Allenes, addition reactions

Most often in these reactions, however, the monoadduct is isolated and used to prepare the alkylidenecyclopropane, as in the case of the 1 1 allene-2-diazopropane adduct, which on thermolysis generated the singlet trimethylenemethane diradical 59, which closed to give a mixture of l,l-dimethyl-2-methylenecyclopropane (60) and isopropylidenecyclopropane (61). Although a wide variety of alkylidenecyclopropanes have been made in this way (see Table 2), the involvement of trimethylenemethane intermediates in the deazetization step means that mixtures of isomers are normally produced. Attempted syntheses of alkylidenecyclopropanes by this route have sometimes been frustrated by tautomerism and autoxidation of the intermediate 4,5-dihydro-3i/-pyrazole or by a reversal in the regiochemistry of the diazoalkane and allene addition reaction leading to 3-alkylidene-4,5-dihydro-3//-pyrazoles. ... 

Like butadiene, allene undergoes dimerization and addition of nucleophiles to give 1-substituted 3-methyl-2-methylene-3-butenyl compounds. Dimerization-hydration of allene is catalyzed by Pd(0) in the presence of CO2 to give 3-methyl-2-methylene-3-buten-l-ol (1). An addition reaction with. MleOH proceeds without CO2 to give 2-methyl-4-methoxy-3-inethylene-1-butene (2)[1]. Similarly, piperidine reacts with allene to give the dimeric amine 3, and the reaction of malonate affords 4 in good yields. Pd(0) coordinated by maleic anhydride (MA) IS used as a catalyst[2]. 

Snider and coworkers125 have reported the Lewis acid catalyzed [2 + 2]cycloaddition of (phenylsulfonyl)allene 112. The reaction with methylenecyclohexane in dichloro-methane gives a 25% yield of an 8 1 mixture of 210 and 211 (equation 132). An addition reaction of l-(p-tolylsulfonyl)ethylene with enamines gives aminocyanobutanes via the zwitterionic intermediate (212) as shown in equation 133126. 

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. 

Air-stable palladium(O) catalyst, [(Cy3P)2Pd(H)(H20)]BF4, catalyses carbonylation of propargylic alcohols to generate dienoic acids and esters (equation 167)286. Since propar-gyl alcohols are obtained from carbonyl compounds by acetyhde addition reactions, this sequence constitutes a three-carbon homologation. a-Allenic alcohols are converted to tt-vinylacrylic acids under similar conditions (equation 168)287. 

In addition to alkenes and alkynes, allenes have attracted considerable interest due to their unique reactivity and multireaction sites. Therefore, transition-metal-catalyzed nucleophilic addition reaction of amines and imines to allenes has been extensively studied to prepare biologically important amines and nitrogen-heterocycles.31,31d... 

We initially observed an addition reaction of tertiary phosphines to unactivated alkynes. The method was then applied to reactive alkenes, allenes and 1,3-dienes, and finally to unactivated alkenes (Scheme 4). Such a step-up methodology turned out to be effective in this study. 

Similar models explain the 1,8-, 1,10- and 1,12-addition reactions to the extended Michael acceptors 91, 93 and 95, respectively (Schemes 2.32 and 2.33). Again, these transformations start with the formation of a cuprate Jt-complex at the double bond neighbouring the acceptor group [61a]. Subsequently, an equilibrating mixture of a-copper(III) intermediates is presumably formed and the regioselectivity of the reaction may then be governed by the different relative rates of the reductive elimination step of these intermediates. Consequently, the exclusive formation of allenic prod-... 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

The attack of the nucleophile on the acceptor-substituted allene usually happens at the central sp-hybridized carbon atom. This holds true also if no nucleophilic addition but a nucleophilic substitution in terms of an SN2 reaction such as 181 — 182 occurs (Scheme 7.30) [245]. The addition of ethanol to the allene 183 is an exception [157]. In this case, the allene not only bears an acceptor but shows also the substructure of a vinyl ether. A change in the regioselectivity of the addition of nucleophilic compounds NuH to allenic esters can be effected by temporary introduction of a triphenylphosphonium group [246]. For instance, the ester 185 yields the phos-phonium salt 186, which may be converted further to the ether 187. Evidently, the triphenylphosphonium group induces an electrophilic character at the terminal carbon atom of 186 and this is used to produce 187, which is formally an abnormal product of the addition of methanol to the allene 185. This method of umpolung is also applicable to nucleophilic addition reactions to allenyl ketones in a modified procedure [246, 247]. 

Less common addition reactions such as the bromination of trifhioromethyl-substi-tuted butatrienes [30] or the reaction of tetrafluoroallene with boron trifluoride have also been reported [283]. Especially the interaction of phosphorylated allenes with electrophiles was summarized in a review by Alabugin and Brel [8], whereas Smadja [284] published a more general overview about the electrophilic addition to allenic derivatives. 

In general, sulfur-substituted allenes are accessible starting from alkyne precursors by a variety of transformations such as isomerization, rearrangement or addition reactions. The standard method for the synthesis of donor-substituted allenes is again the base-induced isomerization of alkynes. This very first method was applied for the preparation of achiral [11, 162, 163] and chiral [164] S-functionalized 1,2-dienes (Scheme 8.78). 

The examples illustrated in the almost 100 schemes in this chapter demonstrate how versatile donor-substituted allenes can be in synthetic processes. The major applications concern addition reactions and cycloadditions to the allenic double bonds, which furnish products with valuable functional groups. Of particular interest are metalations - usually at C-l of the allene unit - followed by reactions with electrophiles that deliver compounds which can often be used for cyclization reactions. A variety of highly substituted and functionalized heterocycles arises from these flexible methods, which cannot be obtained by other reactions. Many of these transformations proceed with good regioselectivity and excellent stereoselection. 

A related method of preparation involves the oxidative addition of a tin(II) salt to propargylic iodides, which yield mixtures of allenyl- and propargyltin halides on treatment with SnCl2 in DMF-DMI (l,3-dimethylimidazol-2-one) (Eq. 9.75) [68], These intermediates react in situ with aldehydes to afford mixtures of propargylic and allenic carbinols via a cyclic SE2 process (Eqs. 9.76 and 9.77). As explained in the Introduction, the ratio of these two products reflects the relative transition-state energies of the addition reactions. 

Recently, Trost et al. reported the vanadium-catalyzed addition reaction of 2,3-allenols [180], Here the oxygen in 401 served as an intramolecular nucleophile to attack the center carbon atom of allene to form a vanadium enolate 402. Aldol condensation of 402 with an aldehyde afforded (2-hydroxy)alkyl vinylic ketones 403. 

Similar 1,2-addition reactions were also observed in the reaction of 2,3-allenal with organolithiums, Grignard reagents or the reduction of 1,2-allenyl ketones with metal hydrides [190],... 

In recent years, interest in radical-based transformations of allenes has been renewed for two major reasons. First, a number of useful intramolecular additions of carbon-centered radicals to 1,2-dienes have been reported, which allowed syntheses of complex natural product-derived target molecules to be accomplished in instances where other methods have failed to provide similar selectivities. Further, a large body of kinetic and thermochemical data has become accessible from results of experimental and theoretical investigations in order to predict selectivities in addition reactions to allenes more precisely. Such contributions originated predominantly from (i) studies directed towards an understanding of the incineration process,... 

Additional routes to a-allenic-a-amino acids were described more recently and utilize radical [136] or transition metal-catalyzed [137] allenylations, in addition to copper-promoted Michael additions [15b]. Thus, sterically demanding amino acid derivatives (e.g. 151) are accessible via a 1,6-addition reaction of lithium di-tert-butyl-cyanocuprate with acceptor-substituted enynes of type 150 (Scheme 18.48). 

As in the case of addition reactions of carbon nucleophiles to activated dienes (Section HA), organocopper compounds are the reagents of choice for regio- and stereoselective Michael additions to acceptor-substituted enynes. Substrates bearing an acceptor-substituted triple bond besides one or more conjugated double bonds react with organocuprates under 1,4-addition exclusively (equation 51)138-140 1,6-addition reactions which would provide allenes after electrophilic capture were not observed (cf. Section IV). 

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