Carbenes rotation

A lateral approach allows positive overlaps, so the reaction is allowed. As the reaction progresses, the carbene rotates (see arrows). 

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... 

Barriers to carbene rotation in A -heterocyclic carbenes (NHCs) appear to be dominated by steric interactions, and they have been observed in square-planar complexes using the diastereotopic protons in benzyl, butyl, and ethyl derivatives. For example, 30 shows diastereotopic methylenes owing to a lack of a mirror plane through the Ir-C(carbene) plane, and averaging of the methylene protons at higher temperatures indicates a 67 kcal mol barrier to rotation." Steric interactions influencing barriers have also been observed in other cases and allow observation of rotamers with unsymmetrically substituted NHCs or triazolinylidenes. 

The approaching carbene rotates about the symmetry axis, taking the system into C2 - the kernel of 02, and then moves parallel to x along a coordinate that has the irrep 62 in C2V but maps onto b in the lower C2 symmetry. 

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]- ... 

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... 

The stereochemistry of the resulting cyclopropane product (.s vn vs anti) was rationalized from a kinetic study which implicated an early transition state with no detectable intermediates. Approach of the alkene substrate perpendicular to the proposed carbene intermediate occurs with the largest alkene substituent opposite the carbene ester group. This is followed by rotation of the alkene as the new C—C bonds begin to form. The steric effect of the alkene substituent determines... 

In 1998, Enders et al. reported the use of the rhodium(cod) complexes 54a-f containing chiral triazolinylidenes in the same reaction [41]. Complexes 54 were prepared in THF in 65-95% yield, by reaction of the tri-azolium salts with 0.45 equiv of [Rh(cod)Cl]2 in the presence of NEts (Scheme 31). The carbene ligand in such complexes is nonchelating with possible hindered rotation around the carbene carbon-rhodium bond. Due to... 

In second generation metathesis initiators the NHC ligand is rotating more or less freely, depending on the nature of the active carbene, configuration of ancillary ligands and the steric bulk of the NHC substituents. 

It was proposed that the more labile IR bands corresponded to the exo-carbene, 71-exo, which has the methyl and fluoro substituents aligned properly for rearrangement. The slower decaying IR bands were assigned to c do-carbene, 71-endo, which was presumed to require rotation to the exo-conformer before undergoing ring expansion. 

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

This may indicate that the reaction to form the carbene is concerted or at least indicates that if a diradical is involved, reaction must be too fast to permit rotation around the C-2-C-3 bond,... 

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). 

The EfZ ratio of stilbenes obtained in the Rh2(OAc)4-catalyzed reaction was independent of catalyst concentration in the range given in Table 22 357). This fact differs from the copper-catalyzed decomposition of ethyl diazoacetate, where the ratio diethyl fumarate diethyl maleate was found to depend on the concentration of the catalyst, requiring two competing mechanistic pathways to be taken into account 365), The preference for the Z-stilbene upon C ClO -or rhodium-catalyzed decomposition of aryldiazomethanes may be explained by the mechanism given in Scheme 39. Nucleophilic attack of the diazoalkane at the presumed metal carbene leads to two epimeric diazonium intermediates 385, the sterically less encumbered of which yields the Z-stilbene after C/C rotation 357,358). Thus, steric effects, favoring 385a over 385 b, ultimately cause the preferred formation of the thermodynamically less stable cis-stilbene. 

Reaction of 3 with Ph3C+PF6" resulted in the formation of methylidene complex [(n-C5H5)Re(N0)(PPh3)(CH2)]+ PF6 (8) in 88-100% spectroscopic yields, as shown in Figure 11. Although 8 decomposes in solution slowly at -10 °C and rapidly at 25 °C (She decomposition is second order in 8), it can be isolated as an off-white powder (pure by H NMR) when the reaction is worked up at -23 °C. The methylidene H and 13C NMR chemical shifts are similar to those observed previously for carbene complexes [28]. However, the multiplicity of the H NMR spectrum indicates the two methylidene protons to be non-equivalent (Figure 11). Since no coalescence is.observed below the decomposition point of 8, a lower limit of AG >15 kcal/mol can be set for the rotational barrier about the rhenium-methylidene bond. 

In this state the addition of methylene occurs in the triplet state and starting from each a mixture of cis and trans products is obtained. This is because in the triplet state, the two electrons have parallel spins and carbene behaves as diradical. To explain non stereospecificity it is assumed that rotation about the single bond occurs more rapidly than spin inversion. The entire mechanism can be written in the following manner ... 

Although it is not known whether this is a dynamic or a static disorder, it is clear from Fig. 2 that the two units result from an inversion at the central carbon, followed by a 180° rotation around the P1-C2 bond. The observation that this carbene has difficulty in maintaining one discrete form in the solid state could well be consistent with the low value for the inversion barrier at carbon, calculated by Hegarty et al.13 (see Section II). 

Carbenes, Structure of (Qoss) sp -sp Carbon-Carbon Single Bonds, Rotational Isomerism about 3 193... 

Skell s hypothesis proved to be extremely useful in carbene chemistry even though it was frequently opposed. The principal significance of these rules is represented in the scheme below. The singlet reaction occurs in a concerted step, the cis-addition product being formed in a stereospeciiic manner. In the triplet addition, which is a two-step reaction, rotation is thought to be faster than intersystem crossing (spin inversion) and ring closure, i.e., which would... 

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