Caibocation primary

Neopentyl (2,2-dimethylpropyl) systems are resistant to nucleo diilic substitution reactions. They are primary and do not form caibocation intermediates, but the /-butyl substituent efiTectively hinders back-side attack. The rate of reaction of neopent>i bromide with iodide ion is 470 times slower than that of n-butyl bromide. Usually, tiie ner rentyl system reacts with rearrangement to the /-pentyl system, aldiough use of good nucleophiles in polar aprotic solvents permits direct displacement to occur. Entry 2 shows that such a reaction with azide ion as the nucleophile proceeds with complete inversion of configuration. The primary beiuyl system in entry 3 exhibits high, but not complete, inversiotL This is attributed to racemization of the reactant by ionization and internal return. 

One important experimental fact is that the rate of reaction of alcohols with hydrogen halides increases in the order methyl < primary < secondary < tertiary. This reactivity order parallels the caibocation stability order and is readily accommodated by the mechanism we have outlined. 

As noted eaiTier (Section 4.10) primary caibocations aie too high in energy to be intennediates in most chemical reactions. If primaiy alcohols don t fonn primary caibocations, then how do they undergo elimination A modification of our general mechanism for alcohol dehydration offers a reasonable explanation. For primary alcohols it is... 

Unbranched primary alcohols and tertiary alcohols tend to react with hydrogen halides without reanangement. The alkyloxonium ions from primary alcohols react rapidly with bromide ion, for exfflnple, in an Sn2 process. Tertiaiy alcohols give tertiaiy alkyl halides because tertiaiy caibocations are stable and show little tendency to reanange. 

The vinylsilane-terminated cyclization strategy has been extended to the preparation of eight-membered cyclic ethers. Oxocenes (Scheme 17) with A -unsaturation (3,6,7,8-tBtrahydro-2ff-oxocins) were prepared efficiently by the SnCU-catalyzed cyclization of the mixed acetals (39) with conqilete re-giochemical control. Electrophilic addition on the 2-(trimethylsUyl)-l-alkene occurs predmninantly at the terminal position of the alkene to form a tertiary a-silyl, rather than a primary -silyl, caibocation. The... 

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