Carbenoid compounds

The Simmons-Smith Reagent and Related Carbenoid Compounds 337... 

The aryl sulfoxide moiety may serve as a good leaving group in the exchange reaction. Thus, 1-haloalkenyl sulfoxide 55 undergo the exchange at —78°C to give carbenoid compounds 56 which can be trapped by electrophiles or converted to acetylenes 57 (equation 39) °. Reaction of carbenoid 58 with lithium acetylides leads to the formation of enynes 59 (equation 40). ... 

FIGURE 69. Solid state structure of carbenoid compound 132... 

As mentioned earlier, the carbene ligand in our complexes shows nucleophilic character with respect to the metal fragment. Therefore, we decided to combine it with an electrophilic carbene. For this purpose we treated pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with phenyl(tri-chloromethyl) mercury (85). Compounds of this kind have been studied intensively by Seyferth et al. (86) and are known as a source of dihalogeno-carbenes. The carbene complex reacted with the carbenoid compound at... 

Using either the carbenoid compound 6 or the carbanionic compound 7 as a bulk base in large excess and 5 as catalyst (Scheme 11) gave the same stereoselectivity as the stoichiometric deprotonation of 2 with 5. 

Carbenoid species, such as the Simmons-Smith reagent (Eq. 10.55), undergo facile additions to alkenes to create cyclopropanes. The reactions are stereospecific, making carbenoids synthetically useful versions of carbenes. Carbenoid compounds do not normally perform insertion reactions. 

Treatment of geminal dihalocyclopropyl compounds with a strong base such as butyl lithium has been for several years the most versatile method for cumulenes. The dihalo compounds are easily obtained by addition of dihalocarbenes to double--bond systems If the dihalocyclopropanes are reacted at low temperatures with alkyllithium, a cyclopropane carbenoid is formed, which in general decomposes above -40 to -50°C to afford the cumulene. Although at present a number of alternative methods are available , the above-mentioned synthesis is the only suitable one for cyclic cumulenes [e.g. 1,2-cyclononadiene and 1,2,3-cyclodecatriene] and substituted non-cyclic cumulenes [e.g. (CH3)2C=C=C=C(CH3)2]. 

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

There are several examples of intramolecular reactions of monocyclic /3-lactams with carbenes or carbenoids most of these involve formation of olivanic acid or clavulanic acid derivatives. Thus treatment of the diazo compound (106) with rhodium(II) acetate in benzene under reflux gives (107), an intermediate in the synthesis of thienamycin (80H(14)1305, 80TL2783). 

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

Tlie desulfurization of thiono compounds is another frequently used synthetic approach for the formation of double bonds via carbenoid intermediates. By this methodology, some indigoid 1,3,5,7-tetraazafulvalenes 88 and 90 were synthesized (83BSB781 90JPR949).Tliis dimerization starting from 2,4,5-tris(dimethylamino)imidazolium chloride via the appropriate thione 87 has been realized in the presence of phosphanes or phosphites to... 

From a historical perspective it is interesting to note that the Nozaki experiment was, in fact, a mechanistic probe to establish the intermediacy of a copper carbe-noid complex rather than an attempt to make enantiopure compounds for synthetic purposes. To achieve synthetically useful selectivities would require an extensive exploration of metals, ligands and reaction conditions along with a deeper understanding of the reaction mechanism. Modern methods for asymmetric cyclopropanation now encompass the use of countless metal complexes [2], but for the most part, the importance of diazoacetates as the carbenoid precursors still dominates the design of new catalytic systems. Highly effective catalysts developed in... 

Whereas the utility of these methods has been amply documented, they are limited in the structures they can provide because of their dependence on the diazoacetate functionality and its unique chemical properties. Transfer of a simple, unsubstituted methylene would allow access to a more general subset of chiral cyclopropanes. However, attempts to utilize simple diazo compounds, such as diazomethane, have never approached the high selectivities observed with the related diazoacetates (Scheme 3.2) [4]. Traditional strategies involving rhodium [3a,c], copper [ 3b, 5] and palladium have yet to provide a solution to this synthetic problem. The most promising results to date involve the use of zinc carbenoids albeit with selectivities less than those obtained using the diazoacetates. 

The study of the Schlenk equilibrium for organozinc compounds represents a major chapter in the understanding of these reagents in general [26]. Before elaborating the studies on zinc carbenoids, it is appropriate to briefly review the definitive investigations on organozinc halides themselves. 

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). ° ... 

Muller et al. have also examined the enantioselectivity and the stereochemical course of copper-catalyzed intramolecular CH insertions of phenyl-iodonium ylides [34]. The decomposition of diazo compounds in the presence of transition metals leads to typical reactions for metal-carbenoid intermediates, such as cyclopropanations, insertions into X - H bonds, and formation of ylides with heteroatoms that have available lone pairs. Since diazo compounds are potentially explosive, toxic, and carcinogenic, the number of industrial applications is limited. Phenyliodonium ylides are potential substitutes for diazo compounds in metal-carbenoid reactions. Their photochemical, thermal, and transition-metal-catalyzed decompositions exhibit some similarities to those of diazo compounds. 

Esters of a-diazoalkylphosphonic acids (95) show considerable thermal stability but react with acids, dienophiles, and triphenylphosphine to give the expected products. With olefinic compounds in the presence of copper they give cyclopropane derivatives (96), but with no such compounds present vinylphosphonic esters are formed by 1,2-hydrogen shift, or, when this route is not available, products such as (97) or (98) are formed, resulting from insertion of a carbenoid intermediate into C—C or C—H bonds. The related phosphonyl (and phosphoryl) azides (99) add to electron-rich alkynes to give 1,2,3-triazoles, from which the phosphoryl group is readily removed by hydrolysis. 

Scheme 6.1 Catalytic cycle of metal-catalysed carbenoid cyclopropanation reactions with diazo compounds.

When a reaction appears to involve a species that reacts as expected for a carbene but must still be at least partially bound to other atoms, the term carbenoid is used. Some carbenelike processes involve transition metal ions. In many of these reactions, the divalent carbene is bound to the metal. Some compounds of this type are stable, whereas others exist only as transient intermediates. In most cases, the reaction involves the metal-bound carbene, rather than a free carbene. 

The reactive intermediates under some conditions may be the carbenoid a-haloalkyllithium compounds or carbene-lithium halide complexes.158 In the case of the trichloromethyllithium to dichlorocarbene conversion, the equilibrium lies heavily to the side of trichloromethyllithium at — 100°C.159 The addition reaction with alkenes seems to involve dichlorocarbene, however, since the pattern of reactivity toward different alkenes is identical to that observed for the free carbene in the gas phase.160... 

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