Carbene complexes restricted rotation

H-NMR studies of oligocarbene Ru(II) complexes indicate a substantial barrier to rotation about the metal-carbene carbon and nitrogen-R bonds. This restricted rotation is thought to arise as a consequence of intramolecular non-bonding cis interactions of the carbene nitrogen-R substituents, and not because of any significant double bond character in ruthenium-carbene carbon (76). 

Fischer recognized the first carbene complexes in 1964. They were formed by the attack of an alkyllithium on a metal carbonyl followed by methylation (equations 1 and 2). Resonance form (2), considered as the dominant one in the heteroatom stabilized Fischer carbenes, shows the multiple character of this carbon-heteroatom bond. This effect is responsible for the restricted rotation often observed for this bond in nuclear magnetic resonance (NMR) studies. For example cis and trans isomers (6) and (7) of methoxymethyl carbenes rapidly interconvert at room temperature, but can be frozen out in the proton NMR at -40 °C. By contrast, the M-C bond is close to single and often rotates freely. 

As observed with carbene rotations (see Section 3), conformations of aryls can introduce additional chirality into a complex. Aryls prefer to lie orthogonal to the coordination plane in square-planar complexes. Hindered rotation of aryls in these complexes can produce atropisomerism, although this description is most commonly associated with restricted rotation about C-C single bonds in biaryls. An example of hindered rotation is found in (DIOP)Pt(3,5-Br2C6H3)(I) (43) in Brown s paper which provides an excellent discussion and leading references. The ortho protons of the dibromoaryl are... 

Amines react in general more readily than alcohols and aromatic isocyanides are more reactive than ahphatic ones. The ionic diisocyanide complexes react under milder conditions than the neutral monoisocyanide complexes . No reactions with mercaptanes have been reported. Acyclic aminocarbene complexes occur as isomers when R R due to restricted rotation around the C(carbene)—N bond . Dinuclear carbene complexes are formed when the amine complexes [Au(C6F5)NH2(CH2)nNH2] (n = 1 or 2) or the carbene complexes [(C6F5) ,AuC(NHR)NH(CH2)nNH2] (m = 1 or 3) are treated with the isocyanide compound [(C6F5)mAuCNR] (R = Ph or p-tolyl) . ... 

DNMR also reveals hindered rotations about single bonds in addition to rotation about the M=C bond in some conventional carbene complexes, such as 26-28. Restricted rotation around the C-C bond between the / 3t>-carbon and the a-carbon in both Fe=CHAr and Ru=CHArwas observed with AG 40-50kJ mol . " " ... 

Hindered rotation of metal-aryl complexes can also produce atropisomerism/ and isomerism similar to that observed for restricted rotation of carbenes can be observed. For example, for 34, a 57 43 ratio of rotamers (diastereomers) is observed. 

In the same year, a series of Ci-symmetric chiral triazolium Pd(II) complexes were prepared by Enders et al. As typically reported with the use of Ci-symmet-ric ligands, their NHC-metal complexes were obtained as diastereomeric mixtures due to the restricted rotation around the carbene-metal bond [6]. Without further elaboration, the authors stated that these complexes were used in an enantioselective Heck-type reaction achieving low asymmetric inductions. Soon thereafter, the authors investigated the coordination behavior of chiral triazolium salts 35 with [Rh(COD)Cl]2 and obtained a mixture of axially chiral complexes 36 with a diastereomeric excess of up to 94% (Scheme 3.20). These complexes were used as catalysts in asymmetric hydrosilylation reactions, achieving up to 44% ee for aromatic and ahphatic ketones [38,39]. 

Fischer made the first recognized carbene complexes in 1964 by treatment of Mo or W carbonyl with RLi then Mel (Eq. 11.1). On the Fischer bonding picture, the methoxy substituent helps stabilize the empty p orbital on the carbene carbon by tt donation from one of the lone pairs on oxygen. Resonance form 11.3 is probably dominant in the heteroatom-stabilized Fischer carbenes. The multiple character of the C-OR bond is responsible for the restricted rotation often observed in NMR work and results in a reduced bond order in the M-C bond, often closer to single than double. A C-OR multiple bond in 11.3 implies cis-trans... 

A series of rhodium NHC complexes of the type RhCl(NHC)(cod) were obtained by the direct reaction of [Rh( -Cl)(cod)2l2 with the NHC by insertion of the rhodium atom into the C=C bond of bis(imidazolidin-2-ylidene) olefins, or by transmetallation from the corresponding silveifi) complexes. The restricted rotation about the Rh-C bond was studied and the steric size of the N-alkyl wingtips of the carbene and the size of the auxiliary ligands were shown to be influenced significantly. The uiCO) of the carbonyl derivatives obtained by bubbling CO over solutions of the cod precursors provided an efficient tool to estimate the electron-donor power of the carbene ligands. The preparation of unsymmetrically saturated NHCs by the direct reaction of the carbene with [Rh( -Cl)(cod)2]2 323 also provided examples of coordination to Rh. ... 

Chiral imidazolium or triazolium salt reacted with Pd(OAc)2 in the presence of Nal and KO Bu, giving a mixture of carbene-Pd complexes (Scheme These nucleophilic carbenes are regarded as replacements of phosphines with stronger Lewis basicity and attracted considerable attention recentlyThe palladium complexes formed as mixture of cis and trans isomers, which were separable by column chromatography. Upon heating in DMF at 100 °C, the cis isomer is completely isomerized into the trans isomer. In these complexes, rotations around the carbene-Pd bonds are restricted, probably due to double bond characters of the C—Pd bonds, and several diastereomers were detected. 

Other miscellaneous internal rotations in dinuclear complexes include /f-alkyl rotations in organolanthanide complexes, namely [Lu(/i-Et2SiCpCp")2 (/i-H)(/i-Et)Lu], ° W = C(carbene) rotations in complex (23), and restricted Pt—P rotation in ds-[Pt-Cl2 (/i-dpmp)PtCl2 2]- ... 

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