Secondary "C/"C isotope effects (As') over one bond on the '"Sn nuelear shielding have been determined for 42 organotin compounds. Changes in the magnitude and sign of As' are related to the nature of the carbon atom and the nature of the other substitnents at the tin atom. In addition to the rotational and vibrational effects connected with the Sn--"C bond itself, Asi is affected by the bonding of tin to the other substitnents. In the case of polar bonds this leads to deshieiding of the tin atom in the heavy isotopomer, which compensates the shielding effect normally associated with heavy isotopic substitntion. Therefore, the magnitnde of As' valnes and of the Sn-C bond distances are not related in a simple manner. A relationship exists between J(lfgsnwC) and As' in alkynyltin compounds. The As'(=C/l2C) values for "Si and 207Pb shielding in some representative organo-silanes and -plnmbanes are reported for comparison.

Elimination of H2 from the imidazole-borane adduct 1 leads to a mixture of the imidazaboles 2 and 2'. The symmetrical isomer 2 (two HzBNC fragments) is isolated in pure state and characterized by an X-ray structure analysis (monoclinic, space group P2'). 2-(Trimethylstanny1)thiazole (3a) and 1-methyl-5-(trimethylstannyl)-l,2,4-triazole (3b) react with rize all final products. triethylborane to form adducts 4 from which, in the presence of an excess of Et3B, tetraalkyltin is elimated to give the thiazabole 5a and the triazabole 5b, respectively. Multinuclear 'H-, 'B-, I3C-, I4N-, and l19Sn-NMR spectroscopy serve for following the reactions (compounds 3 to 5) and to characte- In this paper we show that elimination of H2 from the Nmethylimidazole-borane adduct lL4] affords a mixture of the imidazaboles 2 and 2' [eq. (2)]. Another route to azaboles starts with 2-(trimethylstanny1)thiazole (3a) or 1-methyl-5-(trimethylstannyl)-1,2,4-triazole (3b), which are first converted to the triethylborane adducts 4 as intermediates. Elimination of tetraalkyltin leads to the azaboles 5 [eqs. (3) and (4)]. All compounds have been characterized by multinuclear NMR data ('H-, "B-, 13C-, 14N-, and l19Sn NMR), and crystals suitable for an X-ray structure analysis have been obtained from the imidazabole 2. Results and DiscussionThe conversion of the borane adduct 1 into the imidazaboles 2 and 2' requires harsh reaction conditions [eq. (2)] and a catalyst. The catalyst induces intermediate formation of BH21 adducts which react with 1 to form the C(2)-B bond and HI thus closing the catalytic cycle. The compounds 2 and 2' are formed as colorless, air-stable, highmelting solids in an approximate ratio of 2: 1. The amount of 2 increases on heating the melt for several hours to 320°C. This is accompanied by some decomposition. Isomerization of pyrazaboles derived from 3-methylpyrazole has been observed previously, and a sigmatropic shift of the boryl group in the monomer has been assumed to explain the Fast isomerization of monomeric (3,5-di-tertbutylpyrazoly1)boranes has been firmly If monomeric boranes are also involved in the isomerization of 2 and 2', one has to consider the relative stability of the 2-substituted 1-methylimidazole B and the carbene derivative or a zwitterionic type of structure Dt5b36bl. The reaction between B and C would afford 2' whereas two molecules of either B or C would recombine to yield 2. Pure samples of 2 are obtained by crystallization from CH2C12 (suitable for X-ray structure analysis) or from sublimation of the reaction mixture.

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Bis(triorganylstannyl)chalcogenides (O, S, Se) have been studied by 119Sn NMR in solution. In most cases the magnitude of the geminal coupling constants 2J(SnSn) are indicative of a bent arrangement SnXSn (X = O, S, Se). The exception is bis(tri-tert-butylstannyl)oxide for which the linear structure S n - O - S n as determined by X-ray diffraction in the solid state is preserved in solution. The trend of the coupling constants 2J(SnSn) depending on X and on the bond angle SnXSn suggests that 2J(SnOSn) > 0 and 2J(SnSSn), 2J(SnSeSn) <0. The X-ray diffraction study of bis(tribenzylstannyl)sulfide shows a bent arrangement (∢SnSSn 105.52(14)°) in contrast to the corresponding oxide.

Abstract Organoboration, Heterocycles, Multinuclear NMR 1,1-Organoboration of diethylamino-dimethyl-(1-propynyl)stannane (6), formed by an exchange reaction between the bis(propynyl)-(4) and bis-(diethylamino)dimethylstannane (5), with trimethyl-(2a) or triethylborane (2b) leads to highly reactive new heterocyclic compounds (7). Both, the coordinative N→B and the weak Sn-N bond are readily attacked by H-acidic compounds. Ammonia or primary amines are used as examples. Multinuclear NMR (1H, 11B, 13C, 15 N, 119Sn) data serve for the characterization.