Tert rhodium-catalyzed reaction

Cyclohexadiene derivatives are less reactive than butadiene derivatives, thus only a few examples of cycloadditions with these compoimds are known (Figure 4.3) [37 0]. The cyclohexadiene bicychc derivative 32 was synthesized by rhodium-catalyzed reaction of toluene with tert-butyldiazoacetate and cycloadds in about 40% yield to Cjq [39]. The product has anti-cyclopropane orientation relative to the entering dienophile Cjq. Valence isomerization of 33 (Scheme 4.4) leads to the cyclobutene-fused cyclohexene 35 that adds in good yields (50%) at moderate temperatures (110 °C) to Cjq [40]. The reaction of with the electron-deficient cyclohexene 34 is also possible in moderate yields [38]. 

When diallyl diazomalonate or allyl aryldiazoacetates are used as substrates in the rhodium-catalyzed reaction with di(tert-butyl)thioketene, 4-allyl-l,3-oxathiolan-5-one derivatives of type 158 are formed (82). After the initial 1,5-dipolar electro-cyclization, 157a undergoes a subsequent Claisen rearrangement to give the thermodynamically more stable compound 158 (Scheme 5.47). 

The first example of the [3 + 2] cycloaddition of ketocarbenes with a phosphorus-carbon triple bond has also been reported. Thus, the rhodium-catalyzed reaction of 2-diazocyclohexane-l,3-diones with tert-butylphosphaethyne gave 1,3-oxaphosphole 320 (377) (Scheme 8.80) in moderate yield. 

Generally, one would expect that increasing steric hindrance in the catalytically active rhodium complex would result in lower reaction rates. In this respect, the results of Van Leeuwen and Roobeek seemed at first to be contradictory. They used the very bulky tris(ortho tert-butylphenyl)phosphite la (Chart 6.1) as a ligand and found high reaction rates in the rhodium catalyzed hydroformylation of other-... 

Most of the currently applied protocols for rhodium-catalyzed conjugate addition chemistry involve the use of aqueous solvent systems which ensure catalytic turnover by protonation of the intermediate rhodium enolate. Consequently, tandem reaction sequences with electrophiles other than a proton are troublesome. In early investigations, Hayashi reported a rhodium/BINAP-catalyzed conjugate addition-aldol reaction under anhydrous conditions by use of 9-aryl-9-borabicyclo[3.3.1]no nanes (9-Ar-9-BBN) as aryl sources [117]. The reaction between tert-butyl vinyl ketone (145) with 9-(4-fluorophenyl)-9-BBN (146) and propionaldehyde (147) led to the formation of a syn/anti-mixiuve of 148 in a 0.8 to 1 ratio (Scheme 8.39). 

The metal complexes [MCl(CO)(Ph3P)2] (M = Rh, Ir) catalyze epoxidation of tetramethylethylene with tert-hutyl hydroperoxide in good yield and the selectivity was better 90%) with rhodium than iridium, equation (285). In this case a reasonable mechanism for epoxide formation involves epoxidation of unreacted olefin with the intermediate aUylic hydroperoxide, XXXIX. The allylic hydroperoxide was found to reach levels in excess of 11% during the course of the metal catalyzed reactions of tetramethylethylene with oxygen [470], equation (284). 

To probe hydroperoxide reactivity in these systems we studied the reaction of tert-butyl hydroperoxide in the presence of [C5H5V(CO)4]. In contrast to the rhodium(I) and molybdenum complexes, [C5H5V-(CO)4] catalyzed the rapid decomposition of tert-butyl hydroperoxide to oxygen and tert-butyl alcohol in both toluene and TME (Table II). When reaction was done by adding the hydroperoxide rapidly to the vanadium complex in TME, no epoxide (I) was produced. However, when the TME solution of [C5H5V(CO)4] was treated with a small amount (2-3 times the molar quantity of vanadium complex) of tert-butyl hydroperoxide at room temperature, a species was formed in situ which could catalyze the epoxidation of TME. Subsequent addition of tert-butyl hydroperoxide gave I in 13% yield (Table II). This vanadium complex also could catalyze the epoxidation of the allylic alcohol (II) to give tert-butyl alcohol and IV (Reaction 14). Reaction 14 was nearly quantitative, and the reaction rate was considerably faster than with TME. 

Imines derived from N tert butanesulfinamide also undergo diastereoselective rhodium(I) catalyzed addition of arylboronic acids [61] (Scheme 1.16), a reaction that has been primarily developed for catalytic asymmetric processes (see below). 

A diastereoselective approach to these chiral building blocks was developed by Ellman (Scheme 8.15) [40]. Thus, N-tert-butylsulfinyl aldimines 57 were employed as electrophiles in the rhodium/phosphine 58-catalyzed addition of arylboronic acids. Whereas Ellman s procedure requires heating, Batey discovered that an amine base allows the reaction to take place at ambient temperature [41]. However, it is worth noting that Ellman s process can also be carried out in an enantioselec-tive way by using N-diphenylphosphinoyl aldimines 48 as substrate and Degu-PHOS (56) as ligand (87-97% yield, 88-94% ee) (Scheme 8.14 Figure 8.4). 

Cheng and Muralirajan devised a rhodium(III)-catalyzed, chelate-assisted C-H activation/annulation reaction as a means to prepare numerous substituted cinnolinium salts by treating azobenzenes with alkynes in the presence of pentamethylcyclopentadienylrhodium(III) chloride dimer [(RhCp Cl2)2] and copper(II) tetrafluoroborate hexahydrate (Cu(BF4)2 6H20) in tert-butanol in air at 70°C (Scheme 2) (13CEJ6198). The salts were obtained in good-to-high yields and were subsequently... 

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