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Hydrogallation Reactions Involving the Monoalkynes H5C6‐C≡C‐SiMe3 and H5C6‐C≡C‐CMe3 – cis/trans Isomerisation and Substituent Exchange Phenyl‐trimethylsilylethyne, H5C6‐C≡C‐SiMe3, reacted with different dialkylgallium hydrides, R2Ga‐H (R = Me, Et, nPr, iPr, tBu), by the addition of one Ga‐H bond to its C≡C triple bond (hydrogallation). The gallium atoms attacked selectively those carbon atoms, which were also attached to trimethylsilyl groups. The cis arrangement of Ga and H across the resulting C=C double bonds resulted only for the sterically most shielded di(tert‐butyl)gallium derivative, while in all other cases spontaneous cis/trans rearrangement occurred with the quantitative formation of the trans addition products. The diethyl compound Et2Ga‐C(SiMe3)=C(H)‐C6H5 (2) gave by substituent exchange the secondary products EtGa[C(SiMe3)=C(H)‐C6H5]2 (7, Z,Z) and Ga[C(SiMe3)=C(H)‐C6H5]3 (8). Interestingly, compound 8 has two alkenyl groups with a Z configuration, while the third C=C double bond has the cis arrangement of Ga and H (E configuration). The reversibility of the cis/trans isomerisation of hydrogallation products was observed for the first time. tert‐Butyl‐phenylethyne gave the simple addition product, R2Ga(C6H5)=C(H)‐CMe3 (9), only with di(n‐propyl)gallium hydride.

Treatment of trimethylsilylethynylbenzene derivatives with HGaCl(2) afforded products, [C(6)H(6-x){C(H)=C(SiMe(3))GaCl(2)}(x)], in which by a very fast cis/trans-rearrangement the Ga and H atoms occupied opposite sides of the resulting C=C double bonds. The stability of the cis-forms considerably increased upon application of 1,3-dibromo- and pentafluorophenylalkyne derivatives. Two pairs of cis/trans-isomers could be characterized by crystal structure determinations and allow the direct comparison of structural parameters. For the first time an equilibrium was detected between cis- and trans-forms in solution. Treatment of 1,4-di(tert-butylalkynyl)benzene with HAlR(2) (R = CMe(3), CH(2)CMe(3)) afforded cyclophane-type molecules by the release of AlR(3). Only the neopentyl derivative could be isolated and characterized by crystal structure determination. In contrast, the dibromo compound, 1,4-Br(2)-2,5-(Me(3)CC[triple bond]C)(2)C(6)H(2), yielded the simple addition product, C(6)H(2)Br(2){C(AlR(2))=C(H)CMe(3)}(2) (R = CMe(3)). Condensation was hindered in this case by intramolecular Al-Br interactions. Surprisingly, the simple addition product was also isolated from the reaction of 1,4-(Me(3)CC[triple bond]C)(2)C(6)H(4) with the relatively small hydride HAlEt(2). Solid-state NMR spectra of the product revealed strong intermolecular Al-C interactions involving the negatively charged terminal vinylic carbon atoms, to give one-dimensional coordination polymers.

2-(Acyloxy)ethenyl)carbene complexes (CO)5M=C(OEt)-CH=C(OCOR)Ph [(Z)-5] (M = Cr, W; R = Ph, p-Me2NC6H4, CH3) c-C7H7CH2, PhCH=CPh, Me2C=CH, 1,4-C6H4) are obtained by the addition of carboxylic acids R-C02H (4) to alkynylcarbene complexes (CO)sM=C-(OEt)-C=CPh (1) (M = Cr, W) in the presence of Et3N at 20 °C in 71-78% isolated yields. The reaction is regio-and stereochemically uniform. (Z)-5g (R = PhCH=CPh), C3iH220eW, was characterized by X-ray diffraction. It crystallizes in space group PI with cell parameters a = 10.381(6) Á, b = 11.444(6) Á, c = 13.509(7) Á, a = 107.84(4)°, ß = 91.54(4)°, = 108.49-(4)°, Z = 2, Ri = 0.0354, and wR2 = 0.0811. Ligand disengagement from (Z)-5 with tertbutyl isocyanide (8b) at 20 °C results in the formation of [2-(acyloxy)ethenyl]ketene imines i-BuN=C=C(OEt)-CH=C(OCOR)Ph [(Z)-ll] (>95% yields).Alkenylcarbene complexes (CO)sM=C-C=C of chromium and tungsten have gained much interest recently as C3 building blocks for the synthesis of car-bocyclic2 and heterocyclic3 rings. Though this class of compounds has been known since 1967,4 systematic studies have been initiated only recently. Complexes (CO)sM=C-C=C(OR1) of types A and B deserve particular attention due to their structural relationship to ß-keto ester equivalents of the enol ether (R1 = alkyl) and enol ester type (R1 = acyl), respectively (Chart 1). Enol Ethers 3Complexes (CO)5M=C-C=C[0(alkyl)] (M = Cr, W) of the enol ether type are accessible by several methods (Scheme 1): (a) by the condensation of methylcarbene complexes (CO)5M=C(OEt)CH3 with dimethylformamide or other nonenolizable acid amides R1-CONR2 (eq l),5

Aminolysis of the 2H-pyran-2-ylidenetungsten complex 3 affords amino-1-tungsta-1,3,5-hexatrienes with different structures, depending on the reaction temperature and the type of amine involved. Addition of primary amines RNH2 (4a − d) (R = allyl, n-Bu, CH2Ph, i-Pr) to 3 at −15 °C yields salt-type (3Z)-6-amino-1-tungsta-1,3,5-hexatrienes 5a − d by reversible ring opening of the pyranylidene ring. Addition of 4a − c at 20 °C affords (3Z)-2-amino-1-tungsta-1,3,5-hexatrienes 6a − c by an irreversible ring opening of the pyranylidene ring. Secondary amines RR1NH 8a − c [RR1N = Me2N, 2-(hydroxymethyl)pyrrolidine, pyrrolidine] undergo an irreversible ring opening to 2-amino-1-tungsta-1,3,5-hexatrienes 9a − c. Aminolysis of 3 in the presence of Me3SiCl/Et3N affords acetimino pyranylidene complexes 10, from which (3Z)-2,6-diamino-1-tungsta-1,3,5-hexatrienes 11 are derived upon aminolysis. Thermolysis of 6a in THF/pyridine affords the captodative 1-amino-1,3-hexadien-5-one 12. 2H-Pyran-2-ylidene complex 3, C19H12O7W, was characterized by X-ray diffraction.