Rearrangements pivalic acid

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

Although 68e could not be prepared, the less reactive pivalic acid ester analogues 71a-d (Scheme 29) were synthesized. Hydrolysis rate constants at 40°C were pH independent from pH 1.0 to 7.0 and k i,s correlated with to give a of —6.0. The products derived from 71a-c are summarized in Scheme 29. These products are consistent with a nitrenium ion mechanism. In particular, the products and product ratios derived from the meta-hxomo ester 71b were consistent with those previously reported for 68e and the Bamberger rearrangement of N-(3-bromophenyl)hydroxylamine, with the exception of the rearrangement products. The rearrangement products 72 appear to be derived from intramolecular aminolysis of 73. These... 

The para-nitro ester 71d generated only 4-nitroaniline (70%) and 4,4 -dinitroazoxybenzene (10%) when it underwent decomposition (Scheme 29). These products could have been derived from either a triplet nitrene or a triplet nitrenium ion precursor. Homolysis of the N—O bond to generate radical intermediates was ruled out because of the nearly quantitative yield of pivalic acid derived from 71d. The pivaloxy radical would have undergone rapid decarboxylation to generate CO2 and the rert-butyl radical under these conditions. Since no rearrangement product was observed, it was tentatively concluded that this ester underwent direct decomposition to 4-nitrophenyl-nitrene without the intermediacy of a nitrenium ion. ... 

As expected, the reaction products after decompression and quenching with hydrochloric acid and ice water were pivalic acid, terr-butyl isobutenyl ketone (in the absence of a hydride donor, this unsaturated ketone replaced the saturated congener, and no isopropyl methyl ketone by rearrangement of the pivaloyl cation could result). An unexpected crystalline material also appeared in small yield when the aqueous solution was saturated with sodium chloride for facilitating the extraction of organic reaction products. This proved to be a pyrylium salt, as will be shown below. 

Startg. alcohol heated 1 hr. at 138° with 7 equivalents ethyl orthoacetate in the presence of a catalytic amount of propionic acid with removal of the resulting alcohol by distillation product. Y 92%. - This rearrangement is highly geo-selective, giving trans-trisuhst carbon-carbon double bonds. F. e. s. W.S. Johnson et al.. Am. Soc. 92, 741 (1970) s. a. C. A. Henrick, F. Schaub, and J. B. Siddall, ibid. 94, 5374 (1972) N-heterocyclics with pivalic acid as catalyst s. F. E. Ziegler and G. B. Bennett, ibid. 93, 5930 (1971). 

Acid Catalyst for Rearrangements. When guaiacol and triethyl orthoacrylate were reacted in refluxing benzene using propionic acid as a catalyst, only 50% of the desired Claisen rear-rangement/ring closure product was obtained, due to esterification of the catalyst. Use of the more hindered pivalic acid furnished the desired product in quantitative yield (eq 1) see also Sato et al. and Dorsey et ah ... 

Treatment of 3 (see Vol.27, p.248 and also Section 5) with either tris(trimethyl-silyl)silane or tributylstannane - AIBN led to the formation of 4 as major product, via a rearrangement of the initially-formed radical. When similar chemistry was carried out on 5 (the major isomer formed by treatment of the relevant l, 2 -ene with NBS - pivalic acid when Tips, as opposed to Tbdms, protection was used), the p-D-orafeino-nucleoside 6 was formed as major product,... 

TroponesReaction of vinyldiazomcthanes (1) with l-methoxy-l-(trimethyl-silyloxy)butadiene (2) catalyzed by rhodium(H) acetate or rhodium(ll) pivalate results in [3+4]cycloaddition via cyclopropanation/Cope rearrangement to form a cyclohcptadiene (3). Short exposure of 3 to citric acid followed by oxidation (DDQ) provides the... 

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