Chao Chen

Carbon-carbon bond activation by 1,1-carboboration of internal alkynes.

Internal alkynes undergo 1,1-carboboration reactions upon treatment with boranes RB(C(6)F(5))(2) (R = C(6)F(5), CH(3)) to yield trisubstituted alkenylboranes. These products can be used as substrates in Pd-catalyzed cross-coupling reactions.

Exploring the Limits of Frustrated Lewis Pair Chemistry with Alkynes: Detection of a System that Favors 1,1‐Carboboration over Cooperative 1,2‐P/B‐Addition

The zirconocene complex [{(C₆F₅)₂B-(CH₂)₃-Cp}(Cp-PtBu₂)ZrCl₂] (6; Cp=cyclo-C₅H₄) was prepared by hydroboration of [(allyl-Cp)(Cp-PtBu₂)ZrCl₂] (5) with HB(C₆F₅)₂ (Piers borane). It represents a frustrated Lewis pair (FLP) in which both the Lewis acid and the Lewis base were attached at the metallocene framework. Its reaction with 1-pentyne did not result in the 1,2-addition of or deprotonation reaction by the FLP, but rather in the 1,1-carboboration of the triple bond, thereby obtaining a Z/E mixture (1.2:1) of the respective organometallic substituted alkenes 7. The analogous reaction of 1-pentyne with the phosphorous-free system [{(C₆F₅)₂B-(CH₂)₃-Cp)}CpZrCl₂] (9) gave the respective 1,1-carboboration products ((Z)-10/(E)-10≈1.3:1).

1,1-Carboboration of 1-Alkynes: A Conceptual Alternative to the Hydroboration Reaction

Strongly electrophilic boranes R-B(C(6)F(5))(2) react readily with a variety of 1-alkynes by means of a 1,1-carboboration reaction to yield alkenylborane products, which can subsequently be used as reagents in metal catalyzed cross-coupling reactions.

Cyclizations via Frustrated Lewis Pairs: Lewis Acid Induced Intramolecular Additions of Amines to Olefins and Alkynes

An emerging strategy for the activation and reaction of small molecules is based on the concept of “frustrated Lewis pairs” (FLPs). Such systems exploit steric congestion that frustrates classical Lewis acid–base adduct formation. In this fashion the unquenched Lewis acidity and basicity is available for reaction with a third component. This concept was first employed to effect the heterolytic cleavage of H2 by sterically frustrated combinations of phosphines and boranes. This reactivity has subsequently been applied to develop a “metal-free” approach to hydrogenation catalysis of imines, enamines, and silylenol ethers. This strategy of FLP activation of H2 is not limited to P/B combinations. Indeed, suitably sterically hindered carbenes, 13] amines, 15] and pyridines have been shown to generate FLPs that react with H2. Moreover the nature of the Lewis acid can be adjusted so as to impart reversibility to the H2 uptake. 11] FLPs have also been exploited to effect the activation of a variety of small molecules including olefins, dienes, alkynes, 20] B H bonds, disulfides, PhNCO, CO2, [25] and N2O. [26] In the case of olefins and alkynes, Stephan and co-workers 19,20] showed that sterically encumbered phosphines and boranes or alanes, can effect interand intramolecular additions affording zwitterionic phosphonium borates. DFT studies have suggested that such additions to olefins occur via an antarafacial asynchronous concerted addition. For the reactions of alkynes, the trans-olefinic products suggest nucleophilic attack on a Lewis acid activated alkyne. Employing more basic phosphines, C H activation of terminal alkynes affords phosphonium alkynyl borate salts. Recently, the Erker group has shown that the linked phosphine borane (C6H2Me3)2PCH2CH2B ACHTUNGTRENNUNG(C6F5)2, reacts with pentyne to give a related C H activation product. However, this P/B species also reacts with a vinyl ether and norbornene to give the cyclized zwitterionic phosphonium borates. In a very recent example of similar reactivity, Erker and co-workers have also described the addition of an amine to an intramolecular olefinic residue in the presence of B ACHTUNGTRENNUNG(C6F5)3, which affords a thermally unstable zwitterionic ferrocene ammonium borate derivative. Herein, we describe reactions of sterically encumbered amines with intramolecular olefin or acetylene fragments in the presence of a Lewis acid. The resulting fiveand six-membered heterocyclic ammonium borate species demonstrate that FLP reactivity provides a facile route to intramolecular cyclizations. B ACHTUNGTRENNUNG(C6F5)3 was added to a solution of o-(2-propenyl)-N,Ndimethylaniline in CH2Cl2 and the mixture was stored at 32 8C for 48 h. Subsequent solvent removal from the supernatant gave a white powder (1) in 87 % yield. The B NMR spectrum of 1 showed a single resonance at d= 14.3 ppm. The corresponding F NMR resonances were observed at d= 132.0, 162.2 and 166.0 ppm. These data are consistent with the quaternization of B and thus a borate anion. The H NMR spectrum reveals inequivalent methyl resonances at d= 3.25 and 3.00 ppm in addition to broad resonances attributable to methylene protons at d= 2.25 and 1.91 ppm. The latter are consistent with B C bond formation. These data together with C, H/H-COSY, H/C-HSQC, and H/C-HMBC data were consistent with the formulation of the product as a reduced indole derivative C6H4ACHTUNGTRENNUNG(NMe2)ACHTUNGTRENNUNG(CH2CH ACHTUNGTRENNUNG(CH2B ACHTUNGTRENNUNG(C6F5)3) (Figure 1). This zwitterionic formulation of 1, comprising a five-membered, cyclic ammonium fragment with a pendant methylene borate group, was subsequently confirmed by X-ray crystallography. The metric parameters were unexceptional. [a] T. Voss, Dr. C. Chen, Dr. G. Kehr, E. Nauha, Prof. Dr. G. Erker Organisch-Chemisches Institut, Westf lische Wilhelms-Universit t 48149 M nster, Correnstrasse 40 (Germany) E-mail : erker@uni-muenster.de [b] T. Voss, Prof. Dr. D. W. Stephan Department of Chemistry, University of Toronto 80 St. George Street, Toronto, ON, M5S 3H6 (Canada) E-mail : dstephan@chem.utoronto.ca [c] E. Nauha Nanoscience Center, P.O. Box 35 40014 University of Jyv skyl (Finland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200903483.

Remarkably variable reaction modes of frustrated Lewis pairs with non-conjugated terminal diacetylenes

The frustrated Lewis pair P(o-tolyl)(3)/B(C(6)F(5))(3) reacts with 1,7-octadiyne by acetylene C-C coupling to yield the zwitterionic product 2a. In contrast, the P(o-tolyl)(3)/B(C(6)F(5))(3) Lewis pair reacts with 1,6-heptadiyne by a sequence involving 1,1-carboboration of a terminal acetylene to eventually yield the heterocyclic product 4.

Dibenzopentalenes from B(C6F5)3-induced cyclization reactions of 1,2-bis(phenylethynyl)benzenes.

Lene and mean: The strong Lewis acid B(C6F5)3 efficiently converts some bis(arylethynyl)benzenes into dibenzopentalenes through a series of Lewis acid induced cyclization reactions at room temperature. Thus the reaction has the potential to be useful in the synthesis of substituted dibenzopentalene derivatives which are difficult to make by conventional means.

The B(C6F5)3 boron Lewis acid route to arene-annulated pentalenes.

4,5-Dimethyl-1,2-bis(1-naphthylethynyl)benzene (12) undergoes a rapid multiple ring-closure reaction upon treatment with the strong boron Lewis acid B(C6F5)3 to yield the multiply annulated, planar conjugated π-system 13 (50u2009% yield). In the course of this reaction, a C6F5 group was transferred from boron to carbon. Treatment of 12 with CH3B(C6F5)2 proceeded similarly, giving a mixture of 13 (C6F5-transfer) and the product 15, which was formed by CH3-group transfer. 1,2-Bis(phenylethynyl)benzene (8u2009a) reacts similarly with CH3B(C6F5)2 to yield a mixture of the respective C6F5- and CH3-substituted dibenzopentalenes 10u2009a and 16. The reaction is thought to proceed through zwitterionic intermediates that exhibit vinyl cation reactivities. Some B(C6F5)3-substituted species (26, 27) consequently formed by in situ deprotonation upon treatment of the respective 1,2-bis(alkynyl)benzene starting materials (24, 8) with the frustrated Lewis pair B(C6F5)3/P(o-tolyl)3. The overall formation of the C6F5-substituted products formally require HB(C6F5)2 cleavage in an intermediate dehydroboration step. This was confirmed in the reaction of a thienylethynyl-containing starting material 21 with B(C6F5)3, which gave the respective annulated pentalene product 23 that had the HB(C6F5)2 moiety 1,4-added to its thiophene ring. Compoundsu200512-14, 23, and 26 were characterized by X-ray diffraction.