Structure and conformations of 2-substituted and 3-substituted alumolanes in polar solvents: a direct NMR observation T.V. Tyumkina,* D.N. Islamov, L.V. Parfenova, L.M. Khalilov and U.M. Dzhemilev Introduction Five-membered saturated metallacarbocycles represent a large family of organometallic compounds, which are frequently postu- lated as reactive intermediates in catalysis, mainly in alkene oligomerization, [1] polymerization, [2] cycloadditions of olefins [3] and diynes. [4] Rather well-known and spectroscopically character- ized are cyclic compounds of some transition metals, e.g. Ti, [5] V, [6] Fe, [7] and Ni. [8] Among the Rh, Ir, Pd, and Pt five-membered rings, most of the references concern chelated complexes. [9] Metallacarbocycles are also known for Main Group metals such as Al, [10] Mg, [11] Sn, [12] and Ga. [13] Among these, of particular interest are alumolanes (the so-called aluminacyclopentanes), which are supposed to be formed in alkene cycloalumination with AlEt 3 cata- lyzed by Cp 2 ZrCl 2 . [10] The proposed synthetic method was applied to a number of substrates, including alkenes of cyclic and bicyclic structure. Typically, aluminacycles serve as precursors for the syn- thesis of a wide range of functionally substituted compounds: 1,4- butanediols, halogen-substituted derivatives, carbo-(O, N, S, Se, Si, P) and hetero-(O, N, S, Se, Si, P) cycles, and so on [10] (Scheme 1). The related magnesacycles could be synthesized in the same man- ner as well. [11] Unlike acyclic alkylalanes AlR 3 (R¼Me, Et, Pr, i Bu), which have been structurally studied in detail, [14] ring organoaluminum com- pounds (OACs) have received much less attention. As a rule, their structures are established by identification of the products formed upon acid hydrolysis and deuterolysis. Nevertheless, the literature contains 13 C NMR spectral characteristics of a few alumolanes. [15] Because of the lack of reliable 1 H, 13 C, or 27 Al NMR structural infor- mation on synthesized aluminacycles and their systematic analysis, spectral identification criteria for cyclic OACs and the effect of the substituent nature on the NMR parameters have not been defined. Moreover, the dynamics of cyclic OACs in nonpolar and polar sol- vents remains obscure, whereas this problem is well studied for acyclic alkylalanes. [14] Thus, the aims of our work are to find the NMR criteria for the identification of cyclic structure of 3 (2)-substituted alumolanes and to study their conformation and solvent effect [Et 2 O, tetrahy- drofuran (THF), and pyridine] on spectral parameters. The following 2-substituted and 3-substituted alumolanes were chosen as the ob- jects of our study (Scheme 2). Because all the compounds considered in the paper were vis- cous homogeneous liquids, multinuclear 1 H, 13 C, and 27 Al NMR spectroscopy was the only reliable tool for the structural studies. Furthermore, we used quantum chemical approach [density func- tional theory (DFT) and the second-order Moller–Plesset perturba- tion theory (MP2)] for the conformational analysis and the estimation of thermodynamic parameters of cyclic OAC solvation by polar solvents in the gas phase. The theoretical results were compared with the experimental data, as well as with the corre- sponding data for the known alkylalanes of acyclic structure. It is expected that the results of our study will contribute into the understanding of the structure and dynamics of cyclic organ- ometallic and organoelement compounds (borolanes and phospholanes). Results and discussion Multinuclear NMR study of 3-substituted alumolanes (1–8) The detailed assignments of the signals in the 1 H and 13 C NMR spectra of the studied compounds 1–8 were performed using two-dimensional spectroscopy (COSY, HSQC, and HMBC). For ex- ample, in the case of alumolane 1, the signals of carbon atoms in the α-position to the metal atom are broadened to a large extent and located in the low-frequency region at δ(C-2) 13.6 ppm and δ(C-5) 5.7 ppm, which is similar to the literature data. [15] However, they are significantly shifted downfield from the signals of pure AlEt 3 (δ C (Al CH 2 СH 3 )= À0.2 ppm in THF). Three pairs of methylene protons H-2, H-4, and H-5 and methy- lene protons of substituents are diastereotopic (Fig. 1). Diastereotopic effects of the H-2 atoms are much greater (Δδ = 0.98 ppm) than that of H-5 (Δδ = 0.29 ppm) because of the proximity of the stereogenic center at C-3 (δ(C-3) = 43.3 ppm). We found that the 13 C NMR chemical shift of the branching carbon atom C-3 in compound 1 is shifted downfield relative to the tertiary carbon atom in 2,3-disubstituted-1,4-(diethylalumina)butanes (δ C ~38 ppm) of acyclic structure. [16] The other 3-substituted alumolanes 2–8 were identified in the same manner (Table 1). * Correspondence to: T. V. Tyumkina, Institute of Petrochemistry and Catalysis, 141 Pr. Oktyabrya, Ufa, Russia, 450075. E-mail: ttvnmr@gmail.com Institute of Petrochemistry and Catalysis, 141 Pr. Oktyabrya, Ufa, Russia, 450075 Magn. Reson. Chem. 2016, 54, 62–74 Copyright © 2015 John Wiley & Sons, Ltd. Letter - spectral assignments Received: 13 May 2015 Revised: 9 July 2015 Accepted: 17 July 2015 Published online in Wiley Online Library: 25 August 2015 (wileyonlinelibrary.com) DOI 10.1002/mrc.4311 62