E. V. Klimkina

1‐Boraadamantane: Reactivity Towards Di(1‐alkynyl)silicon and ‐tin Compounds: First Access to 7‐Metalla‐2,5‐diboranorbornane Derivatives

1-Boraadamantane (1) reacts with di(1-alkynyl)silicon and -tin compounds 2 (Me2M(C...CR)2: M=Si; R=Me (a), tBu (b), SiMe3 (c); M=Sn, R=SiMe3 (e)) in a 1:1 ratio by intermolecular 1,1-alkylboration, followed by intramolecular 1,1-vinylboration, to give siloles 5a-c and the stannole 5e, respectively, in which the tricyclic 1-boraadamantane system is enlarged by two carbon atoms. Owing to the high reactivity of 1, a second fast intermolecular 1,1-alkylboration competes with the intramolecular 1,1-vinylboration as the second major step in the reaction if the substituent R at the C...C bond is small (2a) and/or if the M-C... bond is also highly reactive, as in 2d (M=Sn, R= Me) and 2e (M=Sn, R=SiMe3). This leads finally to the novel octacyclic 7-metalla-2,5-diboranorbornane derivatives 8a, 8d, and 8e, of which 8e was characterized by X-ray analysis in the solid state. 1,1,2,2-Tetramethyldi(1-propynyl)disilane, MeC...C-SiMe2SiMe2-C...CMe (3), reacts with 1 to give mainly a 1,2-dihydro-1,2,5-disilaborepine derivative 9 and the octacyclic compound 11, which is analogous to 8a but with an Me4Si2 bridge. All new products were characterized in solution by 1H, 11B, 13C, 29Si, and 119Sn NMR spectroscopy. For 8 and 11, highly resolved 29Si and 119Sn NMR spectra revealed the first two-bond isotope-induced chemical shifts, 2delta10/11B(29Si) and 2delta10/11B(119Sn) respectively, to be reported.

1-Boraadamantane: reactivity towards mono-1-alkynyltin, -germanium and -silicon compounds — synthesis of 4-methylene-3-borahomoadamantanes

The reaction of 1-boraadamantane 1 with 1-alkynyltin (3), -germanium (4), and -silicon compounds (5) leads to enlargement of the tricyclic system by formation of 4-methylene-3-borahomoadamantanes (6–9). These are 1,1-organoboration reactions which proceed by cleavage of the MC bond (M=Sn, Ge, Si). There is evidence for 1,1-deorganoboration which apparently take place much more readily than for non-cyclic analogues, most likely as the result of the strained tricyclic system. When 2-ethyl-1-boraadamantane (2) is used, again 3-borahomoadamantanes are formed, the isomers 15–18. The product distribution is sensitive to steric effects. However, it appears that the BC(H)Et bond in 2 is slightly more reactive than the BCH2 bonds. All products were characterised by 1H-, 11B-, 13C-, 29Si- and 119Sn-NMR.

Preparation of trans-2-allyl-6-alkyl(aryl)-1,2,3,6-tetrahydropyridines by reductive trans-2,6-dialkylation of pyridine. Synthesis of (±)-epidihydropinidine

Abstract A convenient method for the preparation of the title compounds involving the sequential treatment of pyridine with RLi ( R = Alk , Ar ), triallylborane and methanol is developed.

Reactivity of mono-1-alkynyltin and -germanium compounds towards triallylborane

Abstract Triallylborane, All3B (4), reacts with trialkyl(1-alkynyl)tin compounds 1 R3SnCCR1 [R=Me, R1=Me (a), tBu (b), Ph (c), SiMe3 (d), SnMe3 (e)] and 2 (R=Bu, R1=ferrocenyl) and also with 1-phenylethynyl(trimethyl)germanium (3c) preferably by 1,1-allylboration to give the organometallic-substituted alkenes 6, 8 and 10. In the cases of 1b and 1d, allyl/alkynyl exchange takes place instead. However, the formation of the alkene 6e was observed at −30°C. In the case of 1c, 1,2-allylboration, leading to the alkene 7c, competes with 1,1-allylboration, the ratio 6c/7c being dependent on the polarity of the respective solvent (more of 6c in a more polar solvent). All3B proved to be much more reactive than triethylborane, Et3B (5). All products were characterised by 1H, 11B, 13C and 119Sn NMR.

trans-cis-Isomerization of trans-2-allyl-6-alkyl(aryl)-1,2,3,6-tetrahydropyridines on heating with triallylborane. Synthesis of (±)-dihydropinidine

Abstract A convenient method for isomerization of trans-2-allyl-6-alkyl(aryl)-1,2,3,6-tetrahydropyridines into the corresponding cis-isomers is presented.

Reductive 1,2‐Allylboration of Indoles by Triallyl‐ and Triprenylborane − Synthesis of 2‐Allylated Indolines

Indoles undergo reductive α-allylation upon treatment with allylic boranes (triallyl- and triprenylborane) to give, after deboronation, the corresponding 2-allylated indolines in 70−85% yield. 1,2-Addition of the allylboron fragment to heterocycles occurs with full rearrangement of the allylic moiety. Reductive prenylboration of 3-substituted indoles, as well as allylboration of 3-isopropylindole, with All3B proceed stereoselectively to produce trans-2,3-disubstituted indolines only, while similar reactions of triallylborane with 3-R-indoles, containing a primary group R, afford a mixture of trans (86−92%) and cis isomers (8−14%). From 1-deuterioindole and triallylborane, a mixture of cis- and trans-2-allyl-3-deuterioindole in a ratio of 1:1 was obtained. Proposed mechanism of the general reaction involves intermediate formation of 3H-indole tautomers followed by fast allylboration of the C=N bond. Structures of trans-indolines 3b and 3c were confirmed by X-ray analysis.

Reductive allylation of pyrrole with allylboranes. Synthesis oftrans- andcis-2,5-disubstituted pyrrolidines

Pyrrole undergoes reductive mono- and diallylation on successive treatment with β,γ-unsaturated organoboron derivatives (triallylborane, allyl(dipropyl)borane, and triprenylborane) and alcohols to give 2-allylated 3-pyrrolines andtrans-2,5-diallylated pyrrolidines. The addition of both the first and second boron-allylic fragment to the heterocycle proceeds with rearrangement. A method for transformation of thetrans-2,5-diallylpyrrolidine into thecis-isomer (heating with triallylborane at 190 °C) was developed and a series ofN-substituted derivatives of these pyrrolidines was synthesized. A method for the preparative synthesis of nonsymmetrically substitutedtrans- andcis-2-alkyl(phenyl)-5-allylpyrrolidines, based on reductive allylboration of pyrrole followed by 1,2-addition of RLi to the 5-allyl-1-pyrroline that formed, was also developed. A direct confirmation of intermediate formation of 2H- and 3H-pyrrole tautomers under the action of allylboranes was obtained. The adduct of 2H-pyrrole with BF3 was detected by NMR spectroscopy.

Preparation oftrans- andcis-2-allyl-6-alkyl(aryl)-1,2,3,6-tetrahydropyridines based on the reductivetrans-2,6-dialkylation of pyridine. Synthesis of (±)-epidihydropinidine and (±)-dihydropinidine

A general method for the preparation of unsymmetricaltrans-2-allyl-6-alkyl(aryl)-1,2,3,6-tetrahydropyridines6 based on a combination of 1,2-addition of RLi to pyridine andtrans-6-allylation with triallylborane in the presence of methanol was elaborated. It was shown thattrans-piperideines (6 (R=Alk, Ph) isomerize into the correspondingcis-2-allyl-6-alkyl(phenyl)-3-piperideines14 on heating with triallylborane followed by deboronation of aminoborane (16) with methanol and an alkali. The stereochemistry of compounds6 and14 was determined by two-dimensional NOE spectroscopy. A possible mechanism of the formation oftrans-amines6 and their transformation intocis-isomers14 is discussed. Alkaloids (±)-epidihydropinidine (trans-2-methyl-6-propylpiperidine2a, 70%) and (±)-dihydropinidine (cis-2-methyl-6-propylpiperidine1d, 71%) were synthesized by hydrogenation of compound6a (R=Me) and14a (R=Me), respectively, over Raney nickel.

Stereoselective synthesis of (+)-indolizidines 167B and 209D and their trans-isomers based on the reductive allylboration of pyridine

A general method for the synthesis of 5-substituted indolizidines based on intramolecular cyclization oftrans- andcis-2-allyl-6-R-1,2,3,6-tetrahydropyridines, obtained from pyridine and triallylborane, has been elaborated. The closure of the five-membered ring is carried out by hydroboration-oxidation followed by cyclization of the resulting δ-amino alcohols in the presence of the Ph3P−CBr4−Et3N system. (Pr2BH)2 and Pr3B are used as the hydroborating reagents, and H2O2 in an acid medium is used for the oxidation of 2-[3-(dipropylboryl]-Δ2-piperideines formed. This method has been used for the synthesis of two natural alkaloids: indolizidine 209D (cis-5-hexylindolizidine) and itstrans-isomer were prepared fromcis- andtrans-2-allyl-6-hexyl-1,2,3,6-tetrahydropiridine, respectively; indolizidine 167B andtrans-5-propylindolizidine were synthesized fromcis- andtrans-2,6-diallyl-1,2,3,6-tetra-hydropyridine, respectively.

Reductivetrans-1,3-dialkylation of isoquinoline on treatment with RLi and triallylborane

The preparative synthesis oftrans-1-alkyl(aryl)-3-allyl-1,2,3,4-tetrahydroisoquinolines based on the 1,2-addition of RLi to isoquinoline andtrans-allylboration is described.