Carbenoid ligand

Subsequently, Burgess and coworkers developed the chiral carbenoid ligands 32 (Scheme 29.10), which were structurally related to the JM-PHOS ligand [39, 40]. 

Transition metal complexes con- 127 taining carbenoid ligands (139)... 

The cationic ylide complex 130 is formed when SMej adds to the carbenoid ligand in 129 (109). The reaction between 2,5-dithia-hex-3-yne... 

Transition Metal Complexes Containing Carbenoid Ligands , F. A. Cotton and C. M. Lukehart, Progr. Inorg. Chem., 1972,16, 487. 

Kreiter and Formacek have obtained C NMR data for several Cr and W carbene complexes (123) (Tables XXXVI and XXXVII). They found that increased shielding of the carbene carbon occurred when R = phenyl was substituted for R = methyl. They attributed this to n donation from the aromatic ring to the vacant p orbital on the carbene carbon. Also the carbene carbons in cis 7V-methyl carbenoid ligands appeared at higher fields than in the corresponding trans fV-methyl derivatives. Finally, the shifts of ail carbons of the R and R" ligands were downfield from their typical positions due to the electron withdrawing power of the electron deficient carbene. 

Particularly useful alkene metathesis catalysts consist of ruthenium complexes with a nucleophilic carbene and another carbenoid ligand, CgHjCH=[M], where [M] is the metal with its ligands. 

Carbenoid complexes with heterocyclic ligands as catalysts in enantioselective cyclopropanation of olefins 97S137. 

Chiral carbenoid complexes with 2,2 -bipyridine ligands for asymmetric synthesis 98YGK764. 

From a historical perspective it is interesting to note that the Nozaki experiment was, in fact, a mechanistic probe to establish the intermediacy of a copper carbe-noid complex rather than an attempt to make enantiopure compounds for synthetic purposes. To achieve synthetically useful selectivities would require an extensive exploration of metals, ligands and reaction conditions along with a deeper understanding of the reaction mechanism. Modern methods for asymmetric cyclopropanation now encompass the use of countless metal complexes [2], but for the most part, the importance of diazoacetates as the carbenoid precursors still dominates the design of new catalytic systems. Highly effective catalysts developed in... 

VAPOL ligand, chiral 25 f. carbenoid center 153 carbenoids 146... 

The porphyrin ligands in the diamagnetic ruthenium and osmium carbene complexes generally exhibit four-fold symmetry by NMR, indicating that the barrier to rotation about the M=C bond is low. The carbenoid protons appear shifted down-field in the H NMR spectra, for example appearing for Ru(TTP)=CHC02Et and Ru(TTP)=CHSiMc3 at 13.43 and 19.44 ppm, respectively, and for the osmium... 

Carbenoid imidazolidene ligands such as C can also be used in conjunction with Pd(dba)2, and this method has been applied to a-arylpropanoic acids (NSAIDS) such as naproxen.175... 

In recent years, much attention has been focused on rhodium-mediated carbenoid reactions. One goal has been to understand how the rhodium ligands control reactivity and selectivity, especially in cases in which both addition and insertion reactions are possible. These catalysts contain Rh—Rh bonds but function by mechanisms similar to other transition metal catalysts. 

Before studying some examples more closely, let us consider some cases which are not listed in Table 13. There are numerous compounds SnX2 which are definitely monomeric but are nevertheless no carbene analogs since their valence electron number at the tin atom is at least eight. These compounds contain chelating ligands which can stabilize the carbenoid tin atom due to intramolecular Lewis acid-base interactions as shown by structure A and B (see also Chapter 3). 

Catalysts of the Co(salen) family incorporating chiral centers on the ligand backbone are useful in asymmetric synthesis and the field has been reviewed.1377,1378 In two examples, the hydroxy-lation reaction (Equation (14)) involving (269) proceeds with 38% ee,1379 whereas the cyclo-propanation reaction with (271) (Equation (15)) proceeds with 75% ee and with 95 5 trans cis.1380 A Co(V) salen carbenoid intermediate has been suggested in these reactions. 

Copper chelates in which the ligands are rigid chiral p-diketonates of type 205 are responsible for the highest optical yields known in carbenoid cyclopropanation reactions 200). The cyclopropane 206 was even obtained enantiomerically pure from 2-diazodimedone and styrene in the presence of CuL (L = 205c). 

By contrast, Con(salen) (100), which has the same ligand as (93), catalyzes cyclopropanation with excellent cis- and enantioselectivity (Scheme IT) 1, 274 The addition of NMI also improves stereoselectivity. It is noteworthy, however, that the sense of enantioselection by (100) is opposite to that by (93). Reversal of the enantioselectivity has been attributed to differences in the substrate s approach to the carbenoid centers.274... 

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