Allene, reactions involving

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. 

Such reactions involve the addition of organometallic reagent to unsaturated systems like alkynes, alkenes, allenes, or related structures in order to create a new C-C bond and C-M bond at the same time. The C-M bond can then be functionalized by an electrophile to generate a wide range of products. 

Various intermolecular coupling reactions involving acetylene hydrocarbons have been reported to lead to vinylallenes. For example, 1-phenylpropyne (93), after activation with Hg(II) chloride, is first metalated by butyllithium treatment, then trans-metalated with zinc bromide and finally coupled with 1-iodo-l-phenylethene (94) in the presence of tetrakis(triphenylphosphine)palladium to provide the diphenylvinyl-allene 95 in moderate yield (Scheme 5.12) [31]. 

A regioselective [3 + 2]-cycloaddition approach to substituted 5-membered carbo-cycles was made available by the use of allenylsilanes [188]. The reaction involves regioselective attack of an unsaturated ketone by (trimethylsilyl)allene at the 3-position. The resulting vinyl cation undergoes a 1,2-silyl migration. The isomeric vinyl cation is intercepted intramolecularly by the titanium enolate to produce a highly substituted (trimethylsilyl)cyclopentene derivative. 

Palladium(II) is one of the most important transition metals in catalytic oxidations of allenes [1], Scheme 17.1 shows the most common reactions. Transformations involving oxidative addition of palladium(O) to aryl and vinyl halides do not afford an oxidized product and are discussed in previous chapters. The mechanistically very similar reactions, initiated by nucleophilic attack by bromide ion on a (jt-allene)pal-ladium(II) complex, do afford products with higher oxidation state and are discussed below. These reactions proceed via a fairly stable (jt-allyl)palladium intermediate. Mechanistically, the reaction involves three discrete steps (1) generation of the jt-allyl complex from allene, halide ion and palladium(II) [2] (2) occasional isomeriza-... 

It is worth noting that in most of the reactions involving allenes with an internal nucleophile, cr-vinyl complexes are formed but their further reaction usually lead to unwanted by-products. 

An exceptionally interesting example of the electrohydrocyclization reaction involves the use of allenes which are tethered to a,/S-unsaturated esters (Table 3) [19]. The chemistry takes place in a manner wherein the new carbon-carbon bond forms between the central carbon of the allene and the /S-carbon of the unsaturated ester. Of particular value is the preservation of one of the double bonds of the original allene, thereby providing functionality for further elaboration. It is important to carry out these transformations in an undivided cell, as the use of a divided cell led to hydrogenation of the olefins instead of cyclization. 

Hibino et al. reported a formal synthesis of murrayaquinone A (107) starting from 2-chloro-3-formylindole (891) by an allene-mediated electrocyclic reaction involving the indole 2,3-bond. The 4-hydroxy-3-methylcarbazole (858), a known precursor for murrayaquinone A (107), and required for this formal synthesis was obtained in seven steps, and 26% overall yield, starting from the 2-chloro-3-formylindole (891) (636,637) (Scheme 5.114). 

The rearrangement occurs more readily when activating groups (aryl, carboxyl, etc.) are attached to the triple bond. Jacobs [38] reports that a reaction involving adsorption of an acetylenic compound on an active basic surface has led to the practical synthesis of arylallenes, allenyl ethers, allenyl halides, and other substituted allenes. 

Virtually all reactions involving cyclobutane formation via cycloaddition of a cumulene to another C —C double-bond system involves excitation of this latter moiety, e.g. an enone or a quinone, and not of the allene or ketene itself.1 Earlier examples of such reactions have been discussed in Houben-Weyl, Vol. 4/5 b, pp 926 931. 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

Most gold-catalyzed reactions involve this pattern of reactivity that has been studied since the eighties. The first examples were found with allenes as substrates and subsequently alkenes and alkynes started to be used, the latter being the most popular in the last five years. 

It should be noted that there is a kinetic isotope effect on the normal reaction (9.11) when the a-deuterated compound is used as the substrate. A similar effect is found when the deuterated suicide inhibitor is used. Thus, both reactions involve a proton transfer in the rate-determining step of the reaction. It has also been shown that a sample of the allenic intermediate that is prepared chemically does in fact irreversibly inhibit the enzyme.18... 

An intriguing variant of this coupling process was reported in 1995. Treatment of acetone with SmD in the presence of 42 resulted in the formation of allene 43 in 81% yield and a 5.7 1 diastereomeric ratio (equation 25)74. Although no mechanism was suggested, it is likely that the reaction involves addition of (CH3)2C=0 to the terminal alkyne,... 

Grigg et al. [61] have recently reported a one-pot reaction involving the initial three-component condensation with a 2-halo-benzylamine 112, ethyl glyoxylate 113 and an aryl boronic acid 114, followed by Pd cyclization in the presence of carbon monoxide to give 116, or in the presence of allene to form dihydroisoquinoline amino acid derivatives 117 (Scheme 7.15). 

Having studied the reactions involving alkynes in some detail we turned our attention to other unsaturated molecules, and in particular simple hydrocarbons. Alkenes appear unreactive, though allenes, as shown in the following section, do yield novel products. 

Allenes are activated by a diphenylphosphine oxide substituent towards nucleophilic substitution at the j3-carbon atom. Lithium dimethyl-cuprate adds quickly to the 1,2-bond to give, on hydrolysis, the olefin in 16-84% yield, according to the nature of the substituents (76). Optimum conditions were not reported. The intermediate a-copper compound resulting from the addition can be dimerized or reacted with methyl iodide [Eq. (106)]. Similar reactions involving methyllithium are complicated. 

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