The regioselectivity in the hydroformylation reaction catalyzed by an unmodified Rh catalyst has been investigated for a number of alpha-methylsubstituted alk-1-enes (3-methylbut-1-ene MB1. 3-methylpent-1-ene MP1, 3,4-dimethylpent-1-ene DMP1, and 3,4,4-trimethylpent-1-ene TMP1) experimentally (at 20 degrees C and 100 atm CO/H-2 total pressure) and theoretically at the B3P86/6-31G* level with Rh described by effective core potentials in the LanL2DZ valence basis set. For all substrates the formation of the linear aldehyde (L) with respect to the branched one (B) in a prevailing amount has been observed (L/B >62/38): the L isomer was formed as the almost exclusive product in the case of TMP1 (L/B = 95/5).H-2 NMR investigations of crude reaction mixtures, coming from analogous deuterioformylation experiments interrupted at partial substrate conversion, showed that in the case of TMP1 only the branched alkyl-rhodium intermediate, precursor of the branched aldehyde, via beta-hydride elimination mainly generates terminal deuterated olefins and, to a lesser extent, internal ones. The reversibility of the branched alkyl-Rh intermediates accounts for the high regioselectivity in favor of the linear aldehyde. Computational studies confirm the importance of the alkyl-Rh transition state (TS) stability to reproduce the experimental regioselectivity, or even to predict it, when the reaction is nonreversible (i.e. for MB1, MP1, and DMP1). In the case of TMP1, the free energy profiles for further reaction steps along branched and linear pathways have been examined to elucidate the origin of reaction reversibility. The TS for the alkyl migratory insertion onto the CO coordinated to rhodium, higher than that for the alkyl-Rh intermediate formation, explains the reason why in deuterioformylation experiments at partial conversion the monodeuterated terminal olefin TMP1-1-d(1) is obtained. This occurs for one out of two most populated reactant conformers of TMP1, although for the Curtin-Hammett principle reactant populations are not particularly important. For the other, the reaction proceeds to the branched aldehyde. Only for a less populated reactant conformer the internal olefin is obtained. Conversely, along the linear pathway the CO addition and alkyl migratory insertion steps occur, respectively, in a practically spontaneous way and with very low TS in any case. Agostic interactions (using the QTAIM theory) and kinetic isotope effects have been evaluated and discussed. The examination of further reaction steps for DMP1 allowed us to demonstrate that the reaction is nonreversible for that substrate, despite the similarity between DMP1 and TMP1. The tert-butyl group exerts its steric hindrance mainly on the very first branched reaction steps, favoring an alkyl-Rh TS arrangement lower in free energy than the alkyl-Rh migratory insertion onto the coordinated CO. In part the branched material returns to the reactant complex, thus enriching the linear fraction. (C) 2011 Elsevier B.V. All rights reserved.