Rhodium, alkynylation

In the reaction of the rhodium alkynyl complexes 7 with excess carbon monoxide at room temperature, the cyclobutenone 8 is isolated in 90 % yield... 

Vinyl boranes add to conjugated ketones in the presence of a rhodium catalyst (with high asymmetric induction in the presence of BINAP) 7 Alkynyl-boranes also add to conjugated ketones, in the presence of... 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

A different approach towards titanium-mediated allene synthesis was used by Hayashi et al. [55], who recently reported rhodium-catalyzed enantioselective 1,6-addition reactions of aryltitanate reagents to 3-alkynyl-2-cycloalkenones 180 (Scheme 2.57). In the presence of chlorotrimethylsilane and (R)-segphos as chiral ligand, alle-nic silyl enol ethers 181 were obtained with good to excellent enantioselectivities and these can be converted further into allenic enol esters or triflates. In contrast to the corresponding copper-mediated 1,6-addition reactions (Section 2.2.2), these transformations probably proceed via alkenylrhodium species (formed by insertion of the C-C triple bond into a rhodium-aryl bond) and subsequent isomerization towards the thermodynamically more stable oxa-jt-allylrhodium intermediates [55],... 

Scheme 2.57 Rhodium-catalyzed enantioselective 1,6-addition of aryltitanium reagents to 3-alkynyl-2-cycloalkenones 180 (ee values referto the corresponding allenic enol pivalates).

An intramolecular allenylidene-alkynyl coupling was also observed in the reaction of the mixed alkynyl-allenylidene rhodium(I) complex 73 with carbon monoxide (Scheme 25). In this case, the initially formed thermally unstable allenyl derivative 74 evolved into the metallated cyclobutenone 75 when an excess of CO was present [276]. 

As would be expected, catalytic hydroboration is effective for alkynes as well as al-kenes, and prior examples have been reviewed [6]. An interesting development has been the diversion of the normal syn- to the anti-addition pathway for a terminal alkyne, with 99% (catechoborane) and 91% (pinacolborane) respectively (Fig. 2.5) [20]. The new pathway arises when basic alkylphosphines are employed in combination with [Rh(COD)Cl]2 as the catalyst in the presence of Et3N. Current thinking implies that this is driven by the initial addition of the rhodium catalyst into the alkynyl C-H bond, followed by [1,3]-migration of hydride and formal 1,1-addition of B-H to the resulting alkylidene complex. The reaction is general for terminal alkynes. 

The Ojima group has extended their studies of silylformylation to include more complex substrates, such as alkenyne, dialkyne, alkynyl nitrile, and ethynyl pyrrolidinone. Use of rhodium or rhodium-cobalt metal complexes catalyzes the silylformylation of these substrates with high chemoselectivity, as the other functionalities present are inert to the reaction.122b,c d... 

Ketones and aldehydes, including activated and enolizable substrates and those containing alcohol or carboxylic acid substituents, can be alkynylated using a rhodium(II) catalyst complexed with a bulky phosphine.194... 

The rhodium-catalysed addition of alkynes to 1,2-diketones, 1,2-keto esters, and aldehydes has provided a method for the synthesis of tertiary alkynyl alcohols under... 

Indenyl)rhodium(I) complexes, preparation, 7, 184—185 Indium complexes acid halide reactions, 9, 683 in alkene and alkyne allyindations, 9, 693 alkyl, aryl, alkynyls, 3, 288 in alkynylations, 9, 720... 

For the synthesis of optically pure building blocks we mainly focused on the synthesis of protected noncoded (R)- and (S)-amino acids, as they can be synthesized reliably in enantiomerically pure form with a large variety of side chains using asymmetric hydrogenation of a-amino-a, 3-didehydroamino acids using cationic diphosphine rhodium catalysts.216,217 As a typical example of a reactophore we present a-alkynyl ketones, which is a representative bis-acceptor molecule. In Scheme 5 are depicted some of the many synthetic applications of acetylenic ketones in heterocyclic synthesis, which have great potential for combinatorial and parallel organic synthesis. 

Vaska s complex, the square planar iridium(I) compound shown in equation 145, readily interacts with alkynyl(phenyl)iodonium triflates in toluene at room temperature21. Such reactions proceed with loss of iodobenzene and deliver octahedral Ir(III) complexes possessing (7-alkynyl and trifluoromethanesulfonato ligands in a Jnms-relationship. The rhodium(I) analog of Vaska s complex behaves in a similar way21. 

Since it is known that halo(phenyl)acetylenes add oxidatively to Vaska s complex to give (7-phenylethynyl iridium(III) halides, 32112, the intervention of phenyliodonium iridium(III) and rhodium(III) intermediates, 33, in the alkynyliodonium reactions seems plausible. In any case, the production of cr-alkynyl complexes with alkynyl(phenyl)-iodonium triflates appears to be both more general and efficient21. 

Alkynyl-2-diazo-3-oxobutanoates (391) when treated with catalytic amounts of rhodium(II) acetate were found to produce 1,3-dihydrofuro[3,4-c]furans (392) in good yield. The reaction proceeds by addition of a rhodium-stabilized carbenoid on to the alkynic n bond to give a vinyl carbenoid (393) which subsequently cyclizes on to the neighboring carbonyl group to produce the furan ring <93JOC2l>. 

Brummond et al. disclosed that acyclic allenynes could be efficiently converted into tetracyclic compounds via consecutive rhodium-catalyzed Alder-ene and double Diels-Alder cycloaddition reactions [47]. The former reaction transforms alkynyl allenes such as 45 into triene-ynes (46) using rhodium biscarbonyl chloride dimer. 

In the same vein, Schmalz has proposed a facile construction of the colchicine skeleton by a rhodium-catalyzed cyclization/cycloaddition cascade [56]. A TMS group has to be introduced on the alkyne moiety of 66 in order to avoid participation of the relatively acidic alkynyl hydrogen atom in undesired proton transfers. The resulting 6,7,7 of 67a and 67b architecture was assembled in a remarkably diastereoselective manner (14 1) and in satisfactory yield (Scheme 30). 

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