Combination of 1,2‐hydroboration and 1,1‐organoboration: synthesis of novel organo‐substituted 1‐silacyclobutenes

Abstract

The reaction of di(alkyn-1-yl)silanes Me(R)Si(C C t Bu) 2 [1; R = Me (a), H (b)] with diethylborane or 9-borabicyclo[3.3.1]nonane in a 1 : 1 ratio affords the 1-silacyclobutene derivatives 6a, 7a,b as a result of intermolecular 1,1-hydroboration followed by intramolecular 1,1-organoboration. Intermediates, in which both an alkenyl and an alkynyl group are linked to silicon (2a, 3a), were identified and prepared independently by the reaction of the corresponding chlorosilane 5a with the lithium alkynide LiC C t Bu. This procedure also opened the way to a stepwise synthesis of 1-silacyclobutenes with a choice of substituents in the 2,4-positions (6b and 9a). All reactions were monitored by 29 Si NMR spectroscopy, and the structural assignment of the final products was based on multinuclear magnetic resonance data ( 1 H, 11 B, 13 C and 29 Si NMR).

Synthesis of 1‐silacyclopent‐2‐ene derivatives using 1,2‐hydroboration, 1,1‐organoboration and protodeborylation

“…[25] Again the proposed structures of these derivatives are supported by NMR spectroscopy, and all the important data are summarized in the Experimental section.…”

Synthesis and Characterization of Spirosilanes – 1,2‐Hydroboration and 1,1‐Carboboration

“…[1][2][3] With silicon, heterocycles such as 1-silacyclobutenes, [4][5][6] 1-silacyclopent-2-enes, [7,8] 1-silacyclohex-2-enes, [7,9] and silole derivatives are particularly noteworthy. [10][11][12] Addressing 1-silacyclopent-2-enes, numerous derivatives bearing functionalities such as Si-organyl, Si-Cl, and Si-H at a chiral silicon atom have become available (Figure 1, example A).…”

“…tions appears to be versatile in synthesis. [4][5][6][7][8][9]20,21] In the present work, further examples are given to emphasize the simple and general route to 5-silaspiro [4,4]nona-1,6-diene derivatives. Furthermore, a route to other types of spirosilanes containing the 1-silacyclopent-2-ene unit is explored.…”

Synthesis and structure of novel spirosilanes. Combination of 1,2‐hydroboration and 1,1‐organoboration

“…This intramolecular process requires less harsh reaction conditions, as has been shown previously for the stepwise synthesis of siloles, [2] and by combining 1,2-hydroboration and 1,1-organoboration for the synthesis of 1-silacyclopent-2-ene [3,4] and 1-silacyclobutene derivatives. [5,6] The 1,2-hydroboration of alkyn-1-ylsilanes is particularly attractive in this context since it is highly regiospecific [7 -9] if the alkyn-1-yl group bears a substituent other than hydrogen (Scheme 1). A bulky alkyl group at the C C bond, such as the t Bu group, or an aryl group, induces complete regiospecifity if 9-borabicyclo[3.3.1]nonane (9-BBN) [10,11] is used as hydroborating reagent.…”

“…Intermediates of type A have already been detected in reaction solutions. [5] Although, the presence of a second alkyn-1-yl groups as in A invites a second 1,2-hydroboration, provided that there is sufficient 9-BBN available, the reactions can be controlled to allow for the desired intramolecular 1,1-organoboration leading to B. This rearrangement proceeds via a borate-like intermediate [1] as in A, in which the dashed line indicates an interaction of the electron-deficient boron atom with the alkynyl carbon atom attached to silicon.…”