Guest complexes ketones

Host-guest complexes such as (67) have been prepared from molecular squares involving Lewis base receptor sites, such as cyclobis[(cw-(dppp)Pt(4-ethynylpyridyl)2)(cM-LM)]Ag2 6+(OTf)6, where M = Pdn or Ptn and L = dppp or 2PEt3, by reaction with pyridine, pyrazine, phenazine, or 4,4 -dipyridyl ketone.519... 

The host (7.15) was found by combinatorial synthesis (from a library of 100 salts) It forms host-guest complexes with several alcohols, ketones, xylenes, and others (e.g., 2 3 with methanol, 1 1 with ethanol, 2 2 with acetone, and 2 1 with acetonitrile).76... 

There are two directions in the development of supramolecular catalytic compositions, that is, (1) creation of systans based on macrocyclic compounds as host molecules that bind substrates with their hydrophobic cavity and (2) development of the systems that bind substrates using aggregates formed by am-phiphihc compounds. Compounds that form host-guest complexes like modified cahxarenes are able to aid transport of substrates into the aqueous phase. This approach has been implemented in the Wacker oxidation [40,41], oxidation of alkylaromatic compounds [42], hydroxylation of aromatic compounds [43], hydrogenation [44,45], hydroformylation [45-48], and carbonylation [49]. In this case, the substrate is transported into the aqueous phase in the form of the corresponding inclusion complex. This not only affects the activity of the catalyst, but also provides selectivity of the process. Thus, in the Wacker oxidation of 1-alkenes the maximum yield of methyl ketone was achieved when 1-hexene is used, and for systems based on calix[6]arene with 1-octene among catalytic systems with modified calix[4]arenes [50]. 

The wheel-and-axle design as source for host-guest compounds was originally proposed by Toda and Hart in 1981 for hosts containing hydroxyl functions 481 (see Ch. 3, Sect. 2.1 of Vol. 140). The l,l,6,6-tetraphenylhexa-2,4-diyne-l,6-diol (24) provides a representative compound. It forms 1 2 crystalline inclusion complexes with a large number of small guest molecules, including a variety of ketones, amines, amides and a sulfoxide 48). 

The majority of the studies in this series were performed using aliphatic and aromatic ketones as guests. The steroid sites that undergo functionalization vary with the guest used. Thus, irradiation of the DCA-diethyl ketone complex affords the product, 165, of addition of the ketone to the 6-equatorial position. Exposure of the DCA-cyclohexanone complex to light brings about formation of steroid functionalized in the D-ring, 166, while irradiation of the acetophenone complex affords the 5-fi-DCA adduct, 167. 

Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentasil zeolite) provides at least two types of sites for guest molecules [234-236], The triplet states of several arylalkyl ketones and diaryl ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na + -X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (t varies between 20/is and 30 min) [237]. 

Comparatively, the walls of a reaction cavity of an inclusion complex are less rigid but more variegated than those of a zeolite. Depending upon the constituent molecules of the host lattice, the guest molecules may experience an environment which is tolerant or intolerant of the motions that lead from an initial ketone conformation to its Norrish II photoproducts and which either can direct those motions via selective attractive (NB, hydrogen bonding) and/or repulsive (steric) interactions. The specificity of the reaction cavity is dependent upon the structure of the host molecule, the mode of guest inclusion, and the mode of crystallization of the host. 

Several interesting examples of solid inclusion complexes with ketone guests which undergo the Norrish II reactions have been examined. They illustrate the breadth of reaction cavity types and resultant selectivities that can be expected in such systems. 

Solid inclusion complexes of photoreactive ketones with cyclodextrins as hosts (Figure 6) provide interesting examples of how a fairly stiff, somewhat heterogeneous (in terms of polarity, size, and guest orientation) reaction... 

Although, at that time, the term supramolecular chemistry had not yet been coined, the practical potential for inclusion complexation for acetylene alcohol guests 1 and 2 was recognized back in 1968 [12], Spectroscopic studies showed that 1 and 2 formed molecular complexes with numerous hydrogen-bond donors and acceptors, i.e. ketones, aldehydes, esters, ethers, amides, amines nitriles, sulfoxides and sulfides. Additionally, 1 formed 1 1 complexes with several n-donors, such as derivatives of cyclohexene, phenylacetylene, benzene, toluene, etc. The complexation process investigated by IR spectrometry revealed the presence of OH absorption bands at lower frequencies than those for uncomplexed 1 and 2 [12], These data, followed by X-ray studies, confirmed that the formation of intermolecular hydrogen bonds is the driving force for the creation of complexes [13],... 

Structure-Reactivity Relationship in Deoxycholic Acid Complexes.—The three-channel motifs offer a variety of host-guest arrangements that may be exploited for the performance of solid-state reactions. Two kinds of reagents were occluded (a) peroxides, hydroperoxides, and peresters, which were activated thermally or by irradiation, (b) ketones, which were activated photochemically. 

Table 4 DCA.-ketone complexes. Host-guest distances (A), angles (°), the points of guest addition and of hydroxylation on DCA, and the corresponding chemical yield... 

Insight into the photochemical reactions between deoxycholic or apocholic acid ( choleic acids ) and guest molecules in crystalline inclusion complexes has been obtained by X-rzy studies. The choleic acids form channels with wall structures determined by the nature of the guest molecule. Guest ketones of various types react photochemically by addition to the choleic acid at a site determined by the orientation of the ketone molecule in relation to the host (e.g. deoxycholic acid reacts at C-5 or C-6 with linear aliphatic ketones, but at C-16 with cyclohexanone).12... 

The use of cyclodextrins14 has provided the ability to conduct the Strecker reaction with TMSCN in water via supramolecular catalysis involving reversible guest-host interactions. Activation of imine 16 by complexation with the hydroxyl groups present in cyclodextrins was found to work best with p-cyclodextrin. This chemically green reaction could be applied to ketones as well as aldehydes. 

Recently an improvement in this separation technique was reported, which seemed to indicate that enantioselective inclusion in the lattices of chiral hosts could be employed on a large scale. [11] When crystalline hosts such as R,R)-(-)-S (m.p. 196 °C), [12] (/ ,/ )-(-)-9 (m.p. 165 °C), [12] and (5,5)-(-)-10 (m.p. 128 °C) are suspended in hexane or water, chiral guest molecules form the same inclusion compounds as from solution. This is by no means self-evident, since inclusion compounds have different crystal lattices than the pure host crystals. Thus crystal/liquid reactions occur, and phase rebuildings analogous to those observed in gas/solid reactions [13] must take place. Yet this suspension technique is more selective and more effective than the initially developed solution technique. Numerous racemic alcohols like 11, -hydroxy esters like 12, epoxy esters like 13, and epoxy ketones like 14 were stirred a few hours with appropriate hosts (suspensions of 8, 9, and 10) and formed 1 1 complexes that could be filtered off in yields of > 85 % and with ee values of > 97 % (the complex of 12 and 9 formed in hexane only 80% ee in one step). Recrystallization of the inclusion... 

The participation of the secondary alcoholic functions to the increase in association constant of the ketone recalls the basic catalysis of various esters in aqueous medium. Indeed, this hydrolysis is clearly assisted by j6-CD and Tee et al. present a mechanism in which an ionized hydroxy group of the cyclodextrin acts as a nucleophile towards the guest ester [56]. In several cases the kinetics studies are consistent with a hydrolysis process in which a complex formed from the ester and two molecules of cyclodextrin is involved [57]. 

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