Unstable carbene

Carbenes have divalent carbon with a lone pair and hence only six electrons in the outer shell of the carbon atom. They are normally electrophilic and can form two bonds at once with a re-system.7 One way to make carbenes is by loss of nitrogen from diazocompounds such as diazoketones 33. The formation of very stable nitrogen is initiated by heat or light and compensates for the formation of the unstable carbene 30. Diazoketones are easily made by acylation of diazomethane with an acid chloride 31. Loss of a very acidic proton from the diazonium salt 32 gives 33. Normally the diazoketone and the alkene are combined and treated with heat or light.8... 

You will be familiar with -elimination that leads to alkenes (chapter 15) but a-elimination gives carbenes. The simplest example is dichlorocarbene 44 made by treatment of chloroform 42 with base. The carbanion 43 decomposes by loss of chloride ion to give the neutral but unstable carbene 44 that should be released in the presence of the alkene. So cyclohexene 45 gives the dichloro-cyclopropane 46 using NaOH as the base and an ammonium salt as a phase-transfer catalyst.10... 

Productive cleavage, Eq. (17 b), gives the unstable carbene structure 8 that is trapped by excess norbornene to form a,p,a -trisubstituted titanacyclobutane structure 9. a,b,a -Trisubstituted titanacycle 9 cleaves exclusively in a productive fashion, Eq. (18). The zero-order dependence indicates that the ring-opening of the titanacycle is the rate determining step, similar to the reaction of titanacycle 5 with diphenylacetylene, Eq. (15). 

Diazocarbonyl compounds can be decomposed to carbenes by heat or light. The formation of very stable gaseous nitrogen compensates for the formation of the unstable carbene. 

Thermal or photochemical decomposition of diazo compounds and diazirines gives carbenes. The formation of very stable gaseous nitrogen compensates for the formation of the unstable carbene. Photolysis or thermolysis of a ketene-Hke diazo compound eliminates a stable molecule carbon monoxide (CO) to yield a carbene. The reactions are not widely used since ketenes are not readily available precursors, and tend to polymerize under the reaction conditions (Scheme 5.5). 

An attempt to prepare a phenylmethylcarbene complex led to products attributable to an unstable carbene complex (L. D. Albin and C. P. Casey, unpublished results, 1975). 

Abstract In the examples of the irreversible reactions, the original molecule is broken down on exposure to UV light and releases a part of the molecule to become a new prodnct. The reverse reaction cannot occur. The triplet carbenes are formed from the bis(2,4,6-triphenyl)diazomethane derivatives and the arylnitrenes are produced from the arylazide derivatives on exposure to UV hght The unstable carbenes and nitrenes were observed after the crystal was irradiated with UV light at 80 K. Moreover, the further reaction processes were observed from the intermediate nitrenes to the final products, azobenzene derivatives or five-membered ring derivatives. The acid-base complex formation between arylazides and amine derivatives is very effective to observe the reaction processes of nitrenes. 

The products of a eliminations are unstable divalent carbon species called carbenes. They will be discussed in Chapter 10 of Part B. In this chapter, attention will be focused on fi-elimination reactions. Some representative examples of -elimination reactions are given in Scheme 6.1. 

Various carbene-transfer reactions can be used with both electron-rich and electron-poor alkynes to make fluorinated cyclopropenes [9. 13, 79, 80, 81, 82] (Table 4). Haloacetylenes are too thermally unstable for most cycloaddition conditions, and simple fluorinated cyclopropenes are made by other methods [32, 45, 83, 84] (equations 30-32). 

The reactions of carbenes, which are apparently unique in displaying electrophilic character in strongly basic solutions, include substitution, addition to multiple bonds, and co-ordination with lone pairs of electrons to form unstable ylides. This last reaction is of obvious relevance to a consideration of the reactions of heterocyclic compounds with carbenes and will be summarized. 

NITRILE OXIDES. Nitrile oxides are a well known class of compds represented by R.C N- 0, and are usually prepd by treating hydroxamic acid chlorides with a mild alkali, thus eliminating HQ (Ref 2). Wieland (Refs 1 3) was responsible for the first isolation of free nitrile oxides. These compds are somewhat unstable, showing a marked tendency to dimerize to (he corresponding furoxanes (1,3-dipolar addition) (Refs 2 3). The nitrile oxides add to a considerable number of carbenes, as benzonitrUe oxide (for example) to a large number of olefins in ether at 20° (Ref 3)... 

The structurally simplest silicon reagent that has been used to reduce sulphoxides is the carbene analog, dimethylsilylene (Me2Si )29. This molecule was used as a mechanistic probe and did not appear to be useful synthetically. Other silanes that have been used to reduce sulphoxides include iodotrimethylsilane, which is selective but unstable, and chlorotrimethylsilane in the presence of sodium iodide, which is easy to use, but is unselective since it cleaves esters, lactones and ethers it also converts alcohols into iodides. To circumvent these complications, Olah30 has developed the use of methyltrichlorosilane, again in the presence of sodium iodide, in dry acetonitrile (equation 8). A standard range of sulphoxides was reduced under mild conditions, with yields between 80 and 95% and with a simple workup process. The mechanism for the reaction is probably very similar to that given in equation (6), if the tricoordinate boron atoms in this reaction scheme are replaced... 

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

An interesting carbene, 1-oxobutatrienylidene [25], having cumulated double bonds, has been found by IR spectroscopy in the photolysis (A>230nm) products of matrix-isolated l,2,3,4-pentatetraene-l,5-dione [26] (Maier et al., 1988) (in its turn the unstable dione [26] was generated by thermo- or photo-destruction of compound [27]). The second product was carbon monoxide. The linear structure of the carbene [25] has been suggested on the basis of two intense IR bands at 2222 cm and 1923 cm indicating respectively ketene and allene fragments. 

In spite of all efforts, only very few (fluoromethyl)silicon compounds (CH3.nFn)Si= have been described up to now. In contrast to their CH3Sis analogues they are both difficult to obtain, and are very unstable because of ease of carbene elimination, with concomitant formation of SiF bonds. 

A recent reinvestigation of this reaction has substantiated the intermediacy of the 5-coordinate methylene complex 51. Thermally unstable 51 is formed when a THF solution of 46 is treated with CH2N2 at -60°C (56). The iodo analog Ir(=CH2)I(CO)(PPh3)2 has been similarly prepared and is somewhat more stable than 51, with iodide migrating less readily to the carbene carbon than chloride (56). 

Most carbenes are highly unstable compounds that are capable of only fleeting existence. 

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