Crowding

The sodium fusion and extraction, if performed strictly in accordance with the above directions, should be safe operations. In crowded laboratories, however, additional safety may be obtained by employing the follow ing modification. Suspend the hard-glass test-tube by the rim through a hole in a piece of stout copper sheet (Fig. 69). Place 1 -2 pellets of sodium in the tube, and heat gently until the sodium melts. Then drop the organic compound, in small quantities at a time, down — =. the tube, allowing the reaction to subside after each addition before the next is made. (If the compound is liquid, allow two or three small drops to fall at intervals from a fine dropping-tube directly on to the molten sodium.) Then heat the complete mixture as before until no further reaction occurs. 

A is no good because MeMgl would have to attack the more crowded ester first. So, how do we make B ... 

The steric correction term S, correcting for crowding at the /5 position in the case of N Or C as -substituent, is applied once for each non hydrogen/3-substi-... 

A highly diastereoselective alkcnylation of c/s-4-cyclopentene-l,3>diols has been achieved with 0-protected (Z)-l-iodo-l-octen-3-ols and palladium catalyst (S. Torii, 1989). The ( )-isomers yielded 1 1 mixtures of diastcrcomcric products. The (Z)-alkenylpalladium intermediate is thought to undergo sy/i-addition to the less crowded face of the prochiral cyclopentene followed by syn-elimination of a hydropalladium intermediate. 

In the total synthesis of zearaienone (451), the ester 450 was prepared by the carbonylation of the crowded aryl iodide 448. The alkyl iodide moiety in the alcohol molecule 449 is not attacked[306]. Methyl trifluoromethacrylate (453) was prepared by the carbonylation of 3,3,3-trifluoro-2-bromopropylcne (452), The carbonylation in the presence of alkylurea affords 454. which is converted into the trifluoromethyluracil 455[307],... 

Observed deviations from ideality are attributable to thiazole selfassociation. Such self-association is influenced by steric crowding as indicated by the behavior of methylthiazoles. The constants of selfassociation have been estimated for benzene solutions of thiazole (Kassoc = 3.2 at 5.5°C) and 5-methylthiazole at 6.5°C). 

The greater stability of an equatorial methyl group compared with an axial one IS another example of a steric effect (Section 3 2) An axial substituent is said to be crowded because of 1,3 diaxial repulsions between itself and the other two axial sub stituents located on the same side of the ring... 

In this case the relationship between stability and stereochemistry is easily explained on the basis of van der Waals strain The methyl groups on the same side of the ring m cis 1 2 dimethylcyclopropane crowd each other and increase the potential energy of this stereoisomer Steric hindrance between methyl groups is absent m trans 1 2 dimethylcyclopropane... 

A substituent is less crowded and more stable when it is equatorial than when It IS axial on a cyclohexane ring Ring flipping of a monosubsti tuted cyclohexane allows the substituent to become equatorial... 

FIGURE 5 5 Ball and spoke and space filling models of as and trans 2 butene The space filling model shows the serious van der Waals strain between two of the hydrogens in as 2 butene The molecule ad justs by expanding those bond angles that increase the separation between the crowded atoms The combi nation of angle strain and van der Waals strain makes as 2 butene less stable than trans 2 butene... 

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... 

Tertiary Least reactive most crowded Secondary Primary Methyl Most reactive least crowded... 

Clearly the steric crowding that influences reaction rates in 8 2 processes plays no role in Stvfl reactions The order of alkyl halide reactivity in 8 1 reactions is the same as the order of carbocation stability the more stable the carbocation the more reactive the alkyl halide... 

As crowding at the carbon that bears the leaving group decreases the rate of nude ophilic attack by the Lewis base increases A low level of steric hindrance to approach of the nucleophile is one of the special circumstances that permit substitution to pre dominate and primary alkyl halides react with alkoxide bases by an 8 2 mechanism m preference to E2... 

If however the base itself is a crowded one such as potassium tert butoxide even pn mary alkyl halides undergo elimination rather than substitution... 

Rate IS governed by stability of car bocation that is formed in loniza tion step Tertiary alkyl halides can react only by the SnI mechanism they never react by the Sn2 mecha nism (Section 8 9) Rate IS governed by steric effects (crowding in transition state) Methyl and primary alkyl halides can react only by the Sn2 mecha nism they never react by the SnI mechanism (Section 8 6)... 

The experimental observations combine with the principles of nucleophilic substi tution to give the picture of epoxide ring opening shown m Figure 16 5 The nucleophile attacks the less crowded carbon from the side opposite the carbon-oxygen bond Bond... 

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