Carbene group

The iron atom in [Fe(F2o-TPP)CPh2] is 0.294 A out of the mean porphyrin plane toward the carbene group, whereas [(MeIm)Fe(F2o-TPP)CPh2] exhibits a considerably smaller displacement of the iron atom from the mean plane of the porphyrin ring (0.122 A toward the CPh2 group). In contrast, the iron atom of... 

The lower stability of (phosphino)(silyl)carbene-group 13 (IIIA) element Lewis acid adducts compared to that of the corresponding imidazol-2-ylidene adducts is easily rationalized by considering the inferior ability of phosphorus to stabilize a positive charge in the ot-position. In other words, once complexed, the carbene... 

Note The palladium pincer carbene complex has only one flanking carbene group (on the right-hand side) and the complex is neutral (the left flanking group is an anionic substituent). 

Evidence for the carbene mechanism is now so overwhelming (as discussed below) that the pairwise mechanism is only mentioned in this review for historical reasons. All alkene metathesis reactions are catalyzed by a metal carbene complex of some description, and the widely variable compositions used as catalysts are necessary to generate an active metal carbene group. 

The discovery that Ti complex (4) was an effective catalyst for living ROMP chemistry resulted in the synthesis of several new types of polymers that were inaccessible with conventional catalysts. Narrow polydispersity polymer and di- and triblock copolymers were synthesized soon after living ROMP was discovered. Because polymer chains that are formed in a living ROMP reaction are terminated with metal carbene groups, fimctionalization of the chain ends is possible. Reaction of the carbene with an aldehyde occurs in a Wittiglike fashion, making a metal oxo complex that is inert to finther metathesis chemistry and terminates the chain with the alkylidene group of the aldehyde. 

Casey and co-workers 178, 179) also studied the photochemistry of olefin-substituted carbene complexes and observed CO dissociation. For example, irradiation of a 1.38 1 mixture of the Z and E isomers of 145 led to CO substitution by the allyl-carbene group to give the isolable carbene-olefin complex 146 [Eq. (138)], a reaction which demonstrates CO dissociation. (Z)-145 has the proper stereochemistry to yield 146 upon photoinduced CO loss. However, the E isomer does not. Yet, the authors observed simultaneous disappearance of the E isomer upon irradiation of... 

Insertion of isocyanide carbon atoms into the Cr—carbene bond of [(CO)5CrC(OMe)Me] gave aziridinylcarbene complexes (CIV), some reactions of which are summarized in Scheme 2 28, 198). Cyclic carbene groups (CV)-(CVIII), in which the carbene carbon atom is part of an aromatic six-electron Tr-system, have been reported to form pentacarbonyl chromium and tetracarbonyl iron complexes 383, 384). Related to carbene... 

Several techniques are used to prepare dimers bridged by alkylidene (carbene) groups. Reactions with diazomethane provide one route ... 

Within the series of electrophilic carbenes (group I in Table 17), with the only exception of MeCF, chemical substitution at the carbon site results in electrophilic activation (Aw > 0) for the whole series. For the ambiphilic and nucleophilic series on the other hand (groups II and III in Table 17) chemical substitution results in electrophilic deactivation (Aw < 0), without exceptions.139... 

There are examples of all metals from groups 8 to 11 to catalyze the transfer of a carbene group from a diazo compound to organic substrates. One of the most studied transformation is the olefin cyclopropanation reaction, " for which the use of Tp ML catalysts has provided valuable improvement. Thus, the diastereoselectivity of this reaction, that usually leads to mixtures of both cis and trans isomers, was directed toward the d.y-cyclopropane with the complex Tp Cu(thf) (hydrotris [3-mesitylpyrazolyl]borate) as the catalyst, affording a 98 2 cisdrans mixture with styrene (Scheme 5) and ethyl diazoacetate (EDA) as the carbene source. Other olefins were also cyclopropanated with the preferential formation of the cis isomer. The catalysts can be prepared in situ by mixing a Cu(I) source and the MTp salt. Also, the Tp Cu(NCMe) complex has been employed as catalyst in a fluorous phase for the styrene cyclopropanation reaction. ... 

Indeed this is a theorem for the simple VB model [49] as well as the half-filled Hubbard model [50]. A related type of result applies [51] for Ji-network species decorated with carbene groups. Such results enable one to easily imagine various possibilities for high-spin species, as early noted by Ovchinnikov [52]. 

In metathesis, ruthenium catalysts bound to the ends of the polymer chains through the carbene group were used. Although the activity of the complexes remained high in reuse in six cycles, a part of the rhodium was lost in the reaction. The high catalyst activity was assumed to be due to the fact that highly active monophosphine complexes pass into solution [119-121]. 

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