Allenes formation

When the base is NaNH2 1-alkynes predominate (where possible), because this base is strong enough to form the salt of the alkyne, shifting any equilibrium between 1-and 2-alkynes. When the base is OH" or OR", the equilibrium tends to be shifted to the internal alkyne, which is thermodynamically more stable. If another hydrogen is suitably located (e.g., —CRH—CX2—CH2 ), allene formation can compete, though alkynes are usually more stable. 

Fig. 23. Summary of the unimolecular dissociation processes of the ground electronic state ally 1 radical at 115kcal/mol. Two competing pathways, formation of allene and isomerization to 2-propenyl (which subsequently dissociates into allene and propyne), are observed. The experimental (bold) and RRKM (italics) rates are given. The allene formation is the dominating channel. (From Fischer et at.145)...

Novel alkenylphosphonium salts were subjected to the Wittig reaction (Scheme 12). Allylic deprotonation took place for phosphonium salts possessing such protons, and the olefination proceeded after double bond migration. In cases where such protons were absent, allene formation was observed. 

A further example of the reductive allene formation in the synthesis of a non-alle-nic natural product was reported recently by VanBrunt and Standaert (Scheme 2.47) [81]. Treatment of the propargylic silyl ether 147 with LiAlH4 led to the syn-stereose-lective formation of the hydroxyallene 148, albeit with unsatisfactory chemical yield (25-50%). The latter was then transformed into the antibiotic amino acid furanomy-cin (150) by silver-mediated cycloisomerization to dihydrofuran 149 and elaboration of the side-chain. 

A single example of allene formation was reported for a reaction of 2-iodo-3-phe-nyl-1,3-butadiene (98) with MeMgBr (Scheme 3.48). The reaction, however, proceeded with poor selectivity and the terminal allene 99 was obtained in 36% yield together with 36% of a conjugated diene 100 [92]. 

A single example of allene formation was briefly described for a reaction of 2-methylbut-l-en-3-yne (148) with catecholborane (149) [116]. The allenylborane 150 was not isolated but converted into the homopropargyl alcohol 151 in 57% yield by quenching with benzaldehyde (Scheme 3.76). 

Appropriate choice of the substituents at the propynyl, y/>3-carbon atom improves the regiose-lectivity of the process towards allenes. Using propynyl carbamates, high ratios of simple diastereoselectivity (> 95 <5) combined with good stereoselectivity for allene formation have been achieved, particularly when organotitanium intermediates were employed92-94. 

Cuprates are known to react with propargylic derivatives to give allenes58. The allene formation is also observed in the reaction with zincates (equation 34). However, the reaction course is different. As demonstrated by deuterium incorporation in D20 quench... 

Conversion of trifluoropropene to the bromide allows access (via halogen-metal exchange) to organometallic reagents, synthetic equivalents for the tri-fluoropropenyl anion synthon. Clearly, defluorometallation (leading to allene formation [230]) is a potential problem so the electropositivity of the metal... 

For reviews of allene formation, see Schuster Coppola Allenes in Organic Synthesis, Wiley New York, 1984, pp. 9-56 Landor, in Landor The Chemistry of the Allenes, vol. I Academic Press New York, 1982- pp 19-213 Taylor Chem. Rev. 1967, 67, 317-359. 

Allen, J. M., S. Lucas, and S. K. Allen, Formation of hydroxyl radical in illuminated surface waters contaminated with acidic mine drainage , Environ. Tox. Chem., 15,107-113 (1996). 

When the carbene or carbenoid resulting from a dihalocyclopropane is unable to rearrange to the al-lene due to steric or other factors, insertion or addition reactions characteristic of carbenes take place. Thus dibromonorcarane on reaction with methyllithium gives a bicyclobutane derivative by insertion of the carbene into a 0-C—H bond (equation 57).178 Allene formation is sterically unfavorable in this case. Similarly, dibromotetramethylcyclopropane gives l,2,2-trimethylbicyclo[1.1.0]butane instead of tetra-methylallene (equation 58).179 181 An example involving a tricyclic dibromocyclopropane is given in equation (59).182... 

A facile approach to the benzazepinones 284 from the nitrone precursors 283 was reported by Eberbach et al. (Equation 41) <2001EJ03313> (see also Scheme 11). Reaction of 283 with base under mild conditions gave 284 in generally high yields, for example, 284 (R1 = R3 = H, R2 = Ph 84%). A complex series of steps was proposed in the formation of 284 involving allene formation, 1,7-dipolar cyclization, and a series of bond cleavage and formation steps... 

Allenes are reduced further to 1-alkenes with excess ammonium formate at 100 °C. Whereas allene 216 is formed with two equivalents of formate at room temperature in THF, terminal alkene 217 is obtained with excess HCO2NH4 directly from 215. The use of propargyl formates is therefore recommended for clean allene formation without overreduction whenever the formates are available. 

Palladium(0)-catalysed selective allene formation by hydrogenolysis is a convenient synthetic route to allenes from propargylic alcohols. The method offers a new and efficient preparative method for allenes from easily available propargylic alcohols after converting them to formates or carbonates. The best catalyst for smooth hydrogenolysis is prepared by mixing Pd(OAc)2 or Pd(acac)2 with -Bu3P in a ratio... 

Three zirconium/cycloheptadienyne complexes (231a-c) have been prepared by /3-hydrogen elimination from a mixture of cycloheptatrienyl complexes 269-271 (Scheme 33) and have been used as intermediates for the preparation of a zirconaazulene.87 The alkyne complexes are formed to the exclusion of the allene isomer 268. This is believed to be due to the proximity of the /3-vinyl hydrogen that is a result of both the shorter double bond and its forced coplanarity with the metal. Allene formation from 269 might be induced by blocking the vinyl position (see Sections IV,B and IV,C), but this has not been tested. 

The normal proton abstraction which precedes 3-elimination induces allene formation from II. The allene, being a Michael acceptor then inactivates the enzyme via an alkylative process involving a nucleophilic (Nu) residue on the enzyme. Propargylglycine has since been shown to also irreversibly inactivate glutamate-pyruvate transaminase (8). 

Alkynes (acetylenes, RCsCR) may be prepared by the elimination of a hydrogen halide from alkenyl halides under vigorous conditions. This is exemplified by the preparation of phenylacetylene from cinnamic acid via the dibromide and (o-bromostyrene (Scheme 3.26). The contrast between the conditions required for the bromodecarboxylation and for the second elimination to form the alkyne reveals the difference in reactivity between an alkyl and an alkenyl halide. Alternative modes of elimination, such as allene formation or rearrangement reactions, restrict the use of this procedure. 

Since the concentration-time curves for CO2 and allene formation are coincident in the initial stage of the decomposition, it can reasonably be assumed that the two substances are formed in the same bimolecular reaction, viz. 

At 750 K, k 2x 10 cm molecule s for these reactions, compared with ksA 2 X for the analogous reaction of alkyl radicals, but is a factor of about 10" greater than for allene formation from CH2CHCH2. 

Shortly after discovery of a convenient dihalocyclopropane synthesis their conversion to allenes was effected This area has been reviewed very recently ", and therefore only some very new examples will be mentioned Employing a repetitive cyclopropanation/allene formation sequence, 1,2,3-cyclononatriene—presumably the smallest isolable cyclic butatriene—has become available (equation 120). ... 

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