Amino-allenylidenes

Scheme 2 Octet-stabilized amino-allenylidene ligands left), and non-octet-stabilized alleny-lidene ligands right)...

Cp=Cy bond of indenyl-allenylidene complexes 30 which leads to the stereoselective formation of cationic amino-allenylidenes 31. When R = Ph, complexes 31 can be transformed into the secondary derivatives 32 via treatment with LiBHEts and subsequent purification on sUica-gel column. Further insertions of MeC=CNEt2 into 32 allow the preparation of polyunsaturated cumulene chains (related insertion reactions will be discussed in the reactivity section). 

Heteroatom-substituted allenylidene and higher odd-chain cumulenylidene complexes [M]=C(=C) =CR R ( = 1, 3, 5 R /R = NR2, OR, SR, SeR) are directly related to the classical Fischer-type carbene complexes, being regarded as their functional carbo-mers [5-7]. However, although two of the first allenylidene complexes synthesized were the amino-allenylidene derivatives [M =C=C=CPh (NMe2) 1(00)5] (M = Or, W) [8], the chemistry of these compounds has been much less developed [9, 10]. In fact, as aheady discussed in Chapter 6 the chemistry of metallacumulenes is largely dominated by the all-carbon substituted representatives... 

In sharp contrast with the remaining transition metals, Group 6 allenylidenes [M (=C=C=CR R )(C0)5] (M = Cr, Mo, W) containing non-donnor substituents at Cy are in general thermally unstable [11-15]. For this reason, most of the reported examples are substituted derivatives bearing heteroatomic Ti-donor groups [9]. In this sense, the first stable Group 6 allenylidenes reported in the literature were the amino-allenylidene complexes 2, prepared by E. O. Fischer and coworkers by a Lewis-acid induced elimination of ethanol from the 3-dimethylamino-l-ethoxy-3-... 

Scheme 1 Synthesis of the first amino-allenylidene complexes 2...

Scheme 2 Synthesis of amino-allenylidene chromium complexes 5...

Scheme 8 Synthesis of the amino-allenylidene complex 32 through a [3,3] Aza-Cope rearrangement...

Scheme 9 Synthesis of amino-allenylidene ruthenium(II) complex 38...

Scheme 10 Synthesis of alkenyl(amino)allenylidene ruthenium(II) complexes 41...

Otherwise, the reactions of indenyl-ruthenium(II) allenylidenes [RuCty -CgHy) =C=C=C(R )Ph (PPh3)2][PF6] (R = H, Ph) with ynamines R C CNEtj (R = Me, SiMea) have been reported to yield the alkenyl(amino)allenylidene complexes 41 via insertion of the ynamine into the Cp=Cy allenylidene bond (Scheme 10) [52, 53], This insertion process involves an initial nucleophilic addition of the ynamine at Cy atom of the cumulene, which leads to the cationic alkynyl intermediate complexes 39. Further ring closing, involving the Cp atom, generates the [2+2]... 

Otherwise, while the reactions of chromium mono(amino)allenylidene complexes 5 with dimethylamine afforded the aIkenyl(amino)carbene derivatives 49 by means of the expected N-H addition across the C (=Cp double bond (Scheme 13)... 

R = Ph, OMe. NM62, 0-(-)-menthyl Scheme 17 Synthesis of alkenyl(amino)allenylidenes 60... 

Scheme 19 Synthesis of the trienyl(amino)allenylidene complex 64...

As commented previously, alkenyl(amino)allenylidene ruthenium(II) complexes 41 are easily accessible through the reaction of indenyl-Ru(ll) precursors with ynamines (Scheme 10) [52-54]. Based on this reactivity, an original synthetic route to polyunsaturated allenylidene species could be developed (Scheme 19) [52, 53]. Thus, after the first ynamine insertion, complex 41 could be transformed into the secondary derivative 62 by treatment with LiBHEts and subsequent purification on silica-gel column. Complex 62 is able to insert a second ynamine molecule, via the cyclization/cycloreversion pathway discussed above, to generate the corresponding dienyl(amino)allenylidene species. Further transformations of this intermediate in the presence of LiBHEts and Si02... 

Experimental information on the reactivity of pentatetraenylidene complexes is still rather rare. The isolated pentatetraenylidene complexes [Cl (dppe)2Ru=C=C=C=C=CPh2]+ (20) [7] and [(CO)5M=C=C=C=C=C(NMe2)2] (M = Cr, W) (22) [8] tvere found to read with secondary amines by addition of the amine across the C3=C4 bond, very likely via an initial nucleophilic attack at C3, to give alkenyl(amino)allenylidene complexes (Scheme 3.32). Analogously, methanol... 

The observation by Fischer et al.18 that the 4,1-addition of dimethylamine to compound la is thermodynamically controlled at 20°C, whereas 2,1-addition/elimination is kinetically controlled at -115°C, turned out to be limited to few cases.20 It has been shown9a 9b 42 112 113 that for most cases, three competing reaction paths must be considered (i) 2,1-addition/elimina-tion with formation of (l-amino)alkynylcarbene complexes (= 2-amino-l-metalla-l-en-3-ynes) 98 (ii) 4,1-addition to give [(2-amino)alkenyl]carbene complexes (= 4-amino-l-metalla-l,3-butadienes) 96 and (iii) 4,1-addition/ elimination to (3-amino)allenylidene complexes (= 4-amino-l-metalla-1,2,3-butatrienes) 99 (Scheme 33, M = Cr, W). The product ratio 96 98 99 depends on the bulk of substituents R and R1, as well as on the reaction conditions. Addition of lithium amides instead of amines leads to predominant formation of allenylidene complexes 99.112 Furthermore, compounds 99 also can be generated by elimination of ethanol from complexes 96 with BF3 or AlEt3114 and A1C13,113 respectively. 

The alkenyl(amino)allenylidene complex 41 is also prone to undergo electrophilic additions at the Cp atom of the cumulenic chain. Thus, treatment of 41 with HBF4-OEt2 led to the spectroscopically characterized dicationic vinylidene complex 65 (Fig. 10) [52, 53]. Related Cp-protonations of complexes 35 (Fig. 6) have also been described [49]. 

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