Formation of a W(CO)5–furanosylidene complex from ribose without the use of protective groups
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- 作者：Stringfield ; Thomas W. ; Shepherd ; Rex E.
- 关键词：Oxidative addition ; Hydroxy carbene ; Furanosylidene complexes ; Ribose ; W(CO)5 complexes
- 刊名：Inorganica Chimica Acta
- 出版年：2001
- 出版时间：December 3, 2001
- 年：2001
- 卷：325
- 期：1-2
- 页码：51-57
- 全文大小：195 K
文摘
[W(CO)5(acetone)], formed by photolysis of W(CO)6, undergoes a spontaneous reaction at the C-1 position of d-ribose in d6-acetone in 24 h in quantitative yield to form water and the W(CO)5–furanosylidene complex, exhibiting a characteristic carbene 13C resonance at 427 ppm. The reaction proceeds without protection of any of the ribose hydroxyl groups, and occurs only at the C-1 position. The same reaction does not occur for fructose, d(+)-ribonic γ-lactone, or 2-deoxy-d-ribose. No reaction occurred with the pyranose sugars, d-glucose or d-galactose. A pathway via oxidative addition to C
H of the open chain aldehyde form of ribose is proposed. Insertion of W(CO)5 into the CH bond, followed by rearrangement of the W(II)(CO)5–acyl hydride to a hydroxy carbene that recyclizes to the coordinated furanosylidene accounts for the reactivity of d-ribose and the absence of reactivity for the other sugars. Molecular mechanics calculations were carried out using spartan and mmff94 programs for the free sugars d-ribose and d-glucose and their C-1-coordinated carbenes of W(CO)5. The carbene complexes are energetically uphill of the free sugars by 54.8 and 63.3 kcal mol−1 for ribose and glucose, respectively. Therefore, elimination of water is a key factor in the net driving force to form the coordinated carbene of d-ribose. The structures reveal a useful planarity at C-1 which places the filled p-orbital on the O atom alpha to the carbene in the proper perpendicular arrangement to maximize resonance with the carbene carbon. The theoretical structure for the d-glucose analogue adopts sufficient puckering of the chair arrangement of the glucose to cause a misalignment of the alpha O p-orbital, which would decrease the inherent stablity, consistent with the absence of forming such a species.
H of the open chain aldehyde form of ribose is proposed. Insertion of W(CO)5 into the CH bond, followed by rearrangement of the W(II)(CO)5–acyl hydride to a hydroxy carbene that recyclizes to the coordinated furanosylidene accounts for the reactivity of d-ribose and the absence of reactivity for the other sugars. Molecular mechanics calculations were carried out using spartan and mmff94 programs for the free sugars d-ribose and d-glucose and their C-1-coordinated carbenes of W(CO)5. The carbene complexes are energetically uphill of the free sugars by 54.8 and 63.3 kcal mol−1 for ribose and glucose, respectively. Therefore, elimination of water is a key factor in the net driving force to form the coordinated carbene of d-ribose. The structures reveal a useful planarity at C-1 which places the filled p-orbital on the O atom alpha to the carbene in the proper perpendicular arrangement to maximize resonance with the carbene carbon. The theoretical structure for the d-glucose analogue adopts sufficient puckering of the chair arrangement of the glucose to cause a misalignment of the alpha O p-orbital, which would decrease the inherent stablity, consistent with the absence of forming such a species.