Rhodium acetylenes, cycloaddition

Transition metal-mediated cycloaddition and cyclization reactions have played a vital role in the advancement and applications of modem synthetic organic chemistry. Rhodium-catalyzed cycloadditions/cyclizations have attracted significant attention because of their versatility in the transformations of activated and unactivated acetylenes, olefins, allenes, etc. These reactions are particularly valuable because of their ability to increase molecular complexity through a convergent and highly selective combination of acyclic components. In addition, these reactions allow for the preparation of molecules with chemical, biological, and medicinal importance with greater atom economy. Recent developments in rhodium-catalyzed cycloaddition and cyclization reactions are described in this section. 

Aryl acetylenes undergo dimerization to give 1-aryl naphthalenes at 180 °C in the presence of ruthenium and rhodium porphyrin complexes. The reaction proceeds via a metal vinylidene intermediate, which undergoes [4 + 2]-cycloaddition vdth the same terminal alkyne or another internal alkyne, and then H migration and aromatization furnish naphthalene products [28] (Scheme 6.29). 

Harwood and co-workers (105) utihzed a phenyloxazine-3-one as a chiral derived template for cycloaddition (Scheme 4.50). An oxazinone template can be formed from phenylglycinol as the template precursor. The diazoamide needed for cycloaddition was generated by addition of diazomalonyl chloride, trimethyl-dioxane-4-one, or succinimidyl diazoacetate, providing the ester, acetyl, or hydrogen R group of the diazoamide 198. After addition of rhodium acetate, A-methylmaleimide was used as the dipolarophile to provide a product that predominantly adds from the less hindered a-face of the template in an endo fashion. The cycloaddition also provided some of the adduct that approaches from the p-face as well. p-Face addition also occurred with complete exo-selectivity. Mono- and disubstituted acetylenic compounds were added as well, providing similar cycloadducts. 

Maier and Schoffling (37) extended this intramolecular isomiinchnone cycloaddition to a synthesis of fused furans by employing an alkyne dipolarophile (Scheme 10.9). Thus, the diazo acetylenes (66) are smoothly converted to furans (69) via isomtinchnones (67) with catalytic rhodium acetate. 

Hashimoto and co-workers (206,207) recently published enantioselectivities of up to 92% ee in carbonyl ylide cycloadditions to acetylenic esters in the presence of a chiral rhodium catalyst (Scheme 11.58). 

The mechanism of cyclopropenations of alkynes with ethyl diazoacetate, catalysed by (AcO)4Rh2 and (DPTI)3Rh2(OAc), has been studied by a combination of kinetic isotope effects and theoretical calculations. With each catalyst, a significant normal 13C KIE was observed for the terminal acetylenic carbon, while a very small 13C KIE was detected at the internal acetylenic carbon. These isotope effects are consistent with the canonical variational transition structures for cyclopropenations with intact tetrabridged rhodium carbenoids but not with a 2 + 2-cycloaddition on a tribridged rhodium carbenoid structure.99... 

Analysis of 13C distribution in recovered alkynes using C4 atom as an internal standard led to experimental KIEs as collected in Table 4. For both catalysts significant isotope effect was observed for the terminal acetylenic carbon. Experimental KIEs are consistent with cyclopropenation via intact tetrabridged rhodium carbenoids and do so to support [2+2] cycloaddition. DFT calculations using B3LYP functional were complicated and did not give conclusive results. 

A rhodium(III)-catalysed oxidative C-H/N-H activation/annulation sequence has been developed for the addition of NH-sulfoximines (117) to acetylenes (118), providing 1,2-benzothiazines (119). The re-oxidation of the rhodium catalyst is attained in the presence of a catalytic amount of (AcO)2pe with oxygen as a terminal stoichiometric oxidant. In a similar way, the Ru(lll)-catalysed ortho-directed C-H activation of hydrazones (120), followed by cycloaddition to acetylenes (118), resulted in the formation of isoquinolines (121) upon the concomitant cleavage of the N-N bond. The reaction did not require an external oxidant. ... 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

The cross-[2 + 2 + 2] cycloaddition-aromatization sequence of terminal alkynes, dialkyl acetylenedicarboxylates, and enol esters was accomplished by using a cationic rhodium(I)/BINAP catalyst (Scheme 4.15) [25]. In this reaction, commercially available and cheap liquid enol acetates could be used as gaseous acetylene and propyne equivalents, which are difficult to handle using conventional laboratory equipment, due to their explosive and flammable nature. 

Enol ethers could be used as gaseous alkyne (acetylene and propyne) equivalents, and liquid ketene acetal could be used as a stable equivalent of unstable gaseous ethynyl methyl ether in cationic rhodium(I)/BINAP complex-catalyzed [2 -1- 2 - - 2] cycloaddition (Scheme 4.46) [49]. 

A fast entry toward (5)-(-)-3-n-butylphtalide (74), a constituent of celery that is used in Chinese folk medicine, was found in the rhodium-catalyzed crossed [2 + 2 + 2] cycloaddition of chiral diyne ester 73, which can be assembled enan-tioselectively within a few steps, and acetylene as the monoyne component (Scheme 7.15) [25]. Notably, the reaction provided the crossed cyclotrimerization product 74 (72% yield) at room temperature under an acetylene atmosphere (1 atm) without the need for pressurization. 

The enantioselective synthesis of axially chiral P—N ligands was also accomplished by rhodium-catalyzed [2 + 2+-2] cycloaddition. The reactions of 1,6-diynes 75 with diphenylphosphinoyl-substituted isoquinolinyl acetylenes 76 furnished diphenylphosphinoyl-substituted axially chiral 1-arylisoquinolines 77 with high yields and ee values (Scheme 9.28) [23], The new diphenylphosphinoyl-substituted axially chiral 1-arylisoquinoline 77 (Z = NTs, R = Me) was derivatized to the corresponding axially chiral P—N ligand 78 and isoquinoline A-oxide 79 without racemization, which could be used in the rhodium-catalyzed hydroboration and Lewis base-catalyzed allylation, respectively [23],... 

The reaction of [2+2+2] cycloaddition of acetylenes to form benzene has been known since the mid-nineteenth century. The first transition metal (nickel) complex used as an intermediate in the [2+2+2] cycloaddition reaction of alkynes was published by Reppe [1]. Pioneering work by Yamazaki considered the use of cobalt complexes to initiate the trimer-ization of diphenylacetylene to produce hexasubstituted benzenes [54]. Vollhardt used cobalt complexes to catalyze the reactions of [2+2+2] cycloaddition for obtaining natural products [55]. Since then, a variety of transition complexes of 8-10 elements like rhodium, nickel, and palladium have been found to be efficient catalysts for this reaction. However, enantioselective cycloaddition is restricted to a few examples. Mori has published data on the use of a chiral nickel catalyst for the intermolecular reaction of triynes with acetylene leading to the generation of an asymmetric carbon atom [56]. Star has published data on a chiral cobalt complex catalyzing the intramolecular cycloaddition of triynes to generate a product with helical chirality [57]. 

The rhodium-catalyzed chemo-, regio- and enantioselective [2-1-2- -2] cycloaddition of unsymmetrical alkynes to isocyanates leads to a wide range of 2-pyridones [123]. This method is used satisfactorily in the synthesis of axially chiral arylpyridones 2.196 by coupling alkylisocyanates with unsymmetrical a,to-diynes 2.195 that have one o-substituted phenyl group in one acetylene branch and terminal triple bond in another (Scheme 2.68) [123],... 

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