Multiple Mechanistic Pathways for Zirconium-Catalyzed Carboalumination of Alkynes. Requirements for Cyclic Carbometalation Processes Involving C−H Activation

Abstract

The reactions of internal and terminal alkynes with organoalanes containing Et, n-Pr, and i-Bu groups in the presence of Cp2ZrCl2 and MeZrCp2Cl were investigated with the goal of clarifying mechanistic details of some representative cases. Three fundamentally different processes, i.e., (i) C−M bond addition without C−H activation in the alkyl group, (ii) cyclic C−M bond addition via C−H activation, and (iii) hydrometalation, have been observed, and the courses of these reactions significantly depend on (i) the nature and number of alkyl groups in organoalanes, (ii) their amounts, and (iii) solvents. The reaction of alkynes with Et3Al in the presence of 0.1 equiv of Cp2ZrCl2 in nonpolar solvents, e.g., hexanes, proceeds via C−H activation to give the corresponding aluminacyclopentenes. Investigation of the reaction of 5-decyne with 1−3 equiv of Et3Al and 1 equiv of Cp2ZrCl2, which gave mono-, di-, or trideuterated (Z)-5-ethyl-5-decene as shown in Scheme , together with the previously reported structural study on the reaction of Et3Al with Cp2ZrCl2 leading to the formation of well-characterized bimetallic species 9, 10, and 11 (Scheme ), supports a catalytic cycle involving bimetallic species 10 and 18 (Scheme ). In summary, this process requires a zirconocene derivative containing one Zr-bound Et group which is linked to Et3Al (but not to Et2AlCl) through a Cl bridge, i.e., 18, to produce 10 via β C−H activation. In sharp contrast, the reaction of Et2AlCl−Cp2ZrCl2 as well as of (n-Pr)2AlCl−Cp2ZrCl2 does not involve any C−H activation processes. It proceeds well in chlorinated hydrocarbon solvents, e.g., (CH2Cl)2, but it is extremely sluggish in nonpolar solvents, e.g., hexanes. The reaction may well involve direct C−Al bond addition to alkynes, as suggested earlier for Zr-catalyzed Me−Al bond addition to alkynes, but a few other alternatives cannot be ruled out on the basis of the currently available data. The reaction of alkynes with (n-Pr)3Al−Cp2ZrCl2 in nonpolar solvents proceeds partially via C−H activation and partially via hydrometalation. In contrast with the C−H activation process observed with Et3Al, that with (n-Pr)3Al is totally dominated by dimerization of alkynes to give aluminacyclopentadienes rather than aluminacyclopentenes, reflecting a previously established generalization that propene can be much more readily displaced from Zr by alkynes than ethylene. Hydrometalation is the exclusive process with (i-Bu)3Al−Cp2ZrCl2. This hydrometalation reaction, however, reveals a few interesting complications. Alkyl-substituted internal alkynes give double bond migrated products in addition to the expected hydrometalation products. With terminal alkynes the reaction produces the expected hydrometalation products and the 1,1-dimetalloalkanes in comparable yields. Various other related reactions involving other alkynes, e.g., PhC⋮CPh, n-OctC⋮CH, and PhC⋮CH, and other reagents, e.g., Et3Al−MeZrCp2Cl, Et2AlCl−MeZrCp2Cl, and (n-Pr)3Al−MeZrCp2Cl, were also studied.

Discovery of ZACA reaction − Zr-catalyzed asymmetric carboalumination of alkenes

“…We believe that both the discovery of the Dzhemilev ethylmagnesiation and our mechanistic clarification 22 have not only clearly established the existence of both acyclic and cyclic carbozirconation processes but also alerted us to sharply and carefully distinguish some seemingly analogous carbometalation reactions of zirconocene derivatives. We were later further surprised by the existence of bimetallic (involving both Zr and Al) cyclic carbozirconation of alkynes and alkenes that may be viewed as a hybrid of acyclic and cyclic carbozirconation 23 (Scheme 3). We also noted that our bimetallic (Zr-Al) cyclic carbozirconation process closely resembled the corresponding carbotitanation of alkenes with titanium-carbene species that can be viewed as a two-membered titanacycle (Tebbe reagent) generated from a Ti-Al bimetallic system.…”

“…It was indeed this surprising finding that led to the clarification and establishment of the bimetallic cyclic mechanism for carbozirconation of alkynes mentioned earlier (Scheme 3). 23 Fortunately, we soon learned that the use of more polar solvents including CH 2 Cl 2 , CH 3 CHCl 2 , and (CH 2 Cl) 2 almost totally suppressed the undesired cyclic carbometalation process thereby promoting formation of the desired products (Scheme 6).…”

An efficient and general route to reduced polypropionates via Zr-catalyzed asymmetric C—C bond formation

“…A later report (30) on a closely related reaction with ''cationic'' chiral zirconocene derivatives should be noted. Clearly, from the results presented above, the Zr-catalyzed carbometallation is multimechanistic (23,(31)(32)(33)(34)(35)(36) and sensitive to several critical factors including ligands (22), solvents (23), and metal countercations of alkylmetals. With respect to metal countercations, the following generalizations may be presented as useful guidelines (36).…”

Mechanistic aspects of chemo- and regioselectivity in Cp2ZrCl2-catalyzed alkene cycloalumination by AlEt3

“…It was suggested that complex 11A is the precursor of intermediate 11 [10] or they exist in dynamic equilibrium with each other [13]. The structure 11A also can be represented as η 2 -ethylene interaction with zirconium atom (complex 11B, Scheme 5) Thus, before our study there were no comprehensive answers on question how the probable key intermediates interact with the alkenes of various structures and regulate the reaction chemo-and regioselectivity.…”

“…Experimental study on the mechanism of catalytic cycloalumination of alkynes [10] and alkenes [11] showed that the active site of the reaction is cyclic five-membered bimetallic Zr,Al-complex 11 (Scheme 5), which is fully converted to the target aluminacycles when treated with substrate. Earlier the intermediate 11 was found in the system Cp 2 ZrCl 2 -AlEt 3 by W. Kaminsky [12].…”