Formation of a W(CO)(5)-furanosylidene complex from ribose without the useof protective groups

[W(CO)(5)(acetone)], formed by photolysis of W(CO)(6), undergoes a spontane ous reaction at the C-1 position Of D-ribose in d(6)-acetone in 24 h in qua ntitative yield to form water and the W(CO)(5)-furanosylidene complex, exhi biting a characteristic carbene C-13 resonance at 427 ppm. The reaction pro ceeds without protection of any of the ribose hydroxyl groups, and occurs o nly at the C-1 position. The same reaction does not occur for fructose, D()-ribonic gamma -lactone, or 2-deoxy-D-ribose. No reaction occurred with th e pyranose sugars, D-glucose or D-galactose. A pathway via oxidative additi on to C-H of the open chain aldehyde form of ribose is proposed. Insertion of W(CO), into the C-H bond, followed by rearrangement of the W(II)(CO)(5)- acyl hydride to a hydroxy carbene that recyclizes to the coordinated furano sylidene accounts for the reactivity Of D-ribose and the absence of reactiv ity for the other sugars. Molecular mechanics calculations were carried out using SPARTAN and MMFF94 programs for the free sugars D-ribose and D-gluco se and their C-1-coordinated carbenes of W(CO), The carbene complexes are e nergetically uphill of the free sugars by 54.8 and 63.3 kcal mol (-1) for r ibose and glucose, respectively. Therefore, elimination of water is a key f actor 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 arra ngement to maximize resonance with the carbene carbon. The theoretical stru cture for the D-glucose analogue adopts sufficient puckering of the chair a rrangement 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. (C) 2001 Elsevier Science B.V. All rights reserved .