A palladium-catalyzed 1,4-enyne synthesis was developed based the decarboxylative coupling of acetylides with allyl electrophiles. Stereochemical studies have implicated palladium-allyl-acetylides as intermediates. Thus, decarboxylative metalation was established as an environmentally benign alternative to transmetalation from alkynyl tin reagents.

Bicyclic alkenes, including oxa- and azabenzonorbornadienes and their derivatives, can be readily activated by transition metal complexes face-selectively due to their unsymmetrical bicyclic structure and the intrinsic angle strain on the carbon-carbon double bond. We have developed several stereo-, regio-, and chemoselective reactions catalyzed by nickel and palladium complexes using these bicyclic alkenes as substrates, providing a unique means of constructing a variety of synthetically useful carbocycles and heterocycles with high efficiency not generally accessible by traditional methods. This Account outlines these new metal-catalyzed reactions that include couplings, cycloadditions, and cyclization reactions.

An efficient cobalt-catalyzed carbocylization for the synthesis of indenols and indenes and a new method for reductive decyanation are described. 2-Iodophenyl ketones and aldehydes 1a-g undergo carbocyclization with various disubstituted alkynes 2a-k in the presence of Co(dppe)I(2) and zinc powder in acetonitrile at 80 degrees C for 3 h to afford the corresponding indenol derivatives 3a-s and4a-m in good to excellent yields. For some unsymmetrical alkynes, the carbocyclization was remarkably regioselective, affording a single regioisomer. The cobalt-catalyzed carbocyclization reaction was successfully extended to the synthesis of indene derivatives. Thus, the reaction of 2-iodophenyl ketones and aldehydes (1) with acrylates H(2)C=CHCO(2)R (7a-d) and acrylonitrile H(2)C=CHCN (7e) proceeds smoothly in the presence of Co(dppe)Cl(2)/dppe and zinc powder in acetonitrile at 80 degrees C for 24 h to afford the corresponding indenes 8a-k and 9a-c in moderate to good yields. Interestingly, when 7e was employed for the carbocylization, reductive decyanation also occurred to give an indene derivative without the cyano functionality. A possible mechanism for this cobalt-catalyzed carbocyclization reaction is also proposed.

A highly regio- and stereoselective method for the synthesis of various 2-silylallylboronates 7 from allenes 1 and 2-(dimethylphenylsilanyl)-4,4,5,5-tetramethyl[1,3,2]dioxaborolane (5) catalyzed by palladium complexes and initiated by organic iodides is described. Treatment of monosubstituted aryl and alkylallenes RCH=C=CH(2) (1a-m) and 1,1-dimethylallene (1n) with borylsilane 5 in the presence of Pd(dba)(2) (5 mol %) and organic iodide 3a (10 mol %) afforded the corresponding silaboration products 7a-n in moderate to excellent yields. This catalytic silaboration is totally regioselective with the silyl group of 5adding to the central carbon and the boryl group to the unsubstituted terminal carbon of allene. Furthermore, the reactions show very high E stereoselectivity with the Z/E ratios lying in the range from 1/99 to 7/93. In the absence of an organic iodide, silaboration of 1 with 5 still proceeds, but gives products having completely different regiochemistry as that of 7. The silaboration chemistry can be applied to the synthesis of homoallylic alcohols. Treatment of allenes (1) with borylsilane 5 and aldehydes 14 in the presence of Pd(dba)(2) (5 mol %) and 3a (10 mol %) at 80 degrees C in ethyl acetate for 5 h afforded homoallylic alcohols 15a-p in one pot in good to excellent yields, with exceedingly high syn selectivity (>93%). Mechanistic pathways involving an unusual palladium-catalyzed three-component assembling reaction of dimethylphenylsilyl iodide, allene 1, and borylsilane 5 were proposed to account for these catalytic reactions.

No abstract