Carbenes, generation isomerization

The related [1,2]-H shift in the isomeric triplet 1-phenylethylidene (53) was also investigated. The triplet carbene, generated from irradiation of the corresponding diazo compound, was characterized in low temperature inert matrices by EPR, IR, and UV/VIS spectroscopy. In this case, the carbene was stable in Ar up to the temperature limits of the matrix (36 K). Irradiation, however, readily converted the carbene to styrene. 

Butenyl)-, (2,3-dimethyl-3-butenyl)- and (4-pentenyl)-dimethylsilyl)]carbene have been generated by treatment of the corresponding chloromethylsilanes with sodium. Intramolecular [1 + 2] cycloaddition of the carbenic carbon atom to the double bond leads to l-silabicyclo[3.1.0]hexanes and l-silabicyclo[4.1.0]heptanes, respectively, usually in competition with intramolecular C,H insertion (equation 24)56. In contrast, no carbene-derived product could be obtained from (allyldimethylsilyl)carbene. Finally, reaction of chloromethyldimethylvinylsilane with sodium provided, besides the typical products of a Wurtz reaction (103 and 104), a small amount of cyclopropane 106 (equation 26)56. It has been suggested that (dimethylvinylsilyl)carbene (102) isomerizes to silabicyclo[1.1.0]butane 105 by intramolecular cyclopropanation, and nucleophilic ringopening finally leads to 106. 

It was reported that trimethylsilyldiazomethane 492 thermolytically (440°C, 10 torr) (75TL2061) or under FVP conditions (750°C) (86JA7849) produces a carbene that isomerizes to 1,1,2-trimethylsilene 493, thereby furnishing the head-to-tail dimer 494 in 40% yield. Silene generated from disilanyl a-diazoacetate 495 in the presence of 7-norbornanone affords 38% of adduct 496 (82JA6830). 

Whereas the position of the valence equilibrium is of course not affected by the method of carbene generation, a specific disadvantage of the thermal procedure is the possibility of thermally induced structural isomerization of the diazocarbonyl compound itself or of the initially formed norcaradiene a case in point was found when 5-diazouracil or 5-diazo-3-methyluracil were thermally decomposed in benzene. ... 

Addition of chloro(phenyl)carbene, generated from diazirine, to alkenes is stereospecific, yet diethyl (Z)-but-2-enedioate isomerized partially to give a mixture of stereoisomeric cyclopropanes(for a more detailed discussion see Houben-Weyl, Vol. E19b, p995). 

Addition versus rearrangement of chloro(methyl)-, chloro(ethyl)-, benzyl(chloro)-, chlo-ro(methoxymethyl)- and chloro(isopropyl)carbene generated by photolytic or thermal decomposition of 3-alkyl-3-chloro-3//-diazirines has also been investigated.81 However, it is known that chloro(isopropyl)- or ie/7-butylchlorocarbene does not undergo addition to 2,3-dimethyl-but-2-ene, which is used as a carbene trap. Photolysis or thermolysis of 3-arylmethyl-3-chloro-3//-diazirines in the presence of an alkene has also been studied extensively.84 Addition of the ambiphilic benzylchlorocarbene82 to an alkene to give 1-benzyl-1-chlorocyclopropanes is usually accompanied by its rearrangement to co-chlorostyrene (1,2-hydride shift, for details see ref 83). This isomerization was seriously impeded when the decomposition of 3-benzyl-3-chloro-3//-diazirines was performed in the presence of an alkene,85 with 2,3-dimethylbut-2-ene only traces of co-chlorostyrene were observed.81,86 The results of reactions of benzylchlorocar-benes with some alkenes are collected in Table 6.85... 

The carbenes generated from thermal decomposition of allyloxy-silyldiazoacetates at temperatures >140°C provided 2,5-dihydro-1,2-oxasiloles 66 and 67 as major products (eq 15). The authors propose that dihydro-oxasiloles 66 and 67 are generated from bicyclic pyrazolines upon thermal extrusion of N2 followed by isomerization of brradicals (eq 16). 

Nevertheless, a more traditional approach to the stabilization of carbenes and the investigation of their spectral properties deals with the direct generation of carbenes in low-temperature matrices, e.g. by the photolysis of diazo-compounds or ketenes. The method allows stabilization of carbenes in their ground electronic state, prevents intramolecular isomerization and also facilitates direct spectroscopic monitoring of their chemical transformations in low-temperature matrices. 

Cyclopropylchlorocarbene [20] has been generated by UV photolysis (A = 335 nm) of cyclopropylchlorodiazirine [21] frozen in a nitrogen matrix at 12 K (Ho et al., 1989). IR and UV spectra of [20] have been recorded. The reaction of [20] with HCl resulted in the formation of (dichloromethyl)-cyclopropane [22], and annealing of the matrix gave (dicyclopropyl)dichloro-ethene [23]. Subsequent irradiation (A = 450 nm) of the carbene [20] led to its isomerization to 1-chlorocyclobutene [24], which was partialy destroyed to give ethene and chloroacetylene. Ab initio calculations predict the existence of two carbene conformers, but attempts to distinguish them in IR or UV spectra were unsuccessful. 

There are two important rhodium-catalyzed transformations that are broadly used in domino processes as the primary step. The first route is the formation of keto carbenoids by treatment of diazo keto compounds with Rh11 salts. This is then followed by the generation of a 1,3-dipole by an intramolecular cyclization of the keto carbenoid onto an oxygen atom of a neighboring keto group and an inter- or intramolecular 1,3-dipolar cycloaddition. A noteworthy point here is that the insertion can also take place onto carbonyl groups of aldehydes, esters, and amides. Moreover, cycloadditions of Rh-carbenes and ring chain isomerizations will also be discussed in this section. 

This chapter will cover only reactions in which the isomerization to the allene starts from a stable molecule and not from a reactive intermediate generated in situ by reactions which are not isomerizations, such as the Doering-Moore-Skattebol reaction or free carbenes. Metallotropic rearrangements also will not be covered many of these reactions can be found in Chapter 9. Furthermore, the allene should be the final product of the reaction and not only a transient species leading to other products (see, for example, Chapters 6 and 20). 

The proposed catalytic cycle for this reaction begins with the initial attack of the in situ generated thiazolylidene carbene on the epoxyaldehyde followed by intramolecular proton transfer (Scheme 28, XXXII-XXXIII). Isomerization occurs to open the epoxide forming XXXIV which undergoes a second proton transfer forming XXXV. Diastereoselective protonation provides activated carboxylate intermediate XXXVI. Nucleophilic attack of the activated carboxylate regenerates the catalyst and provides the desired P-hydroxy ester. 

The same reaction (RCM) was used as the key step for the formation of a family of potent herbicidal 10-membered lactones. An important aspect from the preparative point of view is the control of stereochemical outcome of the RCM by the choice of catalyst. Thus, the use of the ruthenium indenylidene complex IX always leads to the corresponding ( )-alkenes, whereas the second generation of Grubbs catalyst bearing a N-heterocyclic carbene ligand affords the isomeric (Z)-olefin with good selectivity (Scheme 8.19) [64]. 

Trifluoromethyl)-3//-diazirin-3-yl]arenes, e.g. 36 (R1 = CF3), were introduced as photophores by Brunner et alJ116 to provide a more favorable carbene than the one produced from diazoesters[117 or from aryldiazirines. 118 Both 3//-diazirin-3-ylarenes I8-1201 and the [(3-tri-fluoromethyl)-3//-diazirin-3-yl]arenes (e.g., 36, R = H, CF3, respectively) 116 are more stable in acid than most diazoesters, and photolyze at around 360 nm to generate highly reactive carbenes such as 37. However, these diazirines can photo-isomerize to some extent to the... 

The dihydrothiazol-2-ylidene (4) was generated by photolysis of matrix-isolated thiazol-2-carboxylic acid.12 Calculations suggested that the barrier to isomerization to thiazole is about 42.3 kcal mol 1 and that the carbene resembles the related imidazol-2-ylidene in structure. An ab initio study of hydroxyoxiranone predicted that the decarboxylation of the zwitterion (5) to form hydroxycarbene (6) would be favourable in vacuo but not in water.13 A theoretical study showed that dihalosulfenes (X2C=S02) are best viewed as dihalocarbenc-SO complexes with a carbon-sulfur bond order of approximately zero.14 hi a study directed at the elusive thionformic acid (7), tandem mass spectrometric methods were applied to isomeric ethyl thioformates.15 The results suggest that the radical cations generated have the carbene structure [(HS)C(OH)]+ ... 

In this context, it is worth mentioning that there is only one other, clear-cut example for the simultaneous occurrence of the acyl(silyl)carbene-to-acylsilene and the acylcarbene-to-silylketene rearrangement of an acylcarbene bearing a Si—Si substituent. Carbene 57, generated by photolysis of diazoketone 56 in benzene, isomerized to both 58 and 59 in about equal amounts44. While the acylsilene cyclized to 1,2-silaoxetene 60, the ketene was isolated and structurally characterized by X-ray diffraction analysis of the derived... 

However, the Rh-catalysed hydrosilylation of terminal alkynes affords the thermodynamically unfavorable anti product 583 as the main product [224], The symanti ratio changes depending on the catalysts and solvents. The syn adduct 584 is obtained by a cationic Rh complex in MeCN [225]. The Ru catalyst gives the anti adduct. Formation of the anti adducts is explained by the following mechanism [224, 226]. Insertion of alkyne to the R3S-RI1 bond generates 585 which, due to steric repulsion, isomerizes to 588 via the carbene species 586, or the metallacyclopropene 587, and gives the anti adduct 583. 

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