Hybridized centers

Such reductive ring contractions of sulfones are formally similar to two other methods capable of supplanting a sulfur atom by a carbon-carbon double bond the Ramberg-Backlundand Stevens rearrangements. The distinguishing feature of this novel approach to cyclobutenes consists in the resultant higher level of alkyl substitution at the sp -hybridized centers. 

The CpCo complexes, on the other hand, should be more stable due to the presence of the robust and bulky Cp-shield. Unfortunately, however (tetraiodo-cyclobutadiene)CpCo is not available, and there is no obvious synthetic path to make it. But maybe another way to produce CpCo-stabiHzed tetraethynylated cyclobutadiene complexe exists It is known, that 22 a undergoes a rearrangement to 22 d when subjected to the conditions of flash vacuum pyrolysis at elevated temperatures [24]. The driving force behind this rearrangement is twofold first, the steric strain between the two adjacent TMS groups is removed in 22d and second, the TMS groups in 22d are not bound to an -hybridized center but to an sp-hybridized one, which is a more favorable situation from a thermodynamic point of view. 

Scheme 7.7, ORTEP adapted from reference 32), with [BCCgFj) or [CBnHgBrg] as the counterions. The X-ray structure clearly shows evidence of an sp hybridized center with a C -C -C angle of 178.8° and an abnormally short C -C double bond distance of 1.22 A (compared to 1.32 A, Table 7.1), and a nearly normal C -CA (sp -sp) distance of 1.45 A. The most striking feature of 8, however, is the very long C -Si distance of 1.97 A (compared to 1.87 A, Table 7.1), which is attributed to hyperconjugation, as shown in the scheme. Elongation of the bonds a- to the cation center is a characteristic of hyperconjugation, also observed in the structure of the adamantyl cation. ...

The strongly bent nature of the NBOs of W(CH2)3 is depicted in Fig. 4.19, confirming the inadequacy of a simple a/7t-bonding picture for this species. Both contour plots are plotted in a plane perpendicular to the CH2 plane, showing the expected bending of the nominal a- and 7t-bond-forming hybrids centered both on M and on C in directions above and below the internuclear M—C axis. While the NBOs can still be crudely described with nominal a and 7t labels, incipient banana-bond character is clearly significant. 

More complicated cross-terms between the different intramolecular degrees of freedom are also employed in some force fields, but we will not consider them in the following. The dihedral term may also include four-center improper torsion or out-of-plane bending interactions that occur at sp2 hybridized centers.29... 

Hybridization of substrate at the electrophilic center undergoing reaction—small at sp centers" large a-effects have been reported for reactions at sp and sp hybridized centers ... 

A second reaction involves the chlorin-to-porphyrin conversion. Any chlorin which has hydrogen atoms at the sp -hybridized centers of the reduced ring can be oxidized to the respective porphyrin. Oxidation may be achieved by various oxidants including oxygen . Likewise, rednctions to hydroporphyrins and other reactions of the macrocycle are possible. However, most of these are of interest only for the specialist. Under... 

The hybridization of a carbon determines to a great extent the range within which its 13C signal is found. As illustrated in Fig. 3.3, sp3 carbons resonate at highest field, followed by sp carbons, while sp2 hybridized centers are shifted farthest to low field. The hybridization effect in 13C NMR thus parallels the effect observed in H NMR. The 13C resonances of sp3 carbons are found between — 20 and 100 ppm relative to TMS sp carbons resonate from 70 to 110 ppm the low field sp2 carbon signals occur at 120 to 240 ppm in organic compounds. 

A comparison of diethyl amide 107, 3-pentyl ketone 108, 3-pentyl ether 109, and alcohol 110 was conducted in order to examine the effects of the side-chain conformation on both potency and selectivity in the carboxamide series of inhibitors (Fig. 14).100 The ketone, ether, and alcohol resembled the amide in that all showed selective inhibitory activity towards influenza A NA. An sp2-hybridized center was found to contribute to the potency of 107 and 108. 

The application of the LCAO method to ethene yields two TT-type MOs, since two sp2-hybridized centers build up the -electron system. Butadiene contains four TT-type MOs because four. sp2-hybridized centers build up the conjugated -electron system. The MOs of ethene and butadiene and their occupations are shown in Figure 15.4. In the )t-MO diagrams... 

While the present discussions focus on orbital hybridization relative to bonds between atoms, it is important to recognize that nonbonding electron pairs (lone pairs) also participate in orbital hybridization. Thus, as illustrated in Figure 5.4 and relating to. sp3-hybridized centers, for the purposes of determining orbital hybridization, lone pairs can be treated as bonds between a central atom and nothing. 

Although unexpected, the above reduction and methanol addition process does appear to be well established. For instance, the relevant product, 5.96, has been subject to X-ray diffraction analysis (Figure 5.5.5). The structure so obtained revealed that complex 5,96 is bowl shaped, and does, indeed, possess a meso-position that has been converted into a methoxy-bearing sp -hybridized center. Consistent with this structure and the addition of methanol it implies, is the observed loss of a Soret-like band in the visible spectrum of 5.96. Also consistent with it is the complex nature of the H NMR spectrum. Complex 5.96 actually exists as four stereoisomers (an enantiomeric set of diastereomers). These stereoisomers were found to interconvert readily in solution. This interconversion is slow on the NMR time scale. Thus, all species are seen in the NMR spectrum and this, as well as the loss in s)mimetry, contributes to the complexity of the H NMR spectrum. 

Direct nucleophilic substitution at an sp -hybridized center is not likeiy under common reaction conditions. Thus, nucleophiiic substitution reactions at such centers usually are broken into two steps. (For exceptions to this hint, see Dietze, P. Jencks, W. P. J. Am. Chem. Soc. 1989, III, 5880-5886, and references cited therein. 

There are several reasons why direct substitutions occur at xp -hybridized centers less readily than at sp centers. First, because there is more s character in the bond to the leaving group, this bond is stronger than the... 

HIV protease inhibitors are designed to mimic the transition state of hydroly.sis at the active site the.se compounds are called analogue inhibitors. Hydrolysis of a peptide bond proceeds through a transition state that is sp hybridized and. hence, tetrahedral. The analogue inhibitors possess a preexisting sp hybridized center that will be drawn into the active. site (one hopes with high affinity) but will not be cleavable hy the enzyme. This principle has been u.scd to prepare hundreds of potentially useful transition state inhibitors. Unfortunately, very few of the.se are likely to be clinically successful candidates for the treatment of HIV infection. Since HIV protease inhibitors are aimed at arresting replication of the vims at the maturation. step to prevent the... 

Similarly, when an aldehyde or ketone is treated with p-toluenesulfonylhydrazine (P-CH3C6H4NHNH2), a p-toluenesulfonylhydrazone (tosylhydrazone) (Scheme 9.69) is formed. Now, when this stable, easily purified carbonyl derivative is treated with at least two equivalents of a strong base (such as methyllithium, CH3Li),proton loss on the carbon adjacent to the xp -hybridized center (i.e., the a-carbon) occurs and an organolithiated alkene results. Hydrolysis of the latter results in alkene formation and the consummation of the Shapiro reaction. ... 

Zero or one exomethylene per ring (introduced to provide a single sp2-hybridized center within the ring and two additional vectors (the CH bonds) off the ring)... 

Sn2 reactions are generally not observed at r -hybridized centers, because back-side attack is sterically encumbered. In addition, vinyl halides and aryl halides are also unreactive toward S l, because loss of a leaving group would generate an unstable carbocation ... 

The phenyl position is sp hybridized, and 5 2 processes do not occur at sp -hybridized centers. The other position is a primary position, and Sn2 processes can occur readily at primary substrates. Therefore, we must start with phenol and an ethyl halide. 

The phenol is not further converted into a hafide, because neither S],j2 nor S],jl processes are efficient at r -hybridized centers. 

The reaction mechanism cannot be drawn like this because Sivf2 reactions do not occur readily at r/) -hybridized centers. When drawing a nucleophilic acyl substimtion, make sure to draw the first step, which forms the tetrahedral intermediate, followed by the second step, which shows how the carbonyl group is re-formed. 

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