1- -1,2-propadiene, metallation

The presence of the endothermic triply-bonded acetylene (ethyne) group confers explosive instability on a wide range of acetylenic compounds (notably when halogen is also present) and derivatives of metals (and especially of heavy metals) [1], Explosive properties of butadiyne, buten-3-yne, hexatriyne, propyne and propadiene have been reviewed, with 74 references [2], The tendency of higher acetylenes to explosive decomposition may be reduced by dilution with methanol [3], The class includes the separately treated groups ... 

Propadiene polymerisation during hydrogenation has been observed to occur, particularly with iron, cobalt and nickel. Over these metals, up to approximately 25% of the propadiene has been observed to polymerise [204], although the chemical identity of the polymeric products was not established. 

The behavior of allene (1,2-propadiene) and its derivatives as ligands in transition metal complexes has been investigated for more than 20 years (197,198). Allene may be coordinated by only one C=C double bond to a metal (197,199), or it may link to two metal centers as a 2 2 or 3 1 electron donor. Dinuclear /i-f/2l2-allene complexes are known for Mo (200,201), W (201), Mn (202-203), and Rh (199,204). In [Fe2(CO)7(/ -f/3 1-C3H4)] a bonding mode (205,206). In the course of the reaction, often two or more allene molecules become connected with transition metal complexes. Two allenes linked by C-2 yield tetramethyl-eneethane (207,208), which is stabilized by coordination on Fe (205,208), Ni (270), or Pd (210,211). 

A much shorter route is the Reppe carbonylation [83] of propyne. Propyne is, together with propadiene (allene), part of the C3 stream of the cracking process. The order of metal substrate binding strength is allenes>alkynes>alkenes. Thus the desired reaction can only proceed if the propadiene has been removed from the feed, since it is an inhibitor of the Pd catalyst. Equally important, the alkyne complex reacts much faster than the alkene complex. Thus the product is neither a substrate nor an inhibitor for the catalyst (Scheme 5.46). 

The reaction conditions for the metallation of propadiene show that this compound is readily metallated. Introduction of alkyl substituents causes a marked decrease of the kinetic acidity of the allenic protons. This is reflected in the higher temperature in the lithiation of 1,1-dimethylallene. 

Metallation of methoxyallene with butyllithium in Et20 or THF is complete in a few minutes at temperatures in the region of —25 °C, and gives exclusively the a-metallated intermediate. It is obvious that alkoxyallenes are deprotonated more easily than propadiene [97] and vinyl ethers H2C=CH—OR [9]. All types of electrophiles react with perfect regiospecificity at the a-carbon atom (compare the reactions with metallated allenic hydro carbons discussed in Chap. Ill)... 

Insertion of 1,2-propadiene, allene, into the transition metal-carbon bond gives methylene-substituted polymers (Scheme 1.31) [90],... 

TABLE 8. Kinetic Parameters for the Hydrogenation of Propadiene on Pumice-Supported Metals r oc Pn Pfy... 

Samarium, erbium and ytterbium metal vapor also react with ethene, propene, and 1,2-propadiene at — 196°C. The colored matrices, orange to black, contain up to 80% of the appropriate metal as shown by the infrared absorptions of the coordinated olefinic double bond. The reaction product of erbium metal and propene was characterized by elemental analysis as Er(CH2=CHCH,)3. The predominant volatile products after hydrolysis of the complexes are CH4, C2H, C3Hft/C3Hg and C4Hg/C4H,o for the ethene complexes, CjH /CjHg, H2C=C=CH2, and CH3propene complexes. Since ethene is only a minor reaction product in the hydrolysis of... 

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