Solvolysis skeletal rearrangements

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

A long-established feature of the carbocation intermediates of reactions, such as SnI solvolysis and electrophilic aromatic alkylation, is a skeletal rearrangement involving a 1,2-shift of a hydrogen atom, or an alkyl, or aryl group. The stable ion studies revealed just how facile these rearrangements were. Systems where a more stable cation could form by a simple 1,2-shift did indeed produce only that more stable ion even at very low temperatures (see, e.g., Eq. 3). 

All these fairly similar results are, however, in sharp contrast to solvolysis of cyclopropylmethyl chloride, which is much more reactive than cyclobutyl chloride or 4-chlorobut-2-ene (Roberts and Mazur, 1951 a). There was also positive evidence for internal return in the cyclopropylmethyl chloride solvolysis, although the solvent used (50 aqueous ethanol) is known to be unfavorable for internal return. Another reaction, closely related to the protolysis of cyclopropyldiazomethane mentioned above, is the reaction of that diazoalkane with ethereal benzoic acid. The product ratio cyclopropylmethanol cyclobutanol = 5.8 indicated a strong decrease of skeletal rearrangement (Moss and Shulman, 1968). 

The proposed scheme differs from that of Gray and Hart for the solvolysis of structurally similar epimeric alcohols 386 and 387 in the same medium, benzo-dihydropentalene being obtained as the main product. The configuration of the splitting C—O bond exerts no effect on the reaction course since the cpimerization of the parent alcohols (or esters) occurs faster than the skeletal rearrangement. This process may go via an intermediate classical cation, i.e. along the k, route (the authors do not suggest a mechanism of epimerization) ... 

As stiown recently by Jovanovich and Lambert syn-endo-6-tricyclo[3,2,l,0 ]-octenyl-8-p-nitrobenzoate 576 is solvolysed 10 times faster than compound 220 but 10 times more slowly than 573. There are no reasons for the accelerating effect of the syn-endo ring the solvolysis is not accompanied by skeletal rearrangements and, hence, there is no relief in the steric strain. The authors detected no bridge-flipping in the intermediate ion A which might lead to a more stable ion B. 

Anomalous oxidation products are observed from the oxidation of tetraalkylborate salts (i.e. Li(R2-9-BBN)), which produces bicyclo[3.3.0]octan-l-ol as a co-product through a skeletal rearrangement which occurs during the oxidation process. Moreover, the alkaline hydrogen peroxide oxidation of l,l-di-9-BBN derivatives gives primary alcohols rather than aldehydes because of their solvolysis prior to oxidation. ... 

The synthesis of (+ )-sesquifenchene (450) and of ( )-epi-P-santalene (451) from the common intermediate (449), derived from endo-dicyclopentadiene, has been detailed. Conversion of (449) into the precursor of (450) makes use of the skeletal rearrangement that occurs during solvolysis of active 2-norbornyl esters. In a synthesis of (-H )-hinesol (452) and 10-epi-(-l-)-hinesol, interesting use has been made of a fragmentation reaction.The tosyl derivative (453), which was obtained in several steps from ( —)-P-pinene, was converted into the spiro[4,5]decane (455) on treatment with sodium hydride in DMSO the essential stereoelectronic changes are summarized in the intermediate (454). 

The deltacyclane system 1 has a fusion of nortricyclane and norbomane skeletons and is an interesting substrate to study skeletal rearrangements or a-bond partiei-pations in solvolysis processes. 

Another possibility for the isomerization step is to consider alkyl (Wagner-Meerwein) shifts, which are frequently proposed to account for the skeletal rearrangements in earbocations. In Scheme 40.11 we have indicated a reasonable series of alkyl shifts that could justify the raeemization observed in the solvolysis products. C3 alkyl shift in 17 (from C2 to C8) would lead to cation 20 that by 1,2-carbon shift forms 21. Cations 20 and 21 could be considered as two extreme canonical forms of nonclassical isodeltacyclyl cation 11. We know from previous studies (acetolysis of brosylates 8 in Scheme 40.4) fliat if 11 is formed in the medium, exo-acetate 3 is the main solvolysis product. The isomerization of 20 to 22 occurs by 1,2-alkyl migration, as the formation of 23 from 22. Cations 22 and 23 could also be considered as two extreme canonical forms of nonclassical cation 24, which would give the enantiomer of the exo-acetate 3 by nucleophile attack of the solvent. ... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

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