Alessandro Gambaro

Allylbutyltin halides (CH2CHCH2)SnBu3−nCln (n = 0, 1, 2, 3). Preparation, carbon-13 NMR characterization and allylstannylation ability towards ketones and aldehydes

Abstract Mixed allylbutyltin halides (CH 2 CHCH 2 )SnBu 3-n Cl n ( n = 0–3) have been prepared, and characterized by carbon-13 NMR spectroscopy. Their ability to bring about allylstannylation of ketones and aldehydes, to form organostannoxy compounds, Bu 3-n SnCl n OC(R′)(R″)CH 2 CHCH 2 , has been shown to increase on increasing the value of n , that is on increasing the acceptor ability of the tin centre.

Allylstannation. II. A total "cis-preference" in the addition of n-Bu2ClSnCH(CH3)CHCH2 to aldehydes

Abstract 1-Buten-3-yldi-n-butylchlorotin, formed by redistribution of ( E Z )-2-butenyltri-n-butyltin and Bu2SnCl2, reacts readily with neat RCHO (R  C2H5, C2H5(CH3)CH, (CH3)2CH, (CH3)3C and C6H5) to give high yields (80–100%) of alcohols of the type RCH(OH)CH2CHCHCH3 only in the Z-configuration. This appears to be the first example of total “cis-preference” in the addition of Grignard-like reagents to carbonyl compounds. The results are discussed in terms of steric requirements around the tin centre which is probably five-coordinate in the transition state.

Allylstannation : I. stereochemistry of the addition of trans/cis-2-butenyl-chloro-di-n-butyltin to aldehydes

2-Butenyl-chloro-di-n-butyltin, in various trans/cis ratios, reacts readily with neat RCHO (R = CH3, C2H5, C2H5(CH3)CH, (CH3)2CH, C6H5) at 25°C to give mixtures of threo/erythro-α-methylallylcarbinols in high yields. The same mixtures are obtained from the equilibrated mixtures obtained by redistribution between Bu3SnC4H7 (C4H7 = trans-, cis-crotyl and α-methylallyl group) and Bu2SnCl2. The reactions are characterized by a high degree of stereoselectivity, especially when bulky R groups are present. The complete allylic rearrangement and the stereoselectivity indicate that an exacyclic transition state is involved. Two stereochemically different transition states lead to two diastereoisomers, threo- and ertythro-α-methylallylcarbinol in the enantioforms RS, SR and RR, SS, respectively.

Reversible crotylstannation of carbonyl compounds. Crotylstannation ability of Bu3-nClnSnCH2CHCHCH3 (n = 0, 1, 2) compounds towards acetone and benzaldehyde and 13C NMR characterization

Abstract The crotylstannation reaction: has been found to be reversible. The compounds trans/cis- Bu 3- n Cl n SnCH 2 CH= CHCH 3 ( n = 0, 1, 2) have been prepared by elimination reactions of organostannoxy compounds,Bu 3− n Cl n Sn—O—C(Me)(i-Pr)CH(Me)CHCH2, which were synthesized by means of transalkoxylation between Bu 3- n CI n Sn(OMe) compounds and threo/erithro -2,3,4-trimethyl-5-hexen-3-ol. Under the conditions used the elimination occurs stereospecifically and with complete allylic rearrangement. The ability of the organostannoxy compounds to yield crotyl butylchlorotins via elimination increases in the order, Bu 3 Sn—O—C 2 ClSn—O—C 2 Sn—O—C. In the addition reactions, the sequence of increasing reactivity is Bu 3 SnCrot 2 SnCrot 2 BuSnCrot (Crot = crotyl). The 13 C NMR spectra of the compounds made reveal that the chemical shifts of the allylic carbon atoms are related to the inductive effects of the chloro-substituents.

Metal-stabilized carbanions

Abstract The rate of deprotonation of fluorene and of π-(tricarbonylchromium)-flourene by an excess of KH in THF was measured by monitoring the hydrogen evolution. The pseudo-first order rate constant for the complex is ca. one order of magnitude higher than that for the free ligand. 1 H and 13 C NMR analysis showed that when the anion is produced at -20°C or below, the Cr(C0) 3 group is bonded to one of the phenyl rings (η 6 -anion), whereas ionization at room temperature produces solutions containing mainly the anion with the Cr(CO) 3 bonded to the cyclopentadienyl ring (η 5 -anion) in equilibrium with the η 6 -isomer. The effect of ionization on free and complexed systems, together with the effect of complexation of the free anion, is discussed on the basis of the NMR data. The η 6 π η 5 isomerization equilibrium was followed at various temperatures and different degrees of solvation are deduced for the two isomeric ion pairs from the kinetic and thermodynamic solvation parameters.

Synthesis and structure of tricarbonylchromium mono-, bis- and tris-complexes of 10-methyltribenzotriquinacene

Complexation of 10-methyltribenzotriquinacene (MTBT) a bent triarene of C3v symmetry afforded six mono-, bis- and tris-Cr(CO)3complexed isomers which have been isolated and identified. The results of various synthetic experiments indicate that complexation at the convex side of a free benzene ring is favoured, probably for steric reasons. As compared with the isomer complexed at the convex face (anti), that complexed at the concave one (syn) is less stable. The 1H and 13C NMR spectral data together with the X-ray structures of the two bis- and one tris-complexes account for the lower stability of the syn isomers.

Metal-stabilized carbanions: VI. Formation of some arylmethyl and dibenzotropenyl carbanions complexed with tricarbonylchromium. A kinetic and A 13C NMR study☆

Abstract Rate constants have been measured for the α-metallation of some arylmethanes, of 5 H -dibenzo[ a,d ]cycloheptene and its dihydro derivative, free and complexed with tricarbonylchromium, by potassium hydride in tetrahydrofuran in the presence of 18-crown-6 ether. The stable solutions of the complexed anions have been studied by 13 C NMR spectroscopy. With the free ligands the kinetic acidity depends upon the structure to a great extent, but in the corresponding complexes the rates of metallation do not vary appreciably. On the basis of the 13 C chemical shift data a close similarity is suggested between the electronic structures of the arylmethyl and dibenzotropenyl anions which would account for their very similar metallation rates.

Allylstannation: VI. Allylation and allenylation of aldehydes and ketones by allyl- and allenyl-tin chlorides in the presence of water

Abstract Allyl- and propargyl-carbinols can be readily prepared in one pot reaction, in the presence of water, by addition of allyl- or allenyl-tin chlorides to carbonyl compounds. The reactions of Bu3-n(CH2CHCH2)SnCln, Bu3-n(CH3CHCHCH2)SnCln (n = 1, 2) and Bu2(CH2CCH)SnCl and the carbonyl compounds RCHO (R = H, C2H5, (CH3)2CH, (CH3)3C, (E)-CH3CHCH, C6H5) and RCOR′ (R = R′ = CH3 and R = CH3 and R′ = (CH3)2CH) have been examined. The monochloroorganotin derivatives undergo the following quantitatvive reactions: Bu2(CH2CHCH2)SnCl + RCOR′ + 1 2 H2O → R(HO)C(R′)CH2CHCH2 + 1 2 [Bu2SnCl]2O↓ (1) Bu2(CH2CHCH2)SnCl + RCOR′ + 1 2 H2O → R(HO)C(R′)CH2CHCH + 1 2 [Bu2SnCl]2O↓ (2) Reaction (2), which involves the allenic substrate, is characterized by rearrangement of the unsaturated chain, the propargylic carbinol being isolated as the major product (∼ 90%).

Indenyl hapticity in (η-indenyl-RhL2) and Cr(CO)3(μ-η:η-indenyl-RhL2) complexes. A 1H, 13C and 103Rh NMR spectroscopic study

A series of indenyl- and (Cr(CO)3)indenyl-RhL2 complexes (L2COD, (CO)2) bearing substituents on both the six- and five-membered ring have been synthesized and fully characterized, and their 1H, 13C and 103Rh NMR spectra recorded. The changes of the spectral parameters caused by the introduction of the Cr(CO)3 unit suggest significant modifications of the electronic distribution in the indenyl moiety induced in the ground state. The increased reactivity in the ligand exchange reactions (‘extra-indenyl effect’) and the strong modifications of the catalytic and spectroscopic properties of the Rh center itself indicate a substantial weakening of the coordinative bond between rhodium and the indenyl moiety in the heterobimetallic species as expected on going from an η5 towards a more pronounced η3 coordination mode.

Allylstannation V. cis-stereoconvergent sysnthesis of homoallylic alcohols and 4-chloro-2,6-dialkyl-3-methyltetrahydro-pyrans by the addition reaction of 1-buten-3-yldichloro-n-butyltin and aldehydes

Abstract 1-Buten-3-yl-n-butyldichlorotin, generated in situ by redistribution of ( E / Z )-2-butenyltri-n-butyltin and BuSnCl 3 , reacts readily with neat RCHO (R = CH 3 , C 2 H 5 , (CH 3 ) 2 CH) at 25°C to give linear alcohols RCH(OH)CH 2 CHCHCH 3 and/or 2,3,4,6-tetrasubstituted tetrahydropyrans, CH 2 CH(R)OCH(R)CH(CH 3 )CH-(Cl), which are mainly in the cis -configuration with respect to the CH(CH 3 )-CH(Cl) bond. When R = (CH 3 ) 3 C and C 6 H 5 , only the homoallylic alcohols are obtained. These cis -stereoconvergent syntheses are explained in terms of kinetic control of the formation of adducts obtained by insertion of one or two aldehyde molecules into the organotin substrate.