Rhodium catalysis intermolecular

Rhodium catalysis has played a critical role in the development of this type of reaction. The rhodium-mediated [4 + 2] carbocyclization between dienes and unactivated olefins or alkynes is a notable early example of this concept [2]. Further investigations demonstrated the extension of this methodology to the reaction between a diene and an allene [3]. Expansion of the scope of this strategy, to both the intra- and intermolecular [5-1-2] homologs of the Diels-Alder reaction, was accomplished with a vinylcyclopropane and either an alkyne or an olefin to afford the carbocyclization adducts (Scheme 11.1) [4, 5]. 

Wender s group has also developed rhodium-catalyzed intermolecular [5-1-2] cycloadditions. At first, they found the catalysis system of Rh(PPh3)3Cl for the intramolecular reactions was not effective at all for the intermolecular reactions. To effect the intermolecular [5-1-2] cycloadditions, [Rh(CO)2Cl]2 must be used and oxygen substitution of the cyclopropane was necessary (see (17)) [37-39]. Then they successfully expanded the substrate to unactivated vinylcyclopropanes by adjusting the substituents. For monosubstituted alkynes, the substitution on the olefin terminus directs the formation of single isomer that minimized steric hindrance (see (18)) [40]. The [5-1-2] cycloadditions can also be applied to VCPs with allenes (see (19)) [41]. It should be noted that the alkyne substituent did not interfere with the reaction, indicating that allenes as reaction partners were superior to alkyne in the [5-1-2] cycloadditions. Curiously, the authors didn t report the corresponding intermolecular [5-1-2] cycloadditions of VCPs with alkenes. 

Trost reported the synthesis of 1,4-dienes with ruthenium catalysis through regioselective carbometallation of alkynes with alkenes.51 Di- and trisubstituted olefins can also be obtained with arylboronic acids through an intermolecular process under rhodium,30 52 55 nickel,56 and palladium catalysis.57 Recently, Larock has reported an efficient palladium-catalyzed route for the preparation of tetrasubstituted olefins.58,59... 

Concurrently, Noels had reported that rhodium carboxylates smoothly catalyze the intermolecular C—H insertion of ethyl diazoacetate into alkanes. Following up on this report, Taber demonstrated that the open chain a-diazo 3-keto ester (60) cyclizes smoothly under rhodium acetate catalysis to give the corresponding cyclopentane (61 equation 24). In contrast to the copper-mediated cyclization cited above (equation 22), the six-membered ring product is not observed. The insertion shows significant electronic selectivity. Although there is a 3 1 statistical preference for methyl C—H, only the methylene C—H insertion product (61) is observed (equation 24). 

In the formative years of diazo chemistry it was recognized that copper catalysis both reduced the decomposition temperature of a diazo compound and allowed much more efficient intra- and intermolecular cyclopropanation reactions. With the advent of rhodium- and palladium-based catalysts, the purely thermal method has lost even more ground. 

The effect of ortho- and weto-substitution in the above-mentioned intramolecular Buchner reactions has been examined. When the 2-methoxy-substituted diazoketone 32 is subjected to rhodium(II) acetate catalysis, a single cycloheptatrienone 34 is obtained in 94% yield.This result is consistent with the outcome of the rhodium(II) trifluoroacetate-catalyzed intermolecular reaction of ethyl diazoacetate with anisole, which yields no product arising from addition of the ketocarbenoid on the most hindered site of the anisole. Dihydroazulenone 34 rearranges to tetralone 36 under acidic conditions, and isomerizes to the conjugated ketone 35 under basic conditions. It is interesting that the catalyzed decomposition of the para-methoxy derivative 37 provides exclusively 6-methoxy-2-tetralone 40 with no trace of the putative trienone 39. ... 

---