Nucleophilic additions to allenes

The thiylation of conjugated dienes and allenes proceeds as expected. Thus, the acid-catalyzed reaction of thiols with 1,3-dienes affords the 1,4-addition product,552 whereas the nucleophilic addition to allene provides the product arising from sulfur attack at the central carbon (equations 303 and 304).553... 

In principle, the nucleophile can attack the allene at two different positions, but the products show exclusive attack at the central carbon atom, similarly to other nucleophilic additions to allenes (Eglinton et al., 1954 Stirling, 1964b Taylor, 1967). This may result from the stabilization of the carbanion (194), formed by attack at this position, by the two phenyl groups. The ion (194) may be protonated at either one of the terminal positions of the allenic system, and low amounts of... 

Attempts to employ allenes in palladium-catalyzed oxidations have so far given dimeric products via jr al lyI complexes of type 7i62.63. The fact that only very little 1,2-addition product is formed via nucleophilic attack on jral ly I complex 69 indicates that the kinetic chloropalladation intermediate is 70. Although formation of 70 is reversible, it is trapped by the excess of allene present in the catalytic reaction to give dimeric products. The only reported example of a selective intermolecular 1,2-addition to allenes is the carbonylation given in equation 31, which is a stoichiometric oxidation64. 

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-... 

It was recognized in early examples of nucleophilic addition to acceptor-substituted allenes that formation of the non-conjugated product 158 is a kinetically controlled reaction. On the other hand, the conjugated product 159 is the result of a thermodynamically controlled reaction [205, 215]. Apparently, after the attack of the nucleophile on the central carbon atom of the allene 155, the intermediate 156 is formed first. This has to execute a torsion of 90° to merge into the allylic carbanion 157. Whereas 156 can only yield the product 158 by proton transfer, the protonation of 157 leads to both 158 and 159. 

Different strategies all including nucleophilic addition to acceptor-substituted allenes have been used for the synthesis of cyclic compounds, mostly heterocycles. Thus, it is obvious to release a nucleophile already existing within the allenic compound in a protected form. For example, treatment of silyl ethers 197 with tetrabu-tylammonium fluoride (TBAF) leads to the intermediates 198, which yield the dihy-drofurans 199 by nucleophilic addition (Scheme 7.32) [251]. 

Nucleophilic additions to activated allenes have been reported fairly often. Since there are several good recent reviews of this area,131 only a brief overview is given here. 

The third reaction was related to the hydrocarboxylation of allenes with C02, catalyzed by a tridendate silyl pincer-type palladium complex [108]. In this reaction, a a-allyl palladium species (via hydropallation of allene) was formed, and its trigonal bipyramidal geometry allowed the facile coordination of C02 (presumably in a side-on fashion) and a following nucleophilic addition to realize the carboxylation of allene (Figure 4.16). This reaction proved to be very attractive not only as a C02 fixation reaction, but also as a general method for the synthesis of Py-unsaturated carboxylic acids. 

Hydroboration of allenes 65 with pinacolborane in the presence of Pt(DBA)2 and a trialkylphosphine provides either the allyl boronate 66 or the vinyl boronate 67 regioselectively, depending on the stereoelectronic factors of the phosphine employed (Equation 2) <1999CL1069>. Allyl and vinyl boronates are synthetically important because of their ability to undergo nucleophilic addition to carbonyl compounds as well as transition metal-catalyzed cross-coupling. 

Dienes can bind to metals in an t) or r fashion. Many metal complexes contain t) -diene ligands, and reactions of -ri -diene complexes are described in Section 11.7.2. However, other metals, particularly Pd(II), bind dienes and allenes in an t) fashion. The reactivity of these species has been exploited to develop useful synthetic methods and is described in this section. Backvall has developed chemistry based on nucleophilic addition to palladium-diene complexes to generate allyl products, which subsequently react with a second nucleophile to generate free organic products from 1,4-addition of two nucleophiles across the diene. The palladium(O) byproduct is then re-oxidized to Pd(II) with quinone. 

Palladium(II) complexes of 1,3-dienes undergo attack by nucleophiles to give stable -ri -allyl complexes (Equation 11.48). These reactions are likely to occur by nucleophilic additions to V diene complexes, rattier ttian to V-diene complexes. For this reason, additions of nucleophiles to palladium diene and allene complexes can be understood with the principles presaated in Section 11.5.2 covering the reactions of V"Olefin complexes. 

As the final conclusion, the authors assumed that a mechanism involving nucleophilic addition of an allene double bond to a phosphinegold-complexed phos-phinegold acetylide to be more likely than an oxidative cyclization or simple nucleophilic addition to phosphinegold-complexed substrate. The corresponding catalytic cycle for the more probable pathway is shown in Scheme 4.9. 

The group Fe(CO)4 being isolobal to CH2 (see Sect. 4.4), iron pentacarbonyl may be regarded as an organometallic analog of ketene CH2=C=0 and Eq. (6.5) as comparable to nucleophilic addition of allene-type carbon to the digonal atom. 

Electrophilic Additions to Allenes.—Protonation of tetramethoxyallene, the bis(dimethylacetal) of carbon suboxide, takes place at the central carbon atom the resulting cation reacts with nucleophiles either by addition or by demethylation (Scheme 16). The tetramethoxyallyl cation (356), produced by... 

Areas of acetylenic chemistry reviewed recently include the base-catalysed isomerization of acetylenes, nucleophilic additions to acetylenes, additions to activated triple bonds, synthetic and naturally occurring acetylene compounds as drugs, allenic and acetylenic carotenoids, linear polymers from acetylenes, carbonylation of mono-olefinic and monoacetylenic hydrocarbons, and the combustion and oxidation of acetylene. Several books have also appeared. ... 

The approach to camptothecin by Danishefsky et alP revolves arpund an interesting synthesis of pyridones via nucleophilic addition to 1,3-dicarbethoxy allene. The C ring of camptothecin has been established by a Dieckmann closure followed by attachment of the quinoline ring using the Friedlander synthesis. Construction of ring E gives desoxycamptothecin, which readily undergoes oxidation to camptothecin. 

The intermolecular hydroamination reactions of alkynes and alkenes occur with Markovnikov or anti-Markovnikov selectivity. The nucleophilic addition to aUenes occurs at terminal carbon of allenes not at central one. 

Nucleophilic addition to terminal carbon of allenes and the catalysts Eqs. (30) and (31)... 

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. 

---