Cationic rhodium(I)/BIPHEP complex-catalyzed cross-cyclotrimerization of silylacetylenes and unsymmetrical electron-deficient internal alkynes

Abstract

a b s t r a c tIt has been established that a cationic rhodium(I)/BIPHEP complex catalyzes the chemo-and regioselective intermolecular cross-cyclotrimerization of silylacetylenes with unsymmetrical electron-deficient internal alkynes. Chemoselectivity was highly dependent on steric bulk of the unsymmetrical electrondeficient internal alkynes used. Two molecules of sterically less demanding alkynoates and alkynone reacted with one molecule of the silylacetylene, on the contrary, one molecule of a sterically demanding alkynoate reacted with two molecules of the silylacetylene.Ó 2015 Elsevier Ltd. All rights reserved.The transition-metal-catalyzed intermolecular cross-cyclotrimerization of alkynes has received much attention as a useful method for the synthesis of densely substituted benzenes. 1 Although a number of transition-metal complexes are capable of catalyzing alkyne cyclotrimerization, the transition-metal catalysts that can simultaneously control both chemo-and regioselectivities are limited in number. 2 In the last 15 years, several transitionmetal complexes were found to catalyze the chemo-and regioselective intermolecular cross-cyclotrimerization of two or three different alkynes involving symmetrical internal alkynes. 3,4 For example, our research group found in 2003 that a cationic rhodium(I)/H 8 -BINAP complex catalyzes the intermolecular crosscyclotrimerization of terminal alkynes and symmetrical internal alkynes (dialkyl acetylenedicarboxylates). 4,5 After this Letter, the intermolecular cross-cyclotrimerization of terminal alkynes with unsymmetrical internal alkynes (alkynoates) has also been accomplished using perfluoroalkylacetylene 6 and aryl ethynyl ethers 7 as terminal alkynes, 8 and the cationic rhodium(I)/H 8 -BINAP complex as a catalyst. In this Letter, we have established that a cationic rhodium(I)/BIPHEP complex is capable of catalyzing the intermolecular cross-cyclotrimerization of silylacetylenes and unsymmetrical electron-deficient internal alkynes (alkynoates and an alkynone).In our previous report, the cationic rhodium(I)/H 8 -BINAP complex catalyzed the intermolecular cross-cyclotrimerization of trimethylsilylacetylene (1a) and diethyl acetylenedicarboxylate at room temperature to give the corresponding 2:1 (=1a/diethyl acetylenedicarboxylate) reaction product in moderate yield (57%) with almost perfect regioselectivity along with the corresponding 1:2 reaction product in low yield (Scheme 1).On the other hand, the use of an unsymmetrical electron-deficient internal alkyne, ethyl 2-butynoate (2a), instead of diethyl acetylenedicarboxylate afforded not the corresponding 2:1 (=1a/2a) reaction products A but the corresponding 1:2 reaction products 3aa and 4aa as major products with good yield and regioselectivity (Table 1, entry 1). Screening of biaryl bisphosphine ligands (entries 1-4, Fig. 1) revealed that the use of BIPHEP afforded 3aa and 4aa in the highest yield (entry 4). Sterically demanding biaryl bisphosphine ligand, tol-BINAP, was also tested, but the combined yield of 3aa a...

Selective Catalytic Cross‐Cyclotrimerization En Route to 1,4‐Diborylated Benzenes and Their Synthetic Transformations

“…Besides that, introduction of heteroatom substituents to the triple bond can be hampered by instability of such starting alkynes, which often only exist as protected surrogates. As typical examples of the above‐mentioned processes can serve Rh‐catalyzed co‐cyclotrimerization of aryloxyalkynes [8] or silylalkynes [9] with propynoates leading to mixtures of regioisomeric products.…”

Some Aspects of the Chemistry of Alkynylsilanes

“…Protiodesilylation of a mixture of regioisomers was able to simplify the reaction mixture, but reaction with ICl gave a synthetically challenging mixture of isomers in modest yield (Scheme 48). 57…”