Butatrienylidene

Schwarz and coworkers115 used 1,2,3-butatriene, along with 1,3-butadiyne, as a precursor for the generation of neutral 1,2,3-butatrienylidene in a neutralization/reionization mass spectrometric sequence (C4H4 - C4H2- - C4H2 - C4H2+ ). 

Addition of electrophiles to diynyl complexes is expected to occur at either or C, the latter being favored if sterically demanding ligands shielding C and are present. The products are butatrienylidenes and the chemistry of these species is closely related to the chemistry of the related unsaturated carbene ligands (Section VIILB). " ... 

Main routes to 71-donor-substituted allenylidene complexes include (1) the nucleophilic addition of secondary amines to Fischer-type carbenes [M =C(OR ) C=CR (C0)5] (M = Cr, W) [9], (2) the Lewis-acid induced abstraction of NR2 groups from anionic complexes [M C=CC(NMe2)3](CO)5] (M = Cr, W) [9], and (3) the regioselective addition of protic nucleophiles to metallacumulenylidenes with more extended unsaturated carbon chains, such as butatrienylidenes or penta-tetraenylidenes [10]. In the following sections updated syntheses are presented by Periodic Group. 

This unusual reactivity was subsequently extended to a broad variety of allylic and propargylic amines, in combination with other ruthenium(II) metal fragments, allowing the isolation of aminoaUenylidene complexes 33-35 [47 9] (Fig. 6). In addition, the S3mthesis of the thio-allenylidene [44] and seleno-aUenylidene [50] derivatives 36 from the in situ generated butatrienylidene tra s-[RuCl (=C=C=C=CH2)(L2)2] (L2 = dppm, dppe) and the corresponding aUyl sulfides... 

Until now, the structures of only three butatrienylidene [2-5] and four pentatetraenylidene complexes [6-9] (Chart 3.2) have been established by X-ray structure analysis. 

Chart3.2 Mononuclear butatrienylidene and pentatetraenylidene complexes characterized by X-ray structure analysis. 

The metal-carbon chain in these complexes is either linear or deviates only slightly from linearity. The M=C=C angle varies between 173.5° and 180°, the various C=C=C angles between 172.3° and 180°. The deviation from linearity is most pronounced with the butatrienylidene manganese complexes [4, 5]. 

As expected, the terminal C=C bond in all complexes is significantly longer than the internal double bonds. Whereas in the butatrienylidene iridium complex [2, 3] both internal double bonds are, vhthin the error limit, equal in length, in the manganese complexes the central C=C bond is shorter than the (Mn)C=C bond. 

The majority of butatrienylidene complexes synthesized or generated so far were obtained by following route a and using butadiyne or a butadiyne derivative as the source of the C4 fragment. A few complexes of iridium or manganese were synthesized by the substitution route c. Until now route b has not been (successfijlly) employed. 

The first butatrienylidene complexes were generated by Bruce et al. [14] and Lomprey and Selegue [15, 16[ in 1993. 

The synthesis introduced by Bruce et al. starts from butadiynyl lithium [14]. The addition of HBF4 to solutions of buta-l,3-diynyl ruthenium complex 3 was proposed to afford the butatrienylidene cation 4 by protonation of the terminal carbon atom of the butadiynyl ligand. Complex 4 could neither be isolated nor spectroscopically detected. It readily decomposed by reaction with even traces of water in the air by nucleophilic attack of H2O on the cationic center (Scheme 3.2). 

The first isolable, albeit binuclear, butatrienylidene complexes, the cationic diiron complexes 8, were likewise prepared by addition of an electrophile E+ to neutral butadiynyl complexes. Instead of mononuclear butadiynyl complexes, binuclear C4-bridged butadiyndiyl complexes 7 were used as the starting complexes by Lapinte et al. (Scheme 3.4) [18]. Complexes 8 were characterized by multinuclear NMR, IR, UV-vis, and Mossbauer spectroscopies, mass spectrometry and cyclic voltammetry. 

Scheme 3.4 Synthesis of the first binuclear butatrienylidene complexes by addition of electrophiles to butadiyndiyi diiron...

An alternative route to cationic butatrienylidene complex 4 involves 1,4-H shift in butadiyne complex 9. The formation of 4 as an intermediate in the reaction of [Cp(PPh3)3Ru-Cl] with AgPFg and buta-l,3-diyne was deduced from trapping experiments. Complex 4 thus generated gave with diphenylamine the corresponding diphenylamino(methyl)allenylidene complex (Scheme 3.5) [19]. 

Scheme 3.6 Formation of the butatrienylidene complex 10 by reaction of [RuCl2(dppm)2] with butadiyne.

Rearrangement reactions have turned out to be very convenient and the most used methods for the synthesis of butatrienylidene complexes. 1,4-H shift reactions have been used by Bruce et al. [19, 20] and Winter et al. (e.g.. Scheme 3.6) [21, 22] for generating butatrienylidene ruthenium complexes. Analogously, a butatrienylidene iron complex tvas obtained from [Cp (dppe)Fe-Cl] (dppe = Ph2PCH2CH2PPh2), Mc3Si-C = CC = CH, and NaBPh4 in methanol via 1,4-H shift (Scheme 3.7) [23, 24]. 

Scheme3.7 Formation of a silyl-substituted butatrienylidene iron complex by reaction of [FeCI(Cp )(dppe)j with trimethylsilyl butadiyne.

Scheme 3.8 Synthesis of the first isolable neutral butatrienylidene complex.

Scheme 3.9 Synthetic routes to butatrienylidene manganese complexes.

Recently, isolable bis (triphenylstannyl)-substituted butatrienylidene complexes of manganese (13) were obtained by photolysis of alkynyl(triphenylstannyl)vinylidene complexes 12 (Scheme 3.9) [4, 5]. Treatment of the resulting bis(stannyl)butatrie-nylidene complexes 13 with tetrabutylammonium fluoride and water afforded the first characterizable butatrienylidene complexes (14) containing an unsubstituted [M=C=C=C=CH2] moiety (Scheme 3.9). In contrast to 13, complexes 14 were unstable above —5 °C and were therefore characterized in solution only by NMR spectroscopy at —40°C. Complexes 14 were also formed instantaneously when solutions of 12 were treated at — 30 °C with one equivalent of tetrabutylammonium fluoride. 

A bis(trimethylstannyl)-substituted butatrienylidene complex (17) related to 13 (R = R = Me) was formed in the reaction of cydoheptatrienyl(methylcyclopentadie-nyl)manganese with Me3SnC = C-C = CSnMc3 and dmpe. Complex 17 was obtained... 

DFT calculations indicate that the LUMO in d and d complexes is predominantly localized on the odd carbon atoms independent of the chain length, the substituents R and the metal-ligand fragment L M. Therefore, nucleophiles are expected to add to the odd carbon atoms of the chain in butatrienylidene as well as in pentatetraeny-lidene complexes. 

Allyl amines, allyl thioethers, and allylferrocenylselenide react analogously with butatrienylidene complex 10 by initial addition to C3 and subsequent hetero-Cope or hetero-Claisen rearrangement (Scheme 3.23) [21, 42-45]. 

The reactions of various butatrienylidene complexes with water [14, 19, 20, 48] giving acylethynyl complexes very likely proceed by initial attack of water (or OH ) at C3 of the butatrienylidene ligand. 

Scheme 3.26 Formation of azabutadiene-2-ethynyl complexes in the reaction of butatrienylidene complexes with imines.

For the formation of the 4-ethynylquinoline complexes a mechanism was proposed involving nucleophilic attack of the terminal carbon of the butatrienylidene ligand at the imine carbon, followed by C—C bond formation between the ortho carbon of the N-aryl group and C3 of the butatrienylidene ligand. Deprotonation finally affords 4-ethynylquinoline complexes (Scheme 3.27). Some preference was observed for quinoline formation with the more electron-rich metal centers, whereas... 

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