Allenes compounds

Propargylic substitution reaction is one of the most important routes to allenic compounds [1, 2], As shown in Scheme 3.1, replacement of a leaving group at the propargylic position with an incoming nucleophile via an SN2 pathway rearranges the C=C-C skeleton into a C=C=C moiety to give a propadienyl species. With certain... 

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]. 

The intermolecular PKR of an allenic compound with an alkyne Co2(CO)6 complex takes place under very mild conditions when N-methylmorpholine oxide (NMO) is used as a promoter, giving rise to ( )-4-heptylidene-2,3-dipropyl-2-cyclo-pentenone in 69% yield (Scheme 16.48) [50-53],... 

The stereoselectivity of the allene attack has been demonstrated by the catalytic reaction of cis and trans seven-membered allenic compounds, in which cis- and trans-fused 5,7-ring systems are provided in 76 and 40% yield, respectively, via Jt-allylpalladium intermediates (Scheme 16.91). 

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],... 

The germaallene 194 was prepared by reaction of fluoroalkynylgermane 195 with tert-butyllithium at -78°C in about 85% yield.174 Elimination of LiF occurred at low temperature. 194 was the first allenic compound of germanium structurally characterized (Scheme 41). 

As the simultaneous creation of Ge = C and P = C double bonds are unlikely, the P = C double bond, much less reactive than the Ge = C double bond, was formed first.173196 was prepared by debromofluorination of 197 [obtained by reaction of ArP = C(Br)Li181 with dimesityldifluorogermane] with ft-butyllithium at low temperature (Scheme 42). The reaction, followed by 31P NMR between -90°C and room temperature, showed the immediate formation of the lithio compound 198, which lost LiF at -60°C to give the germaphosphaallene 196 in 65-70% yield. 196 was stable at -50°C and dimerized slowly above this temperature. It was the first allenic compound of germanium to be characterized by physicochemical methods. 

Some acetylenic (with a non-terminal triple bond) or allenic compounds, RCH=C CH2, can be transformed into alkali metal derivatives of terminal acetylenes by treatment with a very strong base. Treatment of an acetylenic compound with the grouping CHjCsC- or CH3C=CCH=CH- with one equivalent of an alkali amide (preferably the soluble potassium... 

Allenes are versatile synthetic precursors in organic chemistry and the synthesis of many natural products involves the use of allenic compounds [27]. However, they have received much less attention than alkenes or alkynes for transition metal catalyzed reactions. The explanation lies in the problems of selectivity that these substrates display as a result of their reactivity and their inherent chirality [28]. 

Thus, the (3-ionone was smoothly deconjugated and ethynylated to give ethynyl-retro-ionol as a mixture of E/Z stereoisomers. Formation of the carbonate and its Pd-catalyzed rearrangement produced straightforward a mixture of aldehydes and a allene compound. After silica-gel chromatography, the allenic-aldehyde was conjugated with a catalytic amount of HBr in acetone. Retinal was obtained as a mixture of E and Z isomers (75/25), which could be converted into the all E isomer by simple equilibration, Fig. (33). 

Chemistry of Allenic Compounds, Tech Rept Univ of Calif, Los Angeles, Calif,... 

Since RN=S=NR and RN=S=0 are cumulated double bond systems they may be considered as heteroallene compounds. In the case of metal-allene compounds the allene may rotate about the metal- 2-allene bond and in addition the metal may jump intramolecularly from one n-C=C bond to the other and vice versa.1... 

Allenic Compounds are derivs of allene or propadiene, also called dimethylenemethone, CH2 C CHr Allene was probably first prepd in 1865(Ref 2), but the hydrocarbon was not actually identified until 1872 (Ref 3). A rather general method for the prepn of allene by treating 1,2-dihalopropene with Zn dust in ale, was first used in 1888(Ref 4). Many other methods of prepn are known and some of them are described in Ref 5... 

Dimethylenemethane. See Allene under Allenic Compounds A133-R Dimethylketone. See Acetone A33-R Di-(3-methylpentynyl)-3 peroxide. See under Acetylene Hydroperoxides A66-R 2,3-Dimethyl- 1-phenyl- 3 pyrazoli n- 5-one. 

Propadiene. See Allene under Allenic Compounds A133-R... 

Ishihara J, Shimada Y, Kanoh N, Takasugi Y, Fukuzawa A, Murai A (1997) Conversion of Prelaureatin into Laurallene, a Bromo-Allene Compound, by Enzymatic and Chemical Bromo-Etherification Reactions. Tetrahedron 53 8371... 

The successful isolation of 1-silaallenes and 1-germaallenes raises the expectation of synthesis of a novel class of allenic compounds having an sp-hybridized silicon unit, that is, 2-silaallene derivatives. Although the generation of 2-silaallene (f-Bu)-CH2-CH=Si=CH-CH2-(t-Bu) was postulated in the reaction of (H2C=CH)2SiCl2 with 2-equivalent of t-BuLi, no distinct evidence on the generation of such a compound has been obtained so far. 

---