Other Carbenes

The success of phase transfer cyclopropanations with dichlorocarbene naturally stimulated a search for related examples. Numerous trihalomethanes besides chloroform were subjected to phase transfer conditions in the expectation that a wide variety of dihalocyclopropanes would result. Notable among these attempts is the phase transfer generation of dibromocarbene which is discussed in Sects. 4.2—4.5. 

In addition to the miscellany of dihalocarbenes, the principal successes have been in the formation of sulfur containing (Sect. 4.6) and unsaturated carbenes (Sect. 4.7). 

The mechanism of carbene formation has been discussed in Sect. 2.2 and phase transfer catalysis has been discussed in general terms in Sects. 1.4 and 1.7-1.9. The principles presented at earlier stages in this book generally apply to the carbenes discussed in this chapter, so detailed mechanistic discussions will be limited to those cases which are either peculiar or non-obvious. 

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Contents Introduction and Principles. - The Reaction of Dichlorocarbene With Olefins. - Reactions of Dichlorocarbene With Non-Olefinic Substrates. -Dibromocarbene and Other Carbenes. - Synthesis of Ethers. - Synthesis of Esters. - Reactions of Cyanide Ion. - Reactions of Superoxide Ions. - Reactions of Other Nucleophiles. - Alkylation Reactions. - Oxidation Reactions. - Reduction Techniques. - Preparation and Reactions of Sulfur Containing Substrates. -Ylids. - Altered Reactivity. - Addendum Recent Developments in Phase Transfer Catalysis. 

Diazoester aziridinations may be carried out in ionic liquids [39]. Other carbene equivalents have been investigated in aziridination reactions, though not to the same extent as diazocarbonyl compounds. Dibromo(tert-butyldimethylsilyl)me-thyllithium, for example, aziridinates N-arylimines to give l-bromo-2-aryl-3-silyla-ziridines these compounds function as useful synthetic intermediates, reacting... 

From Other Carbene Complexes by Exchange of the Carbene Ligand. . 234... 

As to other carbenes, some react as triplets, some as singlets, and others as singlets or triplets, depending on how they are generated. There are, however, molecules that generate persistent triplet carbenes. 

Two other carbenes, cyclopropenylidene(37) and cycloheptatrienyli-dene,<28> which have aromatic resonance hydrids (15) and (16), have been studied ... 

The Os—CcarbPh angle of 139(1)° in 59 is substantially increased from the angle of 120° expected for an sp2 hybridized carbon atom. This distortion is probably a consequence of both steric interactions between the phenyl ring and other equatorial ligands and the minimal steric demands of the proton (the other carbene substituent). 

The pyridine ylide method also allows determination of the rate constants for the intermolecular reactions of carbenes with alkenes, alcohols, or other carbene... 

IV. Other Carbene Reactions Found in the Study of Insertion Reactions. . . 309... 

IV. OTHER CARBENE REACTIONS FOUND IN THE STUDY OF INSERTION REACTIONS... 

BA in neat cyclohexane at room temperature is 75 ns. Rate constants for reaction of cyclohexane with 3BA and other carbenes are reported in Table 5. 

AGst for BA, XA and DMFL are limits based on unobserved reactions for the other carbenes AGST is calculated by assuming a diffusion controlled reaction rate for the singlet carbene with methyl alcohol. All kinetic parameters refer to room temperature k Sitzmann and Eisenthal, 1983... 

Finally, recall that AGST is ca lOkcalmoP1 for methylene. The corresponding value is smaller for BA. This can be viewed as the major consequence of the structural change. It will be a point of special interest to show how this parameter is affected in the other carbenes that are discussed. 

It appears that the reactivity (as well as the stability) of phosphinocar-benes (R2P-C-X) is strongly dependent on the nature of the other carbene substituent (X), and therefore this section will be subdivided with respect... 

The most common method for the generation of the metal alkylidene species seems to be a-elimination from an intermediate dialkyl-metal species. This procedure gives the most active catalysts. Above we mentioned the addition of other carbene precursors, which leads to active catalysts. Other methods to generate the metal alkylidene species involve alkylidene transfer from phosphoranes [16] or ring-opening of cyclopropenes [17], In Chapter 16.4 we will describe a few compounds that are active by themselves as metathesis catalysts. 

Carbenes with two orbitals, occupied by one electron each, have a total spin number of s = 1 the multiplicity is therefore 2s -f-1 =3, i.e., a triplet state. This is the usual ground state of a carbene. If however, the two orbitals are not degenerate, the multiplicity can become 1, i.e., a singlet state is possible. Methylene — and other carbenes — can, in principle, be linear or bent. The binding is achieved by using the 15 hydrogen Is and the 2s and 2p carbon orbitals. 

Fig. 2. Energies of various electronic states of CH2 Generalization from Methylene to Other Carbenes...

It is seen from Table 7 that all these carbenes in solution react nonstereospeci-fically, indicating triplet character. For some carbenes such as CF3CH, PhCH, (CH3)sSiC(C02C2H5), 2,6-di-t-butyl-carbena-cyclohexadienone and 4,4-dimethyl-carbena-cyclohexadienone, the degree of non-stereospecifity is quite small. Other carbenes usually contain 7t-bonds, aromatic rings or heavy atoms as functional groups. 

In Figure 2.2 the most important synthetic approaches to alkoxy- or (acy-loxy)carbene complexes from non-carbene precursors are sketched. Some of these strategies can also be used to prepare amino- and thiocarbene complexes. These procedures will be discussed in detail in the following sections. In addition to the methods sketched in Figure 2.2, many complexes of this type have been prepared by chemical transformation of other heteroatom-substituted carbene complexes. Because of the high stability of most of these compounds, many different reactions can be used to modify the substituents at C without degrading the carbon-metal double bond. The generation of heteroatom-substituted carbene complexes from other carbene complexes will be discussed in Section 2.2. 

The transition metal-catalyzed cyclopropanation of alkenes is one of the most efficient methods for the preparation of cyclopropanes. In 1959 Dull and Abend reported [617] their finding that treatment of ketene diethylacetal with diazomethane in the presence of catalytic amounts of copper(I) bromide leads to the formation of cyclopropanone diethylacetal. The same year Wittig described the cyclopropanation of cyclohexene with diazomethane and zinc(II) iodide [494]. Since then many variations and improvements of this reaction have been reported. Today a large number of transition metal complexes are known which react with diazoalkanes or other carbene precursors to yield intermediates capable of cyclopropanating olefins (Figure 3.32). However, from the commonly used catalysts of this type (rhodium(II) or palladium(II) carboxylates, copper salts) no carbene complexes have yet been identified spectroscopically. 

Other carbene iridium complexes (Cl, C2) were also applied as catalysts of this reaction, but their catalyhc achvity was very low (only 10% yields of I for Cl and 58% for C2 were obtained) [48]. 

The stable carbene 1,3,4-triphenyl-4,5-dihydro-lH-1,2,4-triazol-5-ylidene (143) reacts with bicyclopropylidene (1) to yield the four unexpected products 144-147, none of which resembles the typical mode of [2 +1] cycloaddition observed for reactions of 1 with other carbenes (Scheme 32) [126]. 

Transport polymerization has also been studied with other monomers, including methylene and other carbenes (phenylcarbene, 1,4-phenylenecarbene), silylenes such as Si2 and germy-lenes such as GeCl2 and Ge2 [Lee, 1977-1978 lee and Wunderlich, 1978]. 

Alkoxy carbene complexes are useful starting compounds for other organometallic complexes,1 5 particularly methylidyne (carbyne) complexes.16 By modification of the coordinated (noncarbene) ligands or of the carbene ligand, other carbene complexes can be synthesized. The use of carbene complexes in organic syntheses has been reviewed recently.17 18... 

Complexes were also considered for other carbenes and even suggested for reactions in alkane solution( ) A counterproposal, backed by the inability of theory to find support for such stable complexes, held that the second source was not the carbene-alkene complex but instead was the diazo compound, formed from isomerization of excited diazirine. " Other LFP studies reinforced the need for two intermediates, but could not finally resolve the question of carbene complexes ver-sus diazo compound. However, the question is now settled in this case,... 

The rate constants observed for hydrogen atom abstraction by several other carbenes are summarized in Table 9.9. " The effect of substituents on hydrogen... 

Fluorenylidene and its substituted derivatives are typical examples of the other class. Other carbenes showing similar behavior are monoarylcarbenes such as mononaphthylcarbenes (12) and phenyl(methoxycarbonyl)carbene (53, AGst = 0.2 kcal/mol). 

The reaction patterns of arylcarbenes with solidified alcohol at 77 K are also completely different from those observed in alcohol solution. For example, generation of phenylcarbene (le) in methanol matrices at 77 K results in the formation of alcohol (63) at the expense of benzyl methyl ether (62), which is the exclusive product in the reaction in alcoholic solution at ambient temperatures (Scheme 9.14). A similar dramatic increase in the CH insertion products is observed in the reaction involving other carbenes with alcohols. ... 

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