Brian L. Booth

Metal carbonyl chemistry : XIV. Reactions of rhodium carbonyls with ligands☆

Abstract Dodecacarbonyltetrarhodium [Rh 4 (CO) 12 ] reacts with phosphine and phosphite ligands under an atmosphere of nitrogen to give the compounds Rh 4 (CO) 10 L 2 , Rh 4 (CO) 9 L 3 and Rh 4 (CO) 8 L 4 [where L = Ph 3 P or P(OCH 2 ) 3 CEt (ETPO) 1 ] in which the metal atom cluster is retained. Under similar conditions hexadecacarbonyl-hexarhodium [Rh 6 (CO) 16 ] reacts with Ph 3 P, P(OCH 2 ) 3 CEt or P(OMe) 3 to give complexes of the type Rh 6 (CO) 10 L 6 . When the reaction of either Rh 4 (CO) 12 or Rh 6 (CO) 16 with the ligands Ph 3 P or Ph 3 As is carried out in the presence of carbon monoxide, breakdown of the metal clusters occurs to give the bridged compounds L 2 (CO)Rh(CO) 2 Rh(CO)L 2 (where L = Ph 3 P or Ph 3 As) in high yields; reaction of Rh 4 (CO) 12 with n-Bu 3 P under similar conditions gives the compound [(n-Bu 3 P) 3 -(CO)Rh] 2 . Dodecacarbonyltetrarhodium has also been shown to react with PhC 2 Ph or CF 3 C 2 CF 3 to give the compounds Rh 4 (CO) 10 (RC 2 R) (R = CF 3 or Ph). Some of these ligand-substituted rhodium carbonyls have been shown to be extremely active hydroformylation catalysts for 1-alkenes.

Cyclotrimerization of acetylenes catalyzed by (η5-cyclopentadienyl)rhodium complexes

Abstract The comparative rates of cyclotrimerization of dimethylacetylenedicarboxylate (DMAD) and hex-3-yne have been studied with the catalysts [η5-C5R5)Rh(COD)] (R = H, Me, Cl; COD = cycloocta-1,5-diene), [(η5-C5R5)Rh(CO)2] (R = H, Me), [(η5-C5Me5RhL2] (L = C2H4, PF3) and [(η5-C5H4PPh3)Rh(CO)2]+ PF6− under pseudo-first-order conditions. In these reactions, which all obey the rate law, rate = kobs [acetylene], the rate of cyclotrimerization of DMAD decreases as the π-acceptor strength of the cyclopentadienyl ligand increases, and this trend is reversed in reactions of hex-3-yne. The rates of cyclotrimerization also appear to vary with the nature of the ligand L (where L = C2H4, CO, PF3, 1 2 COD) implying that L remains bound to the rhodium atom throughout the catalytic cycle. Preliminary studies have shown that in some cases selective co-oligomerization occurs with mixtures of two different acetylenes using the catalysts [(η5-C5H5)Rh(COD)] and [(η5-C9H7RheCOD)].

Silica supported cyclopentadienyl-rhodium(I), -cobalt(I), and -titanium(IV) complexes

Abstract The silylated cyclopentadiene derivative, (MeO) 3 Si(CH 2 ) 3 C 5 H 5 , synthesised from commerically-available (MeO) 3 Si(CH 2 ) 3 Cl, has been used to prepare the complexes [η 5 -(MeO) 3 Si(CH 2 ) 3 C 5 H 4 ]Rh(CO) 2 , [η 5 -(MeO) 3 Si(CH 2 ) 3 C 5 H 4 ]Rh(COD) (COD = cyclo-octa-1,5-diene), and [η 5 -(MeO) 3 Si(CH 2 ) 3 C 5 H 4 ] 2 TiCl 2 . The complex [η 5 -(MeO) 3 Si(CH 2 ) 3 C 5 H 4 ]TiCl 3 , prepared by reaction of NaC 5 H 4 (CH 2 ) 3 Si(OMe) 3 with TiCl 4 (1/1 molar ratio) and also by reaction of [η 5 -(MeO) 3 Si(CH 2 ) 3 C 5 H 4 ]Ti(OEt) 3 with CH 3 COCl, proved to be very unstable. Attempts to synthesise the complex [η 5 -(MeO) 3 Si(CH 2 ) 3 C 5 H 4 ](η 5 -C 5 H 5 )TiCl 2 , either by reaction of [η 5 -(MeO) 3 Si(CH 2 ) 3 C 5 H 4 ]TiCl 3 with NaC 5 H 4 or reaction of (η 5 -C 5 H 5 )TiCl 3 with NaC 5 H 4 (CH 2 ) 3 Si(OMe) 3 , gave none of the expected product and only (η 5 -C 5 H 5 ) 2 TiCl 2 could be isolated from these reactions. The cyclo-octadiene rhodium complex supported on silica has been shown to be an efficient cyclotrimerization catalyst, and the silica-supported titanium complex (SIL-(CH 2 ) 3 C 5 H 4 ) 2 TiCl 2 is, after reduction with butyllithium, an efficient and selective catalyst for the hydrogenation of alk-1-enes.

Synthesis of 9-hydroxyalkyl-substituted purines from the corresponding 4-(C-cyanoformimidoyl)imidazole-5-amines

The amino alcohols HO(CH2)n NH2(n= 2, 3 and 5) react readily with ethyl (Z)-N-(2-amino-1,2-dicyanovinyl)formimidate 5 to give the amidines 6a-c, which cyclize in the presence of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) to give the corresponding 4-(cyanoformimidoyl)imidazole-5-amines 7a-c, which can be isolated in the cases where n= 2 or 3.In the presence of aldehydes and ketones, the imidazoles 7a-d lead to the 6-carbamoyl-1,2-di-hydropurines 9a-f which, in some cases, are oxidised to the corresponding 6-carbamoylpurines.The reaction of the imidate 5 with 2-methoxyethylamine leads to the amidine 6d and, on treatment with DBU, the reactive imidazole 7d which can be used directly for further reaction.

Synthesis of 5-amino-4-(cyanoformimidoyl)-1H-imidazole: a reactive intermediate for the synthesis of 6-carbamoyl-1,2-dihydropurines and 6-carbamoylpurines

5-Amino-4-(cyanoformimidoyl)-1H-imidazole (3) has been prepared in good yield by the base-catalysed cyclisation of (Z)-N-(2-amino-1,2-dicyanovinyl)formamidine. Compound (3) reacts with ketones, R1COR2[R1= R2= Et, Me, Bu, –(CH2)5–, PhCH2; R1= Me, R2= Ph] togive2,2-disubstituted-6-carbamoyl-1,2-dihydropurines as the major products, together with minor amounts of compounds believed to be novel 7-amino-1-carbamoyl-3,3-disubstituted 3H-imidazo[1,5-c]imidazole derivatives, which have been isolated when R1= R2= Et, Bu, and PhCH2; when R1= R2= Ph the only product isolated is tentatively assigned the imidazo[1,5-c]imidazole structure. In the reaction with acetylacetone the 1,2-dihydropurine intermediate is unstable and loses acetone to give 2-methyl-6-carbamoylpurine. The aldehydes RCHO [R = Me, Et, (E)-MeCHCH–, c-C4H4O] also react readily with (3) at room temperature to give the corresponding 6-carbamoyl-1,2-dihydropurine derivatives, which can be isolated when R = Me or Et; these oxidise in solution to afford the corresponding 6-carbamoylpurines.

Nickelocene chemistry : II. New methylidyne trinickel cluster compounds

Abstract The new methylidene trinickel cluster complexes, [RCNi 3 (η 5 -C 5 H 5 3 ] (R  CMe 3 or SiMe 3 ) and [Me 3 SiCNi 3 (η 5 -C 5 H 5 )2(η 5 -C 5 H 4 CH 2 SiMe 3 )] have been isolated in low yield from reactions between nickelocene and the corresponding alkyllithium reagents, RCH 2 Li. The compounds [RCNi 3 (η 5 -C 5 H 5 ) 3 ] (R  Ph, CMe 3 or SiMe 3 ) have also been obtained by treatment of the σ-alkylnickel complexes [(η 5 -C 5 H 5 )Ni(CH 2 R)(PPh 3 )] with n-BuLi in the presence of an excess of nickelocene, but under similar conditions [(η 5 -C 5 H 5 )Ni(CH 2 C 1O H 7 -2)-(PPh 3 )] (where C 1O H 7 -2  2-naphthyl) failed to give [2-C 1O H 7 CNi 3 (η 5 -C 5 H 5 ) 3 ]. The attempted synthesis of [(η 5 -C 5 H 5 )Ni(CH 2 CCH)(PPh 3 )] from [(η 5 -C 5 H 5 )-NiBr(PPh 3 )] and CHCCH 2 MgBr gave only [(η 5 -C 5 H 5 )Ni(CCMe)(PPh 3 )] by an unusual rearrangement reaction.

Reactions of hydridocarbonylstris(triphenylphosphine)rhodium with alkynes

Abstract The hydride, RhH(CO)(PPh 3 ) 3 , reacts with the alkynes RC 2 R (where = CO 2 Me, CO 2 Et, CO 2 H or C 6 H 5 ), to give stable adducts of the type RCH=CR-Rh-(CO)(PPH 3 _ 2 . Cleavage of the adducts where R = CO 2 Me or C 6 H 5 with anhydrous HCl gives dimethyl fumarate and trans -stilbene respectively, indicating that these adducts have a trans -configuration about the double bond. The similar reaction of the hydride with perfluoro-1-butyne also gives a stable, crystalline adduct, CF 3 -CH=C(CF 3 )-Rh(CO)(PPH 3 ), 2 , which has been shown by NMR spectroscopy to have a cis -configuration, implying that a different mechanism operates in this reaction.

Some reactions of 3,4-bis(trifluoromethyl)furan and its precursor, 2,3-bis(trifluoromethyl)-7-oxabicyclo[2,2,1]hepta-2,5-diene: novel isocoumarin formation from thermal reaction of the furan with ethyl propynoate

Abstract Thermolysis at 450 °C of 2,3-bis(trifluoromethyl)-7-oxabicyclo[2,2,1]hepta-2,5-diene ( 2 ) gives 3,4-bis(trifluoromethyl)furan ( 1 ). Treatment of oxanorbornadiene ( 2 ) with triflic acid (in CH 2 Cl 2 under reflux) and then with water affords a mixture of 2,3-bis(trifluoromethyl)phenol ( 4 ) (6%), tris-(6-trifluoromethylsalicylide) ( 5 ) (45%) and 2-trifluoromethyl-6-hydroxybenzoic acid ( 6 ) (16%); analogous reaction with conc. sulphuric acid gives only the phenol 4 (44%). Diels-Alder reaction either of furan 1 with dimethyl acetylenedicarboxylate (DMAD) or of dimethyl 3,4-furandicarboxylate with hexafluorobut-2-yne gives the oxanorbornadiene ( 8 ) (ca. 80%). In contrast, reaction of furan 1 with ethyl propynoate (EP) (1:1 molar ratio) at 150 °C affords a mixture of ethyl 6,7-bis(trifluoromethyl)isocoumarin-3-carboxylate ( 12 ) (52%) and its 4-carboxylate isomer 13 (9%), presumed to arise via a novel Diels-Alder cycloaddition of the dienophile EP to the CCCO system of the 1:1 adduct ethyl 2,3-bis(trifluoromethyl)-7-oxabicyclo[2,2,1]hepta-2,5-diene-5-carboxylate ( 14 ). Diene 14 is conveniently prepared (93%) by the reaction of a large excess of hexafluorobut-2-yne with ethyl 3-furoate at room temperature, and when heated with EP (1:1 molar ratio) at 150 °C, the isocoumarin 12 (84%) is obtained; at 60 °C, however, 14 decomposes to give furan 1 and EP.

Metal carbonyl chemistry IV. The preparation of cobalt and rhodium carbonyls by reductive carbonylation with pentacarbonyliron

Abstract Octacarbonyldicobalt has been obtained in 40% yield from the reaction of cobaltous chloride and pentacarbonyliron; other cobalt salts are less effective in this reaction. The similar reaction of either anhydrous RhCl3 or [Rh(CO)2Cl]2 with pentacarbonyliron has been found to give high yields of Rh6(CO)16 even at atmospheric pressure at the reflux temperature of methanol. Carbonylation of [Rh(CO)2Cl]2 with carbon monoxide in the presence of Ullmann copper-bronze powder or silver can be controlled to give either Rh6(CO)16 or Rh4(CO)12 almost quantitatively.

(Z)-N3-(2-amino-1,2-dicyanovinyl)formamidrazone: a precursor in the synthesis of 1,5-diaminoimidazoles and 6-carbamoyl-1,2-dihydropurines

The reaction of ethyl (Z)-N-(2-amino-1,2-dicyanovinyl)formimidate with hydrazine monohydrate leads to (Z)-N3-(2-amino-1,2-dicyanovinyl)formamidrazone in almost quantitative yield. This compound proved to be an important starting material for the synthesis of the corresponding N1-isopropylidene-and N1-acetyl-formamidrazones.The amidrazones prepared promptly cyclize in the presence of base to give a 1,5-diamino-4-cyanoimidazole or a 1,5-diamino-4-(cyanoformimidoyl)imidazole, depending on the reaction conditions and mainly on the nature of the base used to induce cyclization.1,5-Diamino-4-(cyanoformimidoyl)imidazole was treated with carbonyl compounds and the 1,2-dihydropurines thus isolated indicated that the amino group in position 5 is more reactive than that in position 1.