Journal of the American Chemical Society p. 8089 - 8110 (1991)
Update date:2022-08-04
Topics:
Denmark, Scott E.
Almstead, Neil G.
A systematic examination of the mechanism and origin of stereoselection in the reaction of dioxane acetals with allyltrimethylsilane was undertaken. Experimental tests for two limiting mechanisms, synchronous (SN-like) and dissociative (SN 1 -like) substitution processes, were investigated. The meso 2,4,6-trisubstituted 1,3-dioxane acetals cis- and trans-1 provided an interesting opportunity to test the timing of bond breaking and making in the substitution reaction. The modest and C(2)-substituent-dependent selectivity excluded the possibility of a direct SN2-type attack on a complexed acetal. Further, the enol ethers 3 and 5 and acyclic acetal 7 were studied as precursors of the putative oxocarbenium ion intermediate in the dissociative limit. The weak and inverted selectivity observed with these substrates ruled out the intermediacy of the extended, separated ion in reactions of the cyclic acetals under similar conditions. A unified mechanistic scheme involving three distinct ion pairs is proposed to explain the dependence of allylation selectivity on structural and experimental variables. The three species are analogous to those proposed in the classic Winstein scheme: (1) an intimate ion pair, (2) an external ion pair, and (3) a separated ion. Each of these proposed intermediates has a different stereochemical profile and the ultimate outcome is a composite of those factors that balance the contribution of the different intermediates. The influence of C(2) substituent, acetal configuration, Lewis acid type and stoichiometry, allylsilane stoichiometry, concentration, solvent, and temperature were investigated and integrated in the proposed mechanistic scheme.
View MoreXi'an Galaxy Chemicals CO., Ltd
Contact:86-29-89380370
Address:No.8, Gaoxin three road, Xi'an city.
Xi'an Costrong Pharmaceutical Co., Ltd.
Contact:029- 68576496
Address:Room 2004,Shuibao Building,No.190,South Erhuan Rd, Yanta District,Xi'an,Shaan Xi,China
Changsha Kamer Essence and Flavor Co.,Ltd
Contact:86-731-89832270
Address:No.327 Kangnin Road Changsha Bio Information Industry Park, Changsha city, Hunan province
Shanghai Witshoot Internet Technology Co Ltd
Contact:+86-21-66390020
Address:Room 419, No.285 Luochuan Road (E)
ZHANGJIAGANG FREE TRADE ZONE YONG HAN INTERNATIONAL TRADING CO., LTD(expird)
Contact:86 512 57910558
Address:qianjin M road
Doi:10.1021/ja3031054
(2012)Doi:10.1002/ardp.19933260603
(1993)Doi:10.1021/ja00542a028
(1980)Doi:10.3762/bjoc.7.173
(2011)Doi:10.1016/0022-328X(91)80132-4
(1991)Doi:10.1021/ol300516y
(2012)