Carbon crowded substituents

It is well-known that multiple bonds involving heavier main group elements are unstable, and thus, some stabilizing techniques are needed to prepare compounds with heavy unsaturated skeletons. Kinetic stabilization utilizing steri-cally crowded substituents to stabilize a reactive species is a method to obtain such multiple bonding, and many kinds of kinetically stabilized heavy multiple bonds have been derived so far [Ij. As for phosphorus compounds, in 1978 Bickelhaupt and coworkers reported the first kinetically stabilized phosphorus-carbon double bond I (phosphaethene) [2], and in 1981 Yoshifuji and coworkers reported the first stable phosphorus-phosphorus double bond II (diphosphene)... 

Steric effects may be an even more important factor m controlling the regioselec tivity of addition Boron with its attached substituents is much larger than a hydrogen atom and becomes bonded to the less crowded carbon of the double bond whereas hydrogen becomes bonded to the more crowded carbon... 

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions for both steric and electronic reasons. Sterically, the presence of only one large substituent bonded to the C=0 carbon in an aldehyde versus two large substituents in a ketone means that a nucleophile is able to approach an aldehyde more readily. Thus, the transition state leadingto the tetrahedral intermediate is less crowded and lower in energy for an aldehyde than for a ketone (Figure 19.3). 

In steric terms there is a relief of crowding on going from the initial halide, with a tetrahedral disposition of four substituents about the sp3 hybridised carbon atom, to the carbocation, with a planar disposition of only three substituents (cf. five for the SN2 T.S.) about the now sp2 hybridised carbon atom. The three substituents are as far apart from each other as they can get in the planar carbocation, and the relative relief of crowding (halide - carbocation) will increase as the substituents increase in size (H- Me- Me3C). The SN1 reaction rate would thus be expected to increase markedly (on both electronic and steric grounds) as the series of halides is traversed. It has not, however, proved possible to confirm this experimentally by setting up conditions such that the four halides of Fig. 4.1 (p. 82) all react via the SN1 pathway. 

Reactions of nitrile oxides with 1,1-disubstituted alkenes afford products in which the oxygen atom of the nitrile oxide gets attached to the most crowded carbon atom of the dipolarophile. This high regioselectivity does not seem to depend on the type of substituent present on the alkene (142-152). Some of the results cannot be satisfactorily interpreted on the basis of FMO theory (149,151). Both steric and electrostatic effects often counteract each other and contribute to the regioselectivity actually observed. With trisubstituted alkenes, the orientation of cycloaddition is apparently dominated by this phenomenon. The preference is for the more substituted carbon atom to end up at the 5-position of the heterocyclic product (153,154). However, cases of opposite regiodifferentiation are also found, in particular with donor-substituted alkenes (42,155-157) (Scheme 6.21). 

In agreement with these results a model has been proposed that involves coordination of the lithium of the enolate with the pro-R doublet of the oxygen linked to carbon 5 of the alkylating reagent. It has been established that the methyl substituent of the sulfate is in a very good position for the alkylation of the Si-face of the enolate. From this model, it is expected that the alkylation of the Re-face should become difficult as crowding of the syn-area of the enolate is increased l. 

The helical structure is largely caused by crowding of the bulky lateral substituents, pendant on each backbone carbon atom. In addition, if not relieved by tautomeric structural changes, the accumulation of vicinal dipoles, as are known to destabilize vicinal triketones, should cause a tendency for a molecule to assume a progressively twisted conformation that would allow an orthogonal rather than a parallel situation of neighboring dipoles. The core, then, of the cylinder is made up... 

Increasing crowding of alkyl groups or electron withdrawing substituents at a carbon causes a successive downfield shift of its 13C resonance (Table 3.3). Similarly, crowding of shielding heteroatoms such as iodine successively reinforces upheld shifts (Table 3.3). 

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