Rhodacycle

The replacement of rhodium from a wide range of rhodacycles to form condensed furans, thiophenes, selenophenes, tellurophenes and pyrroles has been widely explored and a range of examples is shown in Scheme 97. The rhodacycles are readily generated from the appropriate dialkyne and tris(triphenylphosphine)rhodium chloride. Replacement of the rhodium by sulfur, selenium or tellurium is effected by direct treatment with the element, replacement by oxygen using m-chloroperbenzoic acid and by nitrogen using nitrosobenzene. 

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... 

It was also found experimentally that the rhodacyclohexenes 27a and 27b can be identified by H NMR when the siloxy-substituted VCP 14 is treated with a stoichiometric amount of [RhCKCO lz (Scheme 16). Addition of DMAD to the mixture of rhodacycles 27a and 27b gives the expected cycloheptadiene 28.47... 

Intermolecular [4+2]-cycloaddition of vinylallenes with alkynes is efficiently mediated by means of an electronically tuned rhodium catalyst (Scheme 16.81) [91]. A five-membered rhodacycle is formed from the vinylallene. Coordination followed by insertion of an alkyne to the rhodacycle generates a seven-membered rhodacycle, from which rhodium(I) is eliminated reductively to produce a cyclohexatriene, leading to the aromatic compound. 

Murakami and colleagues132 studied the Diels-Alder reactions of vinylallenes with alkynes catalyzed by a rhodium complex. When a vinylallene lacking substituents at the vinylic terminus was reacted with a terminal alkyne, 1,3,5-trisubstituted benzenes were obtained, the reaction between vinylallene 197 and 1-hexyne (198) being a representative example (equation 55). The reaction was proposed to proceed via a rhodacycle which afforded the primary Diels-Alder adduct via reductive elimination. Aromatization via isomerization of the exocyclic double bond led to the isolation of 199. 

Equation 8.21 demonstrates another reaction (this time catalytic in the Rh complex and also using a radical scavenger to prevent polymerization of the vinyl group) where C=C insertion into an M-C bond must have occurred. Scheme 8.4 follows up on this reaction by illustrating the results of an analysis of the mechanism of the catalytic cycle that involved first oxidative addition of one of two possible C-C bonds to Rh, followed by C=C insertion. A labeling experiment using 13C indicated that an alternative rhodacyclic intermediate 11 was not involved and suggested that path a was the correct mechanism.29... 

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>. 

Rh(I)-catalyzed cyclizations via rhodacycle intermediates and its application to the synthesis of (+)-epiglobulol 07Y183. 

This reaction can also be used to obtain an annelated phenol 7. The reaction of 2 with 1 under a CO atmosphere provides the phenol 8 via a rhodacycle. 

The cobalt-mediated 6-I-2-cycloaddition of cycloheptatriene and allenes formed bicyclic cycloadducts in high yields and with an excellent E Z selectivity. Rh(I)- (g) catalysed formal intramolecular 6- -2-cycloaddition of the allenal (116) readily produced 5-8- and 6-8-fused bicyclic ketone cycloadducts (118) in excellent yields. A key intermediate in this cycloaddition is the rhodacycle (117) (Scheme 38). The TiCl4-Et2AlCl-catalysed 6-1-2-cycloaddition of 1,2-dienes and 1,3,5-cycloheptatrienes produced endo-bicyclo[4.2.1]nona-2,4-dienes in high yields (80%). ... 

Scheme 97) illustrates the proposed mechanism of this novel process. This reaction is believed to proceed through a series of metallacycles. Thus, it is clearly different from that of the CO-SiCaT reaction, which is a stepwise process involving sequential carbocyclizations. The proposed mechanism includes (i) selective coordination of the diyne moiety of enediyne 216 to the active Rh catalyst species, forming metallacycle N-I [2-1-2-l-M]) (ii) insertion of the olefin moiety of 216 into the Rh-C bond forms the fused 5-7-5 tricyclic rhodacycle N-II ([2-I-2-I-2-I-M]) (iii) coordination of CO to the Rh metal followed by migratory insertion of CO into the Rh—C bond gives 5-8-5 rhodacycle N-III or N-IV ([2-I-2-I-2-I-1-I-M]) and (iv)... 

Based on earlier work in Rh-catalyzed alkyne hydroacylation, the Tanaka group discovered that a rhodacycle generated upon oxidative addition of a 2-alkynylben-zaldehyde derivative underwent efficient dimerization in a formal [4 -h 2] annulation (Scheme 2.46) [92]. This strategy involves formal hydroacylation across one C=C... 

Since simple ketones typically coordinate more weakly to metals than olefins, many Rh-phosphane complexes may show poor activity for the hydrogenation of simple ketones. However, the catalytic properties of cyclometalated half-sandwich Rh(lII) complexes 99-101, isolated or prepared in situ firom metal precursors and optically pure secondary amines, were evaluated, among which the complex 100 showed the highest productivity in the reduction of acetophenone with a i-PrOH/t-BuOK reductant system (Fig. 26) [93]. The complex 99 afforded good yield of alcohol products from acetophenone, but the enantioselectivity was low, while the cyclometalated imine 101 was almost inactive. The rhodacycle 100, which was formed in situ, efficientiy catalysed the reduction of substrates 102 (92 % ee), 103 (93 % ee) and 104 (91 % ee), with substrate 103 showing total chemoselectivity. For substrates 105,106 and 107, however, lower ee values were observed (<85 %). 

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