Cross trimethylenemethane

Interactions polarize bonds. Trimethylenemethane (TMM) and 2-buten-l,4-diyl (BD) dianions (Scheme 6a, b) are chosen as models for hnear and cross-conjngated dianions. The bond polarization (Scheme 7) is shown to contain cyclic orbital interaction (Scheme 6c) even in non-cyclic conjugation [15]. The orbital phase continnity-discon-tinnity properties (Scheme 6d, e) control the relative thermodynamic stabihties. 

Experimental evidence for the six electron systems has been described in Sect. 2.1.4. Skancke reproduced the relative stabihty of the cross conjugated systems relative to the linear isomers by calculating the trimethylenemethane and buta-l,4-diyl dianions [27] and their dilithio salts [28]. For the four electron systems butadiene is more stable than trimethylenemethane. Experimental examination of the relative stabihties of two electron systems using the trimethylenemethane and buta-14-diyl dications needs to overcome the intrinsic instabihties of dications dissatisfying the octet rule. 

Cross-conjugated dienyltricarbonyliron cations have been prepared as follows from trimethylenemethane complexes [Eq. (22)] (186,187) ... 

Both the thermally and the photochemically initiated extrusion of nitrogen from bicyclic diazene (1) leads to the formation of a 1,3-diradical (diyl) which is structurally related to trimethylenemethane. As shown in the cascade mechanism illustrated, two forms of the diyl can be intercepted by alkenes bearing electron-withdrawing groups (diylophiles), viz. the open form of the singlet diyl and the triplet diyl which is formed via intersystem crossing from the singlet (Scheme 1). ... 

Another C HgLij isomer is the dilithium salt of the cross-conjugated dianion of isobutylen the trimethylenemethane dianion 37 (Ds symmetry). This 67c... 

Gund, P. (1972) Guanidine, trimethylenemethane and y-delocalization, Can arylic compounds have aromatic stability. Journal of Chemical Education, 49, 100-103 Klein, J. (1983) Directive effects in allylic and benzylic polymetalations the question of u-stabilization, y-aromaticity and cross-conjugation. Tetrahedron, 39, 2733-2759. 

Trimethylenemethane (1) is a cross-conjugated it system with a diradical ground state. Its chemistry has been reviewed as early as 1972 by Dowd [8]. The Hiickel molecular orbital (MO) scheme resembles that of cyclobutadiene, and accordingly trimethylenemethane can be stabilized as a ligand in transition metal complexes. Trimethylenemethane complexes can generally be obtained by treatment of a trimethylenemethane dianion with a metal halide or by treatment of organic halides with low-valent metal complexes. In addition to these general methods, there are some more specialized procedures. 

The molecules examined so far are conjugated in a linear sense, that is, double bonds are added at the ends of a polyene chain or the chains are tied together so that resonance structures can be drawn that propagate along the chain. Suppose, however, that a polyene is joined to the middle of an existing chain. These polyenes are said to be cross-conjugated and can have unusual properties because of their orbital topology. The simplest example is trimethylenemethane, 12.18. The neutral molecule has four it electrons and a resonance structure cannot be written... 

---