Transition retro-allylation

Base removes bepzylic H, retro- allyl anion+alkene, all-s gives trans-cyclooctene retro [2+2], s on both a-bonds makes trani-double bond. This is an exercise in drawing believable transition structures like 6.42. 

In general, metal-catalyzed retro-allylation of homoallylic fcrf-alcohols 67 proceed by a six-membered transition state 68, and have become a relatively unorthodox method to generate allylmetal species 70 and ketones 69 [52]. 

In 1996, when transition-metal-catalyzed carbon-carbon bond cleavage was just emerging as a current topic in organic chemistry, Kondo and Mitsudo reported the first transition-metal-catalyzed retro-allylation of homoallylic alcohol to simply cleave off the allylic moiety with a ruthenium complex [7]. After 10 years of silence, transition-metal-catalyzed retro-allylation has been rapidly developing since 2006. Nowadays, catalytic retro-allylation can be regio- and stereoselective, hence being a useful tool for modern organic synthesis [8]. 

It is necessary to suppress this isomerization for selective reactions. The allylic transition metal complexes generated in situ can undergo protonation, fi-hydride elimination, nucleophilic addition, or reductive elimination. The fate of the initially formed o-allylic metal species depends on the reaction conditions, mainly on the character of the transition metal used. With proper conditions, retro-allylation is a useful tool in organic synthesis. This section categorizes the chemistry of retro-allylation according to the transition metals used. 

Kondo and Mitsudo are the pioneers of transition-metal-catalyzed retro-allylation [7]. They described this phenomenon as deallylation and fi-allyl elimination . Ruthenium-catalyzed reaction of tertiary homoallylic alcohols proceeds in the presence of allyl acetate under an atmosphere of carbon monoxide in tetrahydro-furan (THF) at a relatively high temperature of 180 C to afford the corresponding ketones (Scheme 5.3). Although the exact roles of allyl acetate and carbon monoxide are not clear, they propose a mechanism involving oxidative addition of the... 

The stereospecificity of the allylation in Scheme 5.12 is rationalized by identifying the most favorable transition state of the retro-allylation (Scheme 5.14). Oxidative addition of Np-Br followed by bromide-alkoxide exchange with 4 generates intermediate 6 or 7. In the case of threo-4,6a should be the more favorable conformation than 6b according to the routine conformational analysis of chairlike transition states. The retro-allylation thus predominantly proceeds from 6a to yield ( )-8. Thanks to bulky tricyclohexylphosphine, the following reductive elimination should take place rapidly prior to o-jt interconversion to retain the E-stereochemistry. A similar picture is applicable to the reaction of erythro-4, where 7a is more favorable than 7b to afford the corresponding Z-product selectively. 

The allyl transfer chemistry is applicable to the synthesis of aryl-substituted vinylic and allylic sUanes of synthetic importance [17]. Homoallyhc alcohol bearing a ferf-butyldimethylsUyl group at the allylic position participates in the allyl transfer to yield ( )-3-aryH-alkenylsilanes (Scheme 5.17). The transition state of the retro-allylation should have the bulky silyl group at the equatorial position, accounting for the -stereoselectivity. 

The reactions of optically active homoallylic alcohol with aryl bromides culminate in excellent chirality transfer to the corresponding crotylsilanes (Scheme 5.19). The chirality transfer is explained by the preference of transition state 12 due to sterics, which leads to fe-face selective retro-allylation, finally giving ( ,S)-products. 

Heteroatom to hydrogen transpositions in allylic systems (equation 43) can be observed in different reactions, namely Ae protonation of allyl metal compounds, in reductions of allyl heterocompounds and in retro-ene reactions. Except for the last reaction, which proceeds by a cyclic transition state, the problem of regioselectivity restricts the synthetic value of such reactions and it is only in specific cases that this can be overcome. 

The same group has applied this imino Diels-Alder reaction in an enantiosel-ective total synthesis of the alkaloid (-)-cannabisativine (Scheme 33) [70b]. An initial Sharpless epoxidation of allylic alcohol 183 provided enantiomerically pure compound 184, which could be converted in four steps to alcohol 185. This compound could then be relayed into the requisite diene 186. Imino Diels-Alder reaction of 186 led to a single cycloadduct 188, presumably via a transition state like 187. It was then possible to homologate the ester functionality of 188 via the corresponding aldehyde to acetal 189. This intermediate could be converted in several steps into 190. Another key step in the strategy was epimeriza-tion via a retro Michael reaction leading to aldehyde 191, which could be transformed in five steps into (-)-cannabisativine. 

Note that there are several modern versions of the Barbier reaction that use transition metals or their derivatives.Examples include indium metal, samarium iodide, and lead. An example using lead reacted benzaldehyde with allyl bromide in the presence of lead metal to give a 99% yield of 45.56 A retro-Barbier fragmentation has also been reported in which an alcohol was treated with 10 equivalents of bromine and 15 equivalents of potassium carbonate in chloroform to give a bromo ketone.5 ... 

M06/6-31-l-l-G calculations in the gas phase and using the CPCM (conductor-like polarizable continuum model) in solution have shown that the formal reaction of imidozirconocene with allylic ethers and an imidotitanium complex with allylic alcohols occurs by a [2 -I- 2] cycloaddition/retro- [2 -I- 2] pathway via central transition states rather than by the proposed [3,3] sigmatropic rearrangement mechanism. ... 

Concerted retro-Diels-Alder decomposition does occur in cyclohexene derivatives that bear substituents in the allylic positions in such compounds the energies of open-ring intermediates are higher than that of the transition state for the concerted process, which occurs near the thermochemical threshold (Turefiek and Havlas, 1986). The critical effect of substituents on the stereochemistry of the retro-Diels-Alder reaction was discussed in Section 8.3 (Turecek and HanuS 1984, Mandelbaum 1983). 

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