Hydrogenation allenes

Similarly, 1,2-cyclononadiene in methanol with 10% palladium on carbon catalyst gave cis-cyclononene122. The cis isomer is not necessarily the primary product of allene hydrogenation, since the initially formed trans isomer is rapidly isomerized under the reaction conditions. Bond and Sheridan showed that allene resembles acetylene in its ease of hydrogenation123. They suggested that it is selectively adsorbed and held more strongly by the catalyst than 1-propene. Allene was selectively hydrogenated with Pd, Pt and Ni in the presence of 1-propene without its further reduction. 

An example of the synthetic use of allene hydrogenation is the preparation of the antibiotic phosphonomycin (equation 47)124. 

The treatment of furfuryl phenyl selenides (178) with n-butyllithium or metallic lithium affords furfuryllithium reagents which undergo ring opening to furnish initially the allenic ketones (180) (80TL3209). Two equivalents of n-butyllithium are required for complete conversion due to the acidic character of the allenic hydrogen. The allenic ketones are readily isomerized to the dienones (181) on p-toluenesulfonic acid treatment (Scheme 38). 

Fluxionality studies have been carried out on 343 (Scheme 44).137 No coalescence of the two allene hydrogens (HI and H6) was observed in temperature ranges where the complex is stable (ca. 100°C). However, spin saturation transfer was observed between 60 and 100°C, which yielded activation parameters (AH = 26.8 1.3 kcal/mol A5J = 15.1 1.3 eu) for migration of the metal between the two double bonds. At 80°C, 343... 

ISOMERIZATION Potassium (-butoxlde. ACETYLENES AND ALLENES Hydrogen fluoride-Boron trifluoride. 

The most important products of decomposition are ethylene, allene, hydrogen, butenes and ethane. Ethylene is found to arise almost entirely from the primary decomposition. Photolysis of cyclo-C3H6-cyclo-C3D6 mixtures gives about 20% HD in the hydrogen, and indicates a significant contribution of an atomic hydrogen process. The formation of propene in a primary step may represent a small fraction of the total primary processes. 

For such systems the constitutional isomerism order is k = 2. Consequently, the simplest ansatz for the description of a point-property of an allenic hydrogen atom conforming to the requirement of completeness only contains one-particle functions (A = 3 - 2). In Ref. 9 it has been shown that for the predictions of CNDO/S electron densities of allenic hydrogens indeed one-particle functions suffice that is, the hydrogen electron densities may be calculated according to Equation 33. 

In Equation 33 is a constant, 0 (R2) and 7 (R/) are ligand-specific parameters for the substituents attached to the ligand sites 2 or / = 3,4, respectively. These parameters have been determined from the CNDO/S hydrogen electron densities of monosubstituted allenes (a (H) = t (H) = 0). They are given in Table 5. From the numerical values one can see that substitution affects both the kinds of allenic hydrogen atoms to a comparable extent which again indicates the pronounced substituent effects across the whole allenic heavy-atom grouping. 

Calculated (8h) and Experimental (8h) H Chemical Shifts of Allenic Hydrogen Atoms... 

Addition Reactions. — A detailed study of the free-radical chlorination of penta-2,3-diene with t-butyl hypochlorite shows that substitution products are obtained by both allylic and allenic hydrogen-abstraction addition... 

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