl. At present only a few data concerning electrophilic 1,3-substitution with allenic and acetylenic tin or silicon compounds are available. 

Instead of alkynes, allenes can also be used as substrates in this type of approach. Finally, one can also apply carbon-nucleophiles such as butadienes in this domino process. Thus, Lu and Xie [145] have treated the alkyne 6/1-303 with an aryl halide 6/1-304 and an amine 6/1-305 to give the substituted pyrrolidinone 6/1-308 via the proposed intermediates 6/1-306 and 6/1-307. As a side product, 6/1-309 is found to have been formed by a cycloaddition of 6/1-303 (Scheme 6/1.81). 

A typical second step after the insertion of CO into aryl or alkenyl-Pd(II) compounds is the addition to alkenes [148]. However, allenes can also be used (as shown in the following examples) where a it-allyl-r 3-Pd-complex is formed as an intermediate which undergoes a nucleophilic substitution. Thus, Alper and coworkers [148], as well as Grigg and coworkers [149], described a Pd-catalyzed transformation of o-iodophenols and o-iodoanilines with allenes in the presence of CO. Reaction of 6/1-310 or 6/1-311 with 6/1-312 in the presence of Pd° under a CO atmosphere (1 atm) led to the chromanones 6/1-314 and quinolones 6/1-315, respectively, via the Jt-allyl-r 3-Pd-complex 6/1-313 (Scheme 6/1.82). The enones obtained can be transformed by a Michael addition with amines, followed by reduction to give y-amino alcohols. Quinolones and chromanones are of interest due to their pronounced biological activity as antibacterials [150], antifungals [151] and neurotrophic factors [152]. 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

A tandem enolate-arylation-allylic cyclisation, in which an essential z-butyldimethylsilyl ether protecting group delays the cyclisation step until the Pd-catalysed arylation is complete, enables 1-vinyl-l//-[2]benzopyrans 54 to be prepared from 2-bromobenzaldehyde (Scheme 32) <00CC1675>. 4-Substituted isochromans 55 are formed from aldehydes by a Pd-catalysed termolecular queuing cascade. The sequence involves cyclisation of an aryl iodide onto a proximate alkyne followed by an allene insertion. Transmetallation with indium then allows addition to the aldehyde (Scheme 33) . 

An interesting annelation reaction of allene-derived 13-dipoles with 3-(IV-aryliminomethyl)chromones 38 affords, in fair yields, after [4 +3] cycloaddition and a subsequent cascade of rearrangements, derivatives of the novel iV-aryl-2,3-dihydro-4-ethoxycarbonylchromano[2,3-h]azepin-6-one system 39 (for example, R = Me, R1 = Cl) (Scheme 9). In the initial cycloaddition, the substituted chromone acts as an azadiene moiety <00OL2023>... 

A variety of optically active 4,4-disubstituted allenecarboxylates 245 were provided by HWE reaction of intermediate disubstituted ketene acetates 244 with homochiral HWE reagents 246 developed by Tanaka and co-workers (Scheme 4.63) [99]. a,a-Di-substituted phenyl or 2,6-di-tert-butyl-4-methylphenyl (BHT) acetates 243 were used for the formation of 245 [100]. Addition of ZnCl2 to a solution of the lithiated phos-phonate may cause binding of the rigidly chelated phosphonate anion by Zn2+, where the axially chiral binaphthyl group dictates the orientation of the approach to the electrophile from the less hindered si phase of the reagent. Similarly, the aryl phosphorus methylphosphonium salt 248 was converted to a titanium ylide, which was condensed with aromatic aldehydes to provide allenes 249 with poor ee (Scheme 4.64) [101]. 

Aryl- and alkylsulfonyl radicals have been generated from the corresponding iodides and added to, e.g., propadiene (la), enantiomerically enriched (P)-(+)-propa-2,3-diene [(P)-(lc)] and (P)-(-)-cyclonona-l,2-diene [(P)-(lk)] [47]. Diaddition of sulfo-nyl radicals may compete considerably with the monoaddition [48,49]. Also, products of diiodination have been purified from likewise obtained reaction mixtures, which points to a more complex reactivity pattern of these substrates towards cumulated Jt-bonds. An analysis of regioselectivities of arylsulfonyl radical addition to allenes is in agreement with the familiar trend that a-addition occurs in propadiene (la), whereas alkyl-substitution at the cumulated Jt-bond is associated with a marked increase in formation of /3-addition products (Scheme 11.7). 

A complementary approach for cross-couplings with allenes was applied by using metallated allene species instead of allenyl halides, which have already been discussed in Sect. 14.2.1. Since allenyllithium compounds are readily available by deprotonation of allenes with n-butyllithium, successful cross-coupling reactions between lithiated allenes such as 54 or 57 and aryl or vinylic halides allowed convenient routes to aryl- and vinyl-substituted allenes, e.g. 55, 58 and 60 (Scheme 14.15) [30],... 

An unusual palladium-catalyzed arylative fragmenation process of /3-hydroxy-substituted allenes was observed by Oh et al. [59]. Compounds such as 8 reacted with aryl bromides and iodides in the presence of Pd(PPh3)4 and K2C03 as base to give 1,3-dienes 120 and aldehydes 121 as second fragment (Scheme 14.28). The initially expected cyclization product, dihydropyran 122 (Scheme 14.29), was usually not formed. 

Knoke and de Meijere [60] recently developed a highly flexible domino Heck-Diels-Alder reaction of a symmetrically substituted cumulene 125, which also involves cross-couplings of an allene at the central position. Both aryl and hetaryl halides react efficiently with l,3-dicyclopropyl-l,2-propadiene (125) and furnish 1,3,5-hexatriene derivatives 126 as intermediates, which are usually trapped by acceptor-substituted olefins in a subsequent cycloaddition, providing adducts 127a/b in moderate to good overall yields (Scheme 14.30). 

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