Organometallic compounds carbenoids

Certain organometallic compounds resemble carbenes m their reactions and are referred to as carbenoids lodomethyizmc iodide (Section 14 12) IS an example... 

Certain organometallic compounds resemble caibenes in their reactions and aie refened to as carbenoids. lodomethylzinc iodide (Section 14.12) is an exfflnple. 

Because most carbenes are so reactive, it is often difficult to prove that they are actually present in a given reaction. The lifetime of formylcarbene was measured by transient absorption and transient grating spectroscopy to be 0.15-0.73 ns in dichloromethane. In many instances where a carbene is apparently produced by an a elimination or by disintegration of a double-bond compound, there is evidence that no free carbene is actually involved. The neutral term carbenoid is used where it is known that a free carbene is not present or in cases where there is doubt. a-Halo organometallic compounds (R2CXM) are often called carbenoids because they readily give a elimination reactions (e.g., see 12-37). ° ... 

Heterocycles treated in this section belong to several structure types. The metal atoms in compounds under discussion can exist not only in the tetravalent but also in the carbenoid divalent state and the heterocycles differ not only in the nature and number of metal atoms but also in the nature of ring bonds of the latter. Thus, besides conventional organometallic compounds having only M—C bonds there are heterocycles with M—M bonds and those with C—M—X or X—M—Y fragments (X and Y—common heteroatoms like O, S, N). The last type is the most numerous and important group of five-membered Ge-, Sn- and Pb-heterocycles in which the metals are involved in M—X and M— Y bonds similar to those in respective metal alkoxides, thiolates and amides. This feature not only affects the structural parameters of these compounds but determines their chemical properties, synthetic routes and applications. 

Carbenoids can be classified into two types (a) complexes of carbene with metals or metallic compounds and (b) organometallic compounds where halogen or another substituent of higher electronegativity is attached to the a-carbon. Carbenes have been shown to act as ligands in transition metal complexes of Fe, Rh, Ru, Co, W, Mo, Cr, Mn, Re, Pd, and Pt (68). Carbenoids of type (a) also include the intermediates in the... 

Carbenoid reactions of certain organometallic compounds may be classed as a-eliminations. The problems encountered in deducing the reaction mechanism are largely similar to those met with in interpreting the base-induced a-eliminations discussed in the previous section. Reactions which fall in this category are those of the a-haloalkyl derivatives of a number of metals and these can be readily prepared, often by treatment of a diazoalkane with a metal halide (for a review, see Seyferth, 1955). Such derivatives have been reported for mercury (Seyferth and Burlitch, 1964 Seyferth, et al., 1965a, b Ledwith and Phillips 1962,... 

Heterocyclic aromatic compounds such as pyrrole are readily metallated with Grignard reagents. The resulting compounds have N"Mg bonds and are, therefore, not organometallic compounds, but on reaction with electrophiles give 2-substituted pyrroles [14] (eq (4)). The reaction of chloroform or bromoform with PrMgCl at -78 °C in THF-HMPA (4 1) is mild and convenient method for the generation of an unstable carbenoid in the solution [15] (eq (5)). 

Carbenes and Carbenoids A common housefly (Musca domestica). Commercially available traps with the attractant muscalure (synthesized via a route using an organometallic compound) combined with a poison are efficient at luring and killing houseflies (Section 15.2B). 

The cyclopropanation of vinyl organometallic and heteroatom substituted vinylic compounds has also been reported using zinc carbenoids. Vinylboronates (equation 37) °, -alanes (equation 38) °, -zincs (equation 39) °, -stannanes (equation 40) ° , -phos-phonates (equation 41) °, -germanes (equation 42) °, and silanes (equation 43) °° "° could be readily converted into cyclopropane derivatives. 

The synthetic potential of silicon substituents in organic and organometallic chemistry has by far not been fully exploited, which is evidenced by the numerous contributions in this chapter. This is examplified for the description of the silyl group as a substituent and as a functional group in carbene and carbenoid chemistry, for the function of the trimethylsilyl substituent in the synthesis of low-valent compounds containing elements ofgroup 15 andfor the influence of a supersilyl ligand to a phosphorous center. 

Rhodium(II)-mediated reactions have found many applications. The literature up to 1985 in intramolecular and intermolecular cyclopropanations, including choice of catalysts and mechanistic aspects, has been thoroughly reviewed by Maas [9]. More recent reviews are available that focus on the ligand effects and mechanism [10], and on their utilization in fine organic synthesis as well as in natural product synthesis [11]. All of these reviews, and in particular McKervey s comprehensive review [11a] on organic synthesis with a-diazocarbonyl compounds, deal with the utilization of functionalized diazo compounds as carbenoid precursors. Herrmann et al. surveyed the organometallic chemistry of diazoalkanes [11c, lid]. 

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