Monoalkoxides

In order to effect stereocontrol over the ROP of lactones, several research groups have examined monoalkoxide aluminum initiators of the general formula LnM(OR). Preliminary studies indicated that dialkylaluminum alkoxides, e.g., Et2Al(OR), initiate the controlled ROP of lactones.772-775... 

As described previously for aluminum, in order to obviate the kinetic complexity arising from aggregation, several groups have examined potentially less complicated single-site monoalkoxides. For example, complexes (299)-(301) are active for the ROP of CL, 6-VL and even /3-PL at 0 °c.888 Polydispersities are low (< 1.10) even up to 90% conversion and Mn increases linearly with conversion for CL, although initiator efficiencies are typically 50-60%. Lanthanocene alkyls, such as (302) and (303), and hydrides, (304), exhibit almost identical reactivity for the polymerization of CL and <5-VL to the alkoxides (299)-(301) (although no activity for /3-PL was observed). 

In a similar way, the grafting-from technique has been applied to the synthesis of poly(chloroethylvinylether) chains by grafted PCL segments, i.e., po-ly(CEVE-g-CL) graft copolymers. Purposely cationically prepared PCEVE were partially modified by the introduction of 5-10% hydroxyl groups [79]. An equimolar reaction of the pendant hydroxyl functions with HAI/BU2 provided diisobutyl aluminum monoalkoxides dispersed along the polyether backbone. 

In this mechanism, the stereoregulation in the propagation reaction is realized faithfully by the steric effect acting between the incoming monomer and the growing end monomeric unit, both of which have interactions with an aluminum atom. The penta-coordinate aluminum atom of the catalyst, which is similar to that shown in the dimeric acetaldehyde-catalyst complex, plays an important role as an intermediate compound or as a transition state. In principle, an identical mechanism may be applied to the polymerization by the dialkyl-aluminum monoalkoxide catalyst. 

Benzylation of cellulose monoalkoxide was expected to give mainly the 2-0-benzyl derivative180 and, in agreement with this expectation, Wolfrom and El-Taraboulsi180 found no detectable 2,3-glycol groups in the product (by the method of Cramer, Hockett, and Purves179). 

It was shown (262) that whereas the tris-/3-diketonates are monomeric with hexacoordinated aluminum, the monoalkoxide bis-/3-diketonates dimerize with alkoxide bridges and can be represented by [(/3-dik)2Al(/i-OR)2Al(/3-dik)2], in which both aluminum atoms are hexacoordinated. The dialkoxide mono-/3-di-ketonate, instead of adopting a symmetric structure [(RO)(/3-dik)Al( -OR)2Al(/3-dik)(OR)] with both aluminum atoms in the five-coordinate state shows a structure that can be represented by the unsymmetric [((3-dik)2Al( -OR)2Al(OR)2], in which one of the aluminum atoms is six coordinate while the other is tetra-coordinate. Although higher (74) coordination for aluminum was reported in a number of glycol derivatives (263), the preferential formation of [ 3-dik)2Al(/i-0-i-Pr)2Al(0-/-Pr)2] may be considered to arise from the stability of Al(0-i-Pr)4 moiety, a very large number of derivatives of which were already described (4-8, 14-20, 25-30, 38, 39). 

The synthesis of a Boc-hke linker oufhned in Scheme 8 was performed in a straightforward maimer. Commercially available 3-methyl-l,3-hutanediol 17 was reacted with one equivalent of potassium tcrt-butoxide, and the resulting monoalkoxide was coupled to a chloromethyl support to create the solid support-bound tertalkyl alcohol 18, which was activated with CDI. In order to enhance the acylating reaction, the imidazole intermediate was methylated before the addition of the amine, leading ultimately to 19. The amine 20 was released by exposure to 10% TFA. 

The sulfur analogues of the alkaline earth monoalkoxides have also been made in a Broida oven by the reactions of Ca and Sr with thiols [111]. 

Scheme 4.2 Proposed activation mechanism for the ROP of -caprolactone promoted by Sn(Oct)2 (only tin monoalkoxide, Oct-Sn-OR, is shown even though the formation of tin dialkoxide cannot be ruled out. Reproduced with Elsevier permission from Ref. 1).

This review will focus on isolated and characterized high-oxidation state molybdenum and tungsten alkylidene and metallacyclobutane complexes. Attention will be directed largely toward monoalkoxide pyrrolide (MAP) complexes because they have yielded the majority of new results in the last several years. MAP species have been found to be especially efficient in several Z-selective olefin metathesis reactions, such as homocoupling, cross-coupling, ethenolysis, and ROMP (see Grubbs, Handbook of Metathesis, 2nd Edition, Volume 2, Chapter 7). Most of what is presented here has appeared since a review in 2009 [4]. 

Tin alkoxides have also been preformed and used as ROP initiators. Alkyl-substituted tin(IV) alkoxides are more commonly used than the tin(II) counterparts. Compared with tin monoalkoxide (R sSnOR), tin(IV) dialkoxides (R 2SnOR2) are preferred because of a higher reactivity (42). Cyclic tin(II) alkoxides deserve interest because they form macrocyclic polyesters, which are precursors of telechelics, upon ring opening by acidic hydrolysis or reaction with acid chlorides (Fig. 7). 

Calculations have revealed the critical influence of the aluminum substituents. Significantly lower activation barriers are predicted for trialkoxides relative to the corresponding monoalkoxides [66]. The sensitivity of aluminum initiators toward steric bulk has also been noted and is attributed to the shortness of the aluminum-oxygen bonds [67]. Accordingly, the activation barrier for the insertion step is found to be about 40% higher for lactide than for glycolide for the... 

Based on the inversion of the stereogenic metal center s absolute configuration after each metathesis cycle, monoalkoxide pyrrolide (MAP) complexes undergo Z-selective ROMP to syndiotactic polynorbornene (Scheme 5.14). 

Not only persistent radicals can be reacted with low-valent metal complexes but also those that are generated in situ by the thermal cleavage of peroxides. To our best knowledge, so far, only samarium [41] and ytterbium [42] compounds were reported into which a tm-butoxide moiety was introduced by reduction of di-rcrr-butylperoxide by the metal center. Here, the first Cp-type titanium monoalkoxide complex synthesized by reacting a titanium(lll) species with di-tert-butylperoxide is reported. 

Barbier-Baudry, D., Heiner, S., Kubicki, M. M., Vigier, E., Visseaux, M., Hafid, A. An easy synthetic route to heteroleptic samarium monoalkoxides for ringopening polymerization initiators. Molecular structures of [(C5H Pr4)SmI(THF)2]2, Sml20t-Bu(THF)4, and (C4Me4P)2SmOt-Bu(THF). Organometallics, 20, 4207-... 

Overall, the isolation and full characterization of products derived from the oxygenation of organoaluminum complexes has proved very rare. In one example, the reaction of the tetranuclear cluster [Al4(p3-8-quinolylimide)2Meg with molecular oxygen selectively afforded the monoalkoxide cluster [Al4(p3-8-... 

An NHC-Os complex 101 was reported to be active in TH of various ketones in refluxing i-PrOH (Figure 13.10) it is worth noting that no base was required for these reactions. Furthermore, the authors observed that, in isopropanol at -30 °C, the hydroxo complex converted into 102, an active monoalkoxide catalyst, and this subsequently led to 103, which was stable enough to be isolated. Thus, this study threw some light into the mechanism of TH of ketones by i-PrOH. ... 

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