H-bonding adducts

Few attempts to determine the thermodynamic characteristics of proton transfer from alcohols to hydrides can be found in the literature. [32a, 47] However, the NMR technique used in those studies has obvious limitations in that it does not allow the determination of the equilibrium concentrations for each intermediate in Eq. 11 and the calculation of their formation constants. These limitations originate from the averaging of the hydride signal in the H NMR spectrum due to a fast interconversion between 1 and 2. Similar technical difficulties also occasionally arise in the IR investigations, resulting for instance from the overlap of the v o bands of the initial hydride 1 and the H-bonded adduct 2. 

Obviously the barriers heights in the energy profile of Fig. 10 are determined by the rates of the corresponding transformations. The H-bonded adduct formation is always a diffusion-controlled process, therefore there is no barrier for this step [3]. 

Scheme 8.24 Photoinduced electron transfer from ketone 56 to the iridium complex occurs coti-certedly with protonation by the chiral phosphate. In the process, the ketyl radical becomes significantly more basic than phosphoric acid, and rapid radical cyciization from the neutral ketyl as an H-bonded adduct to the chiral phosphate occurs. Asymmetric induction results, due to the rapid radical cyciization. Hantsch dihydropyridine esters provide the required proton and hydrogen atoms...

Two points arise when studying the interaction of mildly basic molecifles with the H-Al-MTS surface with the purpose of identifying the acidic Bronsted centers through the formation of H-bonded adducts. A first point is to check whether the missing acidic OH species absorb at the very same frequency of silanols, and are, thus, masked by the latter, being nonetheless characterized by a more marked acidity, as proposed by both Busca and... 

Bordiga S, Regli L, Lamberti C, Zecchina A, Bjprgen M, Lillerud KP. FTIR adsorption studies of H O and CH3OH in the isostructural H-SSZ-13 and H-SAPO-34 Formation of H-bonded adducts and protonated clusters. J Phys Chem B 2005 109 7724-32. 

The affinity towards water of the all-silica counterpart was lower than that of the proton exchanged zeolite, as expected, but it was not negligible. The reported isotherms indicated that hydrophilic sites, responsible for weak and reversible water H-bonding adducts, are developed in zeolites even in the absence of framework A1 atoms. Stmctural defects generating polar species consisting of Si—OH nests (which are characterized by a weak Brpnsted acidic strength),[25, 61] are always present in Al-free zeolites, unless especially prepared in order to obtain hydrophobic, inert materials, as claimed by Flanigen et al.[36]. See also Ref. [24]. 

The room temperature adsorption of CO on protonic zeolites allowed Lewis acidic sites to be calorimetrically discriminated from the Brpnsted acidic ones Al(III) CO adducts of relative stability were formed at the cus Al(III) Lewis sites, whereas only very weak H-bonding adducts were formed at the Brpnsted Si(OH)+Al acidic sites [73],... 

Conversely, the heat of adsorption of CO at polar sites located in the all-silica nanopores (Si-OH nests) was much lower (q llk] mol ) than on H-BEA, and virtually constant over the whole coverage examined in agreement with the formation of very weak H-bonding adducts, as described in Refs. [73, 105], The adsorption capacity of the all-silica zeolite was dramatically lower than that of the proton-exchanged zeolite, as witnessed by the correspondent isotherms reported in the inset... 

The widely used Moifatt-Pfltzner oxidation works with in situ formed adducts of dimethyl sulfoxide with dehydrating agents, e.g. DCC, AcjO, SO], P4O10, CCXTl] (K.E, Pfitzner, 1965 A.H. Fenselau, 1966 K.T. Joseph, 1967 J.G. Moffatt, 1971 D. Martin, 1971) or oxalyl dichloride (Swem oxidation M. Nakatsuka, 1990). A classical procedure is the Oppenauer oxidation with ketones and aluminum alkoxide catalysts (C. Djerassi, 1951 H. Lehmann, 1975). All of these reagents also oxidize secondary alcohols to ketones but do not attack C = C double bonds or activated C —H bonds. 

Insertions into tertiary C—H bonds can be carried out with moderate yields. Among other less common reactions improved by the use of PTC-generated CCI2 are the carbylamine synthesis (RNH2 — R NC) (33). Alkylidene carbene (R2C=C ) and alkenyUdene carbene (R2C=C=C ) adducts have also been prepared (34,35). 

The hydrate N2H4.H2O is an H-bonded molecular adduct and is not ionically dissociated. Two series... 

The strained bicyclic carbapenem framework of thienamycin is the host of three contiguous stereocenters and several heteroatoms (Scheme 1). Removal of the cysteamine side chain affixed to C-2 furnishes /J-keto ester 2 as a possible precursor. The intermolecular attack upon the keto function in 2 by a suitable thiol nucleophile could result in the formation of the natural product after dehydration of the initial tetrahedral adduct. In a most interesting and productive retrosynthetic maneuver, intermediate 2 could be traced in one step to a-diazo keto ester 4. It is important to recognize that diazo compounds, such as 4, are viable precursors to electron-deficient carbenes. In the synthetic direction, transition metal catalyzed decomposition of diazo keto ester 4 could conceivably furnish electron-deficient carbene 3 the intermediacy of 3 is expected to be brief, for it should readily insert into the proximal N-H bond to... 

Finally, concurrently with addition, reduction of tri- or dihalomethyl groups in the adduct can occur under conditions of initiating by metal-complex systems in the presence of hydrogen donor chain transfer at C-H bond, at C-Br one, is also possible to form compounds containing one bromine atom less than adducts. 

The structure of the adducts obtained (addition resulted from homolysis of C-Br bond) is an indirect evidence for radical character of the process. Ionic addition of haloforms is known to occur at C-H bond (ref. 11), this leads to adducts with CX3 group. The highest yield of the adduct with bromoform was obtained for... 

Figure2.2 Products arisingfromC-H bond activation of Au(N)Cl3 adducts (N = mono-ligated 6-substituted 2,2 -bipyridine).

As in the case of 2-substituted pyridine adducts, activation of an aromatic C—H bond is observed when the adducts of 6-thbipy and of 6-Bnbipy are heated under reflux in aqueous media. A cyclic dimer (1) with bridging N,C ligands is obtained in the first case [24], while cyclometallated derivatives [Au(N, N,C)C1] (2) are formed in... 

Under comparable reaction conditions, no C—H bond activation is observed for adducts of 6-Phbipy and 6-Rbipy. Nevertheless, [Au(N,N,C)Cl] derivatives can be obtained with 6-Phbipy [18] and with 6-tBubipy (tBu = CMe3) [20]. The former is obtained by a transmetallation reaction of the arylmercury(II) derivative with [AuClJ, while activation of a C(sp )—H bond of the tert-butyl substituent is accomplished by reaction of the Au(N)Cl3 adduct 3 (N = 6-tBubipy) with AgBp4 in the presence of excess ligand (Scheme 2.2). 

Whereas the Rh2(OAc)4-catalyzed addition of diazoalkanes to propargyl alcohols readily gives the insertion of the carbene into the 0-H bond, with only a small amoimt of cyclopropenation of the resulting propargylic ether [54] the 2-diazopropane 59 reacts at 0 °C with l,l-diphenyl-2-propyn-l-ol 62a in dichloromethane and exclusively gives, after 10 h of reaction, only the adduct 63a isolated in 75% yield and corresponding to the regioselective 1,3-dipolar cycloaddition of the 2-diazopropane to the alkyne C - C bond (Scheme 15). 

NMR monitoring of the reaction of the palladium complex with 1-octyne suggested that the alkyne inserts into the Pd-H bond. Further heating produced a mixture of the two regioisomeric alkenylphosphine oxides, the anti-Markovnikov adduct being the favored product (54 46, 65% yield). 

For consistency, an initial well associated with the formation of an adduct of the metal ion with the alkane should be included in Figure 11. The chemical activation associated with the formation of such an adduct is likely to be essential in overcoming intrinsic barriers associated with insertion into C-H bonds. In comparison to larger hydrocarbons, the weaker interaction of ethane with first row group 8-10 metal ions may be insufficient to overcome intrinsic barriers for insertion. This would explain the failure to observe dehydrogenation of ethane by these metal ions, even though the process is known to be exothermic. The well depths could be determined from high pressure equilibria. Studies in our laboratory and elsewhere have indicated the ease with which many of... 

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