Cumulenes carbene complexes

Cumulene-type carbene complexes of M(CO)s have been studied, including 149 and 150 (Figure 13). In fact, their negative solvatochromism suggests that their structures are better represented by zwitterionic cr-alkyne resonance structures (Figure 13). As with the carbenes 142-148, Cr and W species show similar non-linearities for example, 149 and 150 show /3 (/3o) values of 100 (31) x 10 °esu and 102 (31) x 10 °esu, respectively (HRS, 1,064 nm). 

The basic NRMS method and the auxiliary techniques outlined have enabled the discovery and characterization of a large variety of reactive neutrals that had eluded experimental studies due to their unusual structures and reactivities. The species studied so far are summarized in Table 1 and include radicals, diradicals, carbenes, nitrenes, cumulenes, ylides, hypervalent species, and weak intermolecular complexes. 

Turning to carbene related reactive species, alkylidene carbenoids like 26 (X = halogen, OR, NR2) are particularly valuable for preparative purposes since they can undergo cycloaddition reactions with olefins (to methylenecyclopropanes), isomerizations (to alkynes by the so-called Fritsch-Buttenberg-Wiechell rearrangement), and dimerization (to [3]cumulenes). Although carbenoids have been studied extensively by NMR spectroscopy [23], the first X-ray structural analysis of a stable carbenoid, 27, as a TMEDA 2THF complex has been reported only recently [24]. 

T -Complexes contain a metal-carbon single bond (Figure 1.1). The organic group may be alkyl, vinyl, alkynyl, aryl or acyl. With the exception of the acyl complexes, there are analogous compounds of more familiar metals, such as magnesium and zinc. It is also possible to have complexes with metal-carbon double and triple bonds these are known as carbenes and carbines. Cumulenes are also known, such as in vinylidene complexes. 

Various alkyl- and/or aryl-substituted [3]radialenes couldbe prepared from 1,1-dihaloalkenes via organometallic intermediates. These transformations, hov rever, often give the radialene in low yield due to competing reaction pathways. The first radialene, hexamethyl[3]radialene (35), was obtained from dibromoalkene 32 and -butyl lithium in very low yield, the lithium carbenoid 33 and the [3]cumulene 34 being the likely reaction intermediates (Scheme 4.8) [32]. Preferential cyclo-propanation of the outer double bonds of the [3]cumulene by the carbenoid (or carbene) is a major factor for the complexity of the product mixture. 

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