Wagner-Meerwein isomerization

In a few cases, even the raw oils directly isolated from the plant have been submitted to the transformation with syngas [67]. Product aldehydes possess an antimicrobial activity and have a pleasant smell. Therefore they can be used well with malodorous substances. In most cases, Rh or Co catalysts were submitted to hydroformylation. Also Pt/Sn catalysts should be taken into consideration, since undesired Wagner-Meerwein isomerization is suppressed. All aldehydes and acetals, as well as their mixtures, show woody, floral, and green, or balsam-like odors and can be employed as components of perfume accords. 

Expecting that acidic isomerization involving a double Wagner-Meerwein rearrangement would transform the bicyclic olefin 74 into supposedly stable [10.10]betweena-nene and its (Z)[10.10] isomer, they treated 74 with H2S04-AcOH in benzene only to find that the product was an 85 15 mixture of 75 and 76. Solvolysis of the spiro compound 77 was also found to yield a 60 40 mixture of 75 and 76 which was totally free from the fused (Z)[10.10] and ( )[10.10] olefins. 

Fig. 14.4. Wagner-Meerwein rearrangement in the isomerization of an alkyl halide.

A Wagner-Meerwein rearrangement can be part of the isomerization of an alkyl halide (Figure 14.4). For example, 1 -bromopropane isomerizes quantitatively to 2-bromopropane under Friedel-Crafts conditions. The [l,2]-shift A — B involved in this reaction again is an H atom shift. In contrast to the thermoneutral isomerization between carbenium ions A and B of Figure 14.3, in the present case an energy gain is associated with the formation of a secondary carbenium ion from a primary carbenium ion. Note, however, that the different stabilities of the carbenium ions are not responsible for the complete isomerization of 1-bromopropane into 2-bromopropane. The position of this isomerization equilibrium is determined by thermodynamic control at the level of the alkyl halides. 2-Bromopropane is more stable than 1-bromopropane and therefore formed exclusively. 

Fig. 14.5. Wagner-Meerwein rearrangement as part of an isomerizing El elimination.

In Section 5.2.5, we discussed the Friedel-Crafts alkylation of benzene with 2-chloropentane. This reaction includes a Wagner-Meerwein reaction in conjunction with other elementary reactions. The Lewis acid catalyst A1C13 first converts the chloride into the 2-pentyl cation A (Figure 11.3). Cation A then rearranges into the isomeric 3-pentyl cation B, in part or perhaps to the extent that the equilibrium ratio is reached. The new carbenium ion B is not significantly more stable than the original one (A),... 

Further studies on the isomeric dichlorides of (47) with different Y and Y substitution support the proposal that an electron transfer occurs from the photoexcited n orbital of the aromatic ring to the a orbital of the carbon-chlorine bond [72], When Y = H and Y = CN, COCH3 or N02 no photosolvolysis or photo-Wagner-Meerwein rearrangements were observed. Using the Weller equation it was determined that electron transfer from the presumed triplet states of the COCH3 or N02 substituted compounds or the singlet state of... 

An interesting double ring expansion sequence developed by Vogel involves the initial pinacol coupling of cyclobutanone, isomerization to ketone 79 in acid, followed by reduction and Lewis acid-promoted Wagner-Meerwein rearrangement with dehydration (Scheme 19).129) For preparative purposes, 80 can be more... 

Conversely, however, it is important to notice that the order of re-activity of carbonium ions once they are formed is just reversed. We find, for example, in the isomerization of alkanes (p. 59) and in the alkylation of olefins (p. 143) that a primary or secondary carbonium ion extracts a hydrogen atom with a pair of electrons from an alkane so as to form a secondary or tertiary carbonium ion. For many carbonium ion transformations formation of the ion seems to be the rate-controlling step of the process. The Wagner-Meerwein rearrangement (p. 56) appears to be an exception to this rule. 

Thionyl chloride is the classical reagent for the preparation of alkyl chlorides from alcohols with retention of configuration. This reaction is known to proceed via alkyl chlorosulfinates (7 75) which decompose by an ion pair mechanism, but may be diverted to an SN2 displacement path by addition of pyridine171 Wagner-Meerwein rearrangements have been observed in the course of alkylchlorosulfinate decomposition, e.g. (176) - (777)172). The behavior of the isomeric chlorosulfinates (178) and (179) is consistent with competitive ion pair collapse and 1,2-alkyl shift173. ... 

Collins has made a detailed research into the pinacolic rearrangement of 2-endo-phenyl-2,3-ds-exo-norbomandiol 91. If a 3,2-endo-hydride shift were possible, the diol 91 would yield the ketone 92. The reaction product, however, is an isomeric ketone 93. occuring from the 3,2-hydride shift initiated from the exo side. The absence of exo-H in the initially formed carbocation causes the latter to undergo a series of Wagner-Meerwein rearrangements and 6,2-hydride shifts result near the cation centre and at the exo-position of a hydrogen atom this is subjected to a 3,2-hydride shift. The formation of ketone 93 is not determined by thermodynamic factors because it is less stable than ketone 92. 

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