Allenylidene reactions with amines

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. 

Besides Selegue s methodology, several synthetic alternatives of ruthenium allenylidene complexes have been reported. The most popular involves trapping of transient butatrienylidene or pentatetraenylidene intermediates with nucleophiles [26-29]. Although alcohols, amines, or thiols have been usually employed in these reactions leading to the corresponding heteroatom-substituted allenylidenes, in some cases the use of carbon-centred nucleophiles, such as pyrroles, has been described [185, 186]. Quite recently, a systematic route to prepare sequentially polyalkenyl-allenylidene complexes has also been discovered (Scheme 11) [187— 189]. The first step consists of the insertion of the ynamine MeC=CNEt2 into the... 

Reverse processes, that is reactions of propargyl or allenylidene ligands coordinated to mononuclear centers with water, methanol, amines, and other ligands are known. Tungsten l and platinum complexes give a series of reactions (some of which are mediated by surfaces) comparable with those discussed for clusters. 

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