Synthesis of silenyllithiums Li(R'3Si)Si═C(SiR3)(1-Ad) via transient silyne-silylidene intermediates

The first two lithium silenides, Li(tBu(2)MeSi)Si═C(SiMetBu(2))(1-Ad) (1) and Li(tBuMe(2)Si)Si═C(SiMetBu(2))(1-Ad) (2) were prepared by THF addition to the corresponding lithium-silenolates, [(tBu(2)MeSi)(2)Si═C(OLi)(1-Ad)]·(R(3)SiLi) (3a: R(3)Si═tBu(2)MeSi, 3b: R(3)Si═tBuMe(2)Si). 1 and 2 were crystallized, and their structures were determined by X-ray crystallography. This process requires the presence of both coaggregated silyllithium (R(3)SiLi) (3a and 3b) and THF. Based on reaction products and DFT calculations, it is suggested that elimination of tBu(2)MeSiOLi from 3a (or 3b) produces first the corresponding silyne intermediate which rearranges to the corresponding silylidene, which is then trapped by R(3)SiLi giving 1 (or 2).

Room-temperature dissociation of 1,2-dibromodisilenes to bromosilylenes

A room-temperature dynamic equilibrium between dibromodisilenes and bromosilylenes has been demonstrated by taking advantage of the steric protection using the fused-ring bulky 1,1,3,3,5,5,7,7-octa-R-s-hydrindacen-4-yl (Rind) groups. Although the bromosilylenes cannot be directly observed by spectroscopic methods, the thermal homolytic cleavage of the Si═Si double bond has been confirmed by a pseudo-first-order kinetics for the trapping with bis(trimethylsilyl)acetylene and a crossover reaction using two kinds of Rind-substituted dibromodisilenes. The addition of 4-pyrrolidinopyridine (PPy) to the dibromodisilene leads to an equilibrium mixture between the dibromodisilene and a PPy adduct of bromosilylene, the latter being isolated and characterized. The substitution of the bromine atom in the dibromodisilene by the Grignard reagent is significantly accelerated by the addition of PPy.

Acetylene-expanded dendralene segments with exotopic phosphaalkene units

Bis-TMS protected C,C-diacetylenic phosphaalkene (A(2)PA) 1 (Mes*P=C(C≡CTMS)(2); Mes* = 2,4,6-tBu(3)Ph) has been used as a building block for the construction of butadiyne-expanded dendralene fragments in which phosphaalkenes feature as exotopic double bonds. Treatment of 1 with CuCl gives rise to a Cu(I) acetylide that is selectively formed at the acetylene trans to the Mes* group. The cis-TMS-acetylene engages in similar chemistry, albeit at higher temperatures and longer reaction times. The differentiation between the two acetylene termini of 1 allows for the controlled synthesis of the title compounds by a variety of different Cu- and Pd-catalyzed oxidative acetylene homo- and heterocoupling protocols. Crystallographic characterization of A(2)PA 1 and dimeric Mes*P=C(C≡CR(1))C(4)(R(2) C≡C)C=PMes* (3b, R(1) = R(2) = Ph; 6, R(1) = R(2) = TMS), and 10 (R(1) = R(2) = C≡CPh) verifies that the stereochemistry across the P=C bond is conserved during the coupling reactions, whereas spectroscopic evidence reveals cis/trans isomerization in an iodo-substituted A(2)PA intermediate 4 (Mes*P=C(C≡CTMS)(C≡CI). UV/Vis spectroscopic and electrochemical studies reveal that efficient π conjugation operates through the entire acetylenic phosphaalkene framework, even in the cross-conjugated dimeric structures. The P centers contribute considerably to the frontier molecular orbitals of the compounds, thereby leading to smaller HOMO-LUMO gaps than in all-carbon-based congeners. Phenyl- and/or ethynylphenyl substituents at the A(2)PA framework influence the HOMO and LUMO to a varying degree depending on their relationship to the Mes* group, thus enabling a fine-tuning of the frontier molecular orbitals of the compounds.