Allene derivatives reactions

When allene derivatives are treated with aryl halides in the presence of Pd(0), the aryl group is introduced to the central carbon by insertion of one of the allenic bonds to form the 7r-allylpalladium intermediate 271, which is attacked further by amine to give the allylic amine 272. A good ligand for the reaction is dppe[182]. Intramolecular reaction of the 7-aminoallene 273 affords the pyrrolidine derivative 274[183]. 

As typical examples of crystal-to-crystal thermal reactions, the cyclization of allene derivatives to four-membered ring compounds and the transformation of a racemic complex into a conglomerate complex are described. 

Crystal-to-Crystal Cyclization Reactions of Allene Derivatives... 

The usefulness of allene derivatives has also been revealed in other examples. Thus, the annulated tetrasubstituted furan illustrated in the following scheme was delivered in a moderate yield using the diazoallene as precursor by a two-step reaction in the presence of Rh-catalyst <06S3605>. 

In the presence of triphenylphosphine as a catalyst, benzotriazole adds readily to activated allenes. Its reaction with ethyl 2,3-butadienoate produces a mixture of adducts 149 (54%) and 150 (20%). Both derivatives form exclusively as (Tyi-isomers <2006T3710>. In a reaction of benzotriazole with dibenzoylacetylene and... 

Allenyl cations have been generated by solvolysis of allenic derivatives, by photolysis of allenyl halides and by reaction of metal salts with allenyl and propargyl halides. This review will delineate these reactions. The related butatrienyl cations are not many and they will be only briefly described. 

An interesting annelation reaction of allene-derived 13-dipoles with 3-(IV-aryliminomethyl)chromones 38 affords, in fair yields, after [4 +3] cycloaddition and a subsequent cascade of rearrangements, derivatives of the novel iV-aryl-2,3-dihydro-4-ethoxycarbonylchromano[2,3-h]azepin-6-one system 39 (for example, R = Me, R1 = Cl) (Scheme 9). In the initial cycloaddition, the substituted chromone acts as an azadiene moiety <00OL2023>... 

The lactam 145, bearing a terminal triple bond, is transformed into the corresponding allene derivative 146 through a Crabbe reaction (Equation 7). Using Pd(PPh ()4 as the catalyst and in the presence of phenyl iodide, the corresponding indolizine is obtained. The lactam nitrogen atom is added to the central carbon atom of the allene... 

The 1,4-addition of an appropriate reagent, such as a hydrosilane or a hydroborane, to a conjugated enyne produces an allenic derivative. Occasionally, 1,2-addition to the alkynyl moiety of the substrate competes as an undesirable side reaction to give a conjugated diene (Scheme 3.66). Suppression of the latter reaction path is required for the selective preparation of the allenic products. 

Less common addition reactions such as the bromination of trifhioromethyl-substi-tuted butatrienes [30] or the reaction of tetrafluoroallene with boron trifluoride have also been reported [283]. Especially the interaction of phosphorylated allenes with electrophiles was summarized in a review by Alabugin and Brel [8], whereas Smadja [284] published a more general overview about the electrophilic addition to allenic derivatives. 

This chapter focuses on cycloaddition reactions in which at least two new cr-bonds are formed between allene derivatives and other unsaturated organic molecules. Intramole-cular cycloaddition reactions are also described. The reactions are categorized according to assembly modes, such as [m + n]-cycloaddition, where the variables m and n simply denote the number of atoms that each component contributes to the ring construction. Some electrocyclic reactions of allene derivatives are also included. 

A large number of reaction conditions have been defined for the rearrangement of various allene derivatives into small heterocycles. The rearrangements have been accomplished variously with acids, bases or oxidatively. 

In addition to a-allenic a-amino acids, the corresponding allenic derivatives of y-aminobutyric acid (GABA) have also been synthesized as potential inhibitors of the pyridoxal phosphate-dependent enzyme GABA-aminotransferase (Scheme 18.49) [131,138-142]. The synthesis of y-allenyl-GABA (152) and its methylated derivatives was accomplished through Crabbe reaction [131], aza-Cope rearrangement [138] and lactam allenylation [139], whereas the fluoroallene 153 was prepared by SN2 -reduc-tion of a propargylic chloride [141]. 

As with the reaction of pyrroles, diazoles and triazoles react with propargyl bromide to yield TV-substituted products and, depending upon the base strength, either TV-prop-2-ynylazoles or allenic derivatives are formed [30]. Generally, with potassium carbonate under soliddiquid two-phase conditions at room temperature in the absence of a solvent, the prop-2-ynyl compounds are formed as sole products, whereas with solid potassium hydroxide at elevated temperatures the allenes are obtained as the major products. Benztriazole produces a mixture of the N1- and N2-prop-2-ynyl, and N2-allenic derivatives, whereas with potassium hydroxide only the N -allenic derivative is obtained. The alkynes readily isomerize to the allenes in the presence of base and the quaternary ammonium salt, or upon treatment with methanolic sodium hydroxide. A series of l-(alk-2-ynyl)imidazoles have been prepared, as intermediates in the synthesis of imidazopyridines [31 ] and the reaction of 3-hydroxymethylpyrazoles with propargyl bromide leads to pyrazolooxazines [32]. 

Recent advances in the rhodium-catalyzed [4-1-2] reactions have led to the development of the first highly regioselective intermolecular cyclization, providing access to new classes of carbocycles with both activated and unactivated substrates. The chemo- and stereoselective carbocyclizations of tethered diene-allene derivatives afford new classes of 5,6- and 6,6-bicyclic systems. Additionally, examination of a wide range of factors that influence both diastereo- and enantioselectivity has provided a significant advance in the understanding of catalyst requirements across these systems. 

The reaction is versatile and proceeds with a variety of cyclic and acylic alkenes substituted with alkyl, aryl, vinyl and heteroatom substituents. Allene derivatives also undergo cycloaddition with nitrones ". A variety of cyclic and acyclic aliphatic nitrones bearing aliphatic and aromatic substituents has been tested. The reaction is, however, relatively sensitive to steric constraints and proceeds easily only for mono- and disubstituted alkenes. Steric requirements for a nitrone molecule are similar and, although several reactions with R, R2 are known, good yield has been achieved only with R = H (equation 105). 

Esters of acetylenic alcohols and methanesulfinic and methanesulfonic acid have been used as synthetic intermediates in our laboratory, mainly for Sjsj2 -like substitution reactions leading to allenic derivatives [see for example refs. 210,211]. Although pyridine is recommended as... 

Occasionally the cyclopropylidene to allene isomerization cannot take place for structural reasons. If, for example, the expected allene would be very highly strained, as is the case for certain cyclic allenes, then the reaction is forced to follow an alternative path. A case in point is provided by 1-alkyl-7,7-dibromonorcaranes which undergo a carbene insertion reaction when treated with methyl lithium to provide bicyclobutanes rather than allene derivatives. 

The reactions of type II proceed by transmetallation of the complex 5. The transmetallation of 5 with hard carbon nucleophiles M R (M = main group metals) such as Grignard reagents and metal hydrides MH generates 8. Subsequent reductive elimination gives rise to an allene derivative as the final product. Coupling reactions of terminal alkynes in the presence of Cul belong to Type II. 

Chiral alkoxy allenes derived from 1,3-alkylidene-L-erythritol and -D-threitol have been used in cycloaddition reactions to provide the 4-substituted /3-lactams 418 (R = Me, Ph). Intramolecular alkylation at nitrogen was achieved by the action of potassium carbonate and tetrabutylammonium bromide in dry acetonitrile. The absolute stereochemistry of the product 419 (R = Me, Ph) was assigned on the basis of the CD helicity rule (see Section 2.04.3.5) and NMR spectroscopy. The [2+2] cycloaddition of CSI to threitol vinyl ethers was found to have low stereoselectivity in contrast to the findings with erythritol derivatives <2004CH414, 2005EJ0429>. 

Although a l-(alkylthio)cyclopropene could not be isolated in the above reactions, the corresponding silylated derivative (162) does ring open either on heating or on photolysis, leading to an allene the reaction may involve a 1,2-silyl-shift in an intermediate carbene (163), though in this case the latter could not be trapped by added alkene 82). 

A mechanistic explanation of this transformation of the epoxyhexenynes (325) to furo- and thienofurans (326) and (327) has been proposed <93CB975>. The reaction pathway leading to the products (326) and (327) presumably proceeds via the carbonyl ylides (330) which undergo a 1,7-dipolar cyclization to the allene derivatives (331). These are subsequently transformed into the furo[3,4-6]furans via a pathway involving diradical and carbene intermediates. 

As shown below, allene derivatives have been adopted as starting materials in the synthesis of furans. Palladium(II)-catalyzed cyclization of a-allenic ketones afforded 2,3-disubstituted furans in good yields <07EJO2844>. The substituent at the a-position of a carbonyl group influences the reaction pathway. Cyclization products were provided when the substituent was a methyl group or a phenyl group, while dimerization products were produced when the substituent was a hydrogen atom. 

The reaction of the Simmons-Smith reagent with a carbon-carbon triple bond gives cyclopropene and its isomers (72), along with small amounts of allene derivatives (123, 529). Terminal acetylenic groups give... 

The reaction of allene derivatives with Simmons-Smith reagent gives mono- and dimethylenated products 30, 31, 34, 405, 518, 530). 

---