Abstract:pm-l.' It should nevertheless be noted that the radiationless decay rate of S2 (1, R = H), k,, z 2 X 10" s-l, calculated from = ( 5 f 1) X lo4 and k , z 1 X lo8 s-l, is over 2 orders of magnitude larger than that predicted from an energy gap relation empirically parametrized for polyacene~.~' Inasmuch as this correlation may be considered valid for the heterocycles 34 and 1, this discrepancy may be taken as an indication for the presence of an additional n r * state(s) between the rr* states SI and "S2". Not s… Show more
“…Thereby, the two considerably short SiC bond distances would be probably attributed to a coulombic stabilization resulting from the strongly polarized C1‐Si1‐C2 fragment ( D ; Scheme ). A similar alternation of charges, with a weak allylic resonance contribution, was theoretically predicted for the 2‐sila‐allyl anion,19 as well as for the related bis(methylene)phosphoranes 20…”
Since the first isolation of a stable silene by Brook et al. in 1981, [1] the chemistry of these species featuring a siliconcarbon double bond has been the subject of many studies. [2] The high reactivity of silenes I is mainly due to the SiC double bond, which is naturally polarized towards carbon (Si d+ =C dÀ ), as C is more electronegative than Si. Therefore, the electrophilic silicon center can be stabilized by Lewis bases, leading to adducts II. [3] However, silenes with a reversed SiC bond polarization, arising from the presence of p-donating substituents at C (III [1, 4,5] and IV), [6] or to the fulvalene-type structure V, [7] show a considerably enhanced stability and an elongated SiC double bond. It is clear that the substitution pattern considerably influences the reactivity and the stability of silenes derivatives.Despite intense research efforts over the past three decades, only a few types of stable silenes are available.Particularly, although there are several C-substituted silenes by p-donors III and IV, [6] including anionic 2-silenolates VI, [8] very few studies on related Si-substituted silenes VII have been realized. Only one example of a silene featuring two potentially p-donors on silicon (VIII) has been isolated. However, in this silyl-anion-Si-substituted silene VIII, the pinteraction between the substituents and the silene moiety is not significant. [9] Herein, we report a synthesis of a stable and isolable silene 2 (type VII) by an isomerization of silacyclopropylidene 1. This silene 2 substituted at silicon by two strong p-donating substituents, such as amino-and phosphonium ylide groups, possess unique properties and is an excellent ligand for transition metals.We recently reported the synthesis of the base-stabilized sila-cyclopropylidene 1.[10] Despite its strained cyclic structure, 1 is quite stable and can be easily handled at room temperature under inert conditions. However, simply by heating a few hours a toluene solution of 1 at 150 8C, under pressure, an original isomerization, involving a ring-opening reaction of sila-cyclopropylidene fragment, led to the formation of silene 2, which was isolated as orange crystals in 76 % yield (Scheme 1). The same result was obtained by the photolysis of 1 (l = 300 nm) in toluene solution at room temperature. The stereoselectivity of the rearrangement was indicated by the presence of only one singlet in 31 P NMR at d = 27.4 ppm. The 29 Si NMR spectrum exhibits a doublet signal at d = 74.7 ppm ( 2 J PSi = 24.0 Hz), which is within the range of values previously observed for silenes (d = 41-145 ppm). [2a] However, the resonances observed for the silene carbon (d = 60.9 ppm, 3 J CP = 6.1 Hz), and the C À H proton (d = 3.15 ppm), appear at significantly higher field compared to those observed for other silenes, in 13 C and 1 H NMR, respectively. [11] These NMR data clearly suggest an enhanced polarization of the silene moiety Si=C towards the carbon atom, which is probably due to the presence of the p-donating substituents on the silicon atom...
“…Thereby, the two considerably short SiC bond distances would be probably attributed to a coulombic stabilization resulting from the strongly polarized C1‐Si1‐C2 fragment ( D ; Scheme ). A similar alternation of charges, with a weak allylic resonance contribution, was theoretically predicted for the 2‐sila‐allyl anion,19 as well as for the related bis(methylene)phosphoranes 20…”
Since the first isolation of a stable silene by Brook et al. in 1981, [1] the chemistry of these species featuring a siliconcarbon double bond has been the subject of many studies. [2] The high reactivity of silenes I is mainly due to the SiC double bond, which is naturally polarized towards carbon (Si d+ =C dÀ ), as C is more electronegative than Si. Therefore, the electrophilic silicon center can be stabilized by Lewis bases, leading to adducts II. [3] However, silenes with a reversed SiC bond polarization, arising from the presence of p-donating substituents at C (III [1, 4,5] and IV), [6] or to the fulvalene-type structure V, [7] show a considerably enhanced stability and an elongated SiC double bond. It is clear that the substitution pattern considerably influences the reactivity and the stability of silenes derivatives.Despite intense research efforts over the past three decades, only a few types of stable silenes are available.Particularly, although there are several C-substituted silenes by p-donors III and IV, [6] including anionic 2-silenolates VI, [8] very few studies on related Si-substituted silenes VII have been realized. Only one example of a silene featuring two potentially p-donors on silicon (VIII) has been isolated. However, in this silyl-anion-Si-substituted silene VIII, the pinteraction between the substituents and the silene moiety is not significant. [9] Herein, we report a synthesis of a stable and isolable silene 2 (type VII) by an isomerization of silacyclopropylidene 1. This silene 2 substituted at silicon by two strong p-donating substituents, such as amino-and phosphonium ylide groups, possess unique properties and is an excellent ligand for transition metals.We recently reported the synthesis of the base-stabilized sila-cyclopropylidene 1.[10] Despite its strained cyclic structure, 1 is quite stable and can be easily handled at room temperature under inert conditions. However, simply by heating a few hours a toluene solution of 1 at 150 8C, under pressure, an original isomerization, involving a ring-opening reaction of sila-cyclopropylidene fragment, led to the formation of silene 2, which was isolated as orange crystals in 76 % yield (Scheme 1). The same result was obtained by the photolysis of 1 (l = 300 nm) in toluene solution at room temperature. The stereoselectivity of the rearrangement was indicated by the presence of only one singlet in 31 P NMR at d = 27.4 ppm. The 29 Si NMR spectrum exhibits a doublet signal at d = 74.7 ppm ( 2 J PSi = 24.0 Hz), which is within the range of values previously observed for silenes (d = 41-145 ppm). [2a] However, the resonances observed for the silene carbon (d = 60.9 ppm, 3 J CP = 6.1 Hz), and the C À H proton (d = 3.15 ppm), appear at significantly higher field compared to those observed for other silenes, in 13 C and 1 H NMR, respectively. [11] These NMR data clearly suggest an enhanced polarization of the silene moiety Si=C towards the carbon atom, which is probably due to the presence of the p-donating substituents on the silicon atom...
“…As a result of (a) the weak (subfrontier orbital) interaction of the s orbital (fragment B) with n + and (b) valence angle compression PSiP, the frontier orbitals a 2 (HOMO), a 1 , and b 1 (LUMO) come close to each other. As a consequence of this narrowing of the frontier orbitals, a second-order Jahn−Teller interaction becomes operative, inducing symmetry reduction to C 2 or C s symmetry, as shown in Scheme . 6 Interaction diagram for the formation of the Li complex (C = A − B) from the allylic fragment (A) with a Li atom (B).…”
Trichlorosilanes RSiCl3 (R = t-Bu, Mes, Cp*, Is,
OAr) reacted with a 4-fold excess of lithium
phosphanide ArPHLi (Ar =
2,4,6-tBu3C6H2) to
form 1,3-diphospha-2-silaallylic complexes
3a−f. From the latter, the corresponding
free allylic anions could be liberated by
complexation of the lithium cation with [15]-crown-[5], as could be
established on the basis
of NMR spectra and X-ray structures. In addition, the bonding in
the Li complexes was
further investigated using ab initio calculations of double-ξ
quality. The influence of cation
solvation on the complex geometry was explored as well. The
chemical reactivity of 1,3-diphospha-2-silaallylic anions with electrophiles (E =
H+, ClPR‘R‘‘) was investigated. 1,3-Diphospha-2-silapropenes 4a−d and
1,3,4-triphospha-2-silabutenes-(1) 5a−e and
6a were
prepared.
“…A similar alternation of charges, with a weak allylic resonance contribution, was theoretically predicted for the 2-sila-allyl anion, [19] as well as for the related bis(methylene)phosphoranes. [20] The effects of p-donating substituents on silene fragment are totally different depending on their positions (Si or C atom). Indeed, one or two p-donors, such as amino and phosphonium ylide groups, on the carbon atom considerably alter both structural and electronic properties of the corresponding silenes (4, 5 in Table 1).…”
International audienceA stable and isolable silene with two π-donating groups on the silicon center has been synthesized. This silicon-substituted silene shows totally different π-substituent effects compared to those of related carbon-substituted silenes, and it presents intrinsic silene properties with an enhanced electron-donating character, making it an excellent ligand for transition metals
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