Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.

Transition metal-carboryne complexes: synthesis, bonding, and reactivity.

Abstract: The construction and transformation of metal−carbon (M−C) bonds constitute the central themes of organometallic chemistry. Most of the work in this field has focused on traditional M−C bonds involving tetravalent carbon: relatively little attention has been paid to the chemistry of nontraditional metal−carbon (M−Ccage) bonds, such as carborane cages, in which the carbon is hypervalent. We therefore initiated a research program to study the chemistry of these nontraditional M−Ccage bonds, with a view toward developing synthetic methodologies for functional carborane derivatives. In this Account, we describe our results in constructing and elucidating the chemistry of transition metal−carboryne complexes.Our work has shown that the M−Ccage bonds in transition metal−carboranyl complexes are generally inert toward electrophiles, and hence significantly different from traditional M−C bonds. This lack of reactivity can be ascribed to steric effects resulting from the carboranyl moiety. To overcome this steric p...

Rational Design of Benzyl-Type Protecting Groups Allows Sequential Deprotection of Hydroxyl Groups by Catalytic Hydrogenolysis

Abstract: Benzyl protection of a hydroxyl group is one of the most frequently used procedures in synthesis because of the mild conditions involved in its removal by catalytic hydrogenolysis.1-3 The synthesis of polyhydroxylated compounds often requires orthogonal protecting strategies to distinguish between hydroxyl groups. It would be highly desirable to develop a range of benzyl-type protecting groups with different reactivities that can be sequentially removed via catalytic hydrogenolysis. This requires a detailed understanding of the mechanism of the cleavage of the benzyl oxygen bond by the palladium hydrogen species. Recently, we have determined the amphipolar nature of the palladium hydrogen bond (modes a, Mδ+ Hδ-, or b, MδHδ+) in both homogeneous4 and heterogeneous5 hydrogenation of alkenes. This has led us to test whether the electronic properties of the aromatic group can influence the rate of cleavage, which should in turn guide the development of hydroxyl protecting groups with different reactivities. The results in Table 1 show that the rate of debenzylation can be dramatically affected by the electronic properties of the aromatic ring. The substitution of the electron-withdrawing trifluoromethyl group onto the aromatic ring severely retards debenzylation under 1 atm of hydrogen. In contrast, there is considerable acceleration by electrondonating substituents, which suggests that the benzylic carbon bears a partial positive charge in the transition state. The hydrogenolysis of benzyl alcohols carried out in acetic acid has shown that protonation of the hydroxyl group is essential for the cleavage of the carbon-oxygen bond.6 Under the neutral conditions in our study, the reaction may occur by protonation of the benzyl oxygen atom, through the operation of mode b, MδHδ+, to give a positively charged benzylic carbon. Alternativly, it is possible that palladium could act as a Lewis acid and coordinate to the benzyl oxygen atom to promote the same electron-deficient transition state (mode a, Mδ+ Hδ-). The large difference in reactivity within this range of substituted benzyl groups suggests that they can be sequentially deprotected, therefore proving useful in multistep synthesis. To test the synthetic application of these groups, competition experiments were conducted on model systems with two differently substituted benzyl groups attached to ethanediol (Scheme 1a). Surprisingly, the benzyl group was cleaved first in competition with any of the substituted benzyl groups. This phenomenon has been observed with the 4-methoxybenzyl group (PMB); however, no explanation was proposed.7,8 The results with the linker experiments (Scheme 1a) seem to contradict those obtained when only one benzyl group is involved (Table 1). Surface scientists have determined that the aromatic ring lies flat on the metal surface for optimal coordination.9,10 It is possible that substitution on the aromatic ring could have an adverse steric effect that would interfere with the planar geometry required for effective binding and thus reduce its affinity for the metal surface. The linker experiments show that the limited number of active sites on the palladium surface could lead to a competition for adsorption sites between substituted and unsubstituted benzyl groups. This may explain why the least substituted benzyl group, although not electronically favored, can still be preferentially cleaved. It is clear that for the rational design of selective benzyl type protecting groups both electronic factors and adsorption must be taken into account. For synthetic purposes, it would be desirable to find a more labile group than the benzyl group for protection of the hydroxyl functionality. We anticipated that the 2-naphthylmethyl (NAP) group would fulfill these criteria: it is electron rich and should have a (1) Greene, T. W.; Wuts, P. G. M. In Protective Groups in Organic Synthesis; John Wiley & Sons, Inc.: New York, 1991. (2) (a) Czernecki, S.; Georgoulis, C.; Provelenghiou, C. Tetrahedron Lett. 1976, 3535. (b) Iverson T.; Bundle K. R. J. Chem. Soc., Chem. Commun., 1981, 1240. (3) Czech, B. P.; Bartsch, R. A. J. Org. Chem. 1984, 49, 4076. (4) Yu, J.; Spencer, J. B. J. Am. Chem. Soc. 1997, 119, 5257. (5) Yu, J.; Spencer, J. B. J. Org. Chem. 1997, 62, 8618. (6) Kieboom, A. P. G.; De Kreuk, J. F.; Van Berkum, H. J. Catal. 1971, 20, 58. (7) Srikrishna, A.; Viswajanani, J. A.; Sattigeri, J. A.; Vijaykumar, D. J. Org. Chem. 1995, 60, 5961. (8) Sajiki, H.; Kuno, H.; Hirota, K. Tetrahedron Lett. 1997, 38, 399. (9) Lin, R. F.; Koestner, R. J.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1983, 161. (10) Held, G.; Bessent, M. P.; Titmuss, S.; King, D. A. J. Chem. Phys. 1996, 11305. Table 1a

Ruthenium-Catalyzed Regiospecific Borylation of Methyl C−H Bonds

Abstract: We report the regiospecific, ruthenium-catalyzed borylation of saturated terminal C−H bonds. Alkylboronates were obtained in 78−98% yields. The borylations of alkanes, trialkylamines, protected alcohols, and fluoroalkanes occurred regiospecifically at the methyl group that is least sterically hindered. In contrast to most organometallic C−H activation, the reactions of alkanes occurred in higher yields than the reactions of arenes. Reactions were conducted that probed steric and electronic effects on the alkyl borylation. These reactions showed that the borylation occurred preferentially at the methyl group that is least sterically hindered and most electron-deficient. Ruthenium compounds containing boryl ligands were synthesized, and one was characterized by X-ray crystallography. One of these compounds contained a rare bridging boryl ligand and served as a catalyst precursor for the borylation of octane.

Tuning Nickel with Lewis Acidic Group 13 Metalloligands for Catalytic Olefin Hydrogenation.

Abstract: A series of bimetallic complexes pairing zero-valent nickel with group 13 M(III) ions is reported. Stronger Ni→M(III) dative bonds that render Ni more electron-deficient are seen for larger ions (In > Ga > Al). The larger Ga and In ions stabilize rare, nonclassical Ni–H2 adducts that catalyze olefin hydrogenation. In contrast, neither the Ni–Al complex nor a single nickel center enables H2 binding or olefin hydrogenation. By comparison of the structures, redox properties, and catalytic activities of the Ni–M series, the electronic and steric effects of the supporting metal ion are elucidated.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs.

Abstract: The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or...