Rhodium-Catalyzed Asymmetric Cyclization/Hydroboration of 1,6-Enynes

Abstract

[reaction: see text] Reaction of enyne 1 with catecholborane catalyzed by a 1:1 mixture of [Rh(COD)(2)](+)SbF(6)(-) and (S)-BINAP (5 mol %) followed by Pd-catalyzed arylation with p-IC(6)H(4)CF(3) gave benzylidenecyclopentane 5 in 65% yield with 88% ee. Rhodium-catalyzed asymmetric cyclization/hydroboration followed either by Pd-catalyzed arylation or by oxidation was applied to the synthesis of a number of chiral, nonracemic carbocycles and heterocycles.

Hydroboration of Enynes and Mechanistic Insights

“…Based on their investigation on the innersphere mechanism (Scheme 14), they proposed that first the Pd(0)-complex coordinates with olefin group in enyne substrate (I), followed by the formation of allyl complex (Ia) -36 a). [33] The reaction was performed using [Rh(COD) 2 ]SbF 6 as the catalyst, (S)-BINAP (5 mol %) as the ligand, HBcat as a borylating agent in DCE (1,2-dichloroethane) at 30°C. Borylated alkylidene cyclopentanes (34 b-36 b) were obtained as the single diastereomers in good yields and excellent enantioselectivity.…”

“…Rh-Catalyzed cyclization/hydroboration of 1,6enynes. [33] place on C=C bond of complex B to give B''. This followed by an intramolecular carbometalation could form the rhodium alkenyl boryl species D which will give the desired product by reductive elimination.…”

Addition Reactions: Polar Addition

“…α-Siloxyallyl cyanides also react at the γ -position of a cyano group with both internal and terminal alkynes to give silyl enol ethers, which can be converted into the corresponding aldehydes or ketones upon hydrolysis. 70 Nickel-catalysed coupling of enynes (46) with allyl chlorides has been reported to occur in the presence of zinc; the reaction gives rise to the cyclic products (47) in good yields. 71 …”

Bis[(1,2,5,6-h)-1,5-cyclooctadiene] rhodium(1+), Hexafluoroantimonate(1−)

“…In a transformation similar to that described in the preceding paragraph, mixtures of [Rh(cod) 2 ]SbF 6 and BINAP catalyzed the enantioselective cyclization/hydroboration of 1,6-enynes to form (E)-borylated alkylidenecyclopentanes that were functionalized in situ without isolation. 21 In one example, reaction of 15 and catecholborane with a catalytic 1:1 mixture of [Rh(cod) 2 ]SbF 6 and (S)-BINAP (5 mol %) followed by treatment with p-iodoα,α,α-trifluorotoluene under palladium catalysis led to isolation of benzylidenecyclopentane 16 in 65% yield with 88% ee (eq 11). In a second example, rhodium-catalyzed reaction of the allyl propargyl tosylamide 17 with catecholborane followed by oxidation led to isolation of pyrrolidine 18 in 81% yield as a 4.5:1 mixture of diastereomers with 92% ee of the major diastereomer (eq 12).…”