Diaryl carbinol

The transmetallation from (PhBO)3 to diethylzine has been shown to occur in a two-step process (Scheme 9.16). This catalytic system is highly effective for the enantioselective arylation of aldehydes to give diaryl carbinols. ... 

Compared with asymmetric ethylation, reports on asymmetric phenylation are limited. We disclosed the enantioselective phenylation using diphenylzinc prepared in situ from phenyl Grignard reagent and zinc chloride. This method needs a stoichiometric amount of chiral amino alcohol DBNE 18 but good ee of 82% was achieved [32], A catalytic phenylation was examined using planar chiral compound 1 based on ferrocene, and chiral diaryl carbinols of moderate ee were provided from diphenylzinc and 4-chlorobenzaldehyde (Scheme 10) [33]. A catalytic and highly enantioselective phenylation was realized by binaphthyl-based chiral catalyst 23. In this reaction, the addition of 2 equivalents of diethyl-zinc against catalyst increases the yield and ee [34]. Recently, chiral amino alcohol DPMPM 9 was also reported to be an efficient catalyst for asymmetric phenylation [35]. 

A rhodium-chiral phosphine complex catalyzes the enantioselective addition of phenylboronic acid to 1-naphthaldehyde to give a chiral diaryl carbinol but with moderate ee (Scheme 13) [39]. When considering the introduction of functionalized aryl groups, arylboronic acid is a promising alternative arylating reagent to diarylzinc compounds. 

The asymmetric arylation of aldehydes in the presence of a catalytic amount of chiral amino alcohol111 or of chiral tertiary aminonaphthol112 has been described. The reactive arylzinc species have been generated in situ from a boron-zinc exchange instead of employing the more expensive diphenylzinc. The chiral diaryl carbinols have been obtained in high yields and ees. 

Diferrocenyl- and diaryl-carbinols (11) are reductively coupled when their corresponding lithio-derivatives are treated with titanium trichloride, giving high yields (70—80%) of the ethanes (12), whereas electroreductive dimerization... 

Cheng has published a convenient and synthetically useful alternative method to the NHK reaction for the arylation of aromatic aldehydes in a mild and selective way with nickel(ll) bromide/zinc/dppe mediated protocol for the synthesis of diaryl carbinols. Durandetti reported an electrochemical coupling of aryl halides with aldehydes for the synthesis of diaryl carbinols which was catalytic in chromium and nickel salts. Comins utilized the NHK reaction to prepare 5-(l-hydroxyalkyl)-2,3-dihydro-4-pyridones, which were then explored in reductive, oxidative and substitutive reactions. " The first asymmetric catalytic synthesis of 5y -alk-l-ene-3,4-diols was developed the regio-, diastereo- and enantioselective addition of 3-chloropropenyl pivaloate to aldehydes was made possible by exploiting Salen r(II) species in a catalytic version of the NHK reaction. ... 

Numerous publications have appeared recently disclosing observations on the hydrogenation of aryl or diaryl ketones to yield aromatic carbinols (equation 25)39,338,343-345,348. 

The insertion manifold of MCI reactions is most generally observed with aliphatic CH bonds. However, several diaryl alkynyl carbinols have been reported to react with sodium... 

Oxidation of the carbinol (4) with periodate does not give the expected spiro-epoxy compound but the benzaldehyde acetal of 3,5-di-r-butylcatechol (5). This oxidative rearrangement is apparently limited to o-hydroxy-substituted diaryl- and triaryl-carbinols.3... 

Epoxides. Reaction pathways for the Mitsunobu reaction to convert l,l-diaryl-2-phenylethanediols to epoxides are phosphine-dependent. Retention of configuration at the secondary carbinolic center is observed in reaction mediated by PhsP-DIAD, while mote electron-rich phosphines (e.g., BU3P) favor products with inversion of configuration. ... 

Electrophilic substitution of phenones. The facile formation of C,0-dilithiated diarylmethanols from diaryl ketones and LN, coupled with subsequent reaction with various electrophiles, " solves the compatibility problem of the conventional addition reaction with functional nucleophiles to prepare diaryl r-carbinols. 

A related reaction involves the use of aryl indole-3-carbinols with enamides. Under the influence of acid catalysts the carbinols generate electrophiles. The adducts hydrolyze to products that are the equivalent of conjugate addition to 1,3-diaryl propenones. These reactions can be done in up to 90% ee with chiral BINOL-phosphoric acid catalysts [293]. 

Synthesis of diaryl heteroarotinoids (11) and (12) [27,30,31] began with a Lewis acid-catalyzed cyclization of tertiary alcohol (34) to give dihydroben-zothiophene (36) as the sole isolated product. The chemistry of the ensuing steps was similar to that used to prepare (9) and (10) and other diaryl heteroarotinoids and involved (a) Friedel-Crafts acylation of a fused aromatic-heterocyclic system, (b) reduction of the resulting ketone to a benzylic carbinol, (c) phosponium salt formation, and finally (d), Wittig coupling to methyl 4-formylbenzoate. The free acids (13) and (14) were obtained by saponification. 

Oxidative palladium-catalysed rearrangement of diaryl alkenyl carbinols to /1,/1-diaryl Q ,j8-unsaturated ketones occurs with the geometry of the alkene product determined by the substitution pattern on the aryl rings (Scheme 139). ... 

Lithiation of ethynyl(trifluoromethyl)furans 275 followed by the reaction with diaryl ketones produced diaryl(2-arylethynyl)carbinols 276 and triarylcarbinols 277. Further treatment with HCIO4 yielded the corresponding dyes 278, 279. Similarly, furan 280 was transformed into dye 281 [159]. 

We may now inquire into the reason for the difference between the behavior of carbonium ions and ammonium or oxonium ions assuming, as is reasonable (87), that the difference in activity coefficient ratios does not reside in the free bases. An attractive explanation may be advanced at once that the carbonium ions derived from aryl carbinols or olefins involve a much greater degree of charge delocalization than that in the onium ions and this should have a profound effect on how tightly the ion is solvated. At least three facts eliminate the de-localization theory. Firstly, the small NO+ ion, in which delocalization is restricted, has activity coefficient behavior like a polyaryl carbonium ion (82). Secondly, aryl and diaryl ketones behave for the most part as proper Hammett bases (324,328) although considerable delocalization occurs in their ions. Thirdly, the same acidity function seems to apply equally well to pyrrole bases whether or not they have much charge delocalization in their ions (357). 

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