Armed donors

Although glycosyl triflates have been demonstrated to be intermediates with a number of armed donors, and even with disarmed donors not capable of neighboring-group participation, such as the sulfonate esters, typical disarmed donors with esters in the 2-position function in the anticipated manner through anchimeric... 

Scheme 4.41 Influence of solvent with an armed donor.

First number THF, second alkoxy side-arm donor bond. 

To commence measurements, 1 ml of labeled solution was added to one arm (donor), and at the same time an equal volume of unlabeled solution was dispensed to the remaining arm (receptor). Aliquots of 0.1 ml were withdrawn from both arms at suitable time intervals over 4r-5 days. The activity of samples from the receptor side was determined by liquid scintillation counting, and Wt/Wm vs. y/t plots were constructed. The temperature range studied was 20-50°C. (Note salicylic acid was shown not to penetrate disks composed of polystyrene alone, and self-diffusion kinetics were reproducible between different disks of the same polystyrene/zeolite composition). 

Zirconium and hafnium complexes of an amine bis(phenolate) ligand with a THF side-arm donor have been shown to perform living polymerization of 1-hexene when activated with B(C6F5)3. The Zr catalysts (1) possess very high activities of 21 OOOgnunol cath. In neat olefin, the reaction is very exothermic and accompanied by various termination processes resulting in a higher polydispersity index than observed when the monomer is diluted in chlorobenzene. 

Previous calculations had shown that the relative reactivity of these donors ( 64 200) is 3.2 [111]. Hence, it was expected that 06 mannosylation by the armed donor, 64, would predominate in any trimannan produced. Surprisingly, a single trimannan 212, in which the less reactive donor 200 ended up at 06 was obtained, even in the presence of 2 equiv. of the more reactive 64. 

The hydrazido unit can be used as a Cp side arm donor substituent instead of the amido functionality and to stabilize cationic catalytic species. The synthesis and characterization of amido, chloro, and methyl Cp-hydrazido titanium complexes (C5Me4SiMe2NNMe2)TiR2 (R = NMe2, Cl, Me) (Scheme 313) have been made by general... 

Jensen HH, Pedersen CM, Bois M (2007) Going to extremes super armed donors in giycosyiation chemistry. Chem Eur J 13 7576-7582... 

Figure 11.26 Forcing a glycoside into an all-axial conformation by steric or structural constraints dramatically Increases reactivity towards hydrolysis (note the chair conformation of "super "-armed donor is a simplification - steric hindrance leads to considerable distortions).

In an attempt to clarify the conflict between Scheme 7a and 7b, theoretical studies were carried out using the tetrahydropyran derivatives in Figure 1 as models (79). The ethoxy chlorides, 33 and 36, correspond to armed donors, and acetoxy counterparts, 34 and 37, to the disarmed analogs. The corresponding oxocarbenium ions are 39/43, and 40/44 respectively. 

The transition energies, shown along the broken arrows in Figure 1, affirm that the armed donors should ionize more readily than their disarmed counterparts. However if 34 and 37 should avail themselves of anchimeric assistance, they would proceed to 41/42 and 45 with an energetic advantage of 8-9 Real over their armed competitors 33 and 36. 

Typically, with the manno studies in Scheme 13, a solution containing the NPOE 64 and the diol acceptor was treated with NIS followed by addition of Yb(OTf)3. However, when this procedure was used for gluco and galacto NPOEs, 79 and 81, rapid reairangement to the armed donors, 80 and 82, occurred with the result diat very little glycosidation took place. 

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