Isopinocamphenyl

R = 1-Ad, f-Bu, CHPh2, o-hexyl, hPr, 2,4,6-Me3C6H2, 2-Ad, 2,6-CI2CbH3, 1-isopinocamphenyl, c-CsHis, 0-C12H23... 

Bei dieser asymmetrischen Amin-Synthese dient das Diisopinocamphenyl-boran sowohl als Hydroborierungsreagenz als auch als chirales Hilfsreagenz. Das gleiche trifft bei der folgenden mehrstufigen, mit besserer Ausbeute verlaufenden asymmetrischen Synthese von trans- (1S,2S)-2-Amino-1 -methyl-cyclohexan fur ( —)-Isopinocamphenyl-boran zu3. 

Thus, the hydroboration of 1-phenylcyclopentene with (—)-IpcBH, (99% produces, after crystallization, the chiral organoborane 126 with 94% ee. The reaction of 126 with Et2BH replaces the isopinocamphenyl group with an ethyl substituent (50 C, 16 h) and provides, after the addition of i-Pr2Zn (25 °C, 5 h), the mixed diorganozinc 127. Its stereoselective allylation leads to the fraw5-disubstituted cyclopentane 128 in 44% yield (94% ee trans cis = 98 2) see Scheme 43 ° . This sequence can be extended to open-chain alkenes and Z-styrene derivative 129 is converted to the anf/ -zinc reagent 130, which provides after allylation the alkene 131 in 40% yield and 74% ee (dr = 8 92). 

Fmoc HMPA Ipc KHMDS LDA MCPBA MEM Mes MOM MS- NBS NCS NIS (+)-NLE PCC PDC Ph3P Pht PMB PNB europium 9-fluorenylmethoxycarbonyl hexamethylphosphoric triamide isopinocamphenyl potassium hexamethyldisilazanide lithium diisopropylamide 3- chloroperoxybenzoic acid (2-methoxyethoxy)methyl mesityl methoxymethyl molecular sieves iV-bromosuccinimide iV-chlorosuccinimide iV-iodosuccinimide positive non linear effect pyridinium chlorochromate pyridinium dichromate triphenylphosphane phthaloyl 4- methoxyphenyl 4-nitrobenzyl... 

About 10 years later the catalytic use of boron derivatives was again investigated by Kaufmann et al. Compounds of the type RBBr2 (R = pinanyl) catalyze the Diels-Alder reaction at -78 °C with low ee (Eq. 3) [4]. Modified isopinocamphenyl dibro-moborane, which has been synthesized since then, gives better results in the reaction of cyclopentadiene and methyl acrylate (Eq. 4) [5]. 

With the catalysts in hand, we were able to test their effectiveness in several epoxidation reactions. Initially, we screened the catalysts with our usual test substrates, 1-phenylcyclohexene, a-methylstilbene and triphenylethylene (Table 5.5). Catalyst 33a showed the best reaction profile, being by far the most reactive. For example, in the presence of 33a, 1-phenylcyclohexene oxide was produced in 69% yield with 91% ee in imder 20 min, while the other catalysts (apart from ent-33a) were less selective, and the reactions were slower. The isopinocamphenyl moiety offers little enantiocontrol, leading to epoxides with only moderate ees. The poor reactivity of catalysts 33b-d is highlighted by the attempted epoxidations of a-methyl stilbene and triphenyl ethylene, where no epoxides were formed after 4 h. 

Asymmetric reduction of propargyl ketones with (R) or (S) Alpine borane (B-isopinocamphenyl-9-borabicyclo, [3,3,1] nonane(A))... 

With the exception of reagents 5 and 9 (Table 1, p 4101) the stereochemical outcome can be rationalized by a quasi-planar six-membered transition state A. Steric hindrance is minimized if S and R, and L and Rs oppose each other in this relatively flat, chairlike conformation98. In contrast, for reagents 5 and 9, a boatlike transition state is assumed, as depicted in B98, similar to that postulated for the fl-isopinocamphenyl-9-borabicyclo[3.3.1]nonane (Alpine-Borane)23-35. 

Scheme 6.3 Proposed transition state made for asymmetric reduction with B-isopinocamphenyl-9-borabicyclo[3.3.I]nonane [36,40],...

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