Bottom face

The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface As the molecular model m Figure 6 3 shows one of the methyl groups on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst The bottom face of the double bond is more exposed and both hydrogens are transferred from the catalyst surface to that face... 

Figure 7 7 shows why equal amounts of (R) and (5) 1 2 epoxypropane are formed m the epoxidation of propene There is no difference between the top face of the dou ble bond and the bottom face Peroxyacetic acid can transfer oxygen to either face with equal facility the rates of formation of the R and S enantiomers of the product are the same and the product is racemic... 

In this example addition to the double bond of an alkene converted an achiral mol ecule to a chiral one The general term for a structural feature the alteration of which introduces a chirality center m a molecule is prochiral A chirality center is introduced when the double bond of propene reacts with a peroxy acid The double bond is a prochi ral structural unit and we speak of the top and bottom faces of the double bond as prochiral faces Because attack at one prochiral face gives the enantiomer of the com pound formed by attack at the other face we classify the relationship between the two faces as enantiotopic... 

Approach of borohydnde to the top face of the carbonyl group is sterically hindered by one of the methyl groups The bottom face of the carbonyl group is less congested and the major product is formed by hydride transfer from this direction... 

Acoustical Louvers. Acoustical louvers are used in building mechanical systems when exterior walls are penetrated for fresh air intake, exhaust, or rehef air, in situations where the impact of HVAC noise is of concern in the surrounding environment. The louvers consist of a series of hoUow sheet metal blades. The bottom faces of the louver blades are perforated and the blades are filled with fibrous sound-absorbing material. Typical acoustical louvers are 20 cm (8 in.) to 30 cm (12 in.) in depth. The amount of insertion loss they provide is limited. 

A popular connection system consists of square metal pins, usually 0.064 cm (0.025 in.) in size, that are pressed into holes drilled in a printed circuit board. The holes are copper (qv) plated on the insides and interconnect conductors on the top and bottom faces of the board. Multilayer boards have interior circuits that may also be interconnected in this way. The pias have either a soHd shank or a deformable (compHant) cross section where the pias joia the board (Fig. 2). Separable connectors or soldedess wraps (Fig. 3) engage the ends of the pias. One end of the pia can be the contact and spring of a separable connector. 

Figure 12.7 Ribbon diagram of one subunit of potin from Rhodobacter capsulatus viewed from witbin tbe plane of tbe membrane. Sixteen p strands form an antiparallel p barrel tbat traverses tbe membrane. Tbe loops at tbe top of tbe picture are extracellular whereas tbe short turns at tbe bottom face the periplasm. The long loop between p strands 5 and 6 (red) constricts the channel of the barrel. Two calcium atoms are shown as orange circles. (Adapted from S.W. Cowan, Curr. Opin. Struct. Biol. 3 501-507, 1993.)...

The structure of cholic acid helps us understand how bile salts such as sodium taurocholate promote the transport of lipids through a water-rich environment. The bottom face of the molecule bear s all of the polar groups, and the top face is exclusively hydrocarbon-like. Bile salts emulsify fats by forming micelles in which the fats are on the inside and the bile salts are on the outside. The hydrophobic face of the bile salt associates with the fat that is inside the micelle the hydrophilic face is in contact with water on the outside. 

The assumed transition state of this reaction is shown in Scheme 5.3. Yb(OTf)3, (J )-(-h)-BINOL, and DBU form a complex with two hydrogen bonds, and the axial chirality of (J )-(-h)-BINOL is transferred via the hydrogen bonds to the amine parts. The additive would interact with the phenolic hydrogen of the imine, which is fixed by bidentate coordination to Yb(III). Because the top face of the imine is shielded by the amine, the dienophiles approach from the bottom face to achieve high levels of selectivity. 

Because of their cyclic structures, cycloalkanes have two faces as viewed edge-on, a "top" face and a "bottom" face. As a result, isomerism is possible in substituted cycloalkanes. For example, there are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyl groups on the same face of the ring and one with the methyls on opposite faces (Figure 4.2). Both isomers are stable compounds, and neither can be converted into the other without breaking and reforming chemical bonds. Make molecular models to prove this to yourself. 

Tell whether each of the following substituents on a steroid is axial or equatorial. (A substituent that is Tip" is on the top face of the molecule as drawn, and a substituent that is "down" is on the bottom face.)... 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

What about the configuration at C2, the newly formed chirality center As illustrated in Figure 9.16, the stereochemistry at C2 is established by reaction of H20 with a carbocation intermediate in the usual manner. But this carbocation does not have a plane of symmetry it is chiral because of the chirality center at C4. Because the carbocation has no plane of symmetry, it does not react equally well from top and bottom faces. One of the two faces is likely, for steric reasons, to be a bit more accessible than the other face, leading to a mixture of R and 5 products in some ratio other than 50 50. Thus, two diastereomeric products, (2/L4 K)-4-methyl-2-hexanol and (25,4/ )-4-methyl-2-hexanol, are formed in unequal amounts, and the mixture is optically active. 

The stereochemical course of the reaction may be rationalized by assuming a six-mem-bered chelate which presumably exists in a chair-like conformation. Molecular models support the assumption that the nucleophile preferentially attacks from the bottom face of the complex due to the steric hindrance of the axial A-methyl group. 

Fig. 1. The structure of gas hydrates containing a hydrogen-bonded framework of 46 water molecules. Twenty molecules, arranged at the comers of a pentagonal dodecahedron, form a hydrogen-bonded complex about the comers of the unit cube, and another 20 form a similar complex, differently oriented, about the centre of the cube. In addition there are six hydrogen-bonded water molecules, one of which is shown in the bottom face of the cube. In the proposed structure for water additional water molecules, not forming hydrogen bonds, occupy the centres of the dodecahedra, and...

According to molecular mechanics (MM) calculations, the minimum energy conformation of the enolate is a twist-boat (because the chair leads to an axial orientation of the f-butyl group). The enolate is convex in shape with the second ring shielding the bottom face of the enolate, so alkylation occurs from the top. 

A model for the stereochemical outcome of the dihydroxylation has been proposed (Figure 3.3).72 An olefin positioned according to this model will be attacked either from the top face with DHQD (3.6) ligands or from the bottom face with DHQ (3.7) ligands. 

In 1994, lithium amide 23 was used in the conjugate addition of 2-cyclohexenone to afford optically active adduct with up to 97% ee (Scheme 13).28-29 A dimeric structure was proposed as the intermediate, where the phenyl group in 23 blocked the bottom face and the cyclohexenone substrate approached from the upper face. 

The utility of bis(oxazoline)-Cu(II) complexes as catalysts for the Diels-Alder reaction has been examined in a number of other systems. Aggarwal et al. (205) demonstrated that a-thioacrylates behave as effective two-point binding substrates for these catalysts. With cyclopentadiene, catalyst 271d induces the reaction at -78°C to provide the cycloadduct in 94 6 diastereoselectivity and >95% ee. Aggarwal proposes that the metal binds to the carbonyl oxygen and to the sulfur atom. The sulfur substituent is placed opposite the ligand substituent thereby shielding the bottom face of the alkene. Considerably lower selectivities are observed with 5-Me substrates. 

Second step The new o bond forms between the bottom face of the double bond on the left and the bottom face of the double bond on the right, giving the observed, less thermodynamically stable product. 

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