Rhodacycles, formation

Another approach to the construction of benzofused sultams was also reported by Wang and Li groups [25]. The reaction provides a wide scope of ort/zo-alkylated benzofused sultam analogs in good yields (Eq. (5.40)). The Af-acetyl group is important for rhodacycle formation and subsequent Michael addition reaction, which was finally removed under mild conditions to provide free NH sultams. 

The reaction mechanism leading to advanced intermediate 85 starts with rhodium insertion into the aldehyde moiety. Rhodacycle formation follows to promote hydroacylation into the 4,6-diene providing cycloheptene compound 86. Then, rhodium catalyze a highly regioselective cycloisomerization reaction on the resulting triene to produce the final product 87 (Scheme 7.54 please refer also to Scheme 7.51). 

In order to probe the mechanism, this transformation was conducted under molecular deuterium atmosphere with cationic rhodium(l) complex (Scheme 110). The final compound 440 showed the incorporation of two deuterium atoms in each double bond. This is in agreement with a heterolytic activation of D2. Two different pathways are proposed. The first one involves the formation of a rhodacycle 438 followed by reductive elimination. The second one consists of a deuteriorhodation/carborhodation sequence, affording the same intermediate 437. A vinylrhodium... 

Rhodium(I)-catalyzed reaction of phenyl 2-propylcycloprop-2-en-yl ketone with terminal alkynes gives 2-alkyl-4-propyl-7-phenyloxepines (Scheme 13). The reaction involves the formation of a rhodium-carbene complex, which undergoes a [2 + 2] cycloaddition with a terminal ethyne the resultant rhodacycle rearranges by a 1,5-sigmatropic shift, followed by reductive elimination of rhodium <92JA588l>. 

Reactions with alkynes may lead to the formation of cyclized products. The reaction of iodobenzenes with two equivalents of an alkyne has been shown to give naphthalene derivatives in the presence of cobalt catalyst with a manganese reduc-tant. The process, shown in Scheme 15, is thought to involve oxidative addition of the aryliodide to cobalt followed by double alkyne insertion. The cobalt-catalysed annulation step probably involves an pathway. The cyclopentadienyl-rhodium-catalysed annulation of benzoic acids with alkynes has been used to form isocoumarin derivatives, such as (126). The process is thought to involve cyclorhodation at the ortho-position of a rhodium benzoate intermediate, followed by alkyne insertion to form a seven-membered rhodacycle and reductive elimination The silver-catalysed annulations of diphenylphosphine oxides with alkynes proceed in the absence of rhodium. Benzophosphole oxides such as (127), formed with diphenylethyne, are produced. Here, the proposed mechanism involves homolytic cleavage of the phosphorus-hydrogen bond to give a radical which can add to the alkyne and subsequently cyclize. ... 

The intramolecular reaction of bicyclobutane-ene 17, catalyzed by [RhCl(CH2=CH2)2]2 PPh3, led to the formation of pyrrolidine 18 as the major product (Scheme 2.21). On the other hand, in the presence of a catalyst prepared from [RlCl(CO)2]2-DPPE, (DPPE = l,2-bis(diphenylphosphino)ethane) the reaction resulted in exclusive formation of azepine 19 [32]. This selectivity is determined by the step that forms (allylcarbene)rhodium intermediates from the bicyclic rhodacycle. 

Mukai et al. have found scission of a cyclopentane ring of allenyne 34 in the rhodium-catalyzed cycloisomerization (Scheme 7.11) [14]. When 34 was treated with a rhodium catalyst, the bicyclo [7.4.0]tridecatriene 37 was formed. Mechanistically, initial coordination of 34 to rhodium(I) would occur between an allenic distal double bond and an alkyne to form the intermediary ir-coordinating complex, which undergoes oxidative cyclization to form the rhodabicyclo[4.3.0]nonadiene intermediate 35. Subsequent P-carbon elimination, presumably assisted by release of the ring strain of the cyclopentane (6.3 kcal mol ), results in the formation of the 10-membered rhodacycle 36. Reductive elimination ensues to give the final product 37. 

A plausible reaction mechanism was proposed in Scheme 6.21 after a series of control experiments, and kinetic isotope effect experiments were conducted. First, coordination of [Cp Rh(OPiv)2] to phenoxyacetamide was followed by C-H bond cleavage to give the rhodacyclic intermediate B. Two pathways are possible for the formation of the eight-membered rhodacyclic intermediate G P-carbon elimination of intermediate D or migratory insertion of intermediate E. 

Rhodium catalysis allows the formation of 3,3-disubstituted a-tetralones from l-(2-haloaryl)cyclobutanols. The reaction is likely to involve a five-membered rho-dacycle (94), followed by opening of the four-membered carbocyclic ring to give a seven-membered rhodacycle (95). The tetralone product is formed following reductive... 

A cationic rhodium(I)/BINAP complex catalyses the cyclization reactions of naphthol-or phenol-linked 1,6-enynes to vinylnaphtho- or vinylbenzofurans and vinylnaphtho- or vinylbenzopyrans through the cleavage and formation of sp C-O bonds. The reaction proceeds via a -oxygen elimination cationic rhodacycle intermediate (Scheme 146). ... 

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