Metal allenes

Twenty chapters cover such new and exciting developments as metal-catalyzed synthesis of allenes, strained cyclic allenes, the numerous applications of different metallated allenes in organic synthesis, as well as the many addition and rearrangement reactions of allenes and allene units in natural products like the remarkable enyne-allenes. 

A complementary approach for cross-couplings with allenes was applied by using metallated allene species instead of allenyl halides, which have already been discussed in Sect. 14.2.1. Since allenyllithium compounds are readily available by deprotonation of allenes with n-butyllithium, successful cross-coupling reactions between lithiated allenes such as 54 or 57 and aryl or vinylic halides allowed convenient routes to aryl- and vinyl-substituted allenes, e.g. 55, 58 and 60 (Scheme 14.15) [30],... 

Allenes, H2C=C=CHR (R = alkyl), are metallated regioselectively at the terminal C-atom by butyllithium in THF ai low temperatures [186], Above 0 C, the metallated allenes rearrange to lithium alkynylides ... 

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... 

Structural Parameters op Transition Metal-Allene Complexes 1... 

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. 

Nina A. Nedolya was born in Irkutsk (Russia) and educated in organic chemistry at the Irkutsk State University (Diploma 1972, PhD 1982, DSc 1998). From 1995 to 1999 she was associated with Prof. L. Brandsma at the Utrecht University (The Netherlands). In 1999 she obtained her second PhD from the Utrecht University. She is presently Head of the Research Group of Chemistry of Heterocyclic Compounds at A. E. Favorsky Irkutsk Institute of Chemistry. She is the author of over 210 review articles and research papers. She is also one of the inventors for 112 patents. She is interested in the chemistry of polyfunctional unsaturated heteroatomic systems (vinyl, allenyl, and alkynyl ethers and their derivatives, linear and cyclic heteropolyenes, hetero-cumulenes), including synthesis of important heterocycles, particularly pyrroles, thiophenes, thiazoles, imidazoles, dihydrofurans, dihydropyridines, pyridines, quinolines, dihydroazepines, and azepines, based on metallated allenes or alkynes and/or heterocumulenes. 

The allyl metallation of metallated allenes leads to potential bisnucleophiles [28]. These allylic 1,1-bismetallic reagents may react as geminated bismetals or as vinylic-allylic bismetallic species, so that the location of these two metals has to be taken into consideration (Scheme 7-31). 

Although no pK values of allenic sulfides RCH=C=CHSR/ are known, other experimental data indicate the basicity of the lithiated compounds RCH=C=C(Li)SR/ to be somewhat lower than that of 2-thienyllithium. Competition experiments with butyl bromide in THF or in a THF-HMPT mixture showed the metallated allenic sulfide to be much more reactive than 2-thienyl-lithium [9]. This difference might be due to the higher polarisability of the allenyl system. 

Whereas ethene and butyllithium do not react at all in THF in the temperature range of — 90 to +30 °C, allene is metallated quantitatively in a few minutes at temperatures around — 70 °C. If the reaction with butyllithium is carried out at considerably higher temperatures, isomerization into propynyllithium CH3C=C—Li may take place. pK values of allene and its derivatives have not been determined, but the resonance stabilization in the anions is expected to give rise to a pK value which is lower than that of ethene. Qualitative comparison of the rates of alkylation with alkyl bromides of metallated allenes and sp2-metallated alkenes gives the impression that the allenic intermediates react faster in spite of their (presumed) lower basicity. The yield-reducing dehydrohalogenation of the alkyl halides occurs to a lesser extent with metallated allenes [9]. 

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)... 

Subsequent interaction with electrophilic reagents in general yields a mixture of the allenic and the acetylenic derivative trimethylsilylation and alkylation, however, seem to afford the y-functionalized allenic ether [99]. Under strongly polar conditions, i.e. if lithium is replaced by potassium (by addition of t-BuOK) and HMPT is added as co-solvent, the y-metallated allenic ethers rearrange into the a-metallic derivatives [100]. This remarkable isomerization may take place via a head-to-tail arrangement of two metallated molecules ... 

Although a-metallated allenic thioethers are readily formed from the corresponding thioethers and strong bases, it is in general more practical to generate these intermediates from the isomeric acetylenic sulfides. This can be done either with butyllithium (or BuLi TMEDA) [9] in THF or with alkali amides in liquid ammonia [44] ... 

Functionalization of metallated allenic sulfides has only been investigated with alkyl halides [44], epoxyethane [7] and a few carbonyl compounds [48,102]. Liquid... 

Metallated allenic tertiary amines have been obtained in three ways. In a few cases, direct metallation with BuLi in THF at about — 30 °C is the most attractive way dimethylaminoallene and morpholinoallene can be obtained with a purity of about 95% by base-catalyzed isomerization of the easily accessible propargylic amines [7,103] ... 

Structures of these compounds are analogous to those of olefin and alkyne complexes. In square-planar compounds, the allene molecule is approximately perpendicular to the ML3 plane, while in the remaining types of compounds, the allene molecule lies almost in the ML2 plane. The angle between this plane and the plane containing the allene carbon atoms equals about 10°. The character of the metal-allene bond may be described by means of the Dewar-Chatt-Duncanson model as in the case of alkene and acetylene complexes. The formation of the a(d -n) and n d -n ) bonds causes an increase in the carbon-carbon distance and tilting of the allene substituents away from... 

Like in the case of olefin complexes, the chemical shift t of the coordinated CHR group increases by 2-3 ppm compared to the free allene. This is also in agreement with the Dewar-Chatt-Duncanson model of the metal-allene bond (rehybridization sp -> sp ) according to the valence bond theory. 

Some metal allene compounds undergo migration (insertion) reactions with SO2 and CF3CSCCF3 to give compounds containing [Pg.420]

The cumulenes discussed in this book are subdivided into carbon- and noncarbon cumulenes, and the 1-carbon cumulenes (sulfines, sulfenes, thiocarbonyl S -imides and thiocar-bonyl S -sulfides) are excellent dipolar species. The 2-carbon or the center-carbon cumulenes (carbon dioxide and carbon sulfides) are less reactive but their imides (isocyanates, isothiocyantes and carbodiimides) readily participate in many of the discussed reactions. The 1,2-dicarbon cumulenes (ketenes, thioketenes and ketenimenes) similarly participate in cycloaddition reactions, as well as the more exotic 1,2-dicarbon cumulenes (1-silaalene, 1-phosphaallene and other metal allenes). In contrast, 1,3-dicarbon cumulenes are only... 

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