Allene, reaction with carbonyl compounds

Heterosubstituted allenes are versatile synthetic intermediates. Direct substitution of a heteroatom on the allene confers upon the system an electronic bias that allows the molecule to be deprotonated easily using an organolithium reagent, and also permits regioselective reaction with carbonyl compounds. 

However, the reaction with carbonyl compounds of the a-Grignard or a-zinc halide derivatives (prepared by exchange) affords mixtures of the allenic and acetylenic derivatives [9] ... 

A study of the reactions of butadiene, isoprene, or allene coordinated to nickel in a metallacycle, with carbonylic compounds, has been reported by Baker (example 11, Table IV). In the presence of phosphines, these metallacycles adopt a cr-allyl structure on one end and a ir-allyl structure on the other, as mentioned in Section II,A,1. The former is mainly attacked by aldehydes or electrophilic reagents in general, the latter by nucleophiles (C—H acids, see Table I, or amines, see Table IX). 

The use of organotitanium compounds in the synthesis of allenes involves mainly Wittig-type olefmation reactions of carbonyl compounds [86] with titanium ylides. The formation of allenes according to the scheme Q + Q + Q was described by... 

An interesting novel coupling reaction of allenes with carbonyl compounds mediated by a lanthanide metal species was reported recently [80], The samarium(II) iodide-mediated reaction of various ketones or aldehydes 153 with methoxyallene (56) afforded exclusively y-addition products 4-hydroxy-l-enol ethers 154 in moderate to good yields with low cis/trans selectivity (Scheme 14.39). 

Fluoride ion promoted cleavage of propynylsilanes and subsequent reaction of the carbanion with carbonyl compounds produces allenic compounds. The reaction with formaldehyde and pivaldehyde fails, but both the allenic and acetylenic products are obtained from the reaction with acrolein and benzaldehyde [49]. Allylsilanes react with carbonyl compounds to produce but-3-en-l-ols [50],... 

Indium-promoted reaction of l,4-dibromo-2-butyne with carbonyl compounds gives 1,3-butadiene derivatives via the allenic indium intermediates (Scheme 56).220 Similar indium-mediated l,3-butadien-2-ylation reactions of optically pure azetidine-2,3-diones have been investigated in aqueous media, offering a convenient asymmetric entry to the 3-substituted 3-hydroxy-/ -lactam moiety (Equation (40)). The diastereoselectivity of the addition reaction is controlled by the bulky chiral auxiliary at Q4 221 222... 

The C-H bond cleavage of active methylene compounds with a transition metal catalyst is another method for the functionalization of these C-H bonds. To date, several reactions have been developed. In particular, the asymmetric version of this type of catalytic reaction provides a new route to the enantioselective construction of quaternary carbon centers. One of the most attractive research subjects is the catalytic addition of active methylene C-H bonds to acetylenes, allenes, conjugate ene-ynes, and nitrile C-N triple bonds. The mthenium-catalyzed reaction active methylene compounds with carbonyl compounds involving aldehyde, ketones, and a,y3-unsatu-rated ketones and esters is described in this section. 

Allenic alcohols.3 The reaction of allylsilanes with carbonyl compounds catalyzed by this salt to give homoallylic alcohols (9, 455-446) has been extended to a synthesis of a-allenic alcohols. Use of TiCl4 as catalyst results in a 2-chloro-l,3-diene. 

As for the diols, the symmetric compounds have found most uses for nonsymmetric diols, a versatile synthesis via silyl ketones using the SAMP/RAMP methodology has been developedl5. Both enantiomers of the simplest symmetric diol, 2,3-butanediol (11), are often used in asymmetric synthesis, mostly for the formation of acetals and ketals with carbonyl compounds and subsequent reactions with acidic catalysts (Section D. 1.1.2.2.), Grignard reagents (Section D. 1.3.1.4.) and other carbanions (Sections D. 1.5.1., D. 1.5.2.4.), and diastereoselective reductions (Section D.2.3.3.). Precursors of chiral alkenes for cycloprotonations (Section D.1.6.1.5.) and for chiral allenes (Section B.I.), and chiral haloboronic acids (Section D. 1.1.2.1.) are other applications. The free diol has been employed as a chiral ligand in molybdenum peroxo complexes used for enantioselective epoxidation of alkenes (Section D.4.5.2.2.). 

Indium-mediated allylation of a-chlorocarbonyl compounds with various allyl bromides in aqueous media gave the corresponding homoallylic chlorohydrins, which could be transformed into the corresponding epoxides in the presence of a base (Eq. 4.24). ° The reaction of cyclopentadienylindium(I) with aldehydes gives isomeric mixtures in aqueous rnedia. Indium-promoted reaction of l,4-dibromo-2-butyne with carbonyl compounds gave 1,3-butadien-2-ylmethanols via the allenic intermediates. ... 

Propargylic lithium alanates or lithium borates react with allylic halides or with carbonyl compounds in a regioselective manner to furnish 1,1 -disubstituted allenes (Scheme The reaction between alkyl halides and aluminum metal is the basis of the oldest method for the synthesis of organoaluminum compounds. For example, propargylic bromides react with aluminum in ether giving organoaluminum compounds that on treatment with acetals yield solely a-allenic ethers (equation 1)." However, the reaction of simple alkyl halides with aluminum metal requires a long reaction time. 

Bromoboration Reactions. BBrj does not add to isolated double bonds, but reacts with allene spontaneously even at low temperature to give (2-bromoallyl)dibromoborane, which provides stable (2-bromoallyl)diphenoxyborane by the addition of anisole. The diphenoxyborane derivative reacts with carbonyl compounds to give 2-bromohomoallylic alcohols in high yields (eq 12). Bromoboration of 1-alkynes provides (Z)-(2-bromo-l-alkenyl)dibromoboranes stereo- and re-gioselectively (eq 13), which are applied for the synthesis of trisubstituted alkenes, a,p-unsaturated esters, and 7,8-unsaturated ketones, bromodienes, 1,2-dihalo-l-alkenes, 2-bromoalkanals, and (3-bromo-oi,3-unsaturated amides. ... 

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