Alkenylation, rhodium-catalyzed

In a similar way as described for the hydroformylation, the rhodium-catalyzed silaformylation can also be used in a domino process. The elementary step is the formation of an alkenyl-rhodium species by insertion of an alkyne into a Rh-Si bond (silylrhodation), which provides the trigger for a carbocyclization, followed by an insertion of CO. Thus, when Matsuda and coworkers [216] treated a solution of the 1,6-enyne 6/2-87 in benzene with the dimethylphenylsilane under CO pressure (36 kg cm"2) in the presence of catalytic amounts of Rh4(CO)12, the cyclopentane derivative 6/2-88 was obtained in 85 % yield. The procedure is not restricted to the formation of carbocycles rather, heterocycles can also be synthesized using 1,6-enynes as 6/2-89 and 6/2-90 with a heteroatom in the tether (Scheme 6/2.19). Interestingly, 6/2-91 did not lead to the domino product neither could 1,7-enynes be used as substrates, while the Thorpe-Ingold effect (geminal substitution) seems important in achieving good yields. 

Similarly, ketimines (benzylimines of aromatic ketones) undergo the rhodium-catalyzed ortho-alkenylation with alkynes to give or/ o-alkenylated aromatic ketones after hydrolysis.61 This method is applied to an efficient one-pot synthesis of isoquinoline derivatives by using aromatic ketones, benzylamine, and alkynes under Rh catalysis (Equation (55)). 

The first example of asymmetric rhodium-catalyzed 1,4-addition of organoboron reagents to enones was described in 1998 by Hayashi and Miyaura. Significant progress has been made in the past few years. This asymmetric addition reaction can be carried out in aqueous solvent for a broad range of substrates, such as a,/ -unsaturated ketones, esters, amides, phosphonates, nitroalkenes. The enantioselectivity is always very high (in most cases over 90% ee). This asymmetric transformation provides the best method for the enantioselective introduction of aryl and alkenyl groups to the / -position of these electron-deficient olefins. 

Intramolecular cyclopropanation has a noteworthy advantage. Unlike intermolecular asymmetric cyclopropanation, the intramolecular reaction produces only one diastereomer due to geometric constrains on the fused bicyclic products. Doyle has extensively studied the intramolecular enantioselective reactions of a variety of alkenyl diazoacetates catalyzed by chiral rhodium carboxamides 198 and 200 and has achieved excellent results. 

The rhodium-catalyzed hydroboration has opened the way to cyclization reactions starting from dienes [92], For instance, rhodium-catalyzed hydroboration of the terminal alkenyl group of an os/Tunsaturated lactone followed by reaction with the PTOC-OMe chain transfer reagent afforded the bicyclic a-S-pyridyl lactone in 63% yield (Scheme 39). After oxidation of the sulfide with m-CPBA, thermal elimination of the sulfoxide afforded the corresponding a-methylene lactone in 65% yield. Interestingly, such bicyclic a-methylenelactones are substructures that can be found in many natural products such as mirabolide [93]. 

Scheme 3.23 Rhodium-catalyzed c/ne-substitution of alkenyl sulfones with aryltitanium reagents [41].

Gyclization/hydrosilylation of enynes catalyzed by rhodium carbonyl complexes tolerated a number of functional groups, including acetate esters, benzyl ethers, acetals, tosylamides, and allyl- and benzylamines (Table 3, entries 6-14). The reaction of diallyl-2-propynylamine is noteworthy as this transformation displayed high selectivity for cyclization of the enyne moiety rather than the diene moiety (Table 3, entry 9). Rhodium-catalyzed enyne cyclization/hydrosilylation tolerated substitution at the alkyne carbon (Table 3, entry 5) and, in some cases, at both the allylic and terminal alkenyl carbon atoms (Equation (7)). 

Tan KL, Bergman RG, Ellman JA (2001) Annulation of alkenyl-substituted heterocycles via rhodium-catalyzed intramolecular C-H activated coupling reactions. J Am Chem Soc 123 ... 

Similar, but normally less effective and more complicated, chelation control of regio- and stereochemistry can be achieved in rhodium-catalyzed hydroformylation reactions of open chain alkenyl phosphites82 -84. 

Ellman and co-workers developed a rhodium-catalyzed C-H alkenylation followed by 67t-electrocyclization to give di-, tri-, tetra-, and penta-substituted pyridines. The dihyropyridine intermediates 285 can be directly aromatized via hydrogenolysis to pyridine derivatives 286 in moderate to good yields. ... 

Cheng and co-workers reported a one-pot synthesis of substituted pyridines through a rhodium-catalyzed C-H alkenylation of a,(3-unsaturated ketoximes 287 with symmetrical alkyne substrates 288. 67t-Electro-cyclization of the azatriene intermediates 289 and subsequent loss of water afforded the desired pyridines 290 in moderate to good yields. ... 

Additionally, a rhodium-catalyzed silicon-assisted activation of a C-CN and C-Cl bond was reported in 2008. Under these catalytic conditions, carbon-cyano bonds in aryl, alkenyl, allyl, and benzyl cyanides bearing a variety of functional groups can be silylated. The observation of an enamine side product in the silylation of benzyl cyanides and related stoichiometric... 

Kita, Y, Tobisu, M., Chatani, N. (2010). Rhodium-catalyzed alkenylation of nitriles via silicon-assisted C-CN bond cleavage. Organic Letters, 12, 1864-1867. 

A rhodium-catalyzed one-pot synthesis of substituted pyridine derivatives from a,(3-unsaturated ketoximes and alkynes was developed in 2008 by Cheng and coworkers [99], Good yields of the desired pyri-dines can be obtained (Scheme 3.48). The reaction was proposed to proceed via rhodium-catalyzed chelation-assisted activation of the (3—C—H bond of a,(3-unsaturated ketoximes and subsequent reaction with alkynes followed by reductive elimination, intramolecular electro-cyclization, and aromatization to give highly substituted pyridine derivatives finally [100]. Later on, in their further studies, substituted isoquinolines and tetrahydroquinoline derivatives can be prepared by this catalyst system as well [101]. Their reaction mechanism was supported by isolation of the ort/jo-alkenylation products. Here, only asymmetric internal alkynes can be applied. 

Ellman, Bergman, and coworker reported a rhodium-catalyzed procedure for the synthesis of pyridines from alkynes and a,/ -unsaturated N-benzyl aldimines and ketimines in 2008 [107]. The reaction proceeded via C-H alkenylation/electrocyclization/aromatization sequence through dihydropyridine intermediates. The C-H activated complex was isolated and determination by X-ray analysis. Good yields of highly substituted pyridines were produced in one-pot manner (Scheme 3.50). 

TTie synthetic routes that have been reported for isocarbacycUn (4) and its analogs require at least 15 steps and most allow for veiy little diversification of the key co-side chain. While examining the functionality produced by a rhodium-catalyzed diene-ene [2-t2+l] cycloaddition, which has been previously reported by Croatt and Wender" (Fig. 6), it was noted that alkenyl cyclopentanone core system 16 resulting from this reaction is similar to the isocarbacyclin bicyclic core system. This cycloaddition involves a diene-ene reactant, which, when treated with 10 mol% of a rhodium(I) catalyst under carbon monoxide atmosphere, smoothly converts to the [2-l-2-tl] product with the formation of three stereocenters that have the same relative stereochemical configuration as the isocarbacyclin bicyclic core. We envisioned that... 

Scheme 27 Enantioselective rhodium catalyzed [5+2] cycloaddition of alkenyl-tethered vinylcyclopropanes...

Nishimura T, Katoh T, Takatsu K, Shintani R, Hayashi T (2007) Rhodium-catalyzed asymmetric rearrangement of alkynyl alkenyl carbinols synthetic equivalent to asymmetric conjugate alkynylation of enones. J Am Chem Soc 129(46) 14158-14159. doi 10.1021/Ja076346s... 

Arrayayas and Carretero employed a Af-(2-pyridyl)sulfonyl protecting group to assist in their palladium(II)-catalyzed C2 alkenylation of pyrrole, whereas the research groups of Li and Wang used a A, A-dimethylcarbamoyl group to affect alkenylation of pyrrole at the C2 position under rhodium-catalyzed conditions. ... 

Yu, R. T., Rovis, T. (2006). Enantioselective rhodium-catalyzed [2-I-2-I-2] cycloaddition of alkenyl isocyanates and terminal alkynes application to the total synthesis of (+)-lasubine II. Journal of the American Chemical Society, 128,12370-12371. 

Alkenyl-l-alkynylcyclopropane 33 underwent rhodium-catalyzed tandem Pauson-Khand reaction/spiropentane carbonylation to give 6-hydroxy-l-indanone 34 (Scheme 2.30) [46]. 

The decarboxylative coupling of benzoic acids with internal alkynes in a 1 2 manner was achieved under iridium catalysis to produce the corresponding benzannu-lated products (Scheme 4.31) [36]. The carboxylic group acts as a directing group, as in the rhodium-catalyzed ortho-alkenylation described above (Schemes 4.28 and 4.29). Thus, carboxylic group directed metalation at the ortho C-H bond and subsequent alkyne insertion may take place to form a seven-membered iridacycle intermediate. Then, decarboxylation, the second alkyne insertion, and... 

A unique rhodium-catalyzed oxidative alkenylation of diarylmethanols having a 2-(2-pyridyl) moiety with alkenes using silver carbonate as oxidant was reported (Scheme 4.99) [98]. The diarylmethanols also underwent a rhodium-catalyzed aldehyde-imine exchange reaction [99]. 

Evans et al. recently pubhshed an exciting new rhodium-catalyzed [3+2+1] carbocychzation of alkyhdenecyclopropanes such as 43 with an alkenyl tether and CO resulting in the formation of cyclohexenones such as 44. The authors managed to run the reaction enantioselectively in the presence of the chiral ferrocenylphos-phane hgand 45 (Scheme 10.16). Extensive DFT calculations make a catalytic cycle with rhodium trimethylenemethane intermediates most hkely [47, 48]. 

Lately, Kuwano and coworkers reported on the 1,2-migration in the rhodium catalyzed reaction of alkenyl acetates and arylboron reagents, producing the isomerized frani-alkenes effectively. The catalytic system proved less sensitive to the steric interactions of the Cl substituent resulting in a broad scope and good regioselectivities (Scheme 2.5) [16]. 

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