Chemoselective reactivity

In all of these systems, propagation involves radical species, and therefore chemoselectivities (reactivity ratios and transfer coefficients), regioselectivities (head-to-head units), and stereoselectivities (tacticity) determined for CRPs are similar to those in conventional RP. 

The reaction of acyl chlorides with diazomethane is fast and selective in the presence of a C=C bond, but not in the presence of other carbene acceptors. Knowledge of the chemoselective reactivity of carbenes is of prime importance for planning the retro-Amdt-Eistert disconnection of the target molecule. 

Organoaluminums have received considerable attention in recent years [1] due to their unexpectedly high chemoselective reactivity, but also because of the attractive price of aluminum (available preparation methods and shortly describes the reactivity pattern of these useful organometallic intermediates. Aryl, alkynyl, allylic, and propargylic aluminum derivatives by far have found the most applications. 

The wM-diacetate 363 can be transformed into either enantiomer of the 4-substituted 2-cyclohexen-l-ol 364 via the enzymatic hydrolysis. By changing the relative reactivity of the allylic leaving groups (acetate and the more reactive carbonate), either enantiomer of 4-substituted cyclohexenyl acetate is accessible by choice. Then the enantioselective synthesis of (7 )- and (S)-5-substituted 1,3-cyclohexadienes 365 and 367 can be achieved. The Pd(II)-cat-alyzed acetoxylactonization of the diene acids affords the lactones 366 and 368 of different stereochemistry[310]. The tropane alkaloid skeletons 370 and 371 have been constructed based on this chemoselective Pd-catalyzed reactions of 6-benzyloxy-l,3-cycloheptadiene (369)[311]. 

Sodium cyanoborohydride is remarkably chemoselective. Reduction of aldehydes and ketones are, unlike those with NaBH pH-dependent, and practical reduction rates are achieved at pH 3 to 4. At pH 5—7, imines (>C=N—) are reduced more rapidly than carbonyls. This reactivity permits reductive amination of aldehydes and ketones under very mild conditions (42). 

Addition to Carbonyl Compounds. Unlike Grignard and alkykitliium compounds, trialkylboranes are inert to carbonyl compounds. The air-catalyzed addition to formaldehyde is exceptional (373). Alkylborates are more reactive and can transfer alkyl groups to acyl halides. The reaction provides a highly chemoselective method for the synthesis of ketones (374). 

Me- SiCl also affects the stereoselectivity of 1,2-additions to carbonyl compounds [ 133]. Witli the aid of suitable activators, these mildly reactive reagents show selec-tivities unattainable by the conventional reagents, as ilustrated below for Me- SiCl-dependent Chemoselectivity fEq. 10.13) [134]. 

Hydrolysis of both ester groups and chemoselective re-esterification of the more reactive alkyl acid... 

The catalytic chemistry of M° depends on the elementary properties of M and on the structure and size of the M° nanoclusters ( quantum dots ) [2]. S may play a role as a reactivity enhancer of M°/S as a whole (co-catalytic role) and/or as a promoter of its catalytic chemoselectivity (promotional role) [3,4]. 

The. selective hydrogenation of a nitro group in the presence of other reactive functionalities is a frequently encountered problem in fine chemicals manufacture. Ciba-Geigy (Novartis). scientists developed, in collaboration with a catalyst manufacturer, a new Pt/Pb on CaCO. catalyst that allows the chemoselective hydrogenation of an aromatic nitro group in the presence of C=C, C=0, C=N as well as Cl or Br substituents in selectivities > 95% (even C C groups react very slowly) (Bader et al., 1996). Eqn. (3) shows an example (Bader eJ a/., 1996). 

The 1,4-addition of ZnEt2 to 2-cyclohexenone was also performed by these workers in the presence of thioether-phosphite D-xylose-derived ligands, depicted in Scheme 2.11. In all cases, the chemoselectivities in the 1,4-product were higher than 97% but the enantioselectivities were modest (<41% ee). It was noted that changing the substituent in the thioether moiety produced an elfect on both the reactivity and the enantioselectivity, as shown in Scheme 2.11. 

Mn. Manganese is also effective for mediating aqueous carbonyl ally-lations and pinacol-coupling reactions. Manganese offers a higher reactivity and complete chemoselectivity toward allylation of aromatic aldehydes.178... 

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