Hydrogenation unactivated

Replacement of an unactivated lone chlonne with hydrogen is also accom-phshed in good yield with lithium aluminum hydride. Chlorofluoronorcaranes are selectively dechlormated to give hydro derivatives, largely with retention of configuration The related monodeutcronorcaranes are prepared with lithium aluminum deutende [27] (equations 20a and 20b). 

The scope of heteroaryne or elimination-addition type of substitution in aromatic azines seems likely to be limited by its requirement for a relatively unactivated leaving group, for an adjacent ionizable substituent or hydrogen atom, and for a very strong base. However, reaction via the heteroaryne mechanism may occur more frequently than is presently appreciated. For example, it has been recently shown that in the reaction of 4-chloropyridine with lithium piperidide, at least a small amount of aryne substitution accompanies direct displacement. The ratio of 4- to 3-substitution was 996 4 and, therefore, there was 0.8% or more pyridyne participation. Heteroarynes are undoubtedly subject to orientation and steric effects which frequently lead to the overwhelming predominance of... 

Barton devised this interesting photoinitiated method for functionalizing unactivated carbon-hydrogen bonds in response to a... 

Catalytic hydrogenation seldom breaks unactivated C—C bonds (i.e., R—R + H2 RH + R H), but methyl and ethyl groups have been cleaved from substituted adamantanes by hydrogenation with a Ni-Al203 catalyst at about 250°C. Certain C—C bonds have been cleaved by alkali metals. ... 

In a related reaction, a methylgroup and a hydrogen can be added indirectly to the double bond of an otherwise unactivated allylic alcohol in this manner ... 

A surprising exception has been reported with evidence for a cleavage reaction in the case of divinyl sulphone. In non-aqueous and slightly acidic media, the behaviour of a., ji-unsaturated aromatic sulphones is also complex (see Table 7) since the cleavage and the saturation may compete. Strongly electrophilic double bonds undergo Michael additions in aprotic solvents by slowly protonated anions. Transfer of labile hydrogen may also lead to unactivated bases. It is noteworthy that in numerous cases (Table 6) the saturation is the preferred route. 

Wilkinson reports the first homogeneous hydrogenation of an, <, R Rh(PPh3)3CI unactivated alkene [22-24] h2... 

Finally it should be mentioned that a number of nucleophilic substitution reactions of unactivated halides can be made to proceed in bipolar non-protic solvents such as dimethyl sulphoxide (DMSO), Me2S —Oe. No hydrogen-bonded solvent envelope, as in for example MeOH, then needs to be stripped from Ye before it can function as a nucleophile AG is thus much lower and the reaction correspondingly faster. Rate differences of as much as 109 have been observed on changing the solvent from MeOH to Me2SO. Chlorobenzene will thus react readily under these conditions with Me3COe ... 

The first palladium-catalyzed formation of aryl alkyl ethers in an intermolecular fashion occurred between activated aryl halides and alkoxides (Equation (28)), and the first formation of vinyl ethers occurred between activated vinyl halides and tin alkoxides (Equation (29)). Reactions of activated chloro- and bromoarenes with NaO-Z-Bu to form /-butyl aryl ethers occurred in the presence of palladium and DPPF as catalyst,107 while reactions of activated aryl halides with alcohols that could undergo /3-hydrogen elimination occurred in the presence of palladium and BINAP as catalyst.110 Reactions of NaO-/-Bu with unactivated aryl halides gave only modest yields of ether when catalyzed by aromatic bisphosphines.110 Similar chemistry occurred in the presence of nickel catalysts. In fact, nickel catalysts produced higher yields of silyl aryl ethers than palladium catalysts.108 The formation of diaryl ethers from activated aryl halides in the presence of palladium catalysts bearing DPPF or a CF3-subsituted DPPF was also reported 109... 

As shown in the manganese- and ruthenium-catalyzed intermolecular nitrene insertions, most of these results supposed the transfer of a nitrene group from iminoiodanes of formula PhI=NR to substrates that contain a somewhat activated carbon-hydrogen bond (Scheme 14). Allylic or benzylic C-H bonds, C-H bonds a to oxygen, and very recently, Q spz)-Y bonds of heterocycles have been the preferred reaction sites for the above catalytic systems, whereas very few examples of the tosylamidation of unactivated C-H bonds have been reported to date. 

Conjugated dienes (such as 1,3-cyclohexadiene, cyclopentadiene, 2,4-hexadienoic-sorbic-acid) and polyenes can be selectively hydrogenated to monoenes unactivated alkenes are totally unreactive [20]. Unfortunately, the possibilities for modification of the catalyst by ligand alteration or by the use of additives are very limited [50, 51]. 

Oxidative Functionalization of Unactivated Carbon Hydrogen Bonds in HCTD (22)... 

Many of the limitations of C—C bond formation by C —H insertion outlined for intermolecular reactions (Section 1.2.1.) can be overcome by making the reaction intramolecular. Thus, hydrogen atom abstraction followed by intramolecular radical-radical coupling or radical addition to an alkene are increasingly popular processes. Two-electron carbene insertions, either thermal or transition metal catalyzed, have also been used extensively. In either case, ring construction involves net C—C bond formation at a previously unactivated C-H site. 

Certain hindered alkenes display exceptional behavior. They readily undergo cycloaddition to yield 1,2-dioxetanes despite the presence of allylic hydrogen. The oxidation of 2,2 -biadamantylidene was the first successful synthesis of the dioxetane of an unactivated alkene 400,401... 

Aryl azides react with olefins to give 1,2,3-A2-triazolines.201-203 Whereas unactivated olefins are sluggish toward aryl azides, strained bicyclic systems, on the contrary, are particularly reactive.204-208 The rate of phenyl azide addition to a series of olefins parallels the heat of hydrogenation (Table Vffl).107 This thermodynamic parameter is a measure for the degree of strain associated with these molecules.200 Bicyclo[2.2.1]hep-tenes, such as norbomene (37), react readily at room temperature, and the addition occurs at the less hindered exo side (39).,06 1°7 210 211 A striking example of the increased... 

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