Lewis bases 832 Subject

A valine-derived oxazaborolidine derivative has been found to be subject to activation by Lewis acids, with SnCl4 being particularly effective.98 This catalyst combination also has reduced sensitivity to water and other Lewis bases. 

As mentioned above, the ability of TiCl4 (OR) to function as a Lewis add decreases as n increases. Few adduct complexes of Ti(OR)4 are known, although Ti2(OEt)8 en, Ti2(OPri)8-en and Ti(OPri)4-en have been reported,14 but these complexes almost completely dissodate in solution. However, Ti(OPh)4 readily forms 1 1 adducts with PhOH, NH3, NH2Me, NHMe2, NMe3, py, dioxane, PhNH2, Me2CO- These complexes have been the subject of a review.62 The difference in the behaviour of the Ti(OPh)4 to the Ti(OR)4 (R = Me, Et) towards Lewis bases is undoubtedly related to the monomeric, and thus the coordinatively unsaturated, nature of the phenoxides in solution. 

The role of Lewis bases in MgCl2-supported Ziegler-Natta catalysts for x-olefin polymerisation is still a subject of continuous interest. Recently [67], the use of 1,3-diethers [of the general formula R1OCH2C(R2)(R3)CH20R4], e.g. 2,2-diisobutyl-l,3-dimethoxypropane, as the internal Lewis base during... 

Among the many reactions of these species is the conversion to chiral species such as 18-D-XIX. This compound can be obtained in enantiomerically pure form and can be converted to the CpRe(NO)PPh3 ion.69 This ion is a chiral Lewis base that binds a variety of prochiral molecules (olefins, ketones, aldehydes, amines). With these adducts one may conduct numerous reactions where enantiomeric excesses >98% are obtained. As an example, a prochiral methyl ketone will bind selectively, as in 18-D-XX and is then subject to attack by R X to produce only one enantiomer of the RR MeCO product. 

Both methylphosphine (this synthesis) and dimethylphosphine (the following synthesis) are considerably stronger Lewis bases than phosphine, and their reactions with various Lewis acids, including boranes, have been the subject of recent studies. Both methylphosphine and dimethylphosphine are readily oxidized by air, and thus can serve as sources of methylphos-phonous (methylphosphinic) acid, CH3PHO(OH), and dimethyl-phosphinic acid, (CH3)2PO(OH). ... 

The nature of donor-acceptor complexes has been the subject of various NMR studies conducted as early as the 1960s. Early calorimetric studies showed that boron trihalides are capable of forming donor-acceptor complexes with a number of Lewis bases and the heats of adduct formation for some of these complexes were determined. Gaseous boron trifluoride, for example, was shown to form a ctxnplex with ethyl acetate in a highly exothermic reaction (-A// = 32.9 0.2 kcal mol ). IR and UV analysis of BF3 complexes of aromatic aldehydes indicated a o-complex with a lengthened CVO bond and a highly delocalized ir-system. More detailed structural information, however, was acquired only after closer inspection by low temperature H, B, C and F NMR studies. ... 

Epoxy resins may be cured in the manner of polyadditions, i. e., homogeneously catalyzed by multifunctional amines and isocyanates, or cyclic anhydride, dicyan-diamide, or biguanide derivatives. On the other hand epoxy resins are also subject to homopolymerization. The catalysts represent Lewis bases, preferably tertiary amines, imidazoles, or ureas (the latter exclusively for the dicyandiamide curing)... 

A high degree of stereoselectivity can be realized under chelation control, where an oxygen atom of an ether function (or more generally a Lewis base) in the a-, P- or possibly y-position of carbonyl compounds can serve as an anchor for the metal center of a Lewis acid. Since Cram s pioneering work on chelation control in Grignard-type addition to chiral alkoxy carbonyl substrates [30], a number of studies on related subjects have appeared [31], and related transition state structures have been calculated [32], Chelation control involves Cram s cyclic model and requires a Lewis acid bearing two coordination sites (usually transition metal-centered Lewis acids). 

Most metallaboride clusters are synthesized from metal-rich metallaboranes (Lewis base adducts of BH3 are often used as the source of the B atom) several different methods are used to transform B-H bonds into B-M interactions and hence isolate a naked boron atom. Two recent review articles have been devoted to this subject.l l... 

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