5f, 5g, 6f, 8f, 9g, log, l l g ) of which only 9 g (R = Ph) corresponds to the spirosilanes and -germanes. This is the result of effective competition between intramolecular and intermolecular organoboration in the case of the tetra-1-alkynyltin compounds. The protodeborylation of l a , b and 2d, e with MeCOzH leads to 12a, b and 13d, e, respectively. 12a isomerises by UV irradiation to ally1 isomers 12a'. From 12a with 2 equiv. of maleic anhydride the 1:2 addition compound 14a is obtained, the autaddition of 12a, d leads to 15a, d. Spiro compound 12a reacts with (OC)5Fe or CpCo(C,H,), to give the cyclodiastereomeric q4-complexes [(OC)3Fe]z-12a (16al-a,; X-ray structure analysis of meso16a1), (OC)3Fe-12a (17a) and (CpCo),-12a (18al-a4). -All products were characterized by multinuclear NMR, including measurements of the coupling constants 1J(13C13C), 2J(zgSizgSi), nJ(29Si13C) and nJ("gSn'3C).

Organo‐Substituted Siloles by Twofold Organoboration of Di‐1‐alkynylsilanes Me2Si(CCR)2 [RMe (A), Bu (B), tBu (C), iPent (D), Ph (E), Me3Si (F)], prepared from Me2SiCl2 and MCCR (MLi, Na, K), react with Et3B by 1,1‐ethyloboration to form the organo‐substituted siloles Me2 (1a – f) in high yields with different reaction rates: F > B ≈︁ D > A > C > E. Me2Si(C CiPent)(C  CPh) (H) and Et3B give a 5 : 1 mixture of the two regioisomeric substituted siloles 2 and 2′. – 1a and e are deborylated with H2NCH2CH2OH or MeCO2H to 3a and e, respectively. 1a reacts with maleic anhydride to yield the [4 + 2] addition compound 4a. With (OC)5Fe, Cp‐Co(C2H4)2, and (CDT)Ni the cycloracemic (ligand)transition metal η41‐complexes [(OC)3Fe‐1a (5a), (OC)3Fe‐3a (6a), CpCo‐3a (7a)] and the cyclodiastereomeric η4,η41‐complexes Ni‐(3a)2 (8a/8a′) are formed. All products were characterized by multinuclear NMR, including measurements of J(29Si13C) and J(13C13C).

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Diethynyl(dimethyl)silane, Me2Si(C≡C-H)2 2, reacts with triethylborane, Et3B 1, by 1,1- ethylboration to give a silole, 3-diethylboryl-4-ethyl-1,1-dimethyl-sila-2,4-cyclopentadiene 3, as a short-lived intermediate which reacts further with 2 or dimerises via [4+2]cycloadditions to the 7-silanorbornadiene 4 or the 7-silanorbornene derivatives 5, respectively. Since the silole 3 was not detected by NMR, the analogous reactions were studied using 1-alkynyl(ethynyl)- dimethylsilanes, Me2Si(C≡C-H)C≡C-R 6 [R = Bu (a), iPent (b), tBu (c)]. In these cases, 1,1-ethylboration gave the siloles 7a,b,c and 7'a,b, now detected in the reaction solutions, and also 7-silanorbornadienes as [4+2]cycloaddition products. Major products were identified, and the progress of the reactions was monitored by NMR spectroscopy (1H, 11B, 13C and 29Si NMR).