Regioselectivity rotational barriers

Houk, K.N., Williams Jr., J.C., Mitchell, P.A. and Yamaguchi, K. (1981). Conformational control of reactivity and regioselectivity in singlet oxygen ene reactions Relationship to the rotational barriers of acyclic alkylethylenes. J. Am. Chem. Soc. 103, 949-951... 

In 2002, Winter and coworkers reported that aminoallenylidene complexes trans-[Cl(dppm)2Ru=C=C=C(NRR )(CH3)] were obtained from the regioselective addition of secondary amines to frans-[Cl(dppm)2Ru=C=C=C=CH2] [133]. They also found that unsymmetrically substituted amines gave rise to Z/E isomeric mixtures. To study the Z/E isomeric interconversion, they calculated the rotational barrier around the C—N bond of the model complex shown in Scheme 4.23. The orthogonal... 

However, the barrier to rotation does not always predict the regioselectivity of the ene reaction of O2 with alkenes. As shown latef, it is the non-bonded interactions in the isomeric transition states that control product formation and barriers to rotation are rather irrelevant. The calculated rotational barrier values, with the HF-STO-3G method, for the allylic methyls in a series of trisubstituted alkenes, as well as the experimentally observed ene regioselectivity of a series of selective substrates, are shown in Table 9. ... 

Risk labels, lATA/ICAO, 751-3 Risk phrases, 621, 748, 749 River water, peroxide determination, 642 RNA, ozone disinfection, 616 ROS see Reactive oxygen species Rose Bengal sensitized photooxidation, 890 Rotational barriers, regioselective allylic hydroperoxide formation, 836, 847-9 Rotational isomers, peroxynitrous add, 8-9 Rotational spectra, ozonides, 721, 722-3 RP-HPLC, hydrogen peroxide determination, 627... 

For example, molecular mechanics calculations (MM2) showed that the methyl group geminal to the neopentyl group in 2,3,5,5-tetramethyl-2-hexene (51, Scheme 18) has the lowest rotational barrier and is the most reactive [79], Furthermore, the ethyl group in 2,3-dimethyl-2-pentene (52) has a much higher rotational barrier (5.76 kcal/mol) than the methyl groups and is totally inactive to 102. Similar trends hold with 2-methyl-2-butene (4). However, the barrier to rotation does not always predict the regioselectivity of the ene reaction of 102 with alkenes. As it was shown later [62], it is the nonbonded interactions in the isomeric... 

The calculated rotational barrier values for the allylic methyls, with the HF-STO-3G method [62], in a series of trisubstituted alkenes and the experimentally observed ene regioselectivity of a series of selective substrates are shown in Table 7. 

Orfanopoulos, M., Stratakis, M., Elemes, Y., and Jensen, R, Do rotational barriers dictate the regioselectivity in the ene reactions of singlet oxygen and triazohnedione with alkenes /. Am. Chem. Soc., 113, 3180,1991. 

Coldham and coworkers have shown that the asymmetric deprotonation protocol can be used to regioselectively alkylate the 5-position of imidazolidines (Scheme 35). The process is used as part of a sequence that results in asymmetric alkylation of 1,2-diamines with high stereoselectivity. The yields are limited, in this case, by the barrier to rotation around the carbamate C—N bond. Thus, only the amide rotamer having the carbonyl group syn to C-5 of the heterocycle is deprotonated. There are several examples in this review where this limitation is possible whether it is a factor or not may depend on the temperature at which amide bond rotation occurs versus the stability of the organolithium compound. In this case, the barrier to amide bond rotation was determined as 16.6 kcalmoD at 60 °C. 

The reverse reaction corresponds to intramolecular C—H bond formation. It requires that the alkene rotate from its equilibrium perpendicular orientation toward the less favored parallel orientation. The barrier hindering the rotation of the alkene may be partially or totally offset by the incipient agostic interaction. If the alkene is unsym-metrical, the question of regioselectivity of hydride transfer arises. In the case of an unsymmetrical alkene with an X or C substituent, the donor orbital is polarized away from the substituent (Figure 13.9) and the metal lies closer to that end. Conversely,... 

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