Allenes preparation from

The lithiation of allene can also be carried out with ethyllithium or butyl-lithium in diethyl ether (prepared from the alkyl bromides), using THF as a cosolvent. The salt suspension which is initially present when the solution of alkyllithium is cooled to -50°C or lower has disappeared almost completely when the reaction between allene and alkyllithium is finished. 

The alkylations proceeded much more slowly, when ethyl- or butyllithium in diethyl ether, prepared from the alkyl bromides, had been used for the metallation of allene, in spite of the presence of THF and HMPT as co-solvents. 

A wide variety of compounds with the allene system has been prepared from the... 

To a mixture of 100 ml of THF and 0.10 mol of the epoxide (note 1) was added 0.5 g Of copper(I) bromide. A solution of phenylmagnesium bromide (prepared from 0.18 mol of bromobenzene, see Chapter II, Exp. 5) in 130 ml of THF was added drop-wise in 20 min at 20-30°C. After an additional 30 min the black reaction mixture was hydrolysed with a solution of 2 g of NaCN or KCN and 20 g of ammonium chloride in 150 ml of water. The aqueous layer was extracted three times with diethyl ether. The combined organic solutions were washed with water and dried over magnesium sulfate. The residue obtained after concentration of the solution in a water-pump vacuum was distilled through a short column, giving the allenic alcohol, b.p. 100°C/0.2 mmHg, n. 1.5705, in 75% yield. 

One of the most dramatic developments in the chemistry of N2 during the past 30 years was the discovery by A. D. Allen and C. V. Senoff in 1965 that dinitrogen complexes such as [Ru(NH3)5(N2)1 could readily be prepared from aqueous RUCI3 using hydrazine hydrate in aqueous solution. Since that time virtually all transition metals have been found to give dinitrogen complexes and several hundred such compounds are now characterized.Three general preparative methods are available ... 

A closely related reaction of (—)-(S)-276 with the Grignard reagents obtained from a-acetylenic halides leads to the formation of mixtures of acetylenic sulphoxides 290 and allenic sulphoxides 291363 (equation 161). The latter compounds are most probably formed via transition state 292, which is analogous to 289. On the other hand, hex-l-ynyl p-tolyl sulphoxide 293 is smoothly prepared from hex-1 -ynylmagnesium bromide and (— )-(S)-276363 (equation 162). 

Alkadiene- and alkatrienephosphonates are available from acetylene-allene rearrangement of acetylene phosphites which could easily be prepared from the reaction of carbonyl compounds and 1-alkynes. 

A nickel-chromium catalyst prepared from chromous chloride and (p-diphenylphos-phinopolystyrene)nickel dichloride mediates the ring-closure of the ene-allene 236 (R = H) to a mixture of 3.4 parts of 237 and 1 part of 238 (equation 120)121. An analogous reaction of the t-butyldimethylsilyl ether of 236 yields solely the (E)-isomer 237 (R = t-BuMeaSi). Cyclization of the ene-allene 239 affords the perhydroindane 240 in 72%... 

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) . 

Various nonracemic allenylstannanes have been prepared from nonracemic propargylic mesylates and (Bu3Sn)2CuLi. The stereochemistry of the displacement was shown to be anti by correlation with an allenic stannane prepared through Claisen orthoester rearrangement of a propargylic alcohol of known configuration (Scheme 33)80. 

As noted above, titanocene-alkylidenes can be prepared using various methods and starting materials. Like the methylidene complex, higher alkylidene complexes are useful for the transformation of carbonyl compounds to highly substituted olefins. Ketones and aldehydes are converted into substituted allenes by treatment with titanocene-alkenylidenes prepared by olefin metathesis between titanocene-methylidene and substituted allenes (see Scheme 14.7) [17]. Titanocene-alkenylidene complexes can also be prepared from... 

Organogermanium compounds can be prepared by transmetallation reactions with tin reagents. Examples include Me2PhGeCl (Equation (66)),89 the alkene-functionalized species 26-28, (Equations (67) and (68)),90 and the allenic (Equation (69)) and propargylic (Equation (70)) species 29 and 30.91 A series of aryltrichlorogermanes was prepared from the corresponding tin reagents (Equation (71), Table 9).92 Transmetallation with zirconium species can also be used (Equation (72), Table 10).93... 

The asymmetric synthesis of allenes via enantioselective hydrogenation of ketones with ruthenium(II) catalyst was reported by Malacria and co-workers (Scheme 4.11) [15, 16]. The ketone 46 was hydrogenated in the presence of iPrOH, KOH and 5 mol% of a chiral ruthenium catalyst, prepared from [(p-cymene) RuC12]2 and (S,S)-TsDPEN (2 equiv./Ru), to afford 47 in 75% yield with 95% ee. The alcohol 47 was converted into the corresponding chiral allene 48 (>95% ee) by the reaction of the corresponding mesylate with MeCu(CN)MgBr. A phosphine oxide derivative of the allenediyne 48 was proved to be a substrate for a cobalt-mediated [2 + 2+ 2] cycloaddition. 

A bromoallene 75 was prepared from 74 following the standard procedure and used in the natural product synthesis [31] of 78 (Scheme 4.19) [32]. Crimmins and Emmitte succeeded in the construction of the chiral bromoallene moiety of isolaur-allene 83 by bromination of propargyl sulfonate 81 with LiCuBr2 as a key step (Scheme 4.20) [33] (cf. Section 18.2.3). 

Tillack and co-workers developed a rhodium-catalyzed asymmetric hydrosilylation of butadiyne 258 to afford allenylsilane 260 (Scheme 4.67) [106]. Among more than 30 chiral phosphine ligands investigated, the highest enantioselectivity was observed when the catalyst was prepared from [Rh(COD)Cl]2 (1 mol%) and (S,S)-PPM 259 (2 mol%) to afford the optically active allene 260 with 27% ee. Other metals such as Ir, Pd, Pt or Ni were less effective for example, a nickel catalyst prepared from NiCl2 and (R,R)-DIOP 251 or (S,S)-PPM 259 gave the allene 260 with 7-11% ee. 

Many of the reactions assembled in Scheme 5.4are of undiminished interest in modern allene chemistry when relatively simple alkyl derivatives are the preparative goal. For example, /3-eliminations of enolphosphates prepared from saturated ketones constitute a simple route to 1,3-dialkylated allenes. Thus 3-octanone (49), on LDA treatment followed by quenching the generated enolate ions with diethyl chlor-ophosphate, affords a mixture of the enolphosphates 50. When these are treated with further LDA in THF at low temperatures, 2,3-octadiene (51) is produced in 50% yield (Scheme 5.5) [15]. 

To prepare the parent bisallene 118, 116 is first converted into its Grignard reagent (known from spectroscopic studies to possess the allenic structure), from which, presumably, by the addition of cuprous chloride the organocopper intermediate 117 is generated. Addition of further 116 subsequently provides a mixture of 118 and propargylallene (l,2-hexadien-5-yne) (29) (see below) in a 2 3 isomer ratio [44],... 

Likewise, the sulfoxide-metal exchange reaction of /3-acetoxy sulfoxides 164 (/3-mesyloxy sulfoxides can also be used), which are prepared from alkenyl aryl sulfoxides 163 and aromatic aldehydes, with a Grignard or alkyllithium reagent at low temperatures gives the allenes 162 in good to excellent yield (Scheme 5.24) [65],... 

The analogous transformation of 125, also realized by flash vacuum pyrolysis, gave rise to allenic oximes 126 [165], which are not directly accessible by the classical route starting from allenyl ketones and hydroxylamine (see Section 7.3.2) [122], Because compounds 125 are prepared from allenyl ketones and furan by [4 + 2]-cycloaddition followed by treatment with hydroxylamine, the retro-Diels-Alder reaction 125 —> 126 is in principle the removal of a protecting group (see also Scheme 7.46). 

Acceptor-substituted allenes can be prepared from the corresponding propargyl precursors by prototropic isomerization (see Section 7.2.2). Conversely, such allenes can also be used to synthesize propargyl compounds. For example, treatment of the sulfoxides 417 with 1 equivalent of a lithiation reagent leads to the intermediates 418, which furnish propargyl sulfoxides 419 by hydrolysis (Scheme 7.55) [101]. If the electrophiles used are not protons but primary alkyl halides or carbonyl compounds, the products 420 or 421, respectively, are formed by C,C linkage. 

Sulfur-containing acyclic and cyclic compounds have been prepared from allenyl sulfides in numerous transformations such as substitutions, additions, cydoaddi-tions and other cyclization reactions. Like the other donor-substituted allenes, allenyl sulfides are suitable substrates for regioselective lithiation and substitutions as exemplified in Scheme 8.86 [168, 169,175]. 

Allenylboranes can also be prepared from lithiated propargyl chloride [20]. As noted above, these intermediates react with acetic acid to afford allenes (Table 9.11). 

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