T-butylallene

FIGURE 52. Cl 13C NMR signal of 6Li-labeled 1-lithio-l-ethoxy-t-butylallene (107) in THF-ds at —92 °C. Reprinted with permission from Reference 64. Copyright (1994) American Chemical Society... 

Classic work by Loftfield on the Favorskii reaction showed that cyclopropanones are intermediates in the base-induced rearrangement of a-haloketones (l heme 4). Isolation of such an intermediate was accomplished in the reaction of the sterically hindered a-bromodineopentyl ketone (9) with potassium p-chlorophenyldimethylcarbinolate. The identity of the product (10), rrans-2,3-di-t-butylcyclopropanone, was established by independent synthesis of 1,3-di-t-butylallene oxide (11) which underwent valence isomerization to (10) ... 

An interesting route to the cyclopropanone system involves the rearrangement of allene oxides, usually generated by the epoxidation of allenes. Thus, 1,3-di-t-butylallene oxide (11) may be prepared by the reaction of 1,3-di-t-butylallene with m-chloroperbenzoic acid. Heating 11 to 100 °C leads to isomerization, forming truns-2,3-di-t-butylcyclopropanone (10) (Scheme 4) Similarly, 1,1-di-t-butylallene (15) yields 2,2-di-t-butylcyclopropanone with peracetic acid (equation 7) ... 

With the aid of NMR, Li NMR and HOESY (heteronuclear Overhauser effect spectroscopy) NMR of a-lithiomethoxyallene (106) and l-lithio-l-ethoxy-3-t-butylallene (107) as well as by ab initio model calculations on monomeric and dimeric a-lithiohy-droxyallene, Schleyer and coworkers proved that 106 and 107 are dimeric in THF (106 forms a tetramer in diethyl ether) with a nonclassical 1,3-bridged structure. The NMR spectrum of allenyllithium in IHF is also in agreement with the allenic-type structure the chemical shift of C2 (196.4 ppm) resembles that of neutral allene (212.6 ppm), rather than C2 of propyne (82.4 ppm). 

Thermal dimerization of methylallene gives a mixture of all seven head-to-head dimers. The dimerization of t-butylallene at 220 °C for 72 h gives a mixture of the head-to-head dimers 28 and 29 At more elevated temperatures, in addition to the two cyclodimers, aromatic trimerization products are formed (see Section 6.1.3.). 

Also, from the nickel (o) complex of 1,2-cyclononadiene upon heating to 60 °C mixtures of trimers and oligomers are formed. From t-butylallene at 290 °C, a mixture of the two aromatic trimers 87 and 88 are formed (see also Section 6.1.2.) ... 

Metallation of allerns Allene is metallated at C, by n-butyllithium in THF at — 78°. In the absence of HMPT, the lithio derivative can be alkylated to give a 1-alkylallene (86-93% yield). Under the same conditions, a 1,1-dialkylallene is convertible into a pure 1,1,3-trialkylallene, but a 1-alkylallene is metallated at C, or C3, depending on the size of the alkyl group. When R contains more than four carbons, metallation is largely at C3. Metallation of 1,1,3-trisubstituted allenes requires t-butyllithium or n-butyllithium + 1 equiv. of HMPT 1,1 -dimethyl-3,3-di-w-butylallene can be prepared in this way in 86% yield. 

Allene epoxides. Synthesis of allene epoxides is difficult because of the tendency to undergo further epoxidation and to isomerize to the corresponding cyclopropanone (2, 308-309). The first known one was obtained by Camp and Green (2, 309, ref. 3g) by oxidation of 1,3-di-f-butylallene with m-chloroper-benzoic acid. Crandall and Machlcdcr 1 have now reported successful mono-epoxidation of 1,1,3-tri-t-butvlallene (1), also using m-chloroperbenzoic acid. The epoxide (2) is the main product but is accompanied by an oxetanone (3), undoubtedly formed via an intermediate diepoxide. The epoxide (2) is extraordinarily stable and shows no tendency to isomerize to a cyclopropanone. 

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