Rhodium enediynes

Zhang s group has reported highly enantioselective cycloisomerization processes catalyzed by rhodium(i) chiral complexes (Scheme 53). For instance, (A)-BINAP gives excellent asymmetric induction in the reaction of enediyne 212 to furnish the quasi-enantiopure Alder-ene product 213.219... 

Rhodium and Iridium Vinylidenes in Catalysis 287 Table 9.5 Rh(l)-catalyzed cycloaromatization of enediynes. 

Heteroatom-containing enediynes undergo a carbotricyclization process on exposure to catalytic quantities of rhodium salts and phenyldimethylsilane under a carbon monoxide atmosphere to produce the corresponding fused tricyclic compounds (Equation 140) <2000JA2385>. 

Rhodium carbonyl complexes also catalyze the cascade cyclization/hydrosilylation of 6-dodecene-l,l 1-diynes to form silylated tethered 2,2 -dimethylenebicyclopentanes. For example, reaction of ( )-85 with dimethylphenylsilane catalyzed by Rh(acac)(CO)2 in toluene at 50 °G under GO (1 atm) gave 86a in 55% yield as a single diastereomer (Equation (56)). Rhodium-catalyzed caseade cyclization/hydrosilylation of enediynes was stereospecific, and reaction of (Z)-85 under the conditions noted above gave 86b in 50% yield as a single diastereomer (Equation (57)). Rhodium(i)-catalyzed cascade cyclization/hydrosilylation of 6-dodecene-1,11-diynes was proposed to occur via silyl-metallation of one of the terminal G=G bonds of the enediyne with a silyl-Rh(iii) hydride complex, followed by two sequential intramolecular carbometallations and G-H reductive elimination. ... 

A methodology was reported in 2002 for the synthesis of (3-lactam fused enediynes [224]. When a solution of a diazo enediyne [225] was treated with a catalytic amount of rhodium acetate for 30 min, the (3-lactam fused enediyne was obtained as the only product, (Scheme 101). The yield in the carbene insertion step was about 50%, the rest being decomposition products. 

Shortly after Finn s work came to light a catalytic rhodium(I) system was reported. An acyclic enediyne 40 was heated to 50 °C in the presence of just 0.05 equiv of RhCl(/-Pr2P)2 and EtjN in benzene to provide substituted arene 41 in 58% yield. The latter reaction is presumed to involve Myers-Saito cyclization of an in situ formed vinylidene complex. A catalytic cycle becomes possible due to steps involving /3-hydride elimination and reductive elimination. ... 

Scheme 7.5 Rhodium-catalyzed stereoselective [2-1-2-1-2] cycloaddition of enediyne.

Cycloaddition of an enediyne leads to the corresponding cyclohexadiene along with the possibility of creating one to two consecutive stereogenic centers, depending on the substitution of the alkene inserted. Cobalt, albeit stoichiometric, is a reactant of choice to perform these reactions [8], Thus, the presence of a chiral motif on the enediyne substrate is crucial to achieve a satisfactory level of diastereoselectivity (Scheme 7.4). Stereoselective catalytic versions were also achieved by switching to cationic rhodium in the presence of atropisomeric ligands [9]. Various enan-tioenriched cyclohexadienes were obtained with enantioselectivities up to 97% (Scheme 7.5). 

The first attempt to construct a condensed core was a unique silicon-initiated (CO-SiCaT) reaction of carbonylated carbotricyclization of enediynes 2.314 [154, 161]. The 5-7-5 fused tricyclic products can be produced from the same type enediynes and CO through also by the rhodium-catalyzed intramolecular [2+2+2+1] cycloaddition [154, 160]. Reaction of dodeca-ll-ene-l,6-diyne 2.314 or heteroatomic analogues with a hydrosilane catalyzed by Rh(acac)(CO)2 at ambient temperature and pressure affords the corresponding fused 5-7-5 tricyclic system with a seven-membered lactone ring, 5-oxo-l,3a,4,5,7,9-hexahydro-6H-cyclopenta[e]azulenes 2.315 or related heteroanalogs in a yield from good to excellent (50-98%) (Scheme 2.104) [154]. 

In a further development, enantioselective cyclocondensation of the enediynes 2.385 and 2.387-2.391 (Figure 2.7) in the presence of a chiral rhodium complex leads to chiral cycloadducts [70, 71]. 

Rhodium catalysts also give cyclization products via vinylidene complexes generated in situ (Scheme 3.25) [256, 257]. So enediyne 3.505... 

Aromatization of enediynes with catalytic insertion of C-H bond—In the case of the enediynes 3.718 bearing long alkyl substituents terminating one alkyne branch, the cycloaromatization occurs with radical insertion into a C-H bond of the alkyl group (Scheme 3.80) [257, 263]. The route taken by catalysis with ruthenium differs from that with rhodium [257]. In the rhodium system, cyclization is initiated by a rhodium-vinylidene intermediate which forms OTe i2-diradical naphthalene intermediate A. In the case of ruthenium, the cyclization comprises primary of the formation of a ruthenium-Ti-alkyne, which forms para-dirsidicsil B that converts to the product 3.719. 

Enediynes 3.798 also with a fused p-lactam ring but now with the nitrogen atom as part of the ten membered ring [218, 220] were synthesized by carbene insertion in diazo enediyne 3.797 in 50% yield (Scheme 3.97) [26, 369-371]. This was prepared by acylation of amine 3.795 with ethyl malonyl chloride in the presence of triethylamine to give amide 3.796, followed by the treatment with p-toluene sulfonyl azide on potassium carbonate as solid support. Treatment of solution of the diazo enediyne product 3.797 with a catalytic amount of rhodium acetate for 30 minutes resulted in the lactam-fused enediyne as the only isolable product. 

Bennacer, B., Fujiwara, M., Lee, S.-Y. and Ojima, I. (2005) Silicon-initiated car-bonylative carbotricyclization and [24-24-24-1] cycloaddition of enediynes catalyzed by rhodium complexes. 7o r a/ of the American Chemical Society, 127(50), 17756-17767. 

Manabe, T., Yanagi, S.-L, Ohe, K. and Uemura, S. (1998) New examples of 1,6-and 1,7-hydrogen transfer promoted by an a-silyl group in rhodium(l)-catalyzed radical reactions of acyclic enediynes. Organometallics, 17(14), 2942-2944. 

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