Simple transition-metal complexes of the formula Cp*M(CO),BR, [Cp* = C,(CH,),] containing an electrophilic, covalently bound main-group ligand react with alkanes and release products resulting from selective functionalization of an alkane at the terminal position. These reactions produce alkylboronate esters, which are common reagents in organic synthesis. Thus, the boryl complexes are rare chemical reagents that react selectivity at the terminal position of an alkane to provide simple functionalized products. Mechanistic analysis shows that ligand dissociation is induced photochemically and that thermal reaction of the resulting intermediate occurs with alkanes.

No abstract

Transition-metal boryl complexes of the form Cp‘Fe(CO)LBcat and (CO)5MBcat, where Cp‘ = C5H5, C5Me5, M = Mn, Re, L = CO, PMe3, and cat = 1,2-O2C6H4, were synthesized by reaction of ClBcat with [Cp‘Fe(CO)L]- or [M(CO)5]-. X-ray crystal structures of CpFe(CO)2Bcat, Cp*Fe(CO)2Bcat, and (CO)5MnBcat were obtained. Upon irradiation, these metal boryl complexes reacted with arenes and alkenes to form aryl- and vinylboronate ester products in moderate to high yields. Monosubstituted arenes with methyl, chloro, trifluoromethyl, methoxy, and dimethylamino substituents were used as substrates, and the resulting ratios of ortho- to meta- to para-substituted arene products were measured. No significant electronic effects were observed, indicating that the chemistry is not occurring through a typical electrophilic aromatic substitution pathway. Competition experiments between toluene and other substituted arenes were conducted. Reactivity differences were small, but anisole was found to have the fastest rate of reaction. Kinetic isotope effects were measured for the reaction of CpFe(CO)2Bcat, (CO)5MnBcat, or (CO)5ReBcat with benzene/benzene-d 6 mixtures and were found to be 3.3 ± 0.4, 2.1 ± 0.1, and 5.4 ± 0.4, respectively. This difference in isotope effect along with differences in selectivities with substituted arsenic reagents rules out a mechanism by which a common free Bcat radical attacks free substrate. Several experiments were also conducted to probe for CO loss. A 13CO-labeling experiment, CO inhibition experiment, and PMe3 trapping experiment indicate that the mechanism most likely proceeds through irreversible CO loss to form a 16-electron intermediate. Functionalization of alkenes to form vinylboronate esters was also observed, and mechanistic studies showed the absence of a measurable kinetic isotope effect for reaction of CpFe(CO)2Bcat or (CO)5ReBcat with ethylene/ethylene-d 4 mixtures or for reaction with ethylene-d 2.

octahydro-1,1′-bi-2-naphthol) exhibit higher levels of enantioselectivity than analogous catalysts based on BINOL. A comparison of structures of titanium complexes prepared from H 8 -BINOL and BINOL was, therefore, undertaken. Reaction of (rac)-H 8 -BINOL with 1 equiv of titanium tetraisopropoxide resulted in formation of the dimer (meso)-[(H 8 -BINOLate)Ti-(O-i-Pr) 2 ] 2 [(meso)-6], which was characterized crystallographically. In a similar fashion, use of (rac)-BINOL led to formation of the dimer (meso)-[(BINOLate)Ti(O-i-Pr) 2 ] 2 [(meso)-7]. The torsional angles between the aryl rings of the H 8 -BINOLate and BINOLate ligands in these complexes were 63.2(5)°and 55.7(4)°, respectively. The larger torsional angle of the H 8 -BINOLate ligand results in an increase in the bite angle of the ligand by just over 2°. Upon dissolving dimers (meso)-6 and (meso)-7, equilibria between the homo-and heterochiral dimers were observed. Reaction of H 8 -BINOL with an excess of titanium tetraisopropoxide provided crystals of the dinuclear complex [(H 8 -BINOLate)Ti(O-i-Pr) 2 ]‚Ti(O-i-Pr) 4 (8). Likewise, reaction of 2 equiv of Ti(OCy) 4 (Cy ) cyclohexyl) with H 8 -BINOL furnished [(H 8 -BINOLate)Ti(OCy) 2 ]‚Ti(OCy) 4 ( 9). These compounds were characterized by X-ray crystallography and compared to the known [(BINOLate)Ti(O-i-Pr) 2 ]‚Ti(O-i-Pr) 4 , a proposed intermediate in the asymmetric addition of alkyl groups to aldehydes. The solution behavior of 8 and 9 was studied by NMR spectroscopy, revealing that both complexes in solution are in equilibrium with dimers and free titanium tetraisopropoxide. Nonlinear studies with catalytic H 8 -BINOL and an excess of titanium tetraisopropoxide in the asymmetric addition of ethyl groups to benzaldehyde showed no nonlinearity, suggesting that the equilibrium strongly favors formation of dinuclear 8 under the conditions of the asymmetric addition.

The asymmetric allylation of carbonyl groups to furnish homoallylic alcohols is a fundamental transformation in synthetic organic chemistry. [1±3] Several catalysts will promote the asymmetric allylation of aldehydes to give secondary homoallylic alcohols with excellent enantioselectivities. [4±15] The catalytic asymmetric allylation of ketones, however, has proven to be a more challenging transformation owing to the significant difference in reactivity between aldehydes and ketones. Thus, with one exception, [16] catalysts that promote the enantioselective allylation of aldehydes fail to catalyze the analogous reaction with ketones. In general, the enantioselective formation of quaternary stereocenters, as generated in the asymmetric allylation of ketones, is of considerable difficulty. [17,18] To compensate for the reduced reactivity of ketones, a more reactive allylating agent was needed. Baba and co-workers found that tetraallylstannane added to ketones in the presence of methanol and 200 mol % binol to give the homoallylic alcohol in up to 60 % ee. [19] An important discovery in the asymmetric allylation of ketones was recently reported by Casolari, D©Addario, and Tagliavini. [20] Their catalyst preparation involved the reaction of [Cl 2 Ti(OiPr) 2 ] and binol with allyltributylstannane. After mixing for one hour, tetraallylstannane and the substrate ketone were added.They observed the formation of the ketone allylation product with up to 65 % ee at 20 mol % binol (80 % ee with 40 mol % binol).Based on the results of the Italian team, [20] Maruoka and coworkers [16] recently reported a system for the catalytic asymmetric allylation of aldehydes with a catalyst that is based on titanium, binol, and an achiral diamine spacer (2:2:1 ratio). This catalyst (60 mol % titanium and binol) was examined in the asymmetric allylation of only two ketones, acetophenone and methyl 2-naphthyl ketone, which underwent allylation with 90 and 92 % ee, respectively. [16] More recently, Cunningham and Woodward [21] demonstrated that monothiobinaphthol will promote the allylation of acetophenone derivatives with a mixture of [RSn(allyl) 3 ]/[Sn(allyl) 4 ] (R ¼ Et, Bu) with ee values as high as 92 % (51 % yield).The ketone allylation reaction of Casolari, D'Addario, and Tagliavini [20] attracted our attention because of our interest in the mechanisms of titanium-based asymmetric Lewis acid catalysts [22±24] and the need for a more versatile and enantioselective catalyst for this important process. While investigating the catalyst structure of the Tagliavini system, we made several key observations that allowed us to develop the most general and enantioselective catalyst for the asymmetric allylation of ketones to date.We repeated the catalyst preparation of Tagliavini [20] described above in CDCl 3 to probe the nature of the (binolate)Ti species by NMR spectroscopy. Like Tagliavini and co-workers, [20] we observed the production of tributyltin chloride. However, we were surprised to find that the major titanium-containing pro...