Cyclopropyl carbene complexes

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

Fig. 2.32. Possible mechanism of the formation of cyclopentenones and cycloheptadienones from alkoxy(cyclopropyl)carbene complexes [373],...

The formation of cycloheptadienones from alkoxy(cyclopropyl)carbene complexes and alkynes (Entry 5, Table 2.24) [388,389] proceeds essentially by the same mechanism as the Dotz benzannulation reaction (see Figure 2.32). The cyclopropyl group participates in the electrocyclic rearrangement as the equivalent of a vinyl group. 

A different strategy is utilized to prepare Fischer-type cyclopropylmethoxycarbenium complexes. [LM C=(OMe)(c-Pr)]+ by alkylation of cyclopropylcarbonyl complexes c-PrC(0)ML (vide supra). Thus, treating acyl complex FpCO(c-Pr), obtained from the reaction of NaFp [Fp = Cp(CO)2] and c-PrCOCI, with Me,0+BF4 in CH2C12 gives cyclopropylmethoxycarbenium fluoroborate complex salt [Fp=C(OMe)(c-Pr)]BF4 (equation 68) which can be further reduced to the corresponding Schrock-type cyclopropyl carbene complex (vide infra, Section II.C.2)142. 

The more bulky 1-ethoxycyclopropyl group also induced a complete Z-diastereoselec-tivity to the Michael reaction of the ethynyl carbene complex with dimethylamine (equation 179)246. This is due to the sterically favored arm-position acquired by the bulky group in the intermediate. Unlike the parent cyclopropyl carbene complex, which gave only... 

Cyclopentenones have been also obtained from alkoxy(cyclopropyl)carbene complexes and alkynes via a cyclopentadienone intermediate. [85] The intramolecular version of this type of reaction has been studied intensively. 

Addition of organolithium compounds, RLi, to carbene compounds of structure 71 results in the formation of p-substituted aldehydes with high e.e. (80-97%). 1,5-addition of iodine to the cyclopropyl carbene complexes, [(OC)5Cr=C(SPh)(PrC)], proceeds with a high degree of stereo-and regio-control to form an alkenyl iodide. 5... 

The presence of a cyclopropyl moiety in the carbene complexes makes them useful for synthesis. The cyclopropylcarbene complexes 95 undergo a cycloaddition reaction with alkynes to give the cyclopentenones 96 [51]. The reaction course is explained as being metallacyclopentene fragmentation. (Scheme 34)... 

A variety of geminally substituted cyclopropyl ethers are synthesized employing Fischer carbene complexes [(CO)5M=CR (OR2) M = Cr, Mo, W R1 = alkyl, alkenyl and aryl] as alkoxycarbene sources. Electron-deficient alkenes and conjugated dienes are suitable substrates for the reaction (equation 108)237-245. Electron-rich enol ethers and enamines are also... 

Cyclopropylcarbene complexes of the type L M=C(XR )R2 (X = O, S R1 = alkyl, aryl R2 = cyclopropyl) having a stabilizing heteroalkyl (XR1) group on the electrophilic carbene ligand (Scheme 3) have found widespread application in organic synthesis. These so-called Fischer carbene complexes are best known via their group 6 transition metal carbonyl complexes (CO)5M=(OR )R2(M = Cr, Mo, W)132. Much less abundant are the Schrock-type cyclopropylcarbene complexes L M=CR R2 where no heteroatom is bound to the carbene carbon atom133. 

The rearrangement exhibits some stereochemical preference for c/s-vinyl carbene complex (with respect to the metal) compared to the //ww-isomer. Thus, 2-methyl-2-m-vinyl cyclopropyl (methoxy) carbene chromium pentacarbonyl rearranges to 5-methyl-5-vinyl-2-methoxycyclopentenone approximately 4 times faster (THF, 52 °Q than the trans-isomer, which in turn rearranges faster than phenyl derivatives. This suggests that vinyl complexes undergo initial Cope-type rearrangement to form metallacycloheptadienes, which then rearrange to jt-allyl complexes. Subsequent CO insertion and reductive elimination leads to the vinylcyclopentenones (equation 89)150. 

The proposed mechanism of the ring cleavage reaction of HCI (and other protic acids) with cyclopropyl carbynyl complexes involves addition of HQ across the carbyne triple bond to give a carbene complex as key intermediate. In the absence of a carbonyl ligand this is followed by ring expansion to a metallacyclopentene complex, /J-hydrogen elimination and reductive elimination to the diene complex (equation 111)164. 

Intermolecular trapping of carbene complexes (39) has also been reported. In this case, a second carbene (41) arising from a cyclopropyl shift from (39) can also be trapped to yield (42).39... 

Similarly, thermolysis of cyclopropyl(5-phenyl-4-pentynoxy)carbene complexes of molybdenum and tungsten gave via intramolecular coupling the corresponding 3,4,5,7-tetrahydro-5-phenylcyclohepta[ ]pyran-6(2//)-one (equation inter-... 

XIV) and, in small amounts, CpFe(C0)2S(0)2CH2CH=CH2 is believed to involve the intermediacy of the dipolar carbene complex (XV), which results from electrophilic attack of SO2 at the cyclopropyl ring. 

---