Double bonds in allene

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

Solution (a) The general range of heats of hydrogenation of alkenes is from -27 to -31 kcal/mole. Here we are dealing with the hydrogenation of double bonds between two sp -hybridized carbons and an sp-hybrid zed carbon. We will assume a AH (heat of hydrogenation) value of -28 to -30 kcal/mole for each double bond in allene. 

A detailed discussion of the structural chemistry of allenes and cumulenes (1 b) has revealed that upon arbitrary substitutions the cumulenic double bonds in allenes, butatrienes, and ketenes retain their linear arrangements and their bond lengths within experimental errors. Furthermore, the antiplanar arrangements of the ligands in allenes (and surely also in pentatetraenes) are not affected by substitutions. 

In 2010, our group examined the silylzincation of allenes (Table 13) [92], Since there were two double bonds in allene molecules involving an sp-carbon, the... 

The inertness of ordinary double bonds toward metallie hydrides is quite useful, since it permits reduction of, say, a carbonyl or nitro group, without disturbing a double bond in the same molecule (see Chapter 19 for a discussion of selectivity in reduction reactions). Sodium in liquid ammonia also does not reduce ordinary double bonds, although it does reduce alkynes, allenes, conjugated dienes, and aromatic rings (15-14). 

This method provides a convenient synthesis of alkenes with the double bond in a relatively unstable position. Thus reduction of the p-toluenesulfonylhydrazones of a,(3-unsaturated aryl ketones and conjugated dienones gives rise to nonconjugated olefins. Unsaturated ketones with endocyclic double bonds produce olefins with double bonds in the exocyclic position. The reduction of p-toluenesulfonylhydrazones of conjugated alkynones furnishes a simple synthesis of 1,3-disubstituted allenes. ... 

The unique combination of double bonds in the molecules of those compounds, each with different reactivity along with the easy preparation, makes phosphorylated allenes useful substrates for the synthesis of different cyclic and noncyclic organophosphorus compounds. Recent investigations increase the scope of application of phosphorylated allenes as precursors in organic syntheses. Most of them are accompanied by the formation of five- or six-membered phosphorus heterocycles, which in many cases demonstrate certain biological activity. 

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... 

The double bonds of allene are said to be cumulated because one carbon (the central carbon) participates in two double bonds, i) Hydrocarbons whose molecules have cumulated double bonds are called cumulenes. 

The allene 149 gave by reaction with maleic anhydride (entry 1) and N-phenylmaleimide (entry 2) the [2 + 2] adducts 155a, b as mixtures of two diastereoisomers [36], Nevertheless, their chemical yield was very low and competitive reactions, mostly [4 + 2] cycloadditions on a rearranged al-lylidenecyclopropane and on a primary 1 1 adduct derived from an ene reaction (see Sect. 2.1.2), prevailed. Allenes 149 and 563 cycloadded to tetracyano- and l,l-bistrifluoromethyl-2,2-dicyanoethylene (Table 45, entries 3-6) also selectively at the cyclopropyl substituted double bond in order to remove most of the ring strain [149a],... 

We start with a discussion of allene (propadiene), the simplest diene of all. Its gas phase enthalpy of formation is 190.5 1.2 kJmol-1. We wish to compare this quantity with that of related monoenes. The first comparison addresses the relative stability of one and two double bonds in a 3-carbon chain. Conceptually, this may be expressed as the enthalpy of the formal reaction 9... 

Allene is a versatile functionality because it is useful as either a nucleophile or an electrophile and also as a substrate for cycloaddition reactions. This multi-reactivity makes an allene an excellent candidate for a synthetic manipulations. In addition to these abilities, the orthogonality of 1,3-substitution on the cumulated double bonds of allenes enables the molecule to exist in two enantiomeric configurations and reactions using either antipode can result in the transfer of chirality to the respective products. Therefore, the development of synthetic methodology for chiral allenes is one of the most valuable subjects for the synthetic organic chemist. This chapter serves as an introduction to recent progress in the enantioselective syntheses of allenes. Several of the earlier examples are presented in excellent previous reviews [ ] ... 

The heat of hydrogenation of one carbon-carbon double bond of allene is 41 kcal mol-1, whereas that of an ordinary alkene is around 29 kcal mol-1. Thus the cumulated double bond of allene liberates 12 kcal mol-1 more than that of a simple alkene on hydrogenation. Accumulation of two carbon-carbon double bonds imparts an extra reactivity to allene, making it a remarkably active component participating in a variety of cycloaddition reactions as a two-carbon unit. 

The addition of allenyl ether-derived anions to Weinreb [4] or to morpholino amides [5] follows a slightly different pathway (Eq. 13.2). For example, the addition of lithioallene 6 to Weinreb amide 7 at -78 °C, followed by quenching the reaction with aqueous NaH2P04 and allowing the mixture to warm to room temperature leads to cyclopentenone 9 in 80% yield [6]. The presumed intermediate of this reaction, allenyl vinyl ketone 8, was not isolated, as it underwent cyclization to 9 spontaneously [7]. These are exceptionally mild conditions for a Nazarov reaction and are probably a reflection of the strain that is present in the allene function, and also the low barrier for approach of the sp and sp2 carbon atoms. What is also noteworthy is the marked kinetic preference for the formation of the Z-isomer of the exocyclic double bond in 9. Had the Nazarov cyclization of 8 been conducted with catalysis by strong acid, it is unlikely that the kinetic product would have been observed. 

The Ru-catalyzed cyclocarbonylation of a-allenic sulfonamides proceeds in the presence of Et3N under a CO atmosphere (20 atm) to yield ,/funsaturated lactams (Scheme 16.32) [36], In order to gain an insight into the reaction mechanism, a deuterium-substituted a-allenic sulfonamide was subjected to the carbonylation. The deuterium was found to be totally transferred to the methyl group. Based on this observation, a mechanism has been proposed which involves a ruthenacycle derived from addition of the Ru-H to the terminal double bond of allene (Scheme 16.33). 

Silaboration of 3-substituted 1,2-dienes takes place smoothly at the internal double bond in the presence of the catalytic Pd(acac)2-2,6-xylyl isocyanide complex and the boryl group is regioselectively introduced to the central carbon atom of an allene (Scheme 16.55) [59, 60]. The same regioselectivity is observed with the catalytic system Pd2(dba)3-P(OCH2)3CEt [59]. 

Under palladium catalysis, the < -iodoallenylbenzene 146 first undergoes an intermolecular carbopalladation of the double bond in the added norbornene 116, and only then follows an intramolecular carbopalladation of the allene moiety in 146 ensuing /3-hydride elimination finally provides the tricyclic compound 147 (Scheme 37)7 ... 

Allene ketene cycloadditions are of greater synthetic utility than cither mixed allene dimerization or mixed ketene dimerization. In this class of reaction the ketene is the more reactive species and homodimerization of ketene can be minimized by use of excess allene. Such cycloadditions always result in 2-alkylidenecyclobutanones with the sp carbons of both moieties forming the initial bond. In substituted allenes and ketenes, mixtures of stereoisomers of 2-alkylidenecyclobutanones are obtained with very little stereoselectivity, the stereoisomers arise from cisUrcins isomerism in the cyclobutane ring and EjZ isomerism of the exocyclic double bond. In unsymmetrically substituted allenes some regiochemical preference for ketene cycloaddition is observed. Examples of dimethylketene allene cycloadditions are summarized in Table 1,2... 

In the course of investigations on allylic and acetylene-allene rearrangements of 3-substituted quinuclidines, it was found that by oxidation and ozonolysis of compounds with functional groups at positions allylic to the double bond, not only the double bonds but also the adjacent carbon-carbon bonds are broken. For example, in the oxidation of 3-hydroxy-3-vinylquinuclidine (119), with potassium permanganate under mild conditions, and in its ozonolysis, qui-nuclidin-3-one (2) is formed along with 3-hydroxy quinuclidine-3-carboxylic acid.161 The positions of double bonds in such systems can be firmly established by NMR spectroscopy, but not by oxidative methods.101... 

Orientation of azide addition to allenic ethers is nonspecific addition occurs preferentially to the activated double bond in the j ,y-position to the oxygen atom.254... 

Allenic ketones and esters are found to react with azides by addition, preferentially to the activated double bond in the a, -position to the ester or keto group, although no regioselectivity is observed.254 Allenic nitriles undergo similar additions, and unstable methylene triazolines are obtained (Scheme 78).301... 

Stereocontrolled conjugate addition of lithium dimethylcuprate to the electron deficient 2,3-double bond of allenes 851 leads to 5,6-dihydropyranM-oncs 852 in moderate yield (Equation 343) <2000J(P1)3188>. Similarly, the Ag(l)-catalyzed intramolecular cyclization of the allenic acid 853 is accelerated upon addition of diisopropylethyl-amine to afford the 3,6-dihydropyran-2-one 854, an intermediate during the total synthesis of (—)-malyngolide (Equation 344) <2000JA10470>. 

Dienes are compounds with two double bonds. In the name, each double bond is given the lower number of its two carbon atoms. Allenes are dienes with the two double bonds next to each other, joined at one carbon atom. An allene is chiral if each end has two distinct substituents. 

---