Hofmann-type elimination

The elimination of hydrogen halide from organic halides under basic conditions generates alkenes and is a fundamental reaction in organic chemistry. It is sometimes carried out with a base in aqueous media.14 In contrast, the corresponding Hofmann-type eliminations of quaternary ammonium hydroxides are frequently carried out in aqueous media,15 which will be covered in Chapter 11. 

Exposure of quaternary ammonium salt 163 to NaOMe gives methyl orthoester 37 and ketene acetal 54 as the consequence of a competing Hofmann-type elimination (Equation 33). Elution of 163 as a methanol solution through an anion-exchange resin (Dowex-550-OH, MeOH) leads exclusively to ketene acetal 54 in 89% yield <2004AGE1117>. 

The pyrazolo[l,2-a]pyridazine (55) reacts with sodium methoxide in boiling methanol, presumably via preliminary deprotonation at benzylic carbon. After 8 h a mixture of (56 72%) and the Hofmann-type elimination product (57 12%) is obtained, but after 25 h only (57 75%) is isolated. The imino bond u 1640 cm ) of (57) may be saturated with sodium borohydride. With potassium t-butoxide in t-butyl alcohol at reflux, the rearranged isoindole (58) is obtained (Scheme 22) (74CPB2142). Scheme 23 shows a rearrangement similar to (55) (56) for which a radical mechanism has been suggested. However, here,... 

HOBT = l-hydroxy-l/f-benzotriazole, 145, 231 Hofmann degradation, 139, 308, 331 Hofmann type elimination ... 

In contrast, the corresponding Hofmann-type eliminations of quaternary ammonium hydroxides are frequently carried out in aqueous media, which will be covered in Chapter 11. 

Hofmann-type elimination of aporphine salts such as glaucine methiodide, ethiodide, and benzyl bromide and hydrochloride was achieved in high yield through u.v. irradiation. ... 

In contrast to the relatively intractable laboratory oxidation products primary amines form on air and/or peroxide oxidation, the more stable oxides of tertiary amines have provided a platform for a significant amount of interesting chemistry. Among the reactions that have been explored in some detail are (1) the allylic N O rearrangements of suitably substituted A-oxides (Scheme 10.7), (2) the Cope (or Hofmann) type elimination reactions (vide infra and see Chapter 9) (Scheme 10.8), and (3) the Polonovsid reaction (Scheme 10.9), of particular interest because of its generality. 

Scheme 2.2 Representative example of a Hofmann-type elimination reaction [16].

Scheme 2.4 Activation of a hydroxyethyl group with an A1 site in an organoclay may facilitate a Hofmann-type elimination reaction. Reprinted from Ref. [16], with permission from Elsevier.

Scheme 1. Hofmann-type elimination reaction of OMMT.

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