Allyl cleavage

In approach (a) both synthons present problems the rcgi oselecL i vi ty of ( ) is a serious difficulty and we do not like to use a-bromo aldehydes for (3), Allyl cleavage (Chapter T 26) provides an answer to the last problem. 

III. UNIMOLECULAR ISOMERIZATION AND FRAGMENTATION A. Selected Linear Dienes Allylic Cleavage and Isomer Distinction... 

B. Linear Dienes that Cannot Undergo Allylic Cleavage Allene and Butadienes... 

In this context, the striking difference of the standard 70 eV El mass spectra of the isomeric cyclooctadienes may be mentioned here (see below)87. Whereas the radical cations of the stereoisomeric 1,5-cyclooctadienes, containing two bis-allylic C—C bonds, give the products of the two-fold allyl cleavage as the base peak ([C4H6]+ /[M]+ s=ss 10 1), the cis,cis-l,4 and cis.cis-].3-cyclooctadicnc ions are reluctant to do so (IC4II511 / [M]+ 1 3). Clearly again, 1,2- and 1,3-H shifts cannot efficiently compete with dissociation of the bis-allylic C—C bond. 

Example The spectra of 1-decene, ( )-5-decene, and (Z)-5-decene are shown below (Fig. 6.17). Whereas in the spectrum of 1-decene the basepeak at m/z 41 can easily be explained as a result of allylic cleavage, the base peak of both... 

The next landmark was the synthesis of the germylium, 22, and the stannylium ion, 23, by one-electron oxidations from the corresponding stable radicals with trityl TPFPB by Sekiguchi and co-workers. As in the case of the allyl cleavage to generate the mesityl-substituted cations, the reaction, in this case the oxidation, occurs at the periphery of the molecule and gives the possibility for efficient steric protection of the incipient cation. Both trivalent cations were obtained as their TPFBP salts and the crystal structure show well separated anions and cations. 

In an attempt to establish the limits for ODPM reactivity of (B.y-unsaturated aldehydes, we have extended our studies to a series of aldehydes 65, (Scheme 10) which are monosubstituted at C-2. Triplet-sensitized irradiation of 65 leads to the formation of the corresponding cyclopropanecarbaldehydes 66 [59] (Scheme 10). The diphenyl-substituted aldehydes 65b and 65d yield, in addition to the ODPM products, the corresponding alkenes 67a and 67b, resulting from photodecarbonylation. The formation of these alkenes is probably due to stabilization of the radical, formed by allylic cleavage, by diphenyl conjugation. The ODPM rearrangement of aldehydes 65 is diastereoselective, yielding only one diastereoisomer of 66 (Scheme 10). 

Double bonds favor allylic cleavage and give the resonance-stabilized allylic carbocation. This rule... 

The peak at m/z 93 may be rationalized as a structure of formula CjH9+ formed by isomerization (resulting in increased conjugation), followed by allylic cleavage. 

Double bonds favor allylic cleavage and give the resonance-stabilized allylic carbocation. This rule does not hold for simple alkenes because of the ready migration of the double bond, but it does hold for cycloalkenes. 

The peak at mlz 93 is rationalized in Scheme 1.10 as a structure of formula C7H9+ formed by double bond isomerization (resulting in increased conjugation), followed by allylic cleavage. The ion at mlz 93 has at least two important resonance forms that contribute to its stability. As an exercise, the student is encouraged to draw them. 

Figure 12-20 shows how the radical cation of trans-hex-2-em undergoes allylic cleavage to give the resonance-stabilized cation responsible for the base peak at m/z 55. 

CH3)2CHCH2+ 12.11. 126 loss of water 111 allylic cleavage 87 cleavage next to alcohol. 12.14. (a) about 1660 and 1710 the carbonyl is much stronger (b) about 1660 for both the ether is much stronger ... 

Pyrrolidine, piperidine and morpholine enamines of aldehydes and ketones have been the subject of a pioneering work by D. H. Williams and collaborators8. The most abundant fragment ion is produced by the loss of an alkyl radical R by allylic cleavage (Scheme 3). When R = H the loss of a H atom is more energy demanding and the... 

Enamines prepared from / -diketones and ethyl acetoacetate8 also lose a radical by allylic cleavage. When R = CH3 or OCH3 (Scheme 5), 9, 10 and 11 lose preferentially CH3 or OCH3 radicals9, respectively. 

Allyl Cleavage of 2-Silanorbornenes - A Facile Synthesis of Cyclopentenyl Compounds of Organochlorosilanes and of... 

In chloroform 1 reacts with HCl gas nearly quantitatively (> 91 %) to give the product of the HCl addition to the double bond 10, which is a mixture of two diastereomers (the exolendo ratio of the neopentyl group is the same as in 1 - 60 40). Exclusively the exo-6-chloro-compoimds are formed and this proves the HCl addition to be stereo- and regiospecific. When an HCl/ether solution is used, the trichloroorganosilane 11 (resulting fi om an allylic cleavage) is formed along with the adducts 10 as an... 

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