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for the most efficient reagent that can achieve both these selectivities in a single step have been carried out. 5,8 Hoffmann and coworkers reported the first asymmetric allylboration utilizing a camphorderived auxiliary (Scheme 3). 24 They studied various allyl- and crotylboration reactions using this auxiliary for both single and double asymmetric synthesis. 8a-d Yamamoto and coworkers introduced tartrate esters as chiral auxiliaries for propargylboration (Scheme 4). 25 Later, they utilized this approach for the allenylboration of aldehydes to produce homopropargylic Scheme 1. Preparation of “Crotyl”boronates by a Three-Carbon Homologation. 26 VOL. 35, NO. 1 • 2002 Scheme 2. Preparation of Allylboronates via Haloboration–Homologation. Scheme 3. The First Asymmetric Allylboration. alcohols in very high ee (eq 17). 26 They showed that the ee’s of the alcohols depend on the alkyl group of the tartrate ester: ethyl and isopropyl tartrates lead to lower ee’s, while cyclododecyl and 2,4-dimethylpentyl tartrates lead to higher ee’s. Isopropyl tartrate was used as a chiral auxiliary for allylborations by Roush and coworkers (Scheme 5), 27 who also used tartrates for crotylborations. 28 Tartramidoallylboronates provided better selectivity in allylborations than tartrate esters. 29 Figure 3 shows several “allyl”borating agents. These include allylboronates derived eq 15 eq 16 from chiral 1,2-diols, 30 functionalized allylboronates, 31 oxazaborolane, 32 and diazaborolane. 33 5. Asymmetric Dialkylallylboranes Brown and Jadhav put α-pinene to the test for allylboration and achieved an economical reaction with predictable stereochemistry and high enantioselectivity. 34 B-Allyldiisopinocampheylborane, prepared from either B-chlorodiisopinocampheylborane (DIP- Chloride) 35 or B-methoxydiisopinocampheylborane and allyl Grignard reagent, provided high ee’s for most of the aldehydes
Scheme 4. Tartrate Ester Auxiliary for Propargylboration of Aldehydes. Scheme 5. Tartrate Ester for Allylboration. Figure 3. “Allyl”borating Agents. Scheme 6. Preparation and Reactions of B-Allyldiisopinocampheylborane. VOL. 35, NO. 1 • 2002 eq 17 27
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Page 3 and 4: Aldrich Chemical Co., Inc. 1001 Wes
Page 5 and 6: Boron-Based Reducing Agents for the
Page 7 and 8: Figure 3. Stoichiometric Boron-Base
Page 9 and 10: orane reductions 29,30 of 1,4-diphe
Page 11 and 12: α-Hydroxy Acetals R Reduction Cond
Page 13 and 14: and 5 provided moderate ee’s in t
Page 15 and 16: Other reductions using 14, 64c,d 16
Page 17 and 18: Masuda, H.; Ito, K.; Hirao, A.; Nak
Page 19 and 20: The synthesis of biaryl compounds v
Page 21 and 22: 52,396-8 [87199-18-6] 3-Hydroxyphen
Page 23 and 24: Josiphos is a family of chiral phos
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Page 33 and 34: Pinane-Based Versatile “Allyl”b
Page 35: B-O bond. 15 The mechanism involves
Page 39 and 40: eagent with aldehydes at low temper
Page 41 and 42: from 2- and 3-carene turned out to
Page 43 and 44: Scheme 22. Application of Crotylbor
Page 45: Abstr. 1968, 68, 29743j. (11) Refer
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