1,2,5-THIADIAZOLE 1-OXIDES .3. AN EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE INVERSION BARRIER
1,2,5-THIADIAZOLE 1-OXIDES .3. AN EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE INVERSION BARRIER
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DOI：
10.1021/ja00369a038
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发表时间：
1982-01-01
影响因子：
15
通讯作者：
WEINSTOCK, LM
中科院分区：
文献类型：
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作者：
AMATO, JS;KARADY, S;WEINSTOCK, LM
通过与亚硫酰氯的环缩合反应，合成了1,2,5 -噻二唑-氧化物体系。这些产物的烷氧基和烷基硫基基团被胺取代，如吡咯烷，在室温下由二氧基类似物生成3-乙氧基-4-（l-吡咯烷基）- 1,2,5 -噻二唑1-氧化物。在手性移位试剂的存在下，用13C核磁共振对该产物进行了检查，发现了两个异构体，表明该产物具有稳定的锥体亚砜结构。3,4 -二氧基- 1,2,5 -噻二唑1-氧化物与光学活性胺（如/-麻黄碱）反应，产生易于分离的非对映异构体混合物。非对映异构体仅在高温下发生转化，AG* 120= 33 kcal mol-1，而噻吩-1 -氧化物的转化率仅为14.8 kcal mol-1，二芳基亚砜的转化率为36 kcal mol-1。对3,4 -二（甲基硫）- 1,2,5 -噻二唑1-氧化物的x射线分析表明，该环本质上是非芳族的，并证实了其锥体亚砜结构。硫孤对与二烯的相互作用较小，C3-C4键长更接近于环戊二烯而不是噻吩或噻二唑的键长。理论计算表明，芳香性效应通过稳定平面过渡态和破坏非芳族金字塔结构来降低噻吩亚砜和噻二唑亚砜的反转势垒。然而，噻二唑中势垒的降低被电负性氮原子的作用所抵消，从而将反转势垒提高到正常亚砜的范围。
The 1, 2, 5-thiadiazole1-oxide system was synthesized via cyclocondensation of diethyl oxalimidate or dimethyl thiooxalimidate with thionyl chloride. The alkoxy and alkylthio groupin these products are replaced by amines, eg, pyrrolidine, which produced 3-ethoxy-4-(l-pyrrolidinyl)-1, 2, 5-thiadiazole 1-oxide from the diethoxy analogue at room temperature. Examination of this product by 13C NMR in the presence of a chiral shift reagent showed two isomers, indicative of a stable pyramidal sulfoxide structure. Reaction of 3, 4-diethoxy-1, 2, 5-thiadiazole 1-oxide with optically active amines, eg,/-ephedrine, produces readily separable diastereoisomeric mixtures. The diastereoisomers undergo inversion only at elevated temperature, AG* 120= 33 kcal mol-1, compared to only ca. 14.8 kcal mol-1 for a thiophene 1-oxide and 36 kcal mol-1 for diaryl sulfoxides. X-ray analysis of 3, 4-bis (methylthio)-1, 2, 5-thiadiazole 1-oxide demonstrates that the ring is essentially nonaromatic and confirms the pyramidal sulfoxidestructure. Interaction between the sulfur lone pair and the diene is small, the C3-C4 bond length lying closer to that of cyclopentadiene than of thiophene or thiadiazole. Theoretical calculations indicate that aromaticity effects lower the inversion barrier nearly equally in the thiophene and thiadiazole sulfoxides by stabilizing the planar transition state and destabilizing the nonaromatic pyramidal structure. The reduction of the barrier in the thiadiazole, however, is counteracted by the effect of the electronegative nitrogen atoms, thus raising the inversion barrier back to the range of normalsulfoxides.