Aryl scrambling

Phenylboronic and 4-chloroacetophenone combine efficiently according to eq. (2) at certain palladacycle catalysts (Structure 3), which are easily available from Pd(OAc)2 and appropriate phosphines PR3 (e. g., R = o-tolyl). Turnover numbers (TONs) of 75 000 are achieved with only 0.001 mol% 3 [3]. Several reports on the catalysts of type 3 have substantiated the discovery of 1995 [3a]. As in the Heck coupling, no aryl scrambling is observed with the palladacycle catalysts [4]. 

The reaction is also compatible with the presence of an ortho substituent in the arylboronic acid, while arylboronic acid with two ortho substituents was not reactive. By-products from aryl scrambling and biaryl from homocoupling were not found. The reaction proceeds as previously indicated up to the formation of the biphenylylpalladium species and is terminated by the Suzuki coupling as in Eq. 45. 

Figure 3.7 (a) Aryl scrambling in the triphenylphosphine-modulated Mizoroki-Heck vinylation of 4-bromoanisole. (b) Aryl migration In the oxidative addition complex. 

Non-symmetrical arylphosphines may undergo scrambling of the aryl groups as illustrated in Equation 2.6.[23]... 

Note The pictorial term scrambling is used in mass spectrometry to describe rapid processes of (intramolecular) positional interchange of atoms. Scrambling may occur with hydrogens or may involve the complete carbon skeleton of an ion. Aryl radical ions and protonated aryl compounds are well known for their numerous scrambling processes. [54,55]... 

Perhaloalkanes have been found to scramble halogen atoms via consecutive halophilic reactions following carbanion generation by halophilic attack by base. S l reaction of an allylsilane has been applied in a stereocontrolled synthesis of ( )-dihydronepetalactone, and functionalized aryl and arylmethyl carbanions have been generated by reductive cleavage of aryl and arylmethyl alkyl ethers by electron transfer from alkali metals. ... 

The trend observed with the polycyclic hydrocarbons (see preceding section), namely that the product radical anions (ArNu ) are more stable than those derived from the simple benzene analogs, is even more evident with the heteroaromatic substrates and, as a consequence, fragmentation processes are minimized.41 For example, 2-chloroquinoline is the only substrate of many studied to undergo a substitution reaction with PhCH2S- ion without fragmentation of the benzyl-S bond,103 and to react with diphenyl-arsenide ion without scrambling of the aryl moieties.25... 

In contrast to the above, indenyl compounds such as 160, with a silyl, disilyl or trisilyl group attached to the metal atom, fail to undergo loss of a silylene on photolysis, and simply rearrange to products such as 161, as shown by the example given in equation 19. The locations of aryl groups present in the silyl side chain become scrambled during photolysis78. 

This fragmentation process explains the formation of scrambled products by reaction of diphenylarsenide ions with some aromatic halides and the straightforward substitution products with others [46,47]. On the other hand, only scrambling of aryl rings is obtained with all the substrates studied and diphenylstibide anion [47]. 

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