Rh-C bonds

C2F4 displaces one ethene to give Rh(C2H4)(C2F4)(acac), as does hexa-fluorodewarbenzene, whereas other alkenes (e.g. propene, styrene, vinyl chloride) displace both ethenes. Comparison of the structures of two complexes (Figure 2.28) shows that the Rh-C bonds are shorter to tetra-fluoroethene, because C2F4 is a better 7r-acceptor, with concomitant strengthening of the Rh-C bond. 

NMR spectra show the ethene molecules to undergo a propeller type rotation about the metal-alkene axis the fluxionality is removed on cooling such rotation is not observed with coordinated C2F4, indicating a higher barrier to rotation, in keeping with the stronger Rh—C bonds [66]. 

The coordination of the alkyne to the rhodium catalyst allows the carborhodation of the triple bond to afford the vinylrhodium intermediate 47 (Scheme 14). The rearrangement of this organometallic compound into the 2-(alkenyl)phenylrhodium intermediate 48 is evidenced by one deuterium incorporation resulting from the deuter-iolysis of the Rh-C bond. The addition of the phenylrhodium intermediate 45 must occur before its hydrolysis with water. The 2-(alkenyl)phenylrhodium intermediate 45, generated by the phenylrhodation of an alkyne followed by... 

Coordination of the Rh(I) to the vinyl group and homoallylic rearrangement gives a rhodacyclohexene. Insertion of the alkyne into a Rh-C bond and reductive elimination completes the catalytic cycle. 

A cycloaddition process between the Rh=C bond of the allenylidene derivative 38 and the C=C bond of the terminal alkyne has been evoked in the formation of the zwitterionic 71-aUyl-allenyl complexes 81 (Scheme 28), the initially formed metaUacyclobutenes 80 evolving into 81 by formation of carbene intermediate [RhCl(P/-Pr3)2(=CHCR=C=C=CPh2)] (R = Ph, p-MeC6H4, SiMe3) and subsequent migration of one of the phosphine ligands from the metal to the carbene carbon atom [205]. 

For the cyclization of diazo ester 32 there are four competing diastereomeric chair transition states leading to CH2 insertion products. In the transition state, the Rh-C bond is aligned with the target C-H bond leading to C-C bond formation. The two most stable of these transition states are depicted in Scheme 16.8. The actual product from cyclization is determined as the intermediate carbenoid commits to a particular diastereomeric transition state. If the C-C distance is short at the point of commitment (tight transition state), there will be a substantial steric interaction between the arene and the ester, and 32 b will be disfavored. If the C-C distance is longer, this interaction will not be as severe and more of 32 a will be formed. Thus, it seems reasonable that the ratio of 3 a to 36b is a measure of the C-C bond distance at the point of commitment of the rhodium carbenoid. 

The proposed reaction mechanism is shown in Scheme 9.15. Starting from the phenyl-rhodium complex 87, alkyne rearrangement is expected to furnish the phenyl-vinylidene complex 88. Migration of a phenyl ligand onto the vinylidene moiety of 88 must occur such that the vinyl Rh-C bond and the enone tether of the resultant complex (89) attain a cis-relationship to one another. Intramolecular conjugate... 

Rhodium-catalyzed cyclization/silylation/carbonylation of 1,6-diynes was proposed to occur via a-migratory insertion of CO into the Rh-C bond of dialkylidene cyclopentane intermediate Ik to form acylrhodium alkene intermediate Ilk (Scheme S-endo-[5-M gX2itory insertion of the silylated alkylidene moiety into the Rh-G bond of Ilk would form... 

Phenylacetylene 13 reacts with Rh4(CO)i2 to give 112 under an Ar or GO atmosphere. Complex 112 gives ( )-15 as the sole organic product by the reaction with Mc2PhSiH under GO atmosphere. Under CO pressure, 112 is transformed to dinuclear complex 7, which is directly formed from the reaction of 13 with Rh4(CO)i2 under GO pressure, irrespective of the quantity of 13 used (Scheme 7). No specific indication of CO insertion into the Rh-C bond in 7 is observed at this stage. Coordination of 13 to rhodium metal in 7 is relatively stable and the coordinated 13 is not replaced by the free 13 in solution. ... 

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