It has been established that a cationic rhodium(I)/H8 -binap complex catalyzes the [3+2+2] cycloaddition of 1,6-diynes with cyclopropylideneacetamides to produce cycloheptadiene derivatives through cleavage of cyclopropane rings. In contrast, a cationic rhodium(I)/(S)-binap complex catalyzes the enantioselective [2+2+2] cycloaddition of terminal alkynes, acetylenedicarboxylates, and cyclopropylideneacetamides to produce spiro-cyclohexadiene derivatives which retain the cyclopropane rings.
The transition‐metal‐catalyzed cyclization of alkynals viaoxametallacycle intermediates is a useful method for thestereoselective synthesis of cyclic allylic alcohol derivatives. The reductive cyclization reactions are catalyzed by titanium and nickel complexes using organosilanes, organoboranes, and organozincs as reducing agents. The alkylative, arylative, and alkenylative cyclization reactions are catalyzed by nickel complexes using organozincs and alkenylzirconiums. On the other hand, the recently developed rhodium‐catalyzed reductive cyclization reactions of alkynals allow using dihydrogen as a reducing agent without the use of organometalloid and organometallic reagents. Furthermore, very recently, the rhodium‐catalyzed acylative cyclization reactions of alkynals were accomplished by using aldehydes and acyl phosphonates as acylating reagents. Importantly, the use of the rhodium catalysts realized the enantioselective cyclization reactions of alkynals with excellent levels of enantioselection.
The reaction of allyltitanocenes with five- to seven-membered cyclic enones proceeded with good to high diastereoselectivity depending on the ring size of enones. The stereochemistry of the major isomers produced by the reaction of cinnamyltitanocene was opposite to that of crotyltitanocene.
It has been established that ac ationic rhodium(I)/ H 8 -binap complex catalyzesthe [3+ +2+ +2] cycloaddition of 1,6-diynes with cyclopropylideneacetamides to produce cycloheptadiene derivatives through cleavage of cyclopropane rings. In contrast, acationic rhodium(I)/(S)-binap complex catalyzes the enantioselective [2+ +2+ +2] cycloaddition of terminal alkynes,a cetylenedicarboxylates,a nd cyclopropylideneacetamides to produce spiro-cyclohexadiene derivatives which retain the cyclopropane rings.Transition-metal-catalyzed [3+ +2+ +2] cycloaddition reactions involving cyclopropylidene compounds are valuable methods for the construction of seven-membered rings.[1] Previously, ac yclopropylideneacetate as ac ycloaddition partner and anickel(0)/phosphine complex as acatalyst were used. [2,3] For example,S aito and co-workers reported the nickel(0)/phosphine-complex-catalyzed intermolecular [3+ +2+ +2] cycloaddition of two identical alkynes with the cyclopropylideneacetate.[2h] Subsequently,they developed the cycloaddition of two different alkynes with the cyclopropylideneacetate,a lthough the slow addition of substrates and the large excess of one alkyne component were required.[2g] They also developed the partial intramolecular [3+ +2+ +2] cycloaddition of a1 ,6-diyne with cyclopropylideneacetate.[2e] Ap roposed mechanism of the nickel(0)-catalyzed [3+ +2+ +2] cycloaddition reactions is shown in Scheme 1. [2a,c-h, 4] Tw oalkynes and cyclopropylideneacetate react with the nickel(0) complex to generate the nickelacycloheptadiene intermediate A.Ab-carbon atom elimination affords the nickelacyclooctadiene intermediate B and subsequent reductive elimination furnishes the cycloheptadiene derivative C.A lthough direct reductive elimination from A would furnish the spiro-cyclohexadiene derivative D,s uch reactions are limited to the case using sterically demanding phosphine ligands and proceeded in low yields. [2f, 5] Importantly,both cycloheptane [6] and spiro-cyclohexane [7] skeletons are frequently found in biologically active natural products.T herefore,t heir selective syntheses are important targets for organic synthesis.H erein, we have applied ac ationic rhodium(I)/biaryl bis(phosphine) catalyst, which is known to be ah ighly active catalyst for the [2+ +2+ +2] cycloaddition, [8,9] to the cycloaddition reactions involving cyclopropylidene compounds. [10] Our research group reported that acrylamide derivatives are highly reactive substrates in cationic rhodium(I)-complexcatalyzed [2+ +2+ +2] cycloaddition reactions.[11] Therefore,w e first examined the reaction of the 1,6-diyne 1a and N-methyl-N-phenylcyclopropylidene-acetamide (2a)inthe presence of cationic rhodium(I)/bisphosphine complexes,a ss hown in Table 1. We were pleased to find that the [3+ +2+ +2] cycloaddition of 1awith 2aproceeded using acationic rhodium(I)/ segphos catalyst (20 mol %) to give the [3+ +2+ +2] cycloaddition product 3aa in moderate yield along with the [2+ +2+ +2] cycloaddition product 4aa (entry 1). Screening of bis(phosphine...
The cover picture shows the Japanese famous inverted image of Mt. Fuji (reflected in the water). The scheme shows the general catalytic cycle of the transition‐metal‐catalyzed cyclization of alkynals via oxametallacycle intermediates. This cyclization is a highly efficient method for the stereoselective synthesis of carbocycles and heterocycles. This reaction is catalyzed by Ni, Rh, Ru, and Ti complexes and mediated by organometalloid reagents (X–Y = Si–H, B–C), organometallic reagents (X–Y = Zn–C), dihydrogen (X–Y = H–H), and organic reagents (X–Y = C–H, C–P). Details are presented in the Microreview by K. Tanaka and Y. Tajima on p. 3715 ff.
It has been established that a cationic rhodium(I)/H8‐binap or binap complex catalyzes two different modes of cyclization of γ‐alkynyl aldehydes with carboxylic acid anhydrides to give cyclic aldehyde gem‐dicarboxylates and cyclic alkenyl esters through cleavage of the carboxylic acid anhydride C–O bond. The reaction of a terminal γ‐alkynyl aldehyde with diethyl pyrocarbonate afforded a cyclic allylic carbonate with a high ee value.
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