Carbenes moieties

The fact that only ethylene and tetramethylethylene are evolved from exp-[8]rotane 168 and permethyl-exp-[6]rotane 173 upon thermal decomposition leads to the conclusion that the spirocyclopropane moieties in these expanded [n]rotanes fragment only externally and leave carbene moieties behind. Indeed, the MALDI-TOF mass spectra of several exp-[ ]rotanes show fragment ions with M minus 28. Thus, if this fragmentation in an exp-[n]rotane were to continue n times, a cyclic C carbon cluster would be left over. So far, however, a fragment ion with m/z = 480 corresponding to 182 has not been recorded in the mass spectrum of exp-[8]rotane 168 and it remains to be seen whether a Cgo cluster 183 will be detected in the mass spectrum of exp-[12]rotane 171 (Scheme 35). 

Further improvements in activity of the imidazol-2-ylidene Ru complexes might be attained by the incorporation of a better a-donor substituents with larger steric requirements. The ligands that most efficiently promote catalytic activity are those that stabilize the high-oxidation state (14 e") of the ruthenium metallacyclobutane intermediate [7]. Both ligand-to-metal a-donation and bulkiness of the NHC force the active orientation of the carbene moiety and thus contribute to the rapid transformation into metallacyclobutane species [7b]. Both can be realized by incorporation of alkyl groups in 3,4-position of imidazol-2-ylidene moiety, lyie Me. Me... 

It is assumed that first biradical 5-63 is formed on addition of 5-62 to photochemi-caUy excited 5-61. There follows a ring closure involving the triple bond to yield a-acyl-a, (5-uri saturated carbene 5-64. The carbene moiety then undergoes a final 6jt electrocyclic ring closure on the carbonyl group to give the desired furan 5-65. 

When the o-ethynylphenyl isopropyl ketone 6/4-113 was treated with 1 equiv. of W(CO)5-THF for 2 h in the presence of 4equiv. of 1,1-diethoxyethylene, the bridged compound 6/4-118 was obtained in 73% yield, and not the expected 3-ethoxy-l-isopropylnaphthalene (Scheme 6/4.28). Iwasawa and coworkers [309] proposed a mechanism for their new domino reaction, which includes a [3+2] cycloaddition followed by insertion of the resulting tungsten carbene moiety into the neighboring C-H bond, with 6/4-114-6/4-117 as possible intermediates. These authors have further shown that 10-20 mol% of W(CO) THF is sufficient, and... 

Diels-Alder additions to 17-19 parallel the known Diels-Alder additions to vinyl substituted Fischer carbene complexes (34). It appears that both M(CO)5 carbene moieties, where M is Cr or W, and (ferra-/3-diketonato)BF2 substituents can activate C=C double bonds toward Diels-Alder additions. 

Rh2(OAc)4 has become the catalyst of choice for insertion of carbene moieties into the N—H bond of (3-lactams. Two cases of intermolecular reaction have been reported. The carbene unit derived from alkyl aryldiazoacetates 322 seems to be inserted only into the ring N—H bond of 323 246). Similarly, N-malonyl- 3-lactams 327 are available from diazomalonic esters 325 and (3-lactams 326 297). If, however, the acetate function in 326 is replaced by an alkylthio or arylthio group, C/S insertion rather than N/H insertion takes place (see Sect. 7.2). Reaction of ethyl diazoacetoacetate 57b with 328 also yields an N/H insertion product (329) 298>, rather than ethyl l-aza-4-oxa-3-methyl-7-oxabicyclo[3.2.0]hex-2-ene-2-earboxylate, as had been claimed before 299). 

The reaction of carbenes or carbenoids with compounds containing S—S bonds is likely to begin with sulfonium ylide formation subsequent [1,2] rearrangement then produces a formal insertion product of the carbene moiety into the S—S bond152 b). 

The carbene moieties of methyl diazoacetate 353), dimethyl diazomalonate 3S3) and diazomethane 3541 have been inserted into the Se—Se bond of diaryl diselenides. 

It has been widely accepted that the carbene-transfer reaction using a diazo compound and a transition metal complex proceeds via the corresponding metal carbenoid species. Nishiyama et al. characterized spectroscopically the structure of the carbenoid intermediate that underwent the desired cyclopropanation with high enantio- and diastereoselectivity, derived from (91).254,255 They also isolated a stable dicarbonylcarbene complex and demonstrated by X-ray analysis that the carbene moiety of the complex was almost parallel in the Cl—Ru—Cl plane and perpendicular to the pybox plane (vide infra).255 These results suggest that the rate-determining step of metal-catalyzed cyclopropanation is not carbenoid formation, but the carbene-transfer reaction.254... 

In view of the versatility of A-heterocyclic carbenes as ligands and their structural diversity in silver(i) coordination chemistry, an extension of the work to ligands with two or more carbene moieties was reported. A dinuclear silver(i) complex 52 (Figure 21) with an o-phenylenedimethylene-bridged bis(carbene) ligand has been synthesized in 66% yield from silver(i) oxide and the bis(imidazolium) salt.88 The reaction to synthesize 52 has to be carried out in... 

The reactivity of dichloro carbene towards acetylenic bonds was systematically investigated by Dehmlow19, 20 with respect to substitution of the acetylene, especially those containing additional C-C multiple bonds. It was shown that with aiyl alkyl acetylenes, e.g. 1-phenyl-butyne-l, often the normal cyclopropenone formation occurs only to a minor extent (to yield, e.g. 14), whilst the main reaction consists of an insertion of a second carbene moiety into the original acetylene-alkyl bond (giving, e.g. 15) ... 

Various alkyl- and aryl-substituted [3]radialenes could be prepared from 1,1-dihaloal-kenes using organometallic pathways. Hexamethyl-[3]radialene (25), the first [3]radialene to be synthesized, was obtained in a very low yield by treatment of l,l-dibromo-2-methyl-1-propene (22) with butyllithium8,9. The lithium carbenoid 23 and the butatriene 24 are likely intermediates of this transformation (Scheme 2), the former being the source of an unsaturated carbene moiety which is transferred onto the latter. However, the outer double bonds of 24 are more readily cyclopropanated than the central one. 

In a formal sense, complexes 1 represent pre-catalysts that convert in the first turn of the catalytic cycle (vide infra) into ruthenium methylidene species of type 3 which are believed to be the actual propagating species in solution (Schemes 2,4). The ease of formation of 3 strongly depends on the electronic properties of the original carbene moiety in 1. In addition to complexes la-c with R1=CH=CPh2, ruthenium carbenes with Rx=aryl (e.g. Id, Scheme 3) constitute another class of excellent metathesis pre-catalysts, which afford the methylidene complex 3 after an even shorter induction period [5]. In contrast, any kind of electron-withdrawing (e.g. -COOR) or electron-donating substitu-... 

The axial alignment of Rh2(5R-MEPY)4 leads to probable structures for the carbene intermediate as shown in Figure 17.15. Approach of styrene will occur with the phenyl group pointing away from the rhodium complex, and also in a trans (anti) fashion with respect to the ester group of the carbene moiety. The 2-phenylcyclopropane-l-carboxylic ester resulting from this is indeed the 1R,2R (1R-trans) diastereomer. 

The particular behavior of these anions is difficult to explain. It could be related to the unusually short bond lengths in the carbene moiety . 

As expected, for the metalated carbon atom a trigonal planar coordination geometry is achieved. Both lateral carbene moieties which stabilize the carbon(O) atom are planar but are tilted relative to each other to relieve allylic strain [20, 21]. 

Another approach to substituted bicyclopropylidenes is by carbene addition onto butatrienes and alkenylidenecyclopropanes bringing the substituents in with the allene or with the carbene moiety as well as with both fragments. This way, Skattebol et al. [41] and later Kostikov et al. [42] have prepared tetrahalote-... 

Transfer of a metal carbene moiety from a metal carbene complex to a heteroatom other than oxygen, sulfur, and nitrogen is possible. One such example is the report by Uemura and co-workers, who disclosed catalytic asymmetric reaction of ethyl diazoacetate with ( )-cinnamyl phenyl selenide 193 (Equation (30))." The reaction afforded 194 as a diastereomeric mixture (58 42). Using Rh2(5[Pg.173]

Direct reaction of the bis(isonitrile)gold(I) cations with alcohols or amines also gave the biscarbene complexes [Eq. (38)] (132, 141, 142). With bulky C6HnNC, only one carbene moiety was formed (141, 142) under the conditions employed in the preparation of the other biscarbene compounds. 

Transition-metal catalyzed transfer of acylcarbenes to nitriles leads to 1,3-oxazoles via nitrile ylide intermediates123. The corresponding nitrile ylide chemistry derived from acyl(silyl)carbenes still awaits a closer look, but it has been shown that the rhodium-catalyzed decomposition of 198 in the presence of methyl cyanoformate and benzaldehyde provides 1,3-oxazole 221 (equation 71) exclusively120. This implies that the carbene moiety has been transferred only to the nitrile but not to the aldehyde. 

Reactions of rhodium porphyrins with diazo esters - According to Callot et al., iodorhodium(III) porphyrins are efficient catalysts for the cyclopropanation of alkenes by diazo esters [320,321], The transfer of ethoxycarbonylcarbene to a variety of olefins was found to proceed with a large syn-selectivity as compared with other catalysts. In their study to further develop this reaction to a shape-selective and asymmetric process [322], Kodadek et al. [323] have delineated the reaction sequences (29, 30) and identified as the active catalyst the iodoalkyl-rhodium(III) complex resulting from attack of a metal carbene moiety Rh(CHCOOEt) by iodide. 

The product of reaction (30) is thought to coordinate a further molecule of ethyldiazoacetate trans to the iodoalkyl group which looses dinitrogen, yielding a hexacoordinate rhodium carbene complex according to Eq. (31) which transfers its carbene moiety to an attacking alkene molecule. 

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