Diisopropyl 2-Allyl-1,3,2-dioxaborolane-4,5-dicarboxylate1,2


[99417-55-7]  · C13H21BO6  · Diisopropyl 2-Allyl-1,3,2-dioxaborolane-4,5-dicarboxylate  · (MW 284.12) D-(S,S)


(reagent for the asymmetric allylboration of aldehydes to produce homoallylic alcohols2)

Alternate Name: tartrate allylboronate.

Physical Data: bp 88-90 °C/0.03 mmHg.

Solubility: sol toluene, THF, ether, CH2Cl2.

Analysis of Reagent Purity: 11B NMR (d 35, CDCl3); capillary GC;2c solutions are easily standardized via reaction of an aliquot with cyclohexanecarbaldehyde.2c

Preparative Method: prepared by the reaction of Triisopropyl Borate and Allylmagnesium Bromide in Et2O followed by an acidic extractive workup and direct esterification with diisopropyl tartrate (DIPT) (eq 1).2c

Handling, Storage, and Precautions: allylboronates are typically handled as a solution in toluene (0.5-1.0 M) and transferred by syringe under an inert atmosphere. The reagent, stored neat or as a solution in toluene over 4Å molecular sieves under an argon atmosphere in a refrigerator (-20 °C), is stable for several months. In the presence of water, (1) rapidly hydrolyzes to DIPT and the achiral allylboronic acid.

Reactions with Achiral Aldehydes.

The reaction of tartrate allylboronates with achiral aldehydes proceeds with moderate to excellent enantioselectivity (60-92% ee) and high yield (80-90%). Simple aliphatic aldehydes give good enantioselectivities (decanal 86% ee, CyCHO 87% ee, eq 2),2 while b-alkoxy and conjugated aldehydes give diminished selectivities (60-80% ee) (eq 3).3 The enantioselectivity is highly temperature and solvent dependent. Best results for reactions with the vast majority of aldehydes are obtained in toluene at -78 °C.2c 4Å molecular sieves are included to ensure that the reaction is anhydrous. Other tartrate esters (e.g. diethyl tartrate) may also be used without loss of enantioselectivity.

An allylboronate reagent with a conformationally rigid tartramide auxiliary was designed to improve the enantioselectivity of the reactions with achiral aldehydes.4 The N,N-dibenzyl-N,N-ethylenetartramide modified allylboronate (2) (R = CH2Ph) is considerably more enantioselective than (1) (CyCHO, 97% ee) but has very poor solubility in toluene at -78 °C. Consequently, reactions of (2) often require up to 48 h. N,N-Bistrifluoroethyl-N,N-ethylenetartramide modified allylboronate (3) (R = CH2CF3) is much more soluble at -78 °C than the dibenzyl derivative and therefore reacts with aldehydes much more efficiently (CyCHO, -78 °C, THF, 5 h, 91%, 94% ee).5

The poor results obtained with various unsaturated aldehydes have been overcome by conversion of these substrates to the corresponding metal carbonyl complexes.6,7 For example, allylboration of 2-decynal proceeds with 72% ee, while that of its cobalt complex proceeds with excellent enantioselection (eqs 4 and 5).6

A second example of the allylboration of a metal carbonyl containing substrate is a highly group- and face-selective allylboration of a meso iron-diene dialdehyde complex (eq 6).7 Efficient kinetic resolutions of racemic diene aldehyde-Fe(CO)3 complexes have also been demonstrated.7

Reactions with Chiral Aldehydes.1,8

The tartrate allylboronates have been shown to serve as highly useful chiral acetate enolate equivalents in the reactions with a-chiral aldehydes. The diastereoselectivities obtained are good to excellent, depending on whether the reaction is a matched or mismatched case (eqs 7 and 8).3 These reagents have been applied to several complex problems in natural product synthesis.8,9 As shown in eq 8, the diastereoselection is significantly improved by using the rigid tartramide reagent (3).5

The tartrate-derived allylboronate reagents in the best cases compare favorably with other allylboration reagents in their reactions with both achiral and chiral aldehydes (e.g. B-Allyldiisopinocampheylborane; b-allyl-2-(trimethylsilyl)borolane; 2,5-dimethyl-b-allylborolane; 1,2-diamino-1,2-diphenylethane modified allylboranes). The advantage of the tartrate-modified allylboronate reagent rests with its ease of preparation and its capability of prolonged storage without noticeable deterioration.

Related Allylboronate Reagents.

A stereoselective synthesis of anti 1,2-diols has been achieved by using a DIPT-modified (E)-g-[(cyclohexyloxy)dimethylsilyl]allylboronate reagent.10 This reagent is best applied in double asymmetric reactions with chiral aldehydes such as D-glyceraldehyde acetonide (eq 9).

A chiral allylic alcohol b-carbanion equivalent has also been developed which utilizes a DIPT-modified (E)-g-(dimethylphenylsilyl)allylboronate reagent.10 This method involves treating the product homoallylic alcohol with Dimethyldioxirane and subjecting the derived epoxide to an acid-catalyzed Peterson elimination. This sequence has been applied in the synthesis of the trioxadecalin ring system of the mycalamides (eq 10).11

Highly stereoselective introduction of a b,b-dimethylhomoallylic alcohol subunit was also accomplished in this synthesis by using a DIPT-modified prenylboronate (eq 11).11

All the reagents discussed above are readily prepared using techniques described for the preparation of the tartrate-modified crotylboronates, and can be handled in a similar manner.

Related Reagents.

B-Allyldiisopinocampheylborane; (E)-1-(N,N-Diisopropylcarbamoyloxy)crotyllithium.

1. Roush, W. R. COS 1991, 2, 1.
2. (a) Roush, W. R.; Walts, A. E.; Hoong, L. K. JACS 1985, 107, 8186. (b) Roush, W. R; Banfi, L.; Park, J. C.; Hoong, L. K. TL 1989, 30, 6457. (c) Roush, W. R.; Hoong, L. K.; Palmer, M. A. J.; Park, J. C. JOC 1990, 55, 4109.
3. Roush, W. R.; Hoong, L. K.; Palmer, M. A. J.; Straub, J. A.; Palkowitz, A. D. JOC 1990, 55, 4117.
4. Roush, W. R.; Banfi, L. JACS 1988, 110, 3979.
5. Roush, W. R.; Grover, P. T. Unpublished results.
6. Roush, W. R.; Park, J. C. JOC 1990, 55, 1143.
7. Roush, W. R.; Park, J. C. TL 1990, 31, 4707.
8. (a) Roush, W. R.; Kageyama, M. TL 1985, 26, 4327. (b) Roush, W. R.; Palkowitz, A. D.; Ando, K. JACS 1990, 112, 6348. (c) Goulet, M. T.; Boger, J. TL 1990, 31, 4845.
9. (a) Roush, W. R.; Straub, J. A.; VanNieuwenhze, M. S. JOC 1991, 56, 1636. (b) Roush, W. R.; Lin, X.; Straub, J. A. JOC 1991, 56, 1649.
10. Roush, W. R.; Grover, P. T. T 1992, 48, 1981.
11. Roush, W. R.; Marron, T. G. TL 1993, 34, 5421.

David J. Madar

Indiana University, Bloomington, IN, USA

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