Neopentyl derivatives, rearrangement

Successful substitutions at neopentyl-type substrates can be performed, but can be accompanied by rearrangements. The best results are obtained with small nucleophiles, for example halides, azide, or cyanide. Some representative examples are shown in Scheme 4.20, to illustrate the reaction conditions required. If electron-rich nucleophiles such as thiolates are used, substitutions at neopentyl derivatives can also occur via SET [95],... 

Dauben and coworkers observed unusual rate accelerations and ring-expansion rearrangements in the solvolysis of bicyclo[2.2.0]hexane-l-methyl / -nitrobenzoate, in agreement with Winstein s proposed nonclassical cyclobutylmethyl cations (Figure 3). ° Thus, the rate of the solvolysis of bicyclo[2.2.0]hexane-l-methyl p-nitrobenzoate is 7 x 10 times faster than that of the corresponding extrapolated rate for neopentyl derivative. The lack of scrambling of the label in the solvolysis of the 0-labeled... 

Esters derived from the primary alcohols are the most stable and those derived from the tertiary alcohols are the least stable. The decomposition temperature is lower in polar solvents, eg, dimethyl sulfoxide (DMSO), with decomposition occurring at 20°C for esters derived from the tertiary alcohols (38). Esters of benzyl xanthic acid yield stilbenes on heating, and those from neopentyl alcohols thermally rearrange to the corresponding dithiol esters (39,40). The dialkyl xanthate esters catalytically rearrange to the dithiol esters with conventional Lewis acids or trifluoroacetic acid (41,42). The esters are also catalytically rearranged to the dithiolesters by pyridine Ai-oxide catalysts (43) ... 

It is also evident from the data of Bly et al. (95), Jacobs and Macomber (91), and Garry and Vessiere (99) that neopentyl-type homoallenic systems do not yield cyclopropyl derivatives upon solvolysis, in contrast to the unsubstituted parent system. If they have no substituents at Cj or C3, neopentyl homoallenic substrates yield rearranged acyclic olefins and rearranged solvent-incorporated products exclusively. If they carry an alkyl substituent at Ci, they give both rearranged and unrearranged acyclic products. If a substituent is present on C3 besides the acyclic derivatives, cyclobutyl products also are formed. 

Rearrangement of the neopentyl cation labeled with deuterium in the 1 position (11) gave only tert-pentyl products with the label in the 3 position (derived from 13), though if 12 were an intermediate, the cyclopropane ring could just as well cleave the other way to give lert-pentyl derivatives labeled in the 4 position (derived from 14). ° Another experiment that led to the same conclusion was the generation, in several ways, of Me3C CH2. In this case, the only tert-pentyl products isolated were labeled in C-3, that... 

Neopentyl and related derivatives of the group-VIII transition metals in low oxidation states favor such a rearrangement (sometimes called y elimination), affording 3,3 -disubstituted metallacyclobutanes the reaction is less available earlier transition-metal analogsThis is an important route to metallacyclobutane, isolable examples of which are still relatively rare, but which are of great interest as reaction intermediates... 

Photochemistry of Thiophens.—Wynberg and co-workers have continued their investigations on the photochemistry of thiophens, and an excellent essay by Wynberg on the photochemistry and other aspects of thiophen chemistry has recently been published. The photoisomerizations of thiophens are also discussed in a detailed review article on the photoisomerization of five-membered heterocyclics. Irradiation of 2-methyl-, 2-benzyl-, 2-neopentyl-, and 2-t-butyl-thiophens leads to their irreversible transformation to the corresponding 3-substituted derivatives in 8— 27% yields. 2,5-Di-t-butylthiophen rearranges irreversibly to the 2,4-isomer, which is remarkably stable under the reaction conditions. Irradiation of 2,2 -bithienyl leads to 2,3 -bithienyl and a small amount of benzo[6]thio-phen. 2,3 -Bithienyl affords 3,3 -bithienyl and benzo[6]thiophen as major products. 5,5 -Dideuterio-2,2 -bithienyl gives 5,5 -dideuterio-2,3 -bithienyl... 

Primaiy alcohols with extensive jS-branching give large amounts of a product derived from rearrangement. For example, treatment of 2,2-dimethyl-l-propanol (neopentyl alcohol) with HBr gives a rearranged product almost exclusively. 

Reactions of this kind formally involve rearrangements of intermediate carbonium ions by a 1,2 migration of alkyl. In this case the carbonium ion initially formed is an unstable primary ion. The rearrangement leads to its conversion to a tertiary carbonium ion in which three — / alkyl groups are attached to the cationic center. The products are then derived from the resulting tert-amyl cation rather than neopentyl. 

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